Water treatment system and method for operating same

Abstract

A water treatment system 1 comprises: a tank 2 for storing to-be-treated water; a water treatment means 3, 4 including a membrane separation device 3 and an ion exchange device 4; a pretreatment means 13 for pretreating the to-be-treated water supplied to the water treatment means 3, 4; a water supply line L1 that extends from the tank 2 to the water treatment means 3, 4 via the pretreatment means 13; a first reflux line L9 that causes the treated water obtained by the water treatment means 3, 4 to circulate and reflux to the tank 2; a second reflux line L21 that branches off from the first reflux line L9 so as to connect to the water supply line L1 on the downstream side of the pretreatment means 13, and causes the treated water flowing in the first reflux line L9 to reflux to the pretreatment means 13; and a discharge line L12 that is connected to the water supply line L21 on the side opposite to the side where the second reflux line L21 is connected to the pretreatment means 13, and discharges, to the outside, the treated water refluxed to the pretreatment means 13.

Description

Water treatment system and method for operating same

[0001] The present invention relates to a water treatment system and a method for operating the same.

[0002] Conventionally, known apparatus for producing pure water (such as purified water or water for injection) for use in pharmaceutical manufacturing and the like includes a combination of a membrane separation apparatus that separates raw water, such as industrial water, well water, or city water, into permeate and concentrate, and an electrodeionization water production apparatus that produces deionized water (pure water) by passing the permeate from the membrane separation apparatus through an ion exchanger. Because such pure water production apparatuses produce pure water for use in pharmaceutical manufacturing and the like, they are periodically sterilized by passing hot water at 60°C or higher through the system to reduce the number of viable bacteria in the system in order to meet the requirements of the Japanese Pharmacopoeia.

[0003] One method for sterilizing the above-mentioned pure water production system involves passing hot water through the membrane separation system and the electrodeionized water production system simultaneously. This method has the advantage of enabling more efficient sterilization by reducing the amount of water used and the number of steps compared to sterilizing the membrane separation system and the electrodeionized water production system separately. However, it also has the following disadvantages. During sterilization, hot water cannot be supplied to the membrane separation system at the same supply pressure as during normal operation. Therefore, the same membrane separation process as during normal operation cannot be performed, which may result in the water supply to the electrodeionized water production system not meeting the water quality standards. Patent Document 1 therefore describes a method in which pure water produced by the pure water production system is heated and passed through the system. Specifically, the method describes a method in which pure water produced by the pure water production system is temporarily stored in a raw water tank, heated, and sequentially supplied to the membrane separation system and the electrodeionized water production system, and then returned to the raw water tank, thereby circulating hot water within the system. According to this method, pure water produced in a pure water production system is used as hot water for heat sterilization, which is expected to reduce the ionic load on the electrodeionized water production system during the sterilization process.

[0004] Japanese Patent Application Laid-Open No. 2004-74109

[0005] However, in the method described in Patent Document 1, raw water flows through the water supply line from the raw water tank to the membrane separation device during the process of storing pure water in the raw water tank, resulting in the accumulation of raw water with a high ionic load. Therefore, if a pretreatment device with a relatively large water capacity, such as a water softener, activated carbon filter, or filter, is installed in the water supply line, raw water with a high ionic load will accumulate therein, increasing the total amount of ions in the system. As a result, the ionic load on the electrodeionization water production device increases during the sterilization treatment, which may adversely affect the quality of the treated water when producing pure water after the sterilization treatment. This type of water quality deterioration can also occur in ion exchange devices equipped with ion exchange resins.

[0006] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a water treatment system and an operating method thereof that suppress deterioration in the quality of treated water.

[0007] In order to achieve the above-mentioned object, the water treatment system of the present invention comprises a tank for storing water to be treated, a water treatment means including a membrane separation device and an ion exchange device, a pretreatment means for pretreatment of the water to be treated supplied to the water treatment means, a water supply line from the tank to the water treatment means via the pretreatment means, a first reflux line for circulating treated water obtained by the water treatment means and returning it to the tank, a second reflux line branching off from the first reflux line and connecting to the water supply line upstream or downstream of the pretreatment means, for returning the treated water flowing through the first reflux line to the pretreatment means, and a discharge line connected to the water supply line on the side opposite to the side where the second reflux line is connected to the pretreatment means, for discharging the treated water returned to the pretreatment means to the outside.

[0008] In addition, the method for operating a water treatment system of the present invention is a method for operating a water treatment system having a tank for storing water to be treated, a water treatment means including a membrane separation device and an ion exchange device, and a pretreatment means for pretreatment of the water to be treated that is supplied to the water treatment means, and includes the steps of supplying the water to be treated to the pretreatment means and the water treatment means from a supply source separate from the tank, and returning the treated water obtained by the water treatment means to the tank, and supplying the water to be treated to the water treatment means from a supply source separate from the tank without passing through the pretreatment means, before or after storing the treated water in the tank, and circulating the treated water obtained by the water treatment means to the pretreatment means.

[0009] According to the present invention, deterioration of the water quality of treated water can be suppressed.

[0010] 1 is a schematic configuration diagram of a pure water production system according to a first embodiment of the present invention, 2 is a schematic configuration diagram of a pure water production system according to a second embodiment of the present invention, and 3 is a schematic configuration diagram of a pure water production system according to a third embodiment of the present invention.

[0011] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Components common to the following embodiments are designated by the same reference numerals in the drawings, and redundant description will be omitted where appropriate. In this specification, the water treatment system of the present invention will be described as an example of a pure water production apparatus that produces pure water used in pharmaceutical production, specifically purified water or water for injection. In this specification, purified water refers to water obtained by purifying tap water through distillation, ion exchange, reverse osmosis, ultrafiltration, or a combination thereof, and water for injection refers to purified water that has been sterilized and conforms to pyrogen (endotoxin) testing and sterility testing. Examples of such purified water and water for injection include those specified in the Japanese Pharmacopoeia.

[0012] 1 is a schematic diagram of a pure water production system according to a first embodiment of the present invention. Note that the configuration of the pure water production system shown in the figure is merely an example and does not limit the present invention, and it goes without saying that it can be modified as appropriate depending on the purpose, application, and required performance of the system.

[0013] The pure water production system 1 sequentially treats water to be treated (raw water) to produce pure water, which is then supplied to a pure water supply system (not shown). It includes a raw water tank 2, a membrane separation system 3 as a water treatment means, and an electrodeionized water production system (hereinafter also referred to as an "EDI system") 4, also as a water treatment means. As will be described in detail later, the pure water production system 1 of this embodiment produces pure water for use in pharmaceutical production and the like, and therefore periodically performs a sterilization treatment using hot water between normal operations to reduce the number of viable bacteria in the system. The pure water production system 1 also includes a control unit (control means) 5 that performs such a sterilization treatment.

[0014] The membrane separation device 3 is a device that removes impurities from raw water to produce treated water. It has a reverse osmosis membrane (RO membrane) that separates the raw water supplied from the raw water tank 2 into concentrated water containing impurities and permeate (treated water) from which the impurities have been removed. The separation membrane of the membrane separation device 3 is not limited to an RO membrane, and can be, for example, a microfiltration membrane (MF membrane), an ultrafiltration membrane (UF membrane), or a nanofiltration membrane (NF membrane) as long as it is heat-resistant to the hot water used for heat sterilization, as described below. However, from the viewpoint of improving the quality of the treated water, it is preferable to use an NF membrane or an RO membrane, and it is more preferable to use an RO membrane. Note that the membrane separation device 3 may have multiple RO membranes, which may be connected in series, in parallel, or in a combination of series and parallel. Here, "connected in series" means that the water to be treated is sequentially treated by multiple RO membranes, and that the permeate separated by the upstream RO membrane between two adjacent RO membranes is supplied as the water to be treated to the downstream RO membrane. In this case, the permeated water separated by the upstream RO membrane is further treated by the downstream RO membrane, so that treated water of better quality can be obtained.

[0015] The membrane separation device 3 is connected to a water supply line L1 that supplies raw water to the membrane separation device 3, a permeate line L2 that circulates permeate from the membrane separation device 3, and an RO concentrate line L3 that circulates concentrated water (hereinafter also referred to as "RO concentrate") from the membrane separation device 3. The water supply line L1 is a line that runs from the raw water tank 2 to the membrane separation device 3 via a water softener 13 (described later), and is connected upstream to the raw water tank 2 via an on-off valve V1. The permeate line L2 is connected downstream to the EDI device 4, and the RO concentrate line L3 is connected downstream to the raw water tank 2. A raw water supply line L4 is connected to the raw water tank 2 via an on-off valve V2, and raw water is supplied as needed, as described later. In addition, a raw water introduction line L5 is provided between the water supply line L1 and the raw water supply line L4 to supply raw water from the raw water supply line L4 to the water supply line L1 without passing through the raw water tank 2. The raw water inlet line L5 branches off from the raw water supply line L4 upstream of the on-off valve V2 and is connected to the water supply line L1 via an on-off valve V3 downstream of the on-off valve V1. Instead of the raw water inlet line L5, another line may be provided that directly connects an external raw water supply source to the water supply line L1. A raw water discharge line L11 is connected to the water supply line L1 between the raw water tank 2 and the on-off valve V1 via an on-off valve V11.

[0016] The water supply line L1 is provided with a water pump 11, a heat exchanger 12, a water softener 13, and a pressure pump 14. The positions of the heat exchanger 12 and the water softener 13 may be reversed.

[0017] The water pump 11, together with the pressure pump 14, functions to supply raw water stored in the raw water tank 2 to the membrane separation device 3, and its rotation speed may be controlled by an inverter (not shown). The heat exchanger 12 is used to stabilize the treated water quality and throughput of the membrane separation device 3 by controlling the temperature of the raw water, which fluctuates depending on the season, to a constant temperature (e.g., 25°C), and is also used to generate hot water for thermal sterilization, as described below. The water softener 13 is filled with an ion exchange resin (e.g., a cation exchange resin, preferably a strongly acidic Na-type anion exchange resin) and functions to remove hardness components such as calcium and magnesium from the raw water supplied to the membrane separation device 3. Although not shown, a line for regenerating the ion exchange resin with saline (restoring the ion exchange capacity) is connected to the water softener 13. The pressure pump 14 may be equipped with an inverter (not shown) that controls its rotation speed, thereby adjusting the pressure of the raw water supplied to the membrane separation device 3.

[0018] An on-off valve V12 is provided in the water supply line L1 between the heat exchanger 12 and the water softener 13, and a replacement water discharge line L12 is connected downstream of the on-off valve V13. An on-off valve V31 is provided in the water supply line L1 between the water softener 13 and the pressure pump 14, and a pure water return line L21 is connected upstream of the on-off valve V32. The on-off valves V12, V13, V31, and V32 function as flow path switching means for switching the communication state between the water softener 13 and each of the lines L1, L13, and L21. The pure water return line (second return line) L21 branches off from the pure water return line L9 and is connected to the water supply line L1 downstream of the water softener 13 to return pure water flowing through the pure water return line L9 (described later) from the water supply line L1 to the water softener 13. Accordingly, the replacement water discharge line L12 is connected to the water supply line L1 upstream of the water softener 13 in order to discharge the pure water returned to the water softener 13 to the outside. However, conversely, the pure water return line L21 may be connected to the water supply line L1 upstream of the water softener 13, and the replacement water discharge line L12 may be connected to the water supply line L1 downstream of the water softener 13. Note that instead of the on-off valves V12 and V13, a three-way valve may be provided at the connection between the water supply line L1 and the replacement water discharge line L12, and instead of the on-off valves V31 and V32, a three-way valve may be provided at the connection between the water supply line L1 and the pure water return line L21.

[0019] A bypass line L22 equipped with an on-off valve V33 is connected to the water supply line L1. The bypass line L22 connects the upstream side of the connection portion of the water supply line L1 with the replacement water discharge line L12 (specifically, the upstream side of the on-off valve V12) to the downstream side of the connection portion of the water supply line L1 with the pure water reflux line L21 (specifically, the downstream side of the on-off valve V31), and is provided to supply raw water to the membrane separation device 3 without passing through the water softener 13. In other words, the bypass line L22 is connected to the water supply line L1 so as to bypass the connection portion of the water supply line L1 and the replacement water discharge line L12, the water softener 13, and the connection portion of the water supply line L1 and the pure water reflux line L21. The on-off valve V33 functions as a flow path switching means for switching the communication state between the water supply line L1 and the bypass line L22.

[0020] The water supply line L1 may be provided with a decarbonation device and an ultraviolet sterilizer. The decarbonation device has the function of decarbonating the raw water and removing carbon dioxide gas contained in the raw water, and the ultraviolet sterilizer is used to sterilize the raw water flowing through the water supply line L1 by irradiating it with ultraviolet light. Examples of decarbonation devices include a decarbonation membrane and a decarbonation tower. The water supply line L1 may also be provided with an activated carbon filter to remove residual chlorine from the raw water supplied to the membrane separation device 3, or with a means for injecting a reducing agent into the raw water. Note that an activated carbon filter may be provided instead of the water softener 13 as a pretreatment means for pretreating the raw water supplied to the membrane separation device 3. In this case, a filter for capturing and removing pulverized coal generated by the activated carbon filter may be provided downstream of the activated carbon filter.

[0021] The permeate line L2 is provided with an on-off valve V4, and a permeate return line L6 is connected downstream of the on-off valve V4. The permeate return line L6 branches off from the permeate line L2 via an on-off valve V5 and is connected to the raw water tank 2. If there is sufficient space above the raw water tank 2, the on-off valves V4 and V5 may be provided near the raw water tank 2. Alternatively, a three-way valve may be provided at the connection between the permeate line L2 and the permeate return line L6 instead of the on-off valves V4 and V5. A membrane degassing device for removing oxygen and carbon dioxide dissolved in the permeate from the membrane separation device 3 may be provided upstream of the connection in the permeate line L2. The RO concentrate line L3 is provided with an on-off valve V14, preferably near the raw water tank 2, and an RO concentrate discharge line L13 is connected upstream of the on-off valve V14 via an on-off valve V15. Instead of the on-off valves V14 and V15, a three-way valve may be provided at the connection between the RO concentrated water line L3 and the RO concentrated water discharge line L13.

[0022] The EDI device 4 is an apparatus that simultaneously deionizes (deminesalises) the water to be treated using an ion exchanger and regenerates the ion exchanger, treating the permeate supplied from the membrane separation device 3 through the permeate line L2 to produce deionized water (pure water). Connected to the EDI device 4 are a pure water line L7 through which pure water from the EDI device 4 flows, and an EDI concentrated water line L8 through which concentrated water (hereinafter also referred to as "EDI concentrated water") from the EDI device 4 flows. The pure water line L7 is connected downstream to a pure water tank (not shown) of a pure water supply device, and the EDI concentrated water line L8 is connected downstream to the raw water tank 2. Also connected to the EDI device 4 is an electrode water discharge line (not shown) through which electrode water from the EDI device 4 is discharged to the outside.

[0023] As an example, the EDI device 4 has an anode and a cathode, a deionization compartment disposed between the anode and the cathode and filled with at least one of a cation exchanger and an anion exchanger, and a pair of concentration compartments disposed on either side of the deionization compartment via an ion exchange membrane. Permeated water from the membrane separation device 3 is supplied to the deionization compartment as water to be treated through the permeate line L2, and the concentration compartment is supplied with the permeated water from the membrane separation device 3 as concentrated water. The electrode compartments housing the anode and cathode are also supplied with the permeated water from the membrane separation device 3 as electrode water. When the permeated water is supplied from the membrane separation device 3 to the deionization compartment, ionic components in the permeated water are removed by ion exchange in the ion exchangers in the deionization compartment. The permeated water from which the ionic components have been removed is discharged as deionized water (pure water) from the deionization compartment via the pure water line L7 and the EDI device 4. At this time, the ionic components removed in the deionization compartment are desorbed from the ion exchanger due to the potential difference generated by the application of a DC voltage between the two electrodes and migrate to the concentrating compartment adjacent to the deionization compartment. The ionic components that have migrated to the concentrating compartment in this way are taken up by the concentrated water flowing through the concentrating compartment and discharged from the EDI device 4 through the EDI concentrated water line L8. Meanwhile, in the deionization compartment, a water dissociation reaction (a reaction in which water dissociates into hydrogen ions and hydroxide ions) is continuously occurring, and these hydrogen ions and hydroxide ions are exchanged for the ionic components held in the ion exchanger in the deionization compartment, thereby regenerating the ion exchanger in the deionization compartment.

[0024] An on-off valve V6 is provided in the pure water line L7, and a pure water return line L9 is connected upstream of the pure water line L7 via the on-off valve V7. The pure water return line (first return line) L9 branches off from the pure water line L7 and is connected to the raw water tank 2 to allow pure water (treated water) from the EDI device 4 to flow and return to the raw water tank 2. This allows for circulating pure water within the pure water production system 1, for example, when the system is started up or restarted, or when there is no demand for pure water at the point of use and the water level in the pure water tank (not shown) of the pure water supply system remains constant. That is, by closing the on-off valve V6 of the pure water line L7 and opening the on-off valve V7 of the pure water return line L9, the pure water produced by the EDI device 4 can be returned to the raw water tank 2 via the pure water return line L9. Note that instead of the on-off valves V6 and V7, a three-way valve may be provided at the connection between the pure water line L7 and the pure water return line L9.

[0025] A filter and an ultraviolet sterilizer may be provided upstream of the connection point of the pure water line L7 with the pure water return line L9. The filter serves to capture and remove crushed resin generated by the EDI device 4, and the ultraviolet sterilizer is used to sterilize the pure water flowing through the pure water line L7 by ultraviolet irradiation. The provision of an ultraviolet sterilizer is preferable when the pure water produced is used as purified water for pharmaceutical production, or when viable cell count control of the pure water is required. The EDI concentrated water line L8 is provided with an on-off valve V16 (although shown separately in the figure), preferably located close to the raw water tank 2, and the EDI concentrated water discharge line L14 is connected upstream of the on-off valve V16 via an on-off valve V17. The pure water return line L9 is provided with an on-off valve V18 downstream of the connection point with the pure water reflux line L21. A pure water discharge line for discharging pure water to the outside during the sterilization preparation process described below may be connected upstream of the on-off valve V18 to the pure water return line L9. Furthermore, instead of the on-off valves V16 and V17, a three-way valve may be provided at the connection between the EDI concentrated water line L8 and the EDI concentrated water discharge line L14.

[0026] Instead of or in addition to the EDI device 4, a non-regenerative or regenerative mixed-bed ion exchange resin tower may be provided as an ion exchange device for treating the permeate from the membrane separation device 3.

[0027] During normal operation (pure water production) of the pure water production system 1, a water sampling process is carried out in which raw water stored in the raw water tank 2 is sequentially treated in the membrane separation device 3, the EDI device 4, etc., and the pure water thus obtained is supplied to a pure water supply device (not shown) via the pure water line L7. Specifically, the raw water stored in the raw water tank 2 is supplied to the heat exchanger 12 and the water softener 13 via the water supply line L1 by the operation of the water pump 11. The raw water adjusted to a constant temperature (e.g., 25°C) in the heat exchanger 12 has hardness components removed in the water softener 13 and is then supplied to the membrane separation device 3 by the operation of the pressure pump 14, where it is separated into permeate and RO concentrate by membrane separation. The permeate is supplied to the EDI device 4 via the permeate line L2, and all of the RO concentrate is discharged to the outside via the RO concentrate line L3 and the RO concentrate discharge line L13. The permeate water supplied to the EDI device 4 is subjected to desalination treatment to remove ionic components, and then sent as pure water to a pure water supply device (not shown) through a pure water line L7. At this time, all of the EDI concentrated water is returned to the raw water tank 2 through an EDI concentrated water line L8. All of the electrode water from the EDI device 4 is constantly discharged to the outside through an electrode water discharge line (not shown), including during the hot water sterilization process described below.

[0028] The raw water tank 2 is provided with a water level sensor (not shown), and during the water sampling process described above, raw water is supplied to the raw water tank 2 through the raw water supply line L4 according to the water level in the raw water tank 2 detected by the water level sensor. Specifically, when the water level in the raw water tank 2 falls below a predetermined lower limit, the on-off valve V2 in the raw water supply line L4 is opened to supply raw water. When the water level in the raw water tank 2 reaches a predetermined upper limit, the on-off valve V2 in the raw water supply line L4 is closed to stop the supply of raw water.

[0029] In the pure water production system 1 of this embodiment, since it produces pure water for use in pharmaceutical manufacturing, etc., a sterilization process using hot water is periodically performed between the water collection steps described above to reduce the number of viable bacteria in the system. This hot water sterilization method will be described in detail below.

[0030] (Sterilization preparation process) The sterilization preparation process is a process of replacing the system of the pure water production apparatus 1 with pure water in preparation for sterilizing the pure water production apparatus 1 with hot water, and consists of three steps, from the first preparation process to the third preparation process.

[0031] [First Preparation Step] The first preparation step is a step of discharging raw water stored in the raw water tank 2 to the outside during normal operation (pure water production) of the pure water production system 1 .

[0032] In the first preparation step, first, the on-off valve V1 of the water supply line L1 is closed, the water pump 11 is stopped, and at the same time, the pressure pump 14 is stopped, thereby stopping normal operation of the pure water production system 1. Then, the on-off valve V11 of the raw water discharge line L11 is opened, and the raw water in the raw water tank 2 is discharged to the outside through the raw water discharge line L11. Thereafter, when a water level sensor (not shown) provided in the raw water tank 2 confirms, for example, that the water level in the raw water tank 2 has fallen below a predetermined lower limit, the on-off valve V11 of the raw water discharge line L11 is closed, and the discharge of raw water is stopped, thereby completing the first preparation step.

[0033] The raw water may be discharged until the raw water tank 2 is completely empty, but in that case, air will be taken into the suction side of the water pump 11. As a result, when the water pump 11 is started in the second preparation step described below, the water containing the air will flow into the water pump 11, which may damage the water pump 11. Therefore, from the perspective of suppressing air contamination when the water pump 11 is started, it is preferable to stop discharging raw water when a predetermined amount of raw water remains in the raw water tank 2 rather than discharging raw water until the raw water tank 2 is completely empty. However, if a water pump 11 equipped with an automatic air vent valve is used, the raw water may be discharged until the raw water tank 2 is completely empty.

[0034] [Second Preparation Step] The second preparation step is a step of storing pure water in the raw water tank 2 that was nearly emptied in the first preparation step. By performing the second preparation step, most of the water in the system is replaced with pure water, except for the water supply line L1 including the water softener 13.

[0035] When the first preparation step is completed by closing the on-off valve V11 of the raw water discharge line L11, the on-off valve V3 of the raw water inlet line L5 is opened, and the water supply pump 11 and the pressure pump 14 are started. At this time, the on-off valves V12 and V31 of the water supply line L1 are open, and the on-off valve V13 of the replacement water discharge line L12, the on-off valve V32 of the pure water return line L21, and the on-off valve V33 of the bypass line L22 are closed. Therefore, the water supply line L1 is connected to the water softener 13, and the communication between the water supply line L1 and the bypass line L22, the communication between the water softener 13 and the replacement water discharge line L12, and the communication between the water softener 13 and the pure water return line L21 are all blocked (first communication state). In this way, pure water is produced in the pure water production apparatus 1 in the same manner as during the water sampling process, except that the raw water flowing through the raw water supply line L4 is supplied to the water supply line L1 through the raw water inlet line L5 without passing through the raw water tank 2, and the second preparation process is started.

[0036] In the second preparation step (first treatment), the on-off valve V6 of the pure water line L7 is closed, and the on-off valves V7 and V18 of the pure water return line L9 are opened, so that the pure water (treated water) obtained in the EDI device 4 is supplied from the pure water line L7 through the pure water return line L9 to the raw water tank 2 and stored therein. In this way, the raw water tank 2, the membrane separation device 3, the EDI device 4, and the lines L2, L6 to L7, and L9 from the membrane separation device 3 and the EDI device 4 to the raw water tank 2 are replaced with pure water (including permeated water). Note that the RO concentrate is discharged to the outside from the RO concentrate line L3 through the RO concentrate discharge line L13, as in the water sampling step. The EDI concentrated water is also discharged to the outside from the EDI concentrated water line L8 through the EDI concentrated water discharge line L14 by closing the on-off valve V16 of the EDI concentrated water line L8 and opening the on-off valve V17 of the EDI concentrated water discharge line L14.

[0037] In the second preparation step, similarly to the water sampling step, the temperature of the raw water is preferably adjusted to a constant temperature (e.g., 25°C) by a heat exchanger (temperature control means) 12 in order to stabilize the treated water quality and throughput of the membrane separation device 3. Furthermore, the supply (storage) of pure water to the raw water tank 2 may be started after a certain amount of pure water flowing into the pure water return line L9 has been discharged to the outside through a pure water discharge line (not shown). The timing may be determined by using at least one of a timer included in the control unit 5 and a water quality detection means (not shown) provided in the pure water distribution path (i.e., the pure water line L7, the pure water return line L9, and the pure water discharge line). The water quality detection means is not particularly limited, and may be, for example, a conductivity meter or a resistivity meter.

[0038] The second preparation process is performed, for example, until a water level sensor (not shown) confirms that the water level in the raw water tank 2 has reached a predetermined upper water level. In the hot water sterilization process described below, the greater the amount of water held in the system, the longer the time required to heat and cool the pure water. Therefore, it is preferable that the upper water level be set as low as possible as long as the amount of water held necessary for hot water sterilization is ensured. However, as described above, since electrode water from the EDI device 4 is constantly discharged to the outside during the hot water sterilization process, the amount of water held in the system continues to decrease. Therefore, it is preferable that the upper water level be set to a water level that minimizes the amount of water held, taking into account such discharge.

[0039] [Third Preparation Step] The third preparation step is a step of passing pure water through the water softener 13 to replace the water in the water softener 13 that was not replaced with pure water in the second preparation step with pure water.

[0040] When it is confirmed that the water level in the raw water tank 2 has reached a predetermined upper water level, a third preparation step is initiated. In the third preparation step (second treatment), the on-off valves V12 and V31 of the water supply line L1 and the on-off valve V18 of the pure water return line L9 are closed, and the on-off valve V33 of the bypass line L22, the on-off valve V13 of the replacement water discharge line L12, and the on-off valve V32 of the pure water return line L21 are opened. This blocks communication between the water supply line L1 and the water softener 13, connects the water supply line L1 to the bypass line L22, and connects the water softener 13 to the replacement water discharge line L12 and the pure water return line L21 (second communication state). In this way, pure water is produced in the pure water production system 1 in the same manner as in the second preparation step, except that the raw water supplied to the water supply line L1 bypasses the water softener 13 (i.e., is not treated by the water softener 13) and is supplied to the membrane separation device 3. The pure water flowing through the pure water return line L9 is returned to the water supply line L1 via the pure water return line L21, flows through the water softener 13 in the opposite direction to that during the water sampling step, and is then discharged from the water supply line L1 through the replacement water discharge line L12 to the outside. In this way, pure water is passed through the water softener 13, which has a relatively large capacity, among the parts of the system that were not replaced with pure water in the second preparation step, and the water in the water softener 13 is replaced with pure water.

[0041] Whether or not to terminate the third preparation step is preferably determined based on whether the water in the water softener 13 has been sufficiently replaced with pure water. Whether or not the water in the water softener 13 has been sufficiently replaced with pure water can be determined, for example, based on the detection results of a water quality detection means (not shown) provided in the replacement water discharge line L12. Specifically, if it is determined that the water quality of the wastewater from the replacement water discharge line L12 meets a predetermined water quality, it is determined that the water in the water softener 13 has been sufficiently replaced with pure water, and it can be determined that the third preparation step should be terminated. The water quality detection means is not particularly limited, and for example, a conductivity meter or a resistivity meter can be used. Alternatively, the third preparation step may be terminated when a flow rate sensor (not shown) provided near the outlet of the desalting compartment of the EDI device 4 confirms that the integrated flow rate of pure water has reached a predetermined value or greater, or when a predetermined water flow time has elapsed.

[0042] In the above-described sterilization preparation process, pure water replacement is performed in the system excluding the water softener 13 in the second preparation process, and then pure water replacement is performed in the water softener 13 in the third preparation process. However, the order of pure water replacement may be reversed. That is, after the first preparation process is completed, the third preparation process may be performed to replace the water in the water softener 13 with pure water, and then the second preparation process may be performed to replace the water in the system excluding the water softener 13 with pure water. However, in this case, since the water in the water softener 13 has already been replaced with pure water in the second preparation process, raw water that bypasses the water softener 13 is supplied to the membrane separation device 3, unlike the above-described case. Specifically, the on-off valves V12 and V31 of the water supply line L1 and the on-off valve V32 of the pure water return line L21 are closed, and preferably the on-off valve V13 of the replacement water discharge line L12 is closed, and the on-off valve V33 of the bypass line L22 and the on-off valve V18 of the pure water return line L9 are opened. As a result, communication between the water supply line L1 and the water softener 13 is interrupted, the water supply line L1 is connected to the bypass line L22, and preferably communication between the water softener 13 and the replacement water discharge line L12 is interrupted, and communication between the water softener 13 and the pure water return line L21 is interrupted (third communication state). In this way, raw water supplied to the water supply line L1 is supplied to the membrane separation device 3, bypassing the water softener 13, and pure water from the EDI device 4 is supplied to the raw water tank 2 through the pure water line L7 and the pure water return line L9 and stored therein.

[0043] (Hot Water Sterilization Step) The hot water sterilization step is a step of sterilizing the inside of the system of the pure water production apparatus 1 with hot water. Specifically, this step involves circulating the pure water stored in the raw water tank 2 within the system while heating it to 60°C or higher, preferably 80°C or higher, using the heat exchanger 12, maintaining this temperature for a certain period of time, and then cooling it until the temperature within the system becomes suitable for membrane separation treatment by the membrane separation device 3.

[0044] When the third preparation step is completed, the on-off valve V3 of the raw water inlet line L5 is closed, stopping the supply of raw water to the feedwater line L1 through the raw water inlet line L5, and at the same time, the on-off valve V1 of the feedwater line L1 is opened, starting the supply of pure water to the feedwater line L1 from the raw water tank 2. Then, the on-off valves V12 and V31 of the feedwater line L1, the on-off valve V4 of the permeated water line L2, and the on-off valves V7 and V18 of the pure water return line L9 are opened, and the on-off valve V33 of the bypass line L22, the on-off valve V6 of the pure water line L7, the on-off valve V32 of the pure water reflux line L21, and the on-off valve V13 of the replacement water discharge line L12 are closed. This initiates the hot water sterilization process, and the pure water stored in the raw water tank 2 is circulated through the water supply line L1 including the water softener 13, the membrane separation device 3, the permeate line L2, the EDI device 4, the pure water line L7, and the pure water return line L9. At the same time, a heat medium (e.g., steam) is supplied to the heat exchanger 12, and the circulating pure water is heated to and maintained at 60°C or higher, preferably 80°C or higher. In this way, the circulation of pure water (hot water) maintained at a high temperature sterilizes the inside of the pure water production system 1.

[0045] At this time, the on-off valve V5 of the permeate return line L6 is opened, thereby allowing hot water to flow through the permeate return line L6 as well for sterilization. Furthermore, the on-off valve V14 of the RO concentrate line L3 and the on-off valve V16 of the EDI concentrate line L8 are opened, and the on-off valve V15 of the RO concentrate discharge line L13 and the on-off valve V17 of the EDI concentrate discharge line L14 are closed, thereby allowing hot water to flow through the RO concentrate line L3 and the EDI concentrate line L8 as well for sterilization. During the hot water sterilization step, an air vent valve (not shown) provided at the top of the water softener 13 may be opened constantly or intermittently to discharge gases generated in the water softener 13.

[0046] After the hot water has been circulated for a certain period of time, cooling of the hot water begins. Specifically, the supply of the heat medium to the heat exchanger 12 is stopped, and instead, the supply of a refrigerant (e.g., cold water) to the heat exchanger 12 is started. This cools the circulating hot water, for example, to below 45°C.

[0047] The temperature of the hot water in the hot water sterilization process is adjusted based on the detection value of a temperature sensor (not shown) installed in the hot water circulation path. The location of the temperature sensor is not particularly limited, but it is preferably located as far away from the heat exchanger 12 as possible in the direction of hot water circulation, and more preferably near the inlet of the heat exchanger 12. In other words, the temperature of the hot water is lowest in this vicinity due to heat radiation. Therefore, by adjusting the temperature of the hot water in this vicinity so that it does not fall below the desired sterilization temperature, sufficient sterilization effect within the system can be ensured. Temperature sensors may also be installed in the RO concentrated water line L3 and the EDI concentrated water line L8 to monitor the temperature of the hot water flowing therethrough. The raw water temperature increase rate in the hot water sterilization process is preferably 0.1 to 15°C / min to minimize damage to each device due to sudden temperature changes. The hot water temperature decrease rate is also preferably 0.1 to 15°C / min to reduce the impact of sudden temperature changes.

[0048] (RO Start-Up Process) The RO start-up process is a process carried out as a start-up operation of the membrane separation device 3 before resuming normal operation of the pure water production system 1, and is carried out until permeate satisfying a predetermined water quality is obtained.

[0049] When the circulating hot water is cooled to a temperature suitable for membrane separation treatment by the membrane separation device 3 (for example, less than 45°C), the process shifts from the hot water disinfection process to the RO start-up process, and the same membrane separation process as in the water sampling process is performed in the membrane separation device 3. At this time, the on-off valve V4 of the permeate line L2 is closed and the on-off valve V5 of the permeate return line L6 is opened, so that all of the permeate from the membrane separation device 3 is returned from the permeate line L2 through the permeate return line L6 to the raw water tank 2. Meanwhile, the on-off valve V14 of the RO concentrate line L3 is closed and the on-off valve V15 of the RO concentrate discharge line L13 is opened, so that the RO concentrate is discharged from the RO concentrate line L3 through the RO concentrate discharge line L13 to the outside.

[0050] As described above, the RO start-up process is performed until the quality of the permeate from the membrane separation device 3 meets a predetermined quality. Whether the quality of the permeate meets the predetermined quality can be determined, for example, by whether the conductivity of the permeate detected by a conductivity sensor (not shown) installed near the secondary outlet of the membrane separation device 3 has fallen to a predetermined value (e.g., 15 μS / cm) or less. Furthermore, whether to terminate the RO start-up process may be determined based on whether the flow rate of the permeate and the flow rate of the RO concentrate have each stabilized, in addition to whether the quality of the permeate meets the predetermined quality. Whether the flow rates have each stabilized may be determined using a flow sensor, or the time until the flow rates stabilize may be measured in advance during a trial run and determined based on whether that time has elapsed.

[0051] (EDI startup process) The EDI startup process is a process that is carried out following the RO startup process as startup operation of the EDI device 4 before resuming normal operation of the pure water production system 1, and is carried out until pure water that meets the specified water quality is obtained.

[0052] When it is confirmed that the quality of the permeate from the membrane separation device 3 meets a predetermined water quality, the on-off valve V4 of the permeate line L2 is opened and the on-off valve V5 of the permeate return line L6 is closed, and the permeate from the membrane separation device 3 is supplied to the EDI device 4. When the EDI start-up process is started in this manner, the EDI device 4 performs a desalination process similar to that during the water sampling process, the on-off valve V6 of the pure water line L7 is closed, and the on-off valve V7 of the pure water return line L9 is opened. As a result, the pure water from the EDI device 4 is returned from the pure water line L7 through the pure water return line L9 to the raw water tank 2. In addition, the on-off valve V16 of the EDI concentrate line L8 is opened and the on-off valve V17 of the EDI concentrate discharge line L14 is closed, and the EDI concentrate is also returned to the raw water tank 2 through the EDI concentrate line L8.

[0053] As described above, the EDI start-up process is performed until the quality of the pure water from the EDI device 4 meets a predetermined quality standard. Whether the quality of the pure water meets the predetermined quality standard can be determined, for example, by determining whether the conductivity of the pure water detected by a conductivity sensor (not shown) installed near the outlet of the desalting compartment of the EDI device 4 is equal to or lower than a predetermined value (e.g., 1 μS / cm). Furthermore, whether to terminate the EDI start-up process may be determined based on whether the flow rates of the pure water, the EDI concentrate water, and the EDI electrode water have each stabilized, in addition to whether the quality of the pure water meets the predetermined quality standard. Whether each flow rate has stabilized may be determined using a flow sensor, or the time until the flow rates stabilize may be measured in advance during a trial run and determined based on whether that time has elapsed.

[0054] When the EDI startup process is completed, the on-off valve V6 on the pure water line L7 is opened and the on-off valve V7 on the pure water return line L9 is closed, thereby resuming normal operation of the pure water production system 1, and the pure water produced in the pure water production system 1 is supplied to a liquid supply device (not shown) through the pure water line L7.

[0055] As described above, according to this embodiment, the pure water return line (first return line) L9 is connected to the water supply line L1 downstream of the water softener (pretreatment means) 13 via the pure water return line (second return line) L21, and the replacement water discharge line L12 is connected upstream of the water softener 13. This allows the pure water flowing through the pure water return line L9 to be returned to the water softener 13 and discharged to the outside. Therefore, it is possible to pass pure water through the water softener 13 before performing sterilization treatment with hot water (heated pure water). This allows the inside of the water softener 13, where raw water with a high ionic load would have accumulated in the conventional method, to be replaced with pure water. As a result, the ionic load on the EDI device 4 during sterilization treatment can be reduced, and deterioration of the quality of the treated water from the EDI device 4 during pure water production after sterilization treatment can be prevented.

[0056] The uses of the bypass line L22 and the replacement water discharge line L12 are not limited to those described above, and they may also be used, for example, for backwashing the ion exchange resin in the water softener 13. That is, the bypass line L22 and the replacement water discharge line L12 may be used to allow raw water stored in the raw water tank 2 to flow into the outlet of the water softener 13 as backwash water and to be discharged to the outside through the inlet of the water softener 13. In other words, the water softener 13 is connected to a line for backwashing the ion exchange resin, and such an existing line may be reused as the replacement water discharge line L12 and the bypass line L22.

[0057] Second Embodiment Fig. 2 is a schematic diagram of a pure water production system according to a second embodiment of the present invention. This embodiment differs from the first embodiment in that an activated carbon filter is added as pretreatment means for pretreatment of raw water. The following description will focus on this difference.

[0058] In this embodiment, an activated carbon filter 15 and a filter (not shown) are provided in this order on the water supply line L1 downstream of the water softener 13, specifically, between the connection between the outlet of the water softener 13 and the pure water return line L21. As described above, the activated carbon filter 15 has the function of removing residual chlorine from the raw water supplied to the membrane separation device 3, and the filter has the function of capturing and removing pulverized coal generated by the activated carbon filter 15. Accordingly, in this embodiment, the bypass line L22 is connected to the water supply line L1 so as to bypass the water softener 13 and the activated carbon filter 15 (and the filter). Note that the positions of the water softener 13 and the activated carbon filter 15 may be reversed. However, if the activated carbon filter 15 is provided downstream of the water softener 13, raw water is passed through the water softener 13 before the residual chlorine contained in the raw water is removed by the activated carbon filter 15, thereby suppressing the growth of viable bacteria in the water softener 13. Therefore, it is preferable that the activated carbon filter 15 is located downstream of the water softener 13 as shown in the figure.

[0059] In this embodiment, sterilization using hot water is performed in the same manner as in the first embodiment. That is, after the raw water stored in the raw water tank 2 is discharged to the outside (first preparation step of the sterilization preparation step), pure water is stored in the raw water tank 2, and most of the system is replaced with pure water, except for the water supply line L1 including the water softener 13 and the activated carbon filter 15 (second preparation step of the sterilization preparation step). Then, pure water is passed through the water softener 13 and the activated carbon filter 15, thereby replacing the water in the water softener 13 and the activated carbon filter 15 with pure water (third preparation step of the sterilization preparation step). Then, after the system of the pure water production system 1 is sterilized with hot water (hot water sterilization step), start-up operation of the membrane separation device 3 is performed until permeate satisfying the predetermined water quality is obtained (RO start-up step). Subsequently, start-up operation of the EDI device 4 is performed until pure water satisfying the predetermined water quality is obtained (EDI start-up step), and normal operation of the pure water production system 1 is resumed. Therefore, in this embodiment, even if the number of devices used as pretreatment means is increased compared to the first embodiment, the hot water sterilization method can be carried out without increasing the number of steps.

[0060] (Third Embodiment) Figure 3 is a schematic diagram of a pure water production system according to a third embodiment of the present invention. This embodiment differs from the first embodiment in that two water softeners are provided and an activated carbon filter is added as pretreatment means for pretreatment of raw water. The following description will focus on these differences.

[0061] In this embodiment, two water softeners 13 of the first embodiment are provided in parallel, and accordingly, two water pumps 11 of the first embodiment are also provided. Specifically, the upstream side of the water supply line L1 branches into two branch lines L1a and L1b, which are connected to the raw water tank 2 via on-off valves V1a and V1b, respectively. The first branch line L1a is provided with a first water pump 11a and a first water softener 13a, and the second branch line L1b is provided with a second water pump 11b and a second water softener 13b. As a result, in the first embodiment, which has only one water softener 13, the operation of the pure water production system 1 must be stopped to perform regeneration treatment of the water softener 13. However, in this embodiment, while one of the two water softeners 13a and 13b is performing regeneration treatment, raw water can be treated by the other water softener. Therefore, even if the function of one of the two water softeners 13a, 13b deteriorates, the operation of the pure water manufacturing system 1 can be continued. Note that in this embodiment, since the upstream side of the water supply line L1 branches into two branch lines L1a, L1b, two raw water inlet lines L5 of the first embodiment are also provided. That is, a first raw water inlet line L5a is connected to the first branch line L1a via an on-off valve V3a, and a second raw water inlet line L5b is connected to the second branch line L1b via an on-off valve V3b.

[0062] Furthermore, in this embodiment, since two water softeners 13 of the first embodiment are provided, the components associated with the water softeners 13 of the first embodiment (i.e., the replacement water discharge line L12, the pure water return line L21, the bypass line L22, and the on-off valves V12, V13, V31 to V33) are also provided in the first branch line L1a and the second branch line L1b, respectively, in the same manner as in the first embodiment. Hereinafter, in order to distinguish between the components associated with the first water softener 13a and the components associated with the second water softener 13b, the components associated with the first water softener 13a may be referred to with "first" at the beginning and with the suffix "a" added to their reference numerals, and the components associated with the second water softener 13b may be referred to with "second" at the beginning and with the suffix "b" added to their reference numerals. For example, the replacement water discharge line L12, the pure water return line L21, and the bypass line L22 provided in association with the first water softener 13a are respectively called the first replacement water discharge line L12a, the first pure water return line L21a, and the first bypass line L22a. Also, the replacement water discharge line L12, the pure water return line L21, and the bypass line L22 provided in association with the second water softener 13b are respectively called the second replacement water discharge line L12b, the second pure water return line L21b, and the second bypass line L22b.

[0063] In the first embodiment, the heat exchanger 12 is provided between the water pump 11 and the water softener 13, but in this embodiment, it is provided downstream of the two water softeners 13a and 13b, i.e., downstream of the junction of the two branch lines L1a and L1b of the water supply line L1. However, if costs allow, a heat exchanger 12 may be provided between each of the water pumps 11a and 11b and the water softeners 13a and 13b, as in the first embodiment.

[0064] Furthermore, in this embodiment, an activated carbon filter 15 and a filter (not shown) are provided in this order in the water supply line L1 between the heat exchanger 12 and the pressure pump 14. The activated carbon filter 15 functions as a pretreatment means for pretreating the raw water supplied to the membrane separation device 3, and as described above, is used to remove residual chlorine from the raw water supplied to the membrane separation device 3. The filter has the function of capturing and removing pulverized coal generated from the activated carbon filter 15.

[0065] An on-off valve V21 is provided in the water supply line L1 between the heat exchanger 12 and the activated carbon filter 15, and a third replacement water discharge line L16 is connected downstream of the on-off valve V22. An on-off valve V34 is provided in the water supply line L1 between the activated carbon filter 15 and the pressure pump 14, and a third pure water return line L23 is connected upstream of the on-off valve V35. The on-off valves V21, V22, V34, and V35 function as flow path switching means for switching the communication state between the activated carbon filter 15 and each of the lines L1, L16, and L23. The third pure water return line (second return line) L23 branches off from the pure water return line L9 and is connected to the water supply line L1 downstream of the activated carbon filter 15 to return the pure water flowing through the pure water return line L9 from the water supply line L1 to the activated carbon filter 15. Accordingly, the third replacement water discharge line L16 is connected to the water supply line L1 upstream of the activated carbon filter 15 in order to discharge the pure water returned to the activated carbon filter 15 to the outside. However, conversely, the third pure water return line L23 may be connected to the water supply line L1 upstream of the activated carbon filter 15, and the third replacement water discharge line L16 may be connected to the water supply line L1 downstream of the activated carbon filter 15. Instead of the on-off valves V21 and V22, a three-way valve may be provided at the connection between the water supply line L1 and the third replacement water discharge line L16, and instead of the on-off valves V34 and V35, a three-way valve may be provided at the connection between the water supply line L1 and the third pure water return line L23.

[0066] A third bypass line L24 equipped with an on-off valve V36 is connected to the water supply line L1. The third bypass line L24 connects the upstream side of the connection portion of the water supply line L1 with the third replacement water discharge line L16 (specifically, the upstream side of the on-off valve V21) to the downstream side of the connection portion of the water supply line L1 with the third pure water reflux line L23 (specifically, the downstream side of the on-off valve V34), and is provided to supply raw water to the membrane separation device 3 without passing through the activated carbon filter 15. The on-off valve V36 functions as a flow path switching means for switching the communication state between the water supply line L1 and the third bypass line L24.

[0067] The water sampling process in this embodiment is the same as that in the first embodiment, except that raw water is supplied to one of the two water softeners 13a, 13b, passes through the heat exchanger 12, and then is supplied to the activated carbon filter 15. On the other hand, due to the above-mentioned change in configuration, each step of the hot water sterilization method in this embodiment differs from that in the first embodiment, particularly in the procedure of the third preparation step of the sterilization preparation step. Below, the hot water sterilization method of this embodiment will be described, focusing on the differences from the first embodiment, using as an example a case in which pure water is supplied to the first water softener 13a of the two water softeners 13a, 13b during the water sampling process of the pure water manufacturing apparatus 1.

[0068] In the second preparation step of the sterilization preparation process, the water supply line L1 including the two water softeners 13a, 13b and the activated carbon filter 15 is not replaced with pure water, but in the third preparation step, pure water is first passed through the activated carbon filter 15, and the water inside the activated carbon filter 15 is replaced with pure water.

[0069] Specifically, when the second preparation step is completed, the on-off valves V21 and V34 of the water supply line L1 and the on-off valve V18 of the pure water return line L9 are closed, and the on-off valve V36 of the third bypass line L24, the on-off valve V22 of the third replacement water discharge line L16, and the on-off valve V35 of the third pure water return line L23 are opened. This blocks communication between the water supply line L1 and the activated carbon filter 15, connects the water supply line L1 to the third bypass line L24, and connects the activated carbon filter 15 to the third replacement water discharge line L16 and the third pure water return line L23 (second communication state). Thus, the raw water that has passed through the first water softener 13a and the heat exchanger 12 is supplied to the membrane separation device 3 bypassing the activated carbon filter 15 (i.e., without being treated by the activated carbon filter 15). The raw water supplied to the membrane separation device 3 is treated there, and then further treated in the EDI device 4, before being supplied as pure water to the pure water return line L9. The pure water flowing through the pure water return line L9 is returned to the water supply line L1 via the third pure water return line L23, flows through the activated carbon filter 15 in the opposite direction to that during the water collection process, and is then discharged from the water supply line L1 to the outside via the third replacement water discharge line L16. In this way, the pure water is passed through the activated carbon filter 15, and the water inside the activated carbon filter 15 is replaced with the pure water.

[0070] The supply of pure water to the activated carbon filter 15 can be terminated when a water quality detection means such as a conductivity meter or a resistivity meter determines that the quality of the wastewater from the third replacement water discharge line L16 meets a predetermined water quality. Alternatively, the supply of pure water may be terminated when a flow rate sensor (not shown) provided near the outlet of the deionization compartment of the EDI device 4 confirms that the integrated flow rate of pure water has reached a predetermined value or when a predetermined water supply time has elapsed.

[0071] After the activated carbon filter 15 is replaced with pure water, the pure water is then passed through the second water softener 13b, as in the case of the water softener 13 of the first embodiment, and the pure water is replaced in the second water softener 13b.

[0072] Specifically, the on-off valve V35 of the third pure water return line L23 and the on-off valve V22 of the third replace water discharge line L16 are closed to stop the flow of pure water to the activated carbon filter 15, and the on-off valve V32b of the second pure water return line L21b and the on-off valve V13b of the second replace water discharge line L12b are opened. As a result, the pure water flowing through the pure water return line L9 is returned to the second branch line L1b through the second pure water return line L21b, flows through the second water softener 13b in the opposite direction to that during the water sampling step, and is then discharged from the second branch line L1b through the second replace water discharge line L12b to the outside. In this way, pure water is passed through the second water softener 13b, and the water in the second water softener 13b is replaced with pure water.

[0073] The supply of pure water to the second water softener 13b can be terminated when, for example, a water quality detection means such as a conductivity meter or a resistivity meter determines that the quality of the wastewater from the second replacement water discharge line L12b satisfies a predetermined water quality. Alternatively, the supply of pure water to the second water softener 13b may be terminated when a flow rate sensor (not shown) provided near the outlet of the deionization compartment of the EDI device 4 confirms that the integrated flow rate of pure water has reached a predetermined value or more, or when a predetermined water supply time has elapsed.

[0074] After the water in the second water softener 13b has been replaced with pure water, finally, as in the case of the water softener 13 of the first embodiment, pure water is passed through the first water softener 13a, and the water in the first water softener 13a is replaced with pure water.

[0075] Specifically, first, the on-off valve V32b of the second pure water return line L21b and the on-off valve V13b of the second replacement water discharge line L12b are closed, and the flow of pure water to the second water softener 13b is stopped. Then, the on-off valves V12a and V31a of the first branch line L1a are closed, and the on-off valve V33a of the first bypass line L22a is opened. As a result, the raw water supplied to the first branch line L1a bypasses both the first water softener 13a and the activated carbon filter 15 (i.e., is not treated by either of them) and is supplied to the membrane separation device 3, where it is treated sequentially in the membrane separation device 3 and the EDI device 4, and is supplied as pure water to the pure water return line L9. At this time, the pure water flowing through the pure water return line L9 is returned to the first branch line L1a through the first pure water return line L21a by opening the on-off valve V32a of the first pure water return line L21a and the on-off valve V13a of the first replacement water discharge line L12a. The pure water returned to the first branch line L1a then flows through the first water softener 13a in the opposite direction to that during the water sampling step, and is then discharged from the first branch line L1a through the first replacement water discharge line L12a to the outside. In this way, pure water is passed through the first water softener 13a, and the water in the first water softener 13a is replaced with pure water.

[0076] The supply of pure water to the first water softener 13a can be terminated when, for example, a water quality detection means such as a conductivity meter or a resistivity meter determines that the quality of the wastewater from the first replacement water discharge line L12a meets a predetermined water quality. Alternatively, the supply of pure water may be terminated when a flow rate sensor (not shown) provided near the outlet of the deionization compartment of the EDI device 4 confirms that the integrated flow rate of pure water has reached a predetermined value or greater, or when a predetermined water supply time has elapsed.

[0077] When the sterilization preparation process is completed in this manner, the supply of raw water from the raw water tank 2 is started not only to the first branch line L1a but also to the second branch line L1b, in the same procedure as in the first embodiment. In this manner, the hot water sterilization process is started, and hot water is passed through not only the first branch line L1a but also the second branch line L1b for sterilization. After the hot water sterilization process is completed, the operation of the pure water production system 1 is resumed via the RO start-up process and the EDI start-up process, similar to the first embodiment.

[0078] In the procedure of the third preparation step described above, when pure water is passed through the activated carbon filter 15, the raw water supplied to the membrane separation device 3 bypasses the activated carbon filter 15. Therefore, if the raw water contains residual chlorine, the residual chlorine will flow into the membrane separation device 3 without being removed. Therefore, if the capacity of the raw water tank 2 is sufficient, pure water stored in the raw water tank 2 may be used as the pure water for replacing the contents of the activated carbon filter 15, rather than pure water obtained by sequentially treating the raw water in the membrane separation device 3 and the EDI device 4. That is, after pure water is first passed through the two water softeners 12a, 12b in the procedure described above, the pure water stored in the raw water tank 2 may be supplied to the activated carbon filter 15 through the water supply line L1, thereby replacing the contents of the activated carbon filter 15 with pure water. The pure water passed through the activated carbon filter 15 is sequentially treated in the membrane separation device 3 and the EDI device 4, and then returned to the raw water tank 2 through the pure water return line L9. At this time, the RO concentrated water is discharged from the RO concentrated water line L3 to the outside through the RO concentrated water discharge line L13, while the EDI concentrated water may be discharged from the EDI concentrated water line L8 to the outside through the EDI concentrated water discharge line L14, or may be returned to the raw water tank 2 through the EDI concentrated water line L8. Furthermore, the pure water supplied from the raw water tank 2 to the activated carbon filter 15 may pass through the first water softener 13a or may bypass the first water softener 13a.

[0079] As with the water softeners 13a and 13b, two activated carbon filters 15 may be provided, which allows backwashing of the activated carbon filters 15 without stopping the operation of the pure water production system 1. However, because backwashing of the activated carbon filters 15 generally requires less frequency than regeneration of the water softeners 13a and 13b, it may be possible to perform backwashing during times when there is no demand for pure water at the point of use. Therefore, from a cost perspective, it is not necessary to provide two activated carbon filters 15.

[0080] In each of the above-described embodiments, the second preparation step corresponds to the "step of returning treated water to the tank" of the present invention, and the third preparation step corresponds to the "step of circulating treated water through the pretreatment means" of the present invention. These steps are performed before the hot water sterilization step (a step of sterilizing the water treatment means and the pretreatment means by circulating treated water stored in the tank along a circulation path including the water treatment means and the pretreatment means, heating the water to a predetermined temperature, and holding the temperature for a certain period of time and then cooling the water).

[0081] REFERENCE SIGNS LIST 1 Pure water production apparatus 2 Raw water tank 3 Membrane separation device (water treatment means) 4 EDI device (water treatment means) 5 Control unit (control means) 13, 13a, 13b Water softener (pretreatment means) 15 Activated carbon filter (pretreatment means) L1 Water supply line L5 Raw water introduction line L9 Pure water return line (first reflux line) L13, L13a, L13b, L16 Replacement water discharge line (discharge line) L21, L21a, L21b, L23 Pure water reflux line (second reflux line) L22, L22a, L22b, L24 Bypass line V12, V12a, V12b, V13, V13a, V13b, V21, V22, V31 to V33, V31a to V33a, V31b to V33b, V34 to V36: opening / closing valves (flow path switching means)

Claims

1. A water treatment system comprising: a tank for storing water to be treated; water treatment means including a membrane separation device and an ion exchange device; pretreatment means for pretreatment of water to be treated that is supplied to said water treatment means; a water supply line from said tank to said water treatment means via said pretreatment means; a first return line through which treated water obtained by said water treatment means circulates and returns to said tank; a second return line branching from said first return line and connecting to said water supply line upstream or downstream of said pretreatment means, for returning treated water flowing through said first return line to said pretreatment means; and a discharge line connecting to said water supply line on the side of said pretreatment means opposite to the side where said second return line is connected, for discharging treated water returned to said pretreatment means to the outside.

2. A bypass line connected to the water supply line so as to bypass the pre-treatment means, the connection between the water supply line and the second return line, and the connection between the water supply line and the discharge line; 2. The water treatment system according to claim 1, further comprising: a flow path switching means for selectively switching between a first communication state in which the water supply line communicates with the pretreatment means, the water supply line is blocked from communication with the bypass line, the pretreatment means is blocked from communication with the second return line, and the pretreatment means is blocked from communication with the discharge line; a second communication state in which the water supply line is blocked from communication with the pretreatment means, the water supply line is connected to the bypass line, the pretreatment means is connected to the second return line, and the pretreatment means is connected to the discharge line; and a third communication state in which the water supply line is blocked from communication with the pretreatment means, the water supply line is connected to the bypass line, and the pretreatment means is blocked from communication with the second return line.

3. A water treatment system as described in claim 2, which has a control means that heats the treated water circulating between the tank and the water treatment means via the water supply line and the first return line, and uses the heated treated water to perform a sterilization process that sterilizes the tank, the water supply line including the pre-treatment means, the water treatment means, and the first return line, and the control means controls the flow path switching means to switch to the first communication state or the third communication state before performing the sterilization process, and performs a first process in which the water to be treated supplied to the water supply line is treated by the water treatment means, and a second process in which the water to be treated supplied to the water supply line is treated by the water treatment means, and uses the flow path switching means to switch to the second communication state.

4. The water treatment system of claim 3, wherein the control means controls the flow path switching means to switch to the first communication state before performing the sterilization treatment, and then performs the first treatment, and then controls the flow path switching means to switch to the second communication state and then performs the second treatment, or controls the flow path switching means to switch to the second communication state and then performs the second treatment, and then switches the flow path switching means to the third communication state and then performs the first treatment.

5. The water treatment system according to claim 4, wherein a plurality of the pretreatment means are provided.

6. The water treatment system according to claim 5, wherein the second reflux line, the discharge line and the bypass line are each provided in plurality according to the number of the pretreatment means, and the control means executes the second treatment for each of the pretreatment means.

7. The water treatment system described in claim 5, wherein the plurality of pretreatment means are arranged in series on the water supply line, the second return line is connected to the water supply line downstream of the plurality of pretreatment means, and the discharge line is connected to the water supply line upstream of the plurality of pretreatment means, and the control means performs the second treatment on the plurality of pretreatment means collectively.

8. A water treatment system as described in any one of claims 3 to 7, having a temperature adjustment means provided in the water supply line for adjusting the temperature of the water to be treated supplied to the water treatment means to a predetermined temperature while the second treatment is being performed.

9. The water treatment system according to any one of claims 1 to 7, which is used to produce pure water for use in pharmaceutical manufacturing.

10. A method for operating a water treatment system having a tank for storing water to be treated, a water treatment means including a membrane separation device and an ion exchange device, and a pretreatment means for pretreatment of the water to be treated that is supplied to the water treatment means, the method comprising: a step of supplying the water to be treated to the pretreatment means and the water treatment means from a supply source other than the tank, and returning the treated water obtained by the water treatment means to the tank; and a step of supplying the water to be treated to the water treatment means from a supply source other than the tank without passing through the pretreatment means, before or after storing the treated water in the tank, and circulating the treated water obtained by the water treatment means to the pretreatment means.

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