Pure water production device and its operating method

Abstract

To achieve further water saving.SOLUTION: A pure water production device 1 comprises filtration means 11 comprising a reverse osmosis membrane or a nano filtration membrane, a membrane filtration device 3 treating water to be treated to prepare treated water, and a controller 5 that switches the operation of the pure water production device 1 between a normal operation of supplying a use point with pure water produced by the pure water production device 1, and a circulation operation of recycling treated water prepared by the membrane filtration device 3 or pure water produced by the pure water production device 1 to the upstream of the membrane filtration device 3. The controller 5 carries out, in the normal operation and in the circulation operation, drainage flow rate control of adjusting the flow rate of concentrated water flowing in the drainage line according to the target value of the recovery rate, i.e., the ratio of the flow rate of the permeated water flowing in a permeated water line L2 relative to the sum of the flow rate of the permeated water flowing in the permeated water line L2 and the flow rate of the concentrated water flowing in a drainage line L4, and sets the target value of the recovery rate at a first target value in the normal operation and sets the target value of the recovery rate at a second target value exceeding the first target value, in the circulation operation.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a water purifying apparatus and a method for operating the same. [Background technology]

[0002] A water purifier produces pure water by sequentially treating raw water such as industrial water, well water, and city water, and is generally equipped with a membrane filtration system with a reverse osmosis (RO) membrane or a nanofiltration (NF) membrane that separates the raw water into permeate and concentrated water. To further improve the quality of the treated water, water purifiers often also include an electrodeionization system that treats the permeate produced by the membrane filtration system to produce deionized water (pure water).

[0003] In order to effectively utilize water (water conservation), membrane filtration systems using RO or NF membranes often employ a configuration in which a portion of the concentrated water containing impurities is discharged externally as concentrated wastewater and the remaining portion is returned upstream of the RO or NF membrane as concentrated reflux water. This configuration improves the recovery rate (the ratio of the permeate flow rate to the sum of the permeate flow rate and the concentrated wastewater flow rate) compared to discharging all of the concentrated water as concentrated wastewater, thereby achieving water conservation. At the same time, in these membrane filtration systems, to accommodate changes in the permeate flow rate due to changes in water temperature (i.e., changes in water viscosity), flow control is performed by adjusting the raw water supply pressure to the RO or NF membrane by controlling the rotation speed of the pressure pump to maintain a constant permeate flow rate. When controlling the permeate flow rate, adjusting the raw water supply pressure to maintain a constant permeate flow rate also changes the concentrated water flow rate accordingly. Because such changes in the concentrated water flow rate can lead to membrane clogging due to fouling or scaling, or membrane damage due to increased pressure loss, it is desirable to also control the concentrated water flow rate (concentrated reflux water or concentrated wastewater) when controlling the permeate flow rate. For example, Patent Document 1 describes performing flow control to adjust the flow rate of concentrated wastewater to a set flow rate, and describes calculating the set flow rate of concentrated wastewater based on a target value of recovery rate.

[0004] In such pure water production systems, the quality of the permeate from the membrane filtration system may deteriorate due to factors such as deterioration of the raw water quality or deterioration of the RO or NF membrane, resulting in a failure to meet the water quality standards for the supply water to the electrodeionized water production system. In such cases, as described in Patent Document 1, it is known to perform a circulation operation in which the water supply from the membrane filtration system to the electrodeionized water production system is stopped and the permeate from the membrane filtration system is returned to its upstream side until the quality of the permeate from the membrane filtration system recovers to a certain level. This circulation operation is also performed when the system is started up or restarted, or when there is no demand for pure water at the point of use. Even if the quality of the deionized water (pure water) from the electrodeionized water production system deteriorates, the circulation operation is performed to return the deionized water to the upstream side of the membrane filtration system. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2018-176033 Summary of the Invention [Problem to be solved by the invention]

[0006] In the pure water production apparatus described in Patent Document 1, when setting the target recovery rate, no consideration is given to switching between the above-mentioned circulation operation and normal operation, and there is still room for improvement in order to achieve further water savings.

[0007] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a pure water production system and an operating method thereof that achieve further water conservation. [Means for solving the problem]

[0008] In order to achieve the above-mentioned object, the pure water production apparatus of the present invention is a pure water production apparatus that produces pure water by sequentially treating water to be treated, and is equipped with a filtration means having a reverse osmosis membrane or a nanofiltration membrane that separates the water to be treated into permeate and concentrated water, and a membrane filtration device that treats the water to be treated and produces treated water;an electrodeionization water production device connected downstream of the membrane filtration device to produce deionized water from the treated water produced by the membrane filtration device; Operation of the pure water production equipment electrodeionization Produced in a water production system Deionized water pure water as Normal operation supplies water to the point of use, and treated water generated by the membrane filtration equipment or electrodeionization Produced in a water production system Deionization a control unit for switching to a circulation operation in which water is returned to the upstream side of the membrane filtration device, but a feed line for supplying the water to be treated to the filtration means, a permeate line for circulating the permeate from the filtration means, a concentrated water line for circulating the concentrated water from the filtration means, a drain line branching from the concentrated water line and discharging a portion of the concentrated water flowing in the concentrated water line to the outside, a return water line branching from the concentrated water line and returning the remainder of the concentrated water flowing in the concentrated water line to the upstream side of the membrane filtration device, and a flow rate adjusting means for adjusting the flow rate of the concentrated water flowing in the drain line, The electrodeionization water production apparatus has an electrode chamber consisting of an anode chamber and a cathode chamber, through which a portion of the treated water produced by the membrane filtration device is passed as electrode water; The control unit calculates a target flow rate of the concentrated water flowing through the drain line based on a target value of the recovery rate, which is the ratio of the flow rate of the permeated water flowing through the permeated water line to the sum of the flow rate of the permeated water flowing through the permeated water line and the flow rate of the concentrated water flowing through the drain line, in normal operation and circulation operation, and performs drain flow rate control by controlling the flow rate adjustment means so that the flow rate of the concentrated water flowing through the drain line becomes the target flow rate.In normal operation, the target value of the recovery rate is set to a first target value, and in circulation operation, the target value of the recovery rate is set to a second target value. 、 Exceeding the first target predetermined Set as the second target value The predetermined second target value is set so that the target flow rate of the concentrated water flowing through the drain line is equal to or greater than the flow rate of the electrode water passed through the electrode chamber and discharged to the outside. .

[0009] The method for operating a pure water producing apparatus of the present invention comprises the steps of: A pure water production apparatus for producing pure water by sequentially treating water to be treated, the pure water production apparatus comprising: A filtration means having a reverse osmosis membrane or a nanofiltration membrane that separates the water to be treated into permeate and concentrated water. and an electrodeionization water production device connected downstream of the membrane filtration device to produce deionized water from the treated water produced by the membrane filtration device. The system includes a supply line for supplying the water to be treated to the filtration means, a permeate line for circulating the permeate from the filtration means, a concentrated water line for circulating the concentrated water from the filtration means, a drain line branching from the concentrated water line and discharging a portion of the concentrated water flowing in the concentrated water line to the outside, and a return water line branching from the concentrated water line and returning the remainder of the concentrated water flowing in the concentrated water line to the upstream side of the membrane filtration device. The electrodeionization water production apparatus has an electrode chamber consisting of an anode chamber and a cathode chamber, through which a portion of the treated water produced in the membrane filtration device is passed as electrode water. A method for operating a water purifying apparatus, comprising: electrodeionizationProduced in a water production system Deionized water pure water as The process of supplying treated water to the point of use and the membrane filtration equipment electrodeionization Produced in a water production system Deionization Returning the water to the upstream side of the membrane filtration device; Deionization When supplying water to points of use and when treating water or Deionization When returning water to the upstream side of the membrane filtration device, the target value of the recovery rate, which is the ratio of the flow rate of the permeate flowing through the permeate line to the sum of the flow rate of the permeate flowing through the permeate line and the flow rate of the concentrated water flowing through the drainage line, is set. Discharge Calculate the target flow rate of the concentrated water flowing through the water line. , exclusion Flow rate of concentrated water through the water line The eyes and adjusting the flow rate of the concentrated water flowing through the drain line so as to reach the standard flow rate; Deionization When water is supplied to the point of use, the target value of the recovery rate is set to the first target value, and when treated water or pure water is returned to the upstream side of the membrane filtration device, the target value of the recovery rate is set to the second target value. 、 Exceeding the first target predetermined The second target value is set The predetermined second target value is set so that the target flow rate of the concentrated water flowing through the drain line is equal to or greater than the flow rate of the electrode water passed through the electrode chamber and discharged to the outside. .

[0010] According to such a pure water production system and its operating method, the quality of the treated water is improved through circulation operation, which in turn improves the quality of the water supplied to the membrane filtration system. As a result, the flow rate of concentrated water discharged from the membrane filtration system to the outside can be reduced compared to normal operation without increasing the risk of scale formation. [Effects of the Invention]

[0011] As described above, according to the present invention, further water saving can be achieved. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a schematic configuration diagram of a pure water manufacturing system according to a first embodiment of the present invention. [Figure 2] 1 is a schematic diagram illustrating the configuration of an electrodeionized water production apparatus according to a first embodiment of the present invention. [Figure 3] FIG. 4 is a schematic configuration diagram of a membrane filtration device according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0014] (First embodiment) Fig. 1 is a schematic diagram of a pure water production system according to a first embodiment of the present invention. Fig. 2 is a schematic diagram of an electrodeionized water production system that constitutes the pure water production system of this embodiment. Note that the configurations of the pure water production system and electrodeionized water production system shown in the figures are merely examples and do not limit the present invention. Needless to say, they can be modified as appropriate depending on the purpose, application, and required performance of the system.

[0015] The pure water production system 1 has a raw water tank 2, a membrane filtration system 3, and an electrodeionized water production system (hereinafter also referred to as "EDI system") 4, and sequentially processes the water to be treated (raw water) to produce pure water, which is then supplied to a point-of-use. The pure water production system 1 further has a control unit 5 that controls the operation of the membrane filtration system 3 and the EDI system 4.

[0016] The membrane filtration device 3 has a filtration means 11, and is an apparatus that removes impurities contained in the raw water by treating the raw water stored in the raw water tank 2 with the filtration means 11 to produce treated water (permeated water). The filtration means 11 separates the raw water supplied from the raw water tank 2 into concentrated water containing impurities and permeated water from which the impurities have been removed, and has a reverse osmosis membrane (RO membrane) or a nanofiltration membrane (NF membrane).

[0017] The membrane filtration device 3 also has multiple lines connected to the filtration means 11: a supply line L1 that supplies raw water to the filtration means 11, a treated water line L2 that circulates permeated water from the filtration means 11, and a concentrated water line L3 that circulates concentrated water from the filtration means 11. Additionally, the membrane filtration device 3 has two lines branching off from the concentrated water line L3: a drain line L4 that discharges a portion of the concentrated water flowing through the concentrated water line L3 to the outside, and a return water line L5 that returns the remainder to the raw water tank 2. The supply line L1 is connected upstream to the raw water tank 2, and the treated water line L2 is connected downstream to the EDI device 4. The treated water line L2 is connected to a treated water return line L6 via a three-way valve TV1, and the treated water return line L6 is connected downstream to the raw water tank 2. A raw water supply line L7 is connected to the raw water tank 2, allowing raw water to be replenished as needed. The raw water tank 2 does not necessarily have to be provided, and the reflux water line L5 and the treated water return line L6 may be directly connected to the supply line L1.

[0018] Furthermore, the membrane filtration device 3 has, as components for executing three flow rate controls described below, a pressure pump 12 provided on the supply line L1, a treated water flow meter 13 provided on the treated water line L2, a constant flow valve 14 provided on the concentrated water line L3, a flow control valve CV1 and a drainage flow meter 15 provided on the drainage line L4, and a manual valve MV1 provided on the reflux water line L5. Additionally, the membrane filtration device 3 has a treated water conductivity meter 16 provided on the treated water line L2 as components for determining the timing to switch between normal operation of the pure water manufacturing system 1 and circulation operation described below.

[0019] The pressure pump 12, whose rotation speed is controlled by an inverter (not shown), functions as a pressure adjusting means for adjusting the pressure of the raw water flowing through the supply line L1 (the supply pressure of the raw water to the filtration means 11). The treated water flow meter 13 functions as a flow detecting means for detecting the flow rate of the permeated water flowing through the treated water line L2. The constant flow valve 14 has the function of maintaining a constant flow rate of the concentrated water flowing through the concentrated water line L3. The flow control valve CV1 functions as a flow adjusting means for adjusting the flow rate of the concentrated water (hereinafter also referred to as "concentrated wastewater") flowing through the drainage line L4, and the wastewater flow meter 15 functions as a flow detecting means for detecting the flow rate of the concentrated wastewater. The manual valve MV1 functions as a pressure adjusting valve for adjusting the pressure balance between the concentrated water flowing through the drainage line L4 and the concentrated water flowing through the reflux water line L5. The treated water conductivity meter 16 functions as a water quality detecting means for detecting the water quality (conductivity) of the permeated water flowing through the treated water line L2.

[0020] The EDI device 4 is connected downstream of the membrane filtration device 3 and treats permeate water supplied from the membrane filtration device 3 through a treated water line L2 to produce deionized water (pure water). The EDI device 4 has a pure water line L8 through which pure water from the EDI device 4 flows and is supplied to a point of use, a concentrated water discharge line L9 through which concentrated water from the EDI device 4 (hereinafter also referred to as "EDI concentrated water") is discharged to the outside, and an electrode water discharge line L10 through which electrode water from the EDI device 4 is discharged to the outside. A pure water return line L11 is connected to the pure water line L8 via a three-way valve TV2, and the pure water return line L11 is connected downstream to the raw water tank 2. The concentrated water discharge line L9 is provided with an on-off valve MV2 and is connected to a concentrated water return line L12 via an on-off valve MV3, and the concentrated water return line L12 is connected downstream to the raw water tank 2. All of the EDI concentrated water is returned to the raw water tank 2 through the concentrated water return line L12, but depending on the water quality, some or all of it may be discharged to the outside through the concentrated water discharge line L9. All of the electrode water from the EDI device 4 is always discharged to the outside through the electrode water discharge line L10, including during circulation operation described below. If the raw water tank 2 is not provided, the pure water return line L11 and the concentrated water return line L12 are directly connected to the supply line L1.

[0021] The EDI device 4 is a device that combines electrophoresis and electrodialysis, and is a device that simultaneously deionizes (demines) water to be treated using an ion exchanger and regenerates the ion exchanger. As an example, as shown in Fig. 2, the EDI device 4 has an anode chamber E1 equipped with an anode 21, a cathode chamber E2 equipped with a cathode 22, a deionization chamber D provided between the anode chamber E1 and the cathode chamber E2, and a pair of concentration chambers C1 and C2 disposed on either side of the deionization chamber D. The pair of concentration chambers C1 and C2 includes an anode-side concentration chamber C1 adjacent to the deionization chamber D on the anode 21 side of the deionization chamber D via an anion exchange membrane a1, and a cathode-side concentration chamber C2 adjacent to the deionization chamber D on the cathode 22 side of the deionization chamber D via a cation exchange membrane c1. The anode-side concentrating compartment C1 is adjacent to the anode compartment E1 via a cation exchange membrane c2, and the cathode-side concentrating compartment C2 is adjacent to the cathode compartment E2 via an anion exchange membrane a2.

[0022] The desalting chamber D is filled with at least one of a cation exchanger and an anion exchanger, preferably a mixture of a cation exchanger and an anion exchanger. That is, the cation exchanger and anion exchanger are preferably packed in a so-called mixed bed configuration. Examples of cation exchangers include cation exchange resins, cation exchange fibers, and monolithic porous cation exchangers, with the most commonly used cation exchange resins being preferred. Examples of cation exchangers include weakly acidic cation exchangers and strongly acidic cation exchangers. Examples of anion exchangers include anion exchange resins, anion exchange fibers, and monolithic porous anion exchangers, with the most commonly used anion exchange resins being preferred. Examples of anion exchangers include weakly basic anion exchangers and strongly basic anion exchangers.

[0023] The anode-side concentrating compartment C1 and the cathode-side concentrating compartment C2 are provided to respectively take in the anion components and cation components discharged from the deionization compartment D and discharge them to the outside as concentrated water. Each of the concentrating compartments C1 and C2 is preferably filled with an ion exchanger to reduce the electrical resistance of the EDI device 4. The anode chamber E1 and the cathode chamber E2 are also preferably filled with a conductive material such as an ion exchanger to reduce the electrical resistance of the EDI device 4. The anode 21 housed in the anode chamber E1 is made of a metal mesh or plate, and the cathode 22 housed in the cathode chamber E2 is also made of a metal mesh or plate.

[0024] The treated water line L2 extending from the membrane filtration device 3 branches into three, each connected to a deionization chamber D, an anode-side concentrating chamber C1, and a cathode chamber E2. The deionization chamber D is connected downstream to a pure water line L8, the anode-side concentrating chamber C1 forms a serial flow path with the cathode-side concentrating chamber C2, and the cathode-side concentrating chamber C2 is connected downstream to a concentrated water discharge line L9. In this way, permeated water from the membrane filtration device 3 is supplied to the deionization chamber D as the water to be treated and is supplied from the anode-side concentrating chamber C1 to the cathode-side concentrating chamber C2 as the concentrating chamber inflow. The cathode chamber E2 also forms a serial flow path with the anode chamber E1, and the anode chamber E1 is connected downstream to an electrode water discharge line L10. Therefore, the permeated water from the membrane filtration device 3 is also supplied from the cathode chamber E2 to the anode chamber E1 as the electrode chamber inflow and is discharged to the outside as electrode water.

[0025] The pure water line L8 is provided with a pure water flow meter 23 and a pure water conductivity meter 24. The pure water flow meter 23 is used to perform three flow rate controls, which will be described later, and functions as a flow rate detection unit that detects the flow rate of pure water flowing through the pure water line L8. The pure water conductivity meter 24 is used to determine the timing for switching the pure water production system 1 between normal operation and circulation operation, which will be described later, and functions as a water quality detection unit that detects the water quality (conductivity) of the permeate water flowing through the pure water line L8. Although not shown, the three lines branching from the treated water line L2 may each be provided with a manual valve, if necessary, to adjust the flow rate balance (flow rate ratio) of the treated water, concentrated water, and electrode water flowing through the EDI device 4. Similarly, although not shown, the pure water line L8, the electrode water discharge line L10, and the concentrated water discharge line L9 may each be provided with a manual valve, if necessary, to adjust the pressure balance of the treated water, concentrated water, and electrode water flowing through the EDI device 4.

[0026] As described above, permeate (water to be treated) is supplied to deionization chamber D from membrane filtration device 3 via treated water line L2. Ion components in the permeate are adsorbed to the ion exchanger as they pass through deionization chamber D and removed. The permeate from which the ion components have been removed is supplied as deionized water (pure water) to a point-of-use via pure water line L8. The ion components removed in deionization chamber D are liberated from the ion exchanger and migrate to concentration chambers C1 and C2 adjacent to deionization chamber D due to a potential difference generated by applying a DC voltage between the electrodes 21 and 22. Specifically, cations are attracted to the cathode 22, pass through the cation exchange membrane c1, and migrate to the cathode-side concentration chamber C2. Anion components are attracted to the anode 21, pass through the anion exchange membrane a1, and migrate to the anode-side concentration chamber C1. The ion components that have migrated to concentration chambers C1 and C2 in this way are incorporated into the inflow water to the concentration chambers and discharged to the outside via concentrated water discharge line L9. Meanwhile, in deionization compartment D, the water dissociation reaction (the reaction in which water dissociates into hydrogen ions and hydroxide ions) is continuously progressing. The hydrogen ions are exchanged for cation components adsorbed on the cation exchanger, and the hydroxide ions are exchanged for anion components adsorbed on the anion exchanger. In this way, the cation exchanger and anion exchanger packed in deionization compartment D are each regenerated.

[0027] As mentioned at the beginning, the illustrated configuration of the EDI device 4 is merely an example. The configuration of each compartment (e.g., number, arrangement, etc.) and the flow path configuration can be modified, or valves, measuring instruments, etc. can be added, depending on the purpose, application, and performance requirements of the device. For example, two or more deionization compartments may be provided. In this case, the deionization compartments and the concentration compartments are alternately arranged with cation exchange membranes or anion exchange membranes interposed between them, with the concentration compartment closest to the anode adjacent to the anode compartment, and the concentration compartment closest to the cathode adjacent to the cathode compartment. Alternatively, the electrode compartment may also serve as a concentration compartment by omitting the concentration compartment adjacent to the electrode compartment (anode compartment or cathode compartment). This configuration in which the electrode compartment also serves as a concentration compartment is applicable regardless of the number of deionization compartments. Furthermore, the deionization compartment may be divided into two small deionization compartments that form a serial flow path by an intermediate ion exchange membrane (e.g., a bipolar membrane). The influent of the concentration compartment may be supplied first to the cathode-side concentration compartment, and the influent of the electrode compartment may be supplied first to the anode compartment. Alternatively, the pair of concentration compartments may form parallel flow paths, and the electrode compartments may also form parallel flow paths. Furthermore, the concentration compartment inflow water and the electrode compartment inflow water may each be a portion of deionized water, or, if the deionization compartment is divided into two small deionization compartments, they may be a portion of treated water obtained by passing the water to be treated through one of the small deionization compartments.

[0028] During normal operation of the pure water production system 1, the control unit 5 executes two flow rate controls in parallel: pure water flow rate control, which controls the flow rate of deionized water (pure water) produced by the EDI device 4, and wastewater flow rate control, which controls the flow rate of concentrated wastewater discharged from the membrane filtration device 3 to the outside. Specifically, in the pure water flow rate control, the pressure pump 12 is controlled so that the flow rate of pure water flowing through the pure water line L8 becomes a set flow rate. In the wastewater flow rate control, a target flow rate of concentrated wastewater (concentrated water flowing through the wastewater line L4) is calculated from the flow rate of permeate flowing through the treated water line L2, and the aperture of the flow rate adjustment valve CV1 is controlled so that the flow rate of the concentrated wastewater becomes the target flow rate. These two flow rate controls are described in detail below.

[0029] In the pure water flow rate control, the booster pump 12 of the membrane filtration device 3 is controlled so that the detected flow rate (detected value) of the treated water by the pure water flow meter 23 of the EDI device 4 is kept constant (a predetermined set flow rate). For example, when the water temperature changes, the viscosity of the water changes, causing a change in the flow rate of the permeated water separated by the filtration means 11. As a result, the flow rate of the pure water produced by the EDI device 4 also changes. In response to this change, the control unit 5 controls the rotation speed of the booster pump 12 via an inverter. That is, when the water temperature decreases, the viscosity of the water increases. As a result, the flow rate of the permeated water separated by the filtration means 11 decreases, and the flow rate of the pure water produced by the EDI device 4 also decreases. Therefore, to compensate for this decrease, the control unit 5 increases the rotation speed of the booster pump 12, thereby increasing the supply pressure of the raw water. Furthermore, when the water temperature increases, the viscosity of the water decreases. As a result, the flow rate of the permeated water separated by the filtration means 11 increases, and the flow rate of the pure water produced by the EDI device 4 also increases. Therefore, to offset this increase, the control unit 5 reduces the supply pressure of the raw water by lowering the rotation speed of the pressure pump 12. In this way, the rotation speed of the pressure pump 12, i.e., the supply pressure of the raw water, is adjusted, and the flow rate of the pure water flowing through the pure water line L8 is maintained constant.

[0030] The flow rate of the concentrated water separated by the filtration means 11 changes in accordance with changes in the supply pressure of the raw water to the filtration means 11 (changes in the rotation speed of the pressure pump 12), but the concentrated water line L3 is provided with the constant flow valve 14 as described above. Therefore, the pure water flow rate control can maintain a constant flow rate of the concentrated water flowing through the concentrated water line L3 even when the rotation speed of the pressure pump 12 changes and the supply pressure of the raw water changes. As a result, the pure water flow rate control does not affect the flow rate of the concentrated water flowing through the drain line L4 or the reflux water line L5, and the drain flow rate control, which will be described later, is performed independently without interfering with the pure water flow rate control.

[0031] Here, the specified flow rate of the constant flow valve 14 should be such that clogging of the membrane due to fouling or scaling does not occur, and such that the membrane is not damaged due to an increase in pressure loss. However, setting the specified flow rate of the constant flow valve 14 larger than necessary is not desirable in terms of energy consumption, as it increases the flow rate required of the pressure pump 12 more than necessary, resulting in a larger size of the pressure pump 12. Therefore, the specified flow rate of the constant flow valve 14 is set taking into consideration the permeation flux of the filtration means 11 and the minimum flow rate of concentrated water required for the filtration means 11. For example, when an RO membrane with a diameter of approximately 20.32 cm (8 inches) is used as the filtration means 11, the specified flow rate should be set to 1 to 15 m. 3 / h. The minimum flow rate of concentrated water required for the filtering means 11 means the minimum flow rate of concentrated water that should be passed through the concentrated water line L3 to prevent clogging of the membrane due to fouling or scaling.

[0032] Incidentally, the constant flow valve 14 has a specified operating differential pressure range (the allowable range of the pressure difference between the primary and secondary sides of the constant flow valve) for normal operation of the constant flow valve 14. Therefore, under certain conditions, such as when a medium- to high-pressure RO membrane is used as the filtration means 11 or when the water temperature drops significantly, the supply pressure of the raw water may rise significantly, causing the pressure of the concentrated water to rise and the pressure difference between the primary and secondary sides of the constant flow valve 14 to exceed the operating differential pressure range. In such cases, the flow rate of the concentrated water flowing through the concentrated water line L3 may not be maintained constant.

[0033] Therefore, a pressure reducing valve may be provided in the concentrated water line L3 upstream of the constant flow valve 14 to reduce the pressure of the concentrated water flowing through the concentrated water line L3 (i.e., to lower the pressure on the secondary side below the pressure on the primary side). This allows the constant flow valve 14 to operate normally, even if the supply pressure of raw water to the filtration means 11 significantly increases, by keeping the pressure difference between the primary and secondary sides of the constant flow valve 14 within the operating differential pressure range. This allows the constant flow valve 14 to operate normally and maintain a constant flow rate of the concentrated water flowing through the concentrated water line L3. Furthermore, the provision of a pressure reducing valve prevents the constant flow valve 14 from operating normally and increasing the flow rate of the concentrated water. Therefore, when the flow rate of the concentrated wastewater is adjusted to a target flow rate by the wastewater flow rate control described below, the flow rate of the concentrated water flowing through the reflux water line L5 does not increase, and the discharge flow rate of the pressure pump 12 does not increase. This eliminates the risk of the required permeate flow rate not being obtained due to a decrease in the head of the pressure pump 12. Furthermore, the provision of a pressure reducing valve not only has safety advantages, since peripheral components downstream (such as piping) are not required to have as high a pressure resistance, but also has cost advantages, since it allows the use of inexpensive, general-purpose products that do not have as high a pressure resistance. The type of pressure reducing valve is not particularly limited as long as it can reduce the pressure of the concentrated water to within the operating differential pressure range of constant flow valve 14, but it is necessary to select one that allows a flow rate greater than the specified flow rate of constant flow valve 14, or one whose secondary pressure is greater than the sum of the water flow differential pressure in drain line L4 and return water line L5 and the back pressure on the drain side.

[0034] In the wastewater flow rate control, a target flow rate of concentrated wastewater is calculated taking into account the recovery rate of the filtration means 11 of the membrane filtration device 3 (the ratio of the permeate flow rate to the sum of the permeate flow rate and the concentrated wastewater flow rate). The aperture of the flow control valve CV1 is then adjusted so that the detected flow rate (detected value) of the concentrated wastewater by the wastewater flow meter 15 is equal to the target flow rate. From the viewpoint of effective water utilization (water conservation), the recovery rate is preferably as high as possible. That is, the flow rate of concentrated wastewater is preferably as low as possible. However, because the flow rate of concentrated water is maintained constant by the constant flow valve 14, a decrease in the flow rate of concentrated wastewater naturally increases the flow rate of concentrated water returned from the return water line L5 to the supply line L1. This increases the impurity concentration in the raw water, which can easily lead to scaling, in which impurities (particularly silica or calcium) precipitate on the surface of the RO membrane or NF membrane of the filtration means 11. Therefore, the flow rate of concentrated wastewater is set so that the recovery rate is maximized within a range in which the impurity concentration in the concentrated water does not exceed its solubility, i.e., within a range in which the impurities silica or calcium do not precipitate.

[0035] However, the solubility of impurities varies depending on the water temperature. For example, the solubility of silica increases in proportion to the temperature, while the solubility of calcium (calcium carbonate) decreases as the temperature increases. Therefore, when the water temperature is low, the solubility of silica is relatively low, making it more likely to precipitate (silica scale is more likely to form). However, when the water temperature is high, the solubility of calcium becomes relatively low, making it more likely to precipitate (calcium scale is more likely to form). Therefore, although not shown, the membrane filtration device 3 is provided with a temperature sensor (water temperature detection means) that detects the temperature of any of the raw water, permeate, or concentrated water. The optimal target flow rate of the concentrated wastewater is calculated based on the water temperature detected by this temperature sensor.

[0036] Specifically, first, the theoretical recovery rate at which silica precipitates at the detected water temperature (hereinafter referred to as the "silica precipitation recovery rate") and the theoretical recovery rate at which calcium (calcium carbonate) precipitates at the detected water temperature (hereinafter referred to as the "calcium precipitation recovery rate") are calculated. Note that the methods for calculating the silica precipitation recovery rate and the calcium precipitation recovery rate will be described later. Next, the silica precipitation recovery rate and the calcium precipitation recovery rate are compared, and the smaller precipitation recovery rate is set as the target recovery rate. Then, based on this target recovery rate and the flow rate of the permeate detected by the treated water flow meter 13, the target flow rate of the concentrated wastewater is calculated and set using the following formula (1): (Target flow rate of concentrated wastewater) = (Detected flow rate of permeate water / target recovery rate) - (Detected flow rate of permeate water) (1)

[0037] From the viewpoint of reliably suppressing the occurrence of scaling, a flow rate exceeding the target flow rate calculated by the above formula (1) can be set as the set flow rate of the concentrated wastewater, but from the viewpoint of water conservation, it is preferable to set the calculated target flow rate as the set flow rate of the concentrated wastewater. Note that, although a value expressed as a percentage is usually used as the recovery rate (target recovery rate), it goes without saying that a value expressed as a decimal is used in the above formula (1).

[0038] Here, the methods for calculating the precipitation recovery rate of silica and the precipitation recovery rate of calcium will be described.

[0039] (Calculation method for silica precipitation recovery rate) Silica precipitation recovery rate Y S is the solubility of silica (mg / L) at the detected water temperature, C S The silica concentration (mg / L) of the raw water measured in advance is taken as F S Then, it is calculated using the following formula (2). Y S =(C S -F S ) / C S (2)

[0040] The solubility of silica can be calculated using a method specified in ASTM (American Society for Testing and Materials) D4993-89 or the like.

[0041] (Calculation method for calcium precipitation recovery rate) The calcium precipitation recovery rate is calculated using a method for calculating the Langelier index of the concentrate. Here, the Langelier index (saturation index) is an index indicating the possibility of calcium (calcium carbonate) precipitation and refers to the difference (pH-pH) between the actual pH of the water and the theoretical pH (pH: the pH at which calcium carbonate in water is in an equilibrium state, neither dissolving nor precipitating). That is, the greater the positive absolute value of the Langelier index, the more likely calcium carbonate is to precipitate, whereas a negative value prevents calcium carbonate from precipitating. Therefore, the calcium precipitation recovery rate is calculated as the recovery rate when the Langelier index of the concentrate is zero. To set a more conservative value, the calcium precipitation recovery rate may also be the recovery rate when the Langelier index of the concentrate is negative.

[0042] The Langelier index of the concentrated water is calculated from the pH of the concentrated water, the impurity concentrations (calcium concentration, total alkalinity, and evaporation residue concentration) of the concentrated water, and the detected water temperature. The Langelier index can be calculated, for example, by the method described in JP-A-11-267687 (paragraphs

[0025] to

[0027] ). The impurity concentrations (calcium concentration, total alkalinity, and evaporation residue concentration) of the concentrated water are calculated from the impurity concentrations (calcium concentration, total alkalinity, and evaporation residue concentration) of the raw water measured in advance and the recovery rate. Therefore, the calcium precipitation recovery rate Y C is the impurity concentration (mg / L) of the concentrated water when the Langelier index of the concentrated water becomes zero. C The impurity concentration (mg / L) of the raw water measured in advance is F C Then, the relationship is expressed by the following equation (3). Y C =(C C -F C ) / CC (3)

[0043] The methods for calculating the silica and calcium precipitation recovery rates and the target flow rate of the concentrated wastewater are not limited to the above-described methods if the recovery rates and flow rates are limited by prior constraints on the equipment design, such as the capacity of the pressure pump or the flow rate of the raw water. Furthermore, since the flow rate of the pure water flowing through the pure water line L6 is adjusted to a constant value by the pure water flow rate control, and the flow rate of the permeated water flowing through the treated water line L2 is also adjusted to a substantially constant value, the target flow rate of the concentrated wastewater can be calculated using the actual set flow rate of the permeated water. However, this method is not preferable because the actual recovery rate may deviate from the target recovery rate if the actual set flow rate of the permeated water does not match the actual flow rate. That is, if the actual flow rate of the permeated water is greater than the effective set flow rate, the actual recovery rate may exceed the target recovery rate, resulting in scaling. Alternatively, if the actual flow rate of the permeated water is smaller than the effective set flow rate, the actual recovery rate may fall below the target recovery rate, making water conservation impossible.

[0044] Therefore, as described above, it is preferable to use the flow rate detected by the treated water flow meter 13 to calculate the target flow rate of the concentrated wastewater. This makes it possible to prevent the actual recovery rate from deviating from the target recovery rate even if the pure water flow rate control is not performed appropriately and the permeate flow rate is not adjusted to a substantially constant value. Note that, in the actual calculation, it is preferable to use the average flow rate over a predetermined detection time or a predetermined number of detections to minimize the influence of variations in the detected permeate flow rate.

[0045] However, when the permeate flow rate is unstable and the detected flow rate varies significantly, such as when the apparatus is started up or restarted, the target flow rate of the concentrated wastewater may be calculated using the above-mentioned actual set flow rate of the permeate for a certain period until the permeate flow rate stabilizes. Furthermore, the permeate flow rate used to calculate the target flow rate of the concentrated wastewater may be switched depending on the difference between the actual set flow rate of the permeate and the actual flow rate. That is, when the difference is within a predetermined range, the actual set flow rate may be used for calculation, and when the difference is outside the predetermined range, the actual flow rate may be used for calculation.

[0046] When controlling the recovery rate as described above, it is preferable to use an electric proportional control valve as the flow control valve CV1. This allows for fine adjustment of the opening depending on the resolution of the electric proportional control valve, allowing for smoother adjustment of the recovery rate compared to step-wise adjustment of the opening using a combination of solenoid valves, etc. For example, in a step-wise system that can only control the recovery rate in five steps (50%, 55%, 60%, 65%, 70%) within the range of 50-70%, if the target recovery rate is set to 64%, the recovery rate can only be adjusted to 60%, resulting in wasted concentrated wastewater. Therefore, using an electric proportional control valve as the flow control valve CV1 is advantageous from the perspective of water conservation because it can reduce the waste of concentrated wastewater.

[0047] However, when using an electric proportional control valve as the flow control valve CV1, attention must be paid to the relationship between its opening / closing speed and the calculation speed (computation speed) of the set flow rate of the concentrated wastewater. For example, if the two speeds are significantly different, hunting may occur if the set flow rate is changed before the electric proportional control valve has completed opening and closing and the flow rate of the concentrated wastewater has stabilized. Furthermore, because the target flow rate of the concentrated wastewater is determined based on the flow rate of the permeated water detected by the treated water flow meter 13, the control of the flow rate of the concentrated wastewater may also be affected by the response speed of the inverter that controls the rotation speed of the pressure pump 12. Therefore, when determining the calculation speed of the target flow rate of the concentrated wastewater, it is preferable to take into account the opening / closing speed of the electric proportional control valve and the response speed of the inverter. That is, if the opening / closing speed of the electric proportional control valve is slow, it is preferable to slow down the response speed of the inverter, and if the opening / closing speed of the electric proportional control valve is fast, it is preferable to speed up the response speed of the inverter.

[0048] As described above, in this embodiment, the constant flow rate of the concentrated water is maintained constant by the constant flow valve 14. Therefore, simply by regulating the flow rate of the concentrated water flowing through one of the drain line L4 and the reflux water line L5, the flow rate of the concentrated water flowing through the other line can also be regulated. Therefore, in the illustrated embodiment, the drain line L4 is provided with a flow rate adjustment valve CV1 and a drain flow meter 15 as flow rate control means, and the reflux water line L5 is provided with a manual valve MV1 for adjusting the pressure balance. However, the reverse is also possible. That is, the reflux water line L5 may be provided with a flow rate adjustment valve (proportional control valve) and a flow meter, and the drain line L4 may be provided with a manual valve for adjusting the pressure balance. Alternatively, both the drain line L4 and the reflux water line L5 may be provided with flow rate adjustment valves (proportional control valves) and flow meters. In this case, the flow rates of the concentrated water flowing through both the drain line L4 and the reflux water line L5 can be adjusted, thereby maintaining a constant flow rate of the concentrated water flowing through the concentrate water line L3. Therefore, the constant flow valve 14 is not necessarily provided. On the other hand, even when a flow control means is provided only on the drainage line L4 as in this embodiment, the constant flow valve 14 does not necessarily have to be provided as long as the flow rate of the concentrated water flowing through the concentrated water line L3 can always be ensured to be equal to or greater than the minimum flow rate of the concentrated water required by the filtration means 11, for example, by using a large-sized pressure pump 12.

[0049] However, if the quality of the raw water deteriorates or the filtration means 11 (RO membrane or NF membrane) deteriorates, the quality of the permeate from the membrane filtration device 3 may decline and no longer meet the water quality standards for the water supplied to the EDI device 4. As a result, the EDI device 4 may be unable to produce deionized water (pure water) that meets the required water quality. Therefore, if such a decline in water quality occurs, it is preferable to restore the quality as quickly as possible. To achieve this, the control unit 5 monitors the quality of the permeate flowing through the treated water line L2 and, based on the monitoring results, determines whether to execute operational control to restore the quality of the permeate from the membrane filtration device 3. That is, the control unit 5 determines whether the quality of the permeate flowing through the treated water line L2 meets the required water quality. If it determines that the quality does not meet the required water quality, the control unit 5 stops the water supply from the membrane filtration device 3 to the EDI device 4 and executes a circulation operation in which the permeate from the membrane filtration device 3 is returned to its upstream side. Specifically, by switching the three-way valve TV1, the permeated water separated by the filtration means 11 is returned from the treated water line L2 through the treated water return line L6 to the raw water tank 2 and then returned to the supply line L1. In this way, the permeated water circulation operation is performed, and the quality of the treated water from the membrane filtration device 3 can be restored.

[0050] This permeate circulation operation is also performed when the apparatus is started up or restarted, until the quality of the permeate from the membrane filtration apparatus 3 reaches a certain level or higher, i.e., until it meets the water quality standard for the water supply to the EDI device 4. Then, the supply of water from the membrane filtration apparatus 3 to the EDI device 4 is started or resumed, thereby starting or resuming operation of the EDI device 4. Whether the quality of the permeate from the membrane filtration apparatus 3 meets the predetermined water quality can be determined based on whether the conductivity of the permeate detected by the treated water conductivity meter 16 is below a predetermined value. That is, if the value detected by the treated water conductivity meter 16 is below the predetermined value, it is determined that the quality of the permeate meets the predetermined water quality. If the value detected by the treated water conductivity meter 16 is above the predetermined value, it is determined that the quality of the permeate does not meet the predetermined water quality. Note that a resistivity meter may be installed instead of a conductivity meter to monitor the water quality by detecting the resistivity of the permeate.

[0051] On the other hand, the EDI device 4 may also experience a decline in processing performance due to the accumulation of ions such as silica and deterioration of internal components, resulting in an inability to produce pure water that meets the required water quality. Therefore, the control unit 5 also monitors the quality of the pure water flowing through the pure water line L8. If it determines that the water quality does not meet the required quality, it stops the supply of pure water from the EDI device 4 to the point of use, as in the permeate circulation operation, and performs a circulation operation in which the pure water from the EDI device 4 is returned upstream of the membrane filtration device 3. That is, the control unit 5 switches the three-way valve TV2 to return the pure water produced by the EDI device 4 from the pure water line L8 through the pure water return line L11 to the raw water tank 2 and then to the supply line L1. This pure water circulation operation restores the quality of the pure water from the EDI device 4. In particular, in the EDI device 4, passing pure water through the desalting chamber D promotes the regeneration of the ion exchanger, thereby further restoring the quality of the pure water from the EDI device 4. During the pure water circulation operation, the on-off valve MV2 is closed and the on-off valve MV3 is opened, so that the EDI concentrated water is also returned to the raw water tank 2 through the concentrated water return line L12.

[0052] This pure water circulation operation, like the permeate circulation operation, is also performed when the device is started or restarted. That is, after the permeate circulation operation is completed, when the supply of water from the membrane filtration device 3 to the EDI device 4 is started or resumed, the pure water circulation operation is initiated and continues until the quality of the pure water produced by the EDI device 4 meets the predetermined quality. Whether the quality of the pure water from the EDI device 4 meets the predetermined quality can be determined based on whether the conductivity of the pure water detected by the pure water conductivity meter 23 is equal to or less than a predetermined value. That is, if the value detected by the pure water conductivity meter 23 is equal to or less than the predetermined value, it is determined that the quality of the pure water meets the predetermined quality. If the value detected by the pure water conductivity meter 23 is equal to or less than the predetermined value, it is determined that the quality of the pure water does not meet the predetermined quality. Note that a resistivity meter may be installed instead of a conductivity meter to monitor the water quality by detecting the resistivity of the pure water.

[0053] During the permeate circulation operation, deionized water (pure water) is not produced by the EDI device 4, and pure water does not flow through the pure water line L8. Therefore, the control unit 5 cannot control the pressure pump 12 according to the flow rate of the pure water, i.e., cannot control the pure water flow rate. Therefore, in this case, instead of controlling the pure water flow rate, the control unit 5 can control the pressure pump 12 so that the flow rate of the permeate flowing from the treated water line L2 through the treated water return line L6, specifically, the flow rate of the permeate detected by the treated water flow meter 13, becomes a set flow rate. Alternatively, the control unit 5 can maintain the rotation speed of the pressure pump 12 constant during the permeate circulation operation.

[0054] In contrast, because membrane separation processing is performed by the membrane filtration device 3 during circulation operation of both the permeate and the pure water, the control unit 5 can execute wastewater flow rate control, which controls the flow rate of the concentrated water (concentrated wastewater) flowing through the drainage line L4. In wastewater flow rate control, as described above, the target recovery rate is calculated based on the water temperature, thereby minimizing the risk of scale formation and achieving water conservation by minimizing the flow rate of the concentrated wastewater. However, while such water-saving effects can be expected during normal operation, during circulation operation of the permeate and pure water, in which pure water is not supplied to the point of use, the concentrated wastewater is simply discharged to the outside. Therefore, even if the optimal target flow rate of the concentrated wastewater is calculated based on the water temperature, water will be wasted. Therefore, to achieve further water conservation, there is still room for improvement in setting the target recovery rate (target flow rate of the concentrated wastewater) during circulation operation of the permeate and pure water.

[0055] Therefore, in this embodiment, the target recovery rate of the filtration means 11 is set during the circulation operation of the permeated water and the pure water by a method different from that during normal operation. Specifically, the target recovery rate of the filtration means 11 is set to a first target value (variable value) calculated based on the water temperature as described above during normal operation, while the target recovery rate is set to a predetermined second target value (fixed value) higher than the first target value during circulation operation. This allows the flow rate of concentrated wastewater to be reduced during the circulation operation of the permeated water and the pure water compared to normal operation, thereby reducing water waste and achieving further water conservation. Note that during the circulation operation of the permeated water and the pure water, the quality of the circulating permeated water and the pure water is improved as described above, so the impurity concentration of the supply water to the first filtration means 11 (the water to be treated that is actually supplied to the first filtration means 11) is lower than the impurity concentration of the raw water measured in advance. As a result, the target recovery rate during circulation operation is set to a second target value higher than the first target value calculated based on the water temperature, thereby preventing an increased risk of scale formation even when the flow rate of concentrated wastewater is reduced. The second target value at this time is not particularly limited and can be determined, for example, based on the flow rate range in which the wastewater flow meter 15 provided in the wastewater line L4 operates normally. Alternatively, the second target value may be set to the upper limit of the control range of the recovery rate estimated from the silica concentration and calcium concentration of the raw water.

[0056] If water conservation alone were to be considered, it would be possible to return all of the concentrated water separated by the filtration means 11 to the raw water tank 2. However, in this case, raw water would not be replenished to the raw water tank 2 during the permeate circulation operation, and the quality of the circulating permeate would not be improved, defeating the purpose of the circulation operation. Therefore, during the permeate circulation operation, it is preferable that even a small amount of concentrated water from the filtration means 11 be discharged to the outside through the drain line L4. On the other hand, during the pure water circulation operation, as described above, the electrode water is discharged to the outside through the electrode water discharge line L10. If the flow rate of the concentrated wastewater is lower than this, the concentration of ionic components in the system will proceed. Therefore, during the pure water circulation operation, it is preferable that the flow rate of the concentrated wastewater be equal to or greater than the flow rate of the electrode water discharged to the outside.

[0057] In the embodiment described above, two flow rate controls are performed by one control unit 5, but each flow rate control may be performed by a separate control unit. Note that in this embodiment, since there is one filtration means 11, the permeated water separated by the filtration means 11 corresponds to the treated water generated in the membrane filtration device 3. Therefore, the treated water line L2 in this embodiment corresponds to the permeated water line of the present invention.

[0058] As mentioned above, the illustrated configuration of the pure water production system 1 is merely an example, and it goes without saying that it can be modified as appropriate depending on the purpose, application, and required performance of the system. In particular, the electrodeionized water production system 4 may be omitted as long as the membrane filtration system 3 is provided. In this case, the treated water produced by the membrane filtration system 3, i.e., the permeated water separated by the filtration means 11, corresponds to the pure water produced by the pure water production system 1. Therefore, the treated water line L2 of this embodiment corresponds to the pure water line of the present invention, and the treated water return line L6 corresponds to the pure water return line of the present invention.

[0059] (Second embodiment) 3 is a schematic diagram of a membrane filtration device according to a second embodiment of the present invention. This embodiment is a modified version of the first embodiment, and differs from the first embodiment in that the configuration of the membrane filtration device has been changed. Therefore, the pure water production system of this embodiment has the same configuration as the first embodiment, except for the configuration of the membrane filtration device. Hereinafter, the same components as those in the first embodiment will be assigned the same reference numerals in the drawings and their description will be omitted, and only the components that differ from the first embodiment will be described.

[0060] In this embodiment, in addition to the filtration means (first filtration means) 11 of the first embodiment, another filtration means (second filtration means) 17 is provided downstream thereof. The second filtration means 17 is connected in series to the first filtration means 11 so as to treat the permeated water separated by the first filtration means 11 as water to be treated. As a result, the membrane filtration device 3 of this embodiment can produce treated water of better quality than the first embodiment. Hereinafter, the permeated water and concentrated water separated by the first filtration means 11 will also be referred to as the "primary permeated water" and the "primary concentrated water," respectively, and the permeated water and concentrated water separated by the second filtration means 17 will also be referred to as the "secondary permeated water" and the "secondary concentrated water," respectively.

[0061] Accordingly, in this embodiment, some of the configurations of the first embodiment are changed and some new configurations are added. That is, in this embodiment, the treated water line L2 is connected to the second filtration means 17, and a permeate line L13 is connected between the first filtration means 11 and the second filtration means 17, through which the primary permeate from the first filtration means 11 flows and is supplied to the second filtration means 17. Therefore, in this embodiment, the secondary permeate from the second filtration means 17 is supplied to the EDI device 4 as the treated water generated in the membrane filtration device 3, and a circulation operation is performed in which the secondary permeate is returned to the upstream side of the membrane filtration device 3 as needed. At this time, instead of pure water flow rate control, treated water flow rate control is performed, in which the pressure pump 12 is controlled so that the flow rate of the secondary permeate detected by the treated water flow meter 13 becomes the set flow rate. In addition, a secondary concentrate line L14 is connected to the second filtration means 17, through which the secondary concentrate from the second filtration means 17 flows. In the second filtration means 17, the primary permeate from the first filtration means 11 is further separated into secondary permeate and secondary concentrate, and therefore, from the viewpoint of water quality, it is not necessarily necessary to discharge the secondary concentrate to the outside. Therefore, from the viewpoint of water conservation, the secondary concentrate line L14 is connected to the raw water tank 2 to return all of the secondary concentrate to the raw water tank 2. However, in some cases, part or all of the secondary concentrate may be discharged to the outside, and a drain line for this purpose may be connected to the secondary concentrate line L14.

[0062] The secondary concentrate line L8 is provided with a manual valve MV4 and a concentrate flow meter 18 for adjusting the flow rate of the secondary concentrate flowing through the secondary concentrate line L8. This allows the return rate of the second filtration means 17 (the ratio of the flow rate of the secondary concentrate to the sum of the flow rate of the secondary permeate and the flow rate of the secondary concentrate) to be adjusted as desired. To eliminate the cumbersome task of manually adjusting the return rate, a proportional control valve whose opening can be adjusted based on the flow rate of the secondary concentrate detected by the concentrate flow meter 18 may be provided instead of the manual valve MV4. Alternatively, to maintain the return rate within a certain range, a constant flow valve may be provided instead of the manual valve MV4 and the concentrate flow meter 18. In this case, depending on the conditions, the pressure difference between the primary and secondary sides of the constant flow valve may exceed the operating differential pressure range (the allowable pressure difference range for the constant flow valve to operate normally). To avoid this, a pressure reducing valve may be provided upstream of the constant flow valve. As described above, the second filtration means 17 is supplied with the primary permeate from the first filtration means 11, which has a low impurity concentration, so from the perspective of water conservation, it is preferable to set the return rate of the second filtration means 17 low.

[0063] In this embodiment, as in the first embodiment, pure water flow rate control and wastewater flow rate control are performed in parallel during normal operation and pure water circulation operation, and treated water flow rate control and wastewater flow rate control are performed in parallel during secondary permeate circulation operation. In this embodiment, since no flow meter is provided in the permeate line L13, when calculating the target flow rate of concentrated water based on the target recovery rate during wastewater flow rate control, the flow rate of the primary permeate flowing through the permeate line L13 is indirectly detected using the treated water flow meter 13 and the concentrated water flow meter 18. That is, the detected flow rate of the primary permeate is calculated (acquired) as the sum of the flow rate of the secondary permeate detected by the treated water flow meter 13 and the flow rate of the secondary concentrated water detected by the concentrated water flow meter 18. However, a flow meter (not shown) may be provided in the permeate line L13 to directly detect the flow rate of the primary permeate. Furthermore, as described above, if a constant flow valve is provided instead of the manual valve MV4 and the concentrated water flow meter 18, the flow rate of the primary permeate may be indirectly detected using the specified flow rate of the constant flow valve instead of the value detected by the concentrated water flow meter 18. Alternatively, during the secondary permeate circulation operation, if the flow rate of the secondary permeate is adjusted to a constant value by the treated water flow rate control and the flow rate of the secondary concentrated water is also adjusted to a constant value based on the predetermined target value of the return rate of the second filtration means 17, the primary permeate will also be adjusted to a substantially constant value, and therefore the target flow rate of the concentrated wastewater may be calculated using such a substantial target flow rate of the primary permeate.

[0064] In this embodiment, raw water must be supplied to the two filtration means 11 and 17 using one pressure pump 12, and therefore the supply pressure of raw water to the first filtration means 11 by the pressure pump 12 is higher than in the first embodiment. Therefore, the specified flow rate of the constant flow valve 14 must be set taking this into consideration. For example, if RO membranes with a diameter of approximately 20.32 cm (8 inches) are used as the two filtration means 11 and 17, the applicable temperature range of the first filtration means 11 is 5 to 35°C, and the recovery rate control range is expected to be 50 to 85% based on the silica concentration and calcium concentration of the raw water, then, for example, a constant flow valve manufactured by Keihin Corporation (product number: NSPW-25, set flow rate: 55 L / min) can be used as the constant flow valve 14.

[0065] In this embodiment, two filtration means 11 and 17 are connected in series. However, the number of filtration means is not limited to this. Three or more filtration means may be connected in series. In this case, the permeate line L2 in this embodiment is connected to the most downstream filtration means among the three or more filtration means, and the most upstream filtration means among the three or more filtration means corresponds to the filtration means of the present invention. Note that the set flow rate of the concentrated wastewater from the most upstream filtration means is calculated using the flow rate of the permeate separated by the most upstream filtration means, not the permeate separated by the most downstream filtration means. Furthermore, "connected in series" as used herein means that the water to be treated is sequentially treated by multiple filtration means, and between two adjacent filtration means, the permeate separated by the upstream filtration means is supplied as the water to be treated to the downstream filtration means. Furthermore, each filtration means may be composed of multiple RO or NF membranes. In this case, the multiple RO or NF membranes are connected in series on their primary sides (the sides through which raw water and concentrated water flow) and are ultimately connected to the concentrated water line, and their secondary sides (the sides through which permeate water flows) are connected in parallel and are ultimately connected to the permeate line. [Explanation of symbols]

[0066] 1 Pure water production equipment 2 Raw water tank 3. Membrane filtration equipment 4. EDI equipment (electrodeionized water production equipment) 5. Control section 11 Filtering means (first filtering means) 12 Pressure pump 13 Treated water flow meter 14 Constant flow valve 15 Drain flow meter 16 Treated water conductivity meter 17 Secondary filtration means 21 Anode 22 Cathode 23 Pure water flow meter 24 Pure water conductivity meter D Desalination room C1,C2 Concentration chamber E1,E2 Electrode chamber a1, a2 Anion exchange membrane c1, c2 cation exchange membrane L1 supply line L2 treated water line L3 Concentrated water line (primary concentrated water line) L4 drain line L5 Return water line L6 Treated water return line L7 Raw water supply line L8 Pure water line L9 Concentrated water discharge line L10 Electrode water discharge line L11 Pure water return line L12 Concentrated water return line L13 Permeate line L14 Secondary concentrated water line CV1 flow control valve MV1, MV4 manual valves MV2, MV3 on-off valve TV1, TV2 three-way valve

Claims

1. A pure water producing apparatus for producing pure water by sequentially treating water to be treated, a membrane filtration device that includes a filtration means having a reverse osmosis membrane or a nanofiltration membrane that separates the water to be treated into permeate and concentrated water, and treats the water to be treated to produce treated water; an electrodeionization water production device connected downstream of the membrane filtration device and producing deionized water from treated water produced by the membrane filtration device; a control unit that switches the operation of the pure water production apparatus between a normal operation in which deionized water produced by the electrodeionized water production apparatus is supplied to a point of use as the pure water, and a circulation operation in which treated water produced by the membrane filtration apparatus or deionized water produced by the electrodeionized water production apparatus is returned to the upstream side of the membrane filtration apparatus, The membrane filtration device has a supply line that supplies the water to be treated to the filtration means, a permeate line that circulates permeate from the filtration means, a concentrated water line that circulates concentrated water from the filtration means, a drainage line that branches off from the concentrated water line and discharges a portion of the concentrated water flowing through the concentrated water line to the outside, a return water line that branches off from the concentrated water line and returns the remainder of the concentrated water flowing through the concentrated water line to the upstream side of the membrane filtration device, and a flow rate adjustment means that adjusts the flow rate of the concentrated water flowing through the drainage line, the electrodeionization water production apparatus has an electrode chamber including an anode chamber and a cathode chamber, through which a portion of the treated water produced by the membrane filtration apparatus is passed as electrode water; the control unit, in the normal operation and the circulation operation, calculates a target flow rate of the concentrated water flowing through the drain line based on a target value of a recovery rate, which is the ratio of the flow rate of the permeated water flowing through the permeated water line to the sum of the flow rate of the permeated water flowing through the permeated water line and the flow rate of the concentrated water flowing through the drain line, and performs drain flow rate control to control the flow rate adjustment means so that the flow rate of the concentrated water flowing through the drain line becomes the target flow rate; in the normal operation, the target value of the recovery rate is set to a first target value; and in the circulation operation, the target value of the recovery rate is set to a predetermined second target value that is higher than the first target value; A pure water manufacturing apparatus in which the predetermined second target value is set so that the target flow rate of concentrated water flowing through the drain line is equal to or greater than the flow rate of the electrode water passed through the electrode chamber and discharged to the outside.

2. The membrane filtration device has a water temperature detection means for detecting the water temperature of any of the water to be treated supplied to the filtration means, the permeated water from the filtration means, and the concentrated water from the filtration means, 2. The pure water producing apparatus according to claim 1, wherein during the normal operation, the control unit calculates a maximum recovery rate at which silica or calcium does not precipitate on the membrane surface of the reverse osmosis membrane or nanofiltration membrane of the filtration means based on the value detected by the water temperature detection means, and sets the calculated value as the first target value.

3. A system comprising: a pure water line for circulating deionized water produced by the electrical deionized water production apparatus; and a pure water return line branching off from the pure water line and connected to the upstream side of the membrane filtration apparatus; 3. The pure water producing apparatus of claim 2, wherein the control unit performs the normal operation of supplying the produced deionized water to the point of use through the pure water line when the water quality of the deionized water produced by the electrodeionized water producing apparatus satisfies a predetermined water quality, and performs the circulation operation of returning the produced deionized water from the pure water line through the pure water return line to the upstream side of the membrane filtration apparatus when the water quality does not satisfy the predetermined water quality.

4. The membrane filtration device has a pressure adjusting means for adjusting the pressure of the water to be treated flowing through the supply line, 4. The pure water producing apparatus according to claim 3, wherein the control unit, in parallel with the drainage flow rate control, executes pure water flow rate control by controlling the pressure adjustment means so that the flow rate of deionized water flowing through the pure water line becomes a set flow rate during the normal operation, and so that the flow rate of deionized water flowing from the pure water line to the pure water return line becomes the set flow rate during the circulation operation.

5. The membrane filtration device has a treated water line through which treated water generated in the membrane filtration device flows, and a treated water return line branching from the treated water line and connected to the upstream side of the membrane filtration device, The pure water manufacturing apparatus of claim 3, wherein the control unit performs the circulation operation of returning the generated treated water from the treated water line to the upstream side of the membrane filtration device through the treated water return line when the water quality of the treated water generated by the membrane filtration device does not meet a predetermined water quality.

6. The membrane filtration device has a pressure adjusting means for adjusting the pressure of the water to be treated flowing through the supply line, The pure water manufacturing apparatus of claim 5, wherein, in parallel with the wastewater flow rate control, the control unit performs pure water flow rate control in which, during normal operation, the control unit controls the pressure adjustment means so that the flow rate of deionized water flowing through the pure water line becomes a set flow rate, and during circulation operation, the control unit performs treated water flow rate control in which the control unit controls the pressure adjustment means so that the flow rate of treated water flowing from the treated water line through the treated water return line becomes a set flow rate.

7. 7. The water purification system according to claim 1, wherein the membrane filtration device has at least one other filtration means connected in series downstream of the filtration means via the permeate line.

8. a membrane filtration unit that treats the water to be treated and produces treated water, the membrane filtration unit having a filtration means with a reverse osmosis membrane or a nanofiltration membrane that separates the water to be treated into permeate and concentrate, and an electrodeionization water production unit connected downstream of the membrane filtration unit and producing deionized water from the treated water produced by the membrane filtration unit; the membrane filtration unit having a supply line that supplies the water to be treated to the filtration unit, a permeate line that distributes the permeate from the filtration unit, a concentrate line that distributes the concentrate from the filtration unit, a drain line that branches off from the concentrate line and discharges a portion of the concentrate flowing through the concentrate line to the outside, and a return water line that branches off from the concentrate line and returns the remainder of the concentrate flowing through the concentrate line to the upstream side of the membrane filtration unit; and a method for operating the electrodeionization water production unit, the electrodeionization water production unit having an electrode chamber consisting of an anode chamber and a cathode chamber, through which a portion of the treated water produced by the membrane filtration unit is passed as electrode water, supplying the deionized water produced by the electrodeionized water production apparatus to a point of use as the pure water; a step of returning treated water produced by the membrane filtration device or deionized water produced by the electrodeionized water production device to the upstream side of the membrane filtration device; When the deionized water is supplied to the point of use and when the treated water or deionized water is returned to the upstream side of the membrane filtration device, a target flow rate of the concentrated water flowing through the drainage line is calculated based on a target value of recovery rate, which is the ratio of the flow rate of the permeated water flowing through the permeated water line to the sum of the flow rate of the permeated water flowing through the permeated water line and the flow rate of the concentrated water flowing through the drainage line, and the flow rate of the concentrated water flowing through the drainage line is adjusted so that the flow rate of the concentrated water flowing through the drainage line becomes the target flow rate, When the deionized water is supplied to the point of use, the target value of the recovery rate is set to a first target value, and when the treated water or deionized water is returned to the upstream side of the membrane filtration device, the target value of the recovery rate is set to a predetermined second target value that is higher than the first target value; A method for operating a pure water manufacturing apparatus, wherein the predetermined second target value is set so that the target flow rate of concentrated water flowing through the drain line is equal to or greater than the flow rate of the electrode water passed through the electrode chamber and discharged to the outside.

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