Operation of a two-stage electrodeionization system.

Abstract

To provide a method for operating a two-stage electrodeionization system that reduces power consumption and stabilizes a quality of treated water.SOLUTION: After raw water W is treated in a reverse osmosis membrane device 3, permeate water W1 from the reverse osmosis membrane device 3 is treated in a first electrodeionization device 4 and a second electrodeionization device 5 in this order to obtain second deionized water W3. Conductivity of the water to be treated W1 is measured by a conductivity meter 6, and when the conductivity of the water to be treated W1 increases above a predetermined value, an operating current of the first electrodeionization device 4 is increased. On the other hand, when the conductivity of the water to be treated W1 falls below the predetermined value, the operating current (current density) of the first electrodeionization device 4 is reduced. This allows the amount of power consumption to be reduced while keeping a water quality (e.g., conductivity) of first deionized water W2 within a target range.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an operating method for a two-stage electrodeionization system in which electrodeionization devices for removing ionic components from water to be treated are arranged in two stages in series, and in particular to an operating method for a two-stage electrodeionization system that enables reduction in power consumption and stabilization of treated water quality. [Background technology]

[0002] A typical electrodeionization device has multiple cation exchange membranes and anion exchange membranes arranged alternately between a cathode and an anode to form deionization compartments (D) and concentration compartments (C), with the deionization compartments filled with ion exchange resin. Some devices also have concentration compartments filled with ion exchange resin.

[0003] This electrodeionization device removes cations and anions from the water being treated by applying a current between the cathode and anode, and is usually operated at a constant current. The operating current setting for this constant current operation is set to absorb fluctuations in the feedwater load. However, if this current setting is set to the performance limit of the electrodeionization device, performance will deteriorate when the feedwater load temporarily exceeds the treatment capacity, and recovery will take time (resulting in a deterioration in water quality).

[0004] Recently, electrodeionization systems in which two electrodeionization devices are connected in series have become increasingly popular in order to remove cations and anions from the water being treated to a higher degree. Summary of the Invention [Problem to be solved by the invention]

[0005] However, in a system with two electrodeionization devices connected in series, setting the operating current for constant current operation with a safety factor to absorb fluctuations in the water supply load results in a problem of high power consumption. On the other hand, setting the operating current to a low value reduces performance, so that if the quality of the treated water deteriorates, the quality of the resulting deionized water also deteriorates, and in such cases, it takes time to restore the quality. Furthermore, if the quality of the deionized water (treated water) suddenly deteriorates, it is difficult for constant current operation to keep up with the fluctuations in water quality.

[0006] The present invention has been made in consideration of the above-mentioned problems, and aims to provide an operating method for a two-stage electrodeionization system that enables reduction in power consumption and stabilization of treated water quality. [Means for solving the problem]

[0007] To achieve the above-mentioned object, the present invention provides a method for operating a two-stage electrodeionization system that produces deionized water with a water quality index value within a predetermined range using a system in which two electrodeionization devices are arranged in series, in which the operating current of one or both of the two electrodeionization devices is varied based on the water quality index values ​​of the treated water, the deionized water, and the concentrated water in each electrodeionization device, or on voltage fluctuations during operation of each electrodeionization device (Invention 1).

[0008] According to this invention (Invention 1), when the water quality index value of the treated water, the water quality index value of the deionized water, the water quality index value of the concentrated water in each electrodeionization device, or the voltage fluctuations during operation of each electrodeionization device, the current setting value of one of the two-stage electrodeionization devices can be controlled so that, for example, the current amount is reduced when the water quality index value of the treated water (feed water) is good, and the current amount is increased when the water quality index value of the treated water deteriorates, thereby making it possible to reduce power consumption and stabilize the quality of the deionized water (treated water).

[0009] In the above invention (Invention 1), when the water quality index value of the water to be treated in the two-stage electrodeionization system deteriorates below a predetermined range, it is preferable to increase the operating current of the electrodeionization device in the previous stage (Invention 2).

[0010] According to this invention (Invention 2), if the quality of the water to be treated (feed water) used as raw water for treatment in a two-stage electrodeionization system deteriorates, the operating current value of the electrodeionization device in the previous stage can be increased to stabilize the quality of the deionized water (treated water).

[0011] In the above invention (Invention 1), when the water quality index value of the water to be treated in the two-stage electrodeionization system improves above a predetermined range, it is preferable to reduce the operating current of the electrodeionization device in the previous stage (Invention 3).

[0012] According to this invention (Invention 3), if the quality of the water to be treated (feedwater) used as raw water for treatment in a two-stage electrodeionization system is good, the amount of power consumed can be reduced by reducing the operating current value of the electrodeionization device in the previous stage.

[0013] In the above invention (Invention 1), when the water quality index value of the deionized water from the upstream or downstream electrodeionization device deteriorates below a predetermined range, it is preferable to increase the operating current of the electrodeionization device (Invention 4).

[0014] According to this invention (Invention 4), if the quality of the deionized water from the electrodeionization device deteriorates below a predetermined range due to factors such as the quality of the water to be treated, the operating current value of the electrodeionization device can be increased to stabilize the quality of the deionized water (treated water).

[0015] In the above invention (Invention 1), when the water quality index value of the deionized water of the upstream or downstream electrodeionization device improves above a predetermined range, it is preferable to reduce the operating current of the electrodeionization device (Invention 5).

[0016] According to this invention (Invention 5), when the quality of the deionized water from the electrodeionization device improves above a predetermined range due to factors such as the quality of the water to be treated, the operating current value of the electrodeionization device can be reduced, thereby reducing power consumption.

[0017] In the above invention (Invention 1), when the water quality index of the concentrated water from the upstream or downstream electrodeionization device falls below a predetermined range, it is preferable to increase the operating current of the electrodeionization device (Invention 6).

[0018] According to this invention (Invention 6), if the quality of the concentrated water from the electrodeionization device deteriorates below a predetermined range due to factors such as the quality of the water to be treated, the operating current value of the electrodeionization device can be increased to stabilize the quality of the deionized water (treated water).

[0019] In the above invention (Invention 1), when the water quality index of the concentrated water from the upstream or downstream electrodeionization device rises above a predetermined range, it is preferable to reduce the operating current of the electrodeionization device (Invention 7).

[0020] According to this invention (Invention 7), when the quality of the concentrated water from the electrodeionization device improves above a predetermined range due to factors such as the quality of the water to be treated, the operating current value of the electrodeionization device can be reduced, thereby reducing power consumption.

[0021] In the above invention (Invention 1), when the operating voltage of the preceding or succeeding electrodeionization device increases above a predetermined range, it is preferable to increase the operating current of the electrodeionization device or the electrodeionization device immediately following the preceding electrodeionization device (Invention 8).

[0022] According to this invention (Invention 8), if the resistance value of any of the electrodeionization devices increases, the amount of operating current can be increased to stabilize the quality of the deionized water (treated water). [Brief explanation of the drawings]

[0023] [Figure 1] 1 is a schematic diagram illustrating a two-stage electrodeionization system in which a method of operating a two-stage electrodeionization system according to a first embodiment of the present invention can be implemented. [Figure 2] FIG. 2 is a schematic diagram illustrating a two-stage electrodeionization system in which a method of operating a two-stage electrodeionization system according to a second embodiment of the present invention can be implemented. [Figure 3] FIG. 10 is a schematic diagram illustrating a two-stage electrodeionization system in which a method of operating a two-stage electrodeionization system according to a third embodiment of the present invention can be implemented. [Figure 4] 1 is a graph showing the operating current, voltage, and conductivity of treated water in the operation method of the two-stage electrodeionization system of Example 1. [Figure 5] 1 is a graph showing the operating current, voltage, and conductivity of treated water in the operating method of the two-stage electrodeionization system of Comparative Example 1. [Figure 6] 1 is a graph showing the operating current, voltage, and conductivity of treated water in the operation method of the two-stage electrodeionization system of Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0024] Hereinafter, the method of operating the two-stage electrodeionization system of the present invention will be described in detail based on each embodiment.

[0025] First Embodiment <Two-stage electrodeionization system> 1 shows a two-stage electrodeionization system to which the operating method according to the first embodiment of the present invention can be applied. In this embodiment, the two-stage electrodeionization system 1 includes a reverse osmosis membrane device 3, a first electrodeionization device 4, and a second electrodeionization device 5, which are sequentially arranged in series in a water pipe 2 to which raw water W, such as pretreated water, is supplied. A conductivity meter 6 (water quality measuring means) is provided in the water pipe 2 upstream of the first electrodeionization device 4 to measure the quality of the permeate water W1 from the reverse osmosis membrane device 2, which serves as the water to be treated. In this embodiment, a conductivity meter 7 (water quality measuring means) is provided in the water pipe 2 at the outlet side of the second electrodeionization device 5 to measure the quality of the treated water (second deionized water) W3.

[0026] These conductivity meters 6 and 7 are connected to a control means such as a PLC (not shown) so that they can communicate information with each other. Based on this information, this control means can independently control the operating currents of the first electrodeionization device 4 and the second electrodeionization device 5.

[0027] <Operation method of two-stage electrodeionization system> Next, a method of operating the two-stage electrodeionization system described above will be described.

[0028] First, raw water W is treated in a reverse osmosis membrane device 3, and the permeate from the reverse osmosis membrane device 3 is supplied as treated water W1 to a first electrodeionization device 4. In this first electrodeionization device 4, the treated water W1 is introduced into the inlet of the deionization compartment, and first deionized water W2 is extracted from the outlet of the deionization compartment. Meanwhile, the first deionized water W2 or the treated water W1 is passed through the concentration compartment in the same direction as the deionization compartment or in the opposite direction, and the effluent from the concentration compartment is discharged outside the system as concentrated water. In addition, a portion of the first deionized water W2 or the treated water W1 is fed to the inlet of the anode compartment, the effluent from the anode compartment is fed to the inlet of the cathode compartment, and the effluent from the cathode compartment is discharged outside the system as wastewater.

[0029] Next, this first deionized water W2 is supplied to the second electrodeionization device 5. In this second electrodeionization device 5, the first deionized water W2 is introduced from the inlet side of the deionization compartment, and the second deionized water W3 is taken out from the outlet side of the deionization compartment. Meanwhile, the second deionized water W3 or the water to be treated W1 is passed through the concentration compartment in the same direction as the deionization compartment or in the opposite direction, and the effluent from the concentration compartment is discharged outside the system as concentrated water. In addition, a portion of the second deionized water W3 or the water to be treated W1 is fed to the inlet side of the anode compartment, the effluent from the anode compartment is fed to the inlet side of the cathode compartment, and the effluent from the cathode compartment is discharged outside the system as wastewater.

[0030] In this water flow treatment, the water flow LV of the deionization compartments of the first electrodeionization device 4 and the second electrodeionization device 5 is preferably 100 to 200 m / hr, and the water flow LV of the concentration compartments is preferably 10 to 50 m / hr. The water recovery rate is preferably about 80 to 95%. Furthermore, the current density of the electrodeionization device during the water flow treatment is 1000 mA / dm 2 or more, for example, 1000 to 2000mA / dm 2 It is preferable to set the following.

[0031] In the above-described two-stage electrodeionization system, the conductivity of the water W1 to be treated, which is the feedwater (permeate from the reverse osmosis membrane device 3) to the first electrodeionization device 4, is measured by the conductivity meter 6. If the conductivity of the water W1 increases above a predetermined value (equivalent to when the resistivity decreases below a predetermined value), the operating current (current density) of the first electrodeionization device 4 is increased. This allows the water quality (e.g., conductivity) of the first deionized water W2 to fall within a target range, thereby reducing the subsequent water supply load of the second electrodeionization device 5 and stably producing second deionized water W3 with the desired water quality. On the other hand, if the conductivity of the water W1 to be treated decreases below a predetermined value (equivalent to when the resistivity increases above a predetermined value), the operating current (current density) of the first electrodeionization device 4 is decreased. This reduces power consumption while maintaining the water quality (e.g., conductivity) of the first deionized water W2 within a target range.

[0032] In this embodiment, the conductivity of the second deionized water W3, which is the final deionized water, is measured using a conductivity meter 7. If the conductivity of the second deionized water W3 exceeds a predetermined value, the operating current (current density) of the second electrodeionization device 5 is increased. This allows the water quality (e.g., conductivity) of the second deionized water W3 to be within a target range, and second deionized water W3 with the desired water quality can be stably obtained. On the other hand, if the conductivity of the second deionized water W3 falls below the predetermined value, the operating current (current density) of the second electrodeionization device 5 is decreased. This allows the water quality (e.g., conductivity) of the second deionized water W3 to be within a target range while reducing power consumption.

[0033] In this way, controlling the operating current of the two-stage electrodeionization apparatuses 4 and 5 according to the quality of the water to be treated reduces power consumption and stabilizes the quality of the treated water. In this embodiment, a conductivity meter may be provided between the first electrodeionization apparatus 4 and the second electrodeionization apparatus 5 as a water quality measurement means for measuring the quality of the first deionized water W2. When the conductivity of the first deionized water W2 increases above a predetermined value, the operating current (current density) of the first electrodeionization apparatus 4 and / or the second electrodeionization apparatus 5 is increased. When the conductivity of the first deionized water W2 decreases below the predetermined value, the operating current (current density) of the first electrodeionization apparatus 4 and / or the second electrodeionization apparatus 5 is decreased.

[0034] Second Embodiment <Two-stage electrodeionization system> Figure 2 shows a two-stage electrodeionization system to which the operating method according to the second embodiment of the present invention can be applied. In Figure 2, the same components as those in the first embodiment are designated by the same reference numerals. In the two-stage electrodeionization system 11 of the second embodiment, a reverse osmosis membrane device 3, a first electrodeionization device 4, and a second electrodeionization device 5 are sequentially arranged in series in a water pipe 2 to which raw water W, such as pretreated water, is supplied. A first conductivity meter 13 (water quality measuring means) for measuring water quality is provided in a discharge path 12 for concentrated water W4 from the first electrodeionization device 4, and a second conductivity meter 15 (water quality measuring means) for measuring water quality is provided in a discharge path 14 for concentrated water W5 from the second electrodeionization device 5.

[0035] The first conductivity meter 13 and the second conductivity meter 15 are connected to a control means such as a PLC (not shown) so as to be able to communicate information with each other. Based on this information, the control means can independently control the operating currents of the first electrodeionization device 4 and the second electrodeionization device 5.

[0036] <Operation method of two-stage electrodeionization system> Next, a method of operating the two-stage electrodeionization system described above will be described.

[0037] The process for producing second deionized water W3 using the reverse osmosis membrane device 3, the first electrodeionization device 4, and the second electrodeionization device 5 is the same as that of the first embodiment described above, and therefore will not be described again.

[0038] In this method of operating a two-stage electrodeionization system, the conductivity of the concentrated water W4 in the first electrodeionization device 4 is measured by the first conductivity meter 13. If the conductivity of the concentrated water W4 exceeds a predetermined value (e.g., the resistivity decreases below a predetermined value), the operating current (current density) of the first electrodeionization device 4 is increased. This allows the quality (e.g., conductivity) of the first deionized water W2 to be within a target range, thereby reducing the subsequent water supply load of the second electrodeionization device 5 and stably producing second deionized water W3 with the desired quality. On the other hand, if the conductivity of the concentrated water W4 decreases below a predetermined value (e.g., the resistivity increases above a predetermined value), the operating current (current density) of the first electrodeionization device 4 is decreased. This reduces power consumption while maintaining the quality (e.g., conductivity) of the first deionized water W2 within a target range.

[0039] On the other hand, the conductivity of the concentrated water W5 in the second electrodeionization device 5 is measured by the second conductivity meter 15. If the conductivity of the concentrated water W5 increases above a predetermined value (the same as when the resistivity decreases below a predetermined value), the operating current (current density) of the second electrodeionization device 5 is increased. This allows the water quality (e.g., conductivity) of the second deionized water W3 to fall within the target range. On the other hand, if the conductivity of the concentrated water W5 decreases below a predetermined value (the same as when the resistivity increases above a predetermined value), the operating current (current density) of the second electrodeionization device 5 is decreased. This allows the water quality (e.g., conductivity) of the second deionized water W3 to fall within the target range while reducing power consumption.

[0040] By independently controlling the operating current (current density) of either or both of the first electrodeionization device 4 and the second electrodeionization device 5, it is possible to reduce power consumption while maintaining the water quality (e.g., conductivity) of the second deionized water W3 within a target range.

[0041] Third Embodiment <Two-stage electrodeionization system> 3 shows a two-stage electrodeionization system to which the operating method according to the third embodiment of the present invention can be applied. The two-stage electrodeionization system 21 of the second embodiment has the same configuration as the first embodiment described above, except that it does not have the conductivity meters 6 and 7. The control means constantly monitors the voltages of the first electrodeionization device 4 and the second electrodeionization device 5 during operation, and can independently control the operating currents of the first electrodeionization device 4 and the second electrodeionization device 5 based on the voltage values.

[0042] <Operation method of two-stage electrodeionization system> Next, a method of operating the two-stage electrodeionization system described above will be described.

[0043] The process for producing second deionized water W3 using the reverse osmosis membrane device 3, the first electrodeionization device 4, and the second electrodeionization device 5 is the same as that of the first embodiment described above, and therefore will not be described again.

[0044] In this method of operating a two-stage electrodeionization system, if the quality of the water W1 to be treated (the permeate of the reverse osmosis membrane device 3) to the first electrodeionization device 4 decreases, increasing the water supply load or the flow rate of the water W1, the operating voltage of the first electrodeionization device 4 increases. Therefore, if a high safety margin is not maintained during constant current operation, the water quality will deteriorate. In contrast, in this embodiment, the operating current (current density) of the first electrodeionization device 4 is increased. This allows the water quality (e.g., conductivity) of the first deionized water W2 to be within a target range, thereby reducing the water supply load of the second electrodeionization device 5 and stably producing second deionized water W3 with the desired water quality. On the other hand, if the quality of the water W1 to be treated improves and the water supply load decreases or the flow rate of the water W1 decreases, the operating voltage of the first electrodeionization device 4 decreases, and the operating current (current density) of the first electrodeionization device 4 is therefore reduced. This makes it possible to reduce the amount of power consumption while keeping the water quality (for example, conductivity) of the first deionized water W2 within a target range.

[0045] On the other hand, if the quality of the first deionized water W2, which is the feed water (permeate from the reverse osmosis membrane device 3) to the second electrodeionization device 5, decreases and the water supply load increases, or if the flow rate of the first deionized water W2 increases, the operating voltage of the second electrodeionization device 5 increases, and the operating current (current density) of the second electrodeionization device 5 is increased. This allows the water quality (e.g., conductivity) of the second deionized water W3 to be within a target range, and second deionized water W3 of the desired quality can be stably obtained. On the other hand, if the quality of the first deionized water W2 improves and the water supply load decreases, or the flow rate of the first deionized water W2 decreases, the operating voltage of the second electrodeionization device 5 decreases, and the operating current (current density) of the second electrodeionization device 5 is reduced. This allows the water quality (e.g., conductivity) of the second deionized water W3 to be within a target range, and power consumption can be reduced.

[0046] By independently controlling the operating current (current density) of either or both of the first electrodeionization device 4 and the second electrodeionization device 5, it is possible to reduce power consumption while maintaining the water quality (e.g., conductivity) of the second deionized water W3 within a target range.

[0047] Although the present invention has been described above based on the above-described embodiments, it is not limited to these embodiments and various modifications are possible. For example, the two-stage electrodeionization system 1 can be applied to various systems as long as it has two electrodeionization devices connected in series. Furthermore, the water quality indicator is not limited to conductivity; various indicators such as resistivity, TOC, and particulate matter can be used alone or in combination. Furthermore, the first to third embodiments described above can be combined and applied. Furthermore, the operating method of the present invention can also be applied to systems in which three or more electrodeionization devices are connected in series. [Example]

[0048] The present invention will be described in more detail based on the following specific examples.

[0049] [Example 1] In the two-stage electrodeionization system 1 shown in Figure 1, water W1 to be treated, which had a conductivity of 1 mS / m, was supplied to the first electrodeionization device 4. The electrodeionization system 1 was operated at an initial operating voltage of 8 A. The conductivity of the water W1 was varied between 0.2 and 2 mS / m, and the operating current of the first electrodeionization device 4 was varied according to the conductivity. The operating voltage of the first electrodeionization device 4 was measured during this period. The results of the fluctuations in conductivity, operating current, and operating voltage are shown in Figure 4. Note that the time axis in Figure 4 indicates relative values.

[0050] [Comparative Example 1] In the two-stage electrodeionization system 1 shown in Figure 1, water W1 to be treated, which had a conductivity of 1 mS / m, was supplied to the first electrodeionization device 4. The electrodeionization system 1 was operated at a constant current of 10 A initially, and the conductivity of the water W1 was varied between 0.2 and 2 mS / m. The operating voltage of the first electrodeionization device 4 was measured. The results of the variations in conductivity, operating current, and operating voltage are shown in Figure 5. Note that the time axis in Figure 5 indicates relative values.

[0051] 4 and 5, the method of operating the two-stage electrodeionization system of Comparative Example 1 (conventional example) varied the operating current in response to the conductivity of the water to be treated W1, thereby maintaining the quality of the first deionized water W2. In contrast, the method of operating the two-stage electrodeionization system of Example 1 varied the operating current in response to the conductivity, thereby varying the operating voltage accordingly. This allowed the two-stage electrodeionization system 1 to operate at an appropriate current value while maintaining the quality of the first deionized water W2, thereby reducing power consumption compared to Comparative Example 1.

[0052] [Example 2] In the two-stage electrodeionization system 1 shown in Figure 3, water W1 to be treated, which had a conductivity of 1 mS / m, was supplied to the first electrodeionization device 4 and operated at an initial operating voltage of 8 A. The conductivity of the water W1 was varied between 0.2 and 2 mS / m, and the operating current of the first electrodeionization device 4 was varied in response to the operating voltage of the first electrodeionization device 4 that varied with the conductivity. The results of these variations in conductivity, operating current, and operating voltage are shown in Figure 6. Note that the time axis in Figure 6 indicates relative values.

[0053] 5 and 6, the method of operating the two-stage electrodeionization system of Comparative Example 1 (conventional example) was able to maintain the quality of the first deionized water W2 by varying the operating voltage in accordance with the operating voltage of the first electrodeionization device 4. In contrast, the method of operating the two-stage electrodeionization system of Example 2 was able to operate the two-stage electrodeionization system 1 at an appropriate current value while maintaining the quality of the first deionized water W2 by varying the operating current in accordance with the fluctuations in operating voltage that accompany fluctuations in conductivity, thereby reducing power consumption compared to Comparative Example 1. [Explanation of symbols]

[0054] 1,11,21 Two-stage electrodeionization system 2 Water pipe 3 Reverse osmosis membrane device 4. First electrodeionization device 5. Second electrodeionization device 6 Conductivity meter 7 Conductivity meter 12,14 Concentrated water discharge path 13 First Conductivity Meter 15 Second Conductivity Meter W Raw Water W1 Water to be treated (permeate from reverse osmosis membrane device) W2 First deionized water W3 Second deionized water (deionized water) W4 First electrodeionization unit concentrate W5 Second electrodeionization unit concentrate

Claims

1. A method for operating a two-stage electrodeionization system that produces deionized water with a water quality index value within a predetermined range, comprising: The two-stage electrodeionization system is a system in which a first electrodeionization apparatus and a second electrodeionization apparatus are arranged in series in this order in two stages, the first electrodeionization device and the second electrodeionization device are configured so that their operating currents can be controlled independently; the first electrodeionization device and the second electrodeionization device are configured so that their operating voltages can be controlled independently; The two-stage electrodeionization system is configured to be able to perform all of the following (i) to (iv): A method for operating a two-stage electrodeionization system, comprising controlling the operation of the two-stage electrodeionization system so as to satisfy at least one of the following conditions (i) to (iv): (i) Varying the operating current of the first electrodeionization device based on the water quality index value of the water to be treated by the first electrodeionization device, and / or varying the operating current of the second electrodeionization device based on the water quality index value of the water to be treated by the second electrodeionization device. (ii) varying the operating current of the first electrodeionization device based on the water quality index value of the deionized water from the first electrodeionization device, and / or varying the operating current of the second electrodeionization device based on the water quality index value of the deionized water from the second electrodeionization device. (iii) varying the operating current of the first electrodeionization device based on the water quality index value of the concentrate of the first electrodeionization device, and / or varying the operating current of the second electrodeionization device based on the water quality index value of the concentrate of the second electrodeionization device. (iv) varying the operating current of the first electrodeionization device based on the voltage value of the first electrodeionization device, and / or varying the operating current of the second electrodeionization device based on the voltage value of the second electrodeionization device.

2. 2. The method for operating a two-stage electrodeionization system according to claim 1, wherein, when the condition (i) is satisfied, the operating current of the first electrodeionization device is increased if the water quality index value of the water to be treated in the first electrodeionization device falls below a predetermined range, and / or the operating current of the second electrodeionization device is increased if the water quality index value of the water to be treated in the second electrodeionization device falls below a predetermined range.

3. 2. The method for operating a two-stage electrodeionization system according to claim 1, wherein, when the condition (i) is satisfied, the operating current of the first electrodeionization device is reduced when the water quality index value of the water to be treated in the first electrodeionization device exceeds a predetermined range, and / or the operating current of the second electrodeionization device is reduced when the water quality index value of the water to be treated in the second electrodeionization device exceeds a predetermined range.

4. 2. The method of operating a two-stage electrodeionization system according to claim 1, wherein, when condition (ii) above is satisfied, the operating current of the first electrodeionization device is increased if the water quality index value of the deionized water from the first electrodeionization device deteriorates below a predetermined range, and / or the operating current of the second electrodeionization device is increased if the water quality index value of the deionized water from the second electrodeionization device deteriorates below a predetermined range.

5. 2. The method of operating a two-stage electrodeionization system according to claim 1, wherein, when condition (ii) above is satisfied, the operating current of the first electrodeionization device is reduced when the water quality index value of the deionized water from the first electrodeionization device improves above a predetermined range, and / or the operating current of the second electrodeionization device is reduced when the water quality index value of the deionized water from the second electrodeionization device improves above a predetermined range.

6. 2. The method for operating a two-stage electrodeionization system according to claim 1, wherein, when the condition (iii) is satisfied, the operating current of the first electrodeionization device is increased if the water quality index value of the concentrate from the first electrodeionization device falls below a predetermined range, and / or the operating current of the second electrodeionization device is increased if the water quality index value of the concentrate from the second electrodeionization device falls below a predetermined range.

7. 2. The method for operating a two-stage electrodeionization system according to claim 1, wherein, when the condition (iii) is satisfied, the operating current of the first electrodeionization device is reduced when the water quality index value of the concentrate from the first electrodeionization device is improved above a predetermined range, and / or the operating current of the second electrodeionization device is reduced when the water quality index value of the concentrate from the second electrodeionization device is improved above a predetermined range.

8. 2. The method for operating a two-stage electrodeionization system according to claim 1, wherein, when condition (iv) above is satisfied, the operating current of the first electrodeionization device is increased when the voltage value of the first electrodeionization device increases above a predetermined range, and / or the operating current of the second electrodeionization device is increased when the voltage value of the second electrodeionization device increases above a predetermined range.

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