Operation method of two-stage electric deionization system

Abstract

When raw water W is treated by a reverse osmosis membrane device 3, permeated water W1 of the reverse osmosis membrane device 3 is sequentially treated by a first electric deionization device 4 and a second electric deionization device 5 to obtain second deionized water W3. The conductivity of the water W1 to be treated is measured by a conductivity meter 6, and when the conductivity of the water W1 to be treated increases beyond a predetermined value, the operating current of the first electric deionization device 4 is increased. On the other hand, when the conductivity of the water W1 to be treated decreases below the predetermined value, the operating current (current density) of the first electric deionization device 4 is decreased. Thus, the power consumption can be reduced while maintaining the water quality (for example, conductivity) of first deionized water W2 within a target range. According to such an operation method of a two-stage electric deionization system, the reduction of the power consumption and the stabilization of the treated water quality can be realized.

Description

Method of operating a two-stage electrodeionization system

[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.

[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 apparatuses are arranged in series have become increasingly popular in order to remove cations and anions from the water to be treated to a greater extent.

[0005] However, in a system with two electrodeionization apparatuses 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 view of the above problems, and an object of the present invention is to provide an operating method for a two-stage electrodeionization system that enables reduction in power consumption and stabilization of treated water quality.

[0007] To achieve the above-mentioned objectives, 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 apparatuses are arranged in series. The method varies the operating current of one or both of the two electrodeionization apparatuses based on the water quality index values ​​of the treated water, the deionized water, and the concentrated water in each electrodeionization apparatus, or on voltage fluctuations during operation of each electrodeionization apparatus (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, or fluctuations in the voltage during operation of each electrodeionization device occur, the current setting value of one of the two electrodeionization devices can be controlled so that, for example, the amount of current is reduced when the water quality index value of the treated water (feed water) is good, and the amount of current is increased when the water quality index value of the treated water deteriorates, thereby reducing power consumption and stabilizing the quality of the deionized water (treated water).

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

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

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

[0012] According to this invention (Invention 3), if the quality of the raw water to be treated (feedwater) used in the two-stage electrodeionization system is good, the operating current value of the upstream electrodeionization device can be reduced to reduce power consumption.

[0013] In the above invention (Invention 1), when the water quality index value of the deionized 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 4).

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

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

[0023] 1 is a schematic diagram showing a two-stage electrodeionization system capable of carrying out a method for operating a two-stage electrodeionization system according to a first embodiment of the present invention; 2 is a schematic diagram showing a two-stage electrodeionization system capable of carrying out a method for operating a two-stage electrodeionization system according to a second embodiment of the present invention; 3 is a schematic diagram showing a two-stage electrodeionization system capable of carrying out a method for operating a two-stage electrodeionization system according to a third embodiment of the present invention; 4 is a graph showing the operating current, voltage, and conductivity of treated water in the method for operating a two-stage electrodeionization system of Example 1; 5 is a graph showing the operating current, voltage, and conductivity of treated water in the method for operating a two-stage electrodeionization system of Comparative Example 1; 6 is a graph showing the operating current, voltage, and conductivity of treated water in the method for operating a two-stage electrodeionization system of Example 2;

[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 Figure 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 upstream of the first electrodeionization device 4 in the water pipe 2 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 at the outlet of the second electrodeionization device 5 in the water pipe 2 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, an operation method of 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 taken out 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 a second electrodeionization device 5. In this second electrodeionization device 5, the first deionized water W2 is introduced through the inlet side of the deionization compartment, and second deionized water W3 is taken out through 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%. The current density of the electrodeionization device during the water flow treatment is 1000 mA / dm ２ or more, for example, 1000 to 2000 mA / 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 the case where 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 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 of a desired quality. On the other hand, if the conductivity of the water W1 to be treated decreases below a predetermined value (equivalent to the case where 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 the 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 manner, controlling the operating current of the two-stage electrodeionization apparatuses 4, 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 measuring 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. The two-stage electrodeionization system 11 of the second embodiment 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 supply 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 the 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 the discharge path 14 for concentrated water W5 from the second electrodeionization device 5.

[0035] These first conductivity meter 13 and second conductivity meter 15 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.

[0036] <Operation Method of Two-Stage Electrodeionization System> Next, an operation method of 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 a description thereof will be omitted.

[0038] In this method of operating a two-stage electrodeionization system, the conductivity of the concentrated water W4 from the first electrodeionization device 4 is measured using the first conductivity meter 13. If the conductivity of the concentrated water W4 exceeds a predetermined value (equivalent to a case where the resistivity falls 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 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 water quality. On the other hand, if the conductivity of the concentrated water W4 falls below a predetermined value (equivalent to a case where 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.

[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 and second electrodeionization devices 4 and 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 Figure 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, an operation method of 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 a description thereof will be omitted.

[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 decrease. 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 reduced. This makes it possible to reduce the amount of power consumption while keeping the water quality (e.g., 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 water 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 power consumption to be reduced while the water quality (e.g., conductivity) of the second deionized water W3 is maintained within a target range.

[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.

[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 to be treated W1 with a conductivity of 1 mS / m was supplied to the first electrodeionization device 4, and the electrodeionization system 1 was operated at an initial operating voltage of 8 A. The conductivity of the water to be treated W1 was varied between 0.2 and 2 mS / m, and the operating current of the first electrodeionization device 4 was varied depending on 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 with an initial operating voltage of 10 A, and the conductivity of the water W1 to be treated 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 resulting operating voltage. 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 caused by fluctuations in conductivity, thereby reducing power consumption compared to Comparative Example 1.

[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 Discharge path for concentrated water 13 First conductivity meter 15 Second conductivity meter W Raw water W1 Water to be treated (permeated water from reverse osmosis membrane device) W2 First deionized water W3 Second deionized water (deionized water) W4 Concentrated water from first electrodeionization device W5 Concentrated water from second electrodeionization device

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 using a system in which two stages of electrodeionization devices are arranged in series, wherein the operating current of one or both stages 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 of the electrodeionization devices, or voltage fluctuations during operation of each electrodeionization device.

2. The method for operating a two-stage electrodeionization system according to claim 1, wherein the operating current of the preceding electrodeionization device is increased when the water quality index value of the treated water in the two-stage electrodeionization system falls below a predetermined range.

3. The method for operating a two-stage electrodeionization system according to claim 1, wherein the operating current of the preceding electrodeionization device is reduced when the water quality index value of the treated water in the two-stage electrodeionization system improves above a predetermined range.

4. The method for operating a two-stage electrodeionization system according to claim 1, wherein the operating current of the electrodeionization device in the preceding or following stage is increased when the water quality index value of the deionized water from the electrodeionization device in the preceding stage or following stage deteriorates below a predetermined range.

5. The method for operating a two-stage electrodeionization system according to claim 1, wherein the operating current of the electrodeionization device in the preceding or following stage is reduced when the water quality index value of the deionized water in the electrodeionization device in the preceding stage or following stage improves above a predetermined range.

6. The method for operating a two-stage electrodeionization system according to claim 1, wherein the operating current of the electrodeionization device is increased when the water quality index of the concentrated water from the preceding or succeeding electrodeionization device falls below a predetermined range.

7. The method for operating a two-stage electrodeionization system according to claim 1, wherein the operating current of the electrodeionization device is reduced when the water quality index of the concentrate from the preceding or succeeding electrodeionization device improves above a predetermined range.

8. The method for operating a two-stage electrodeionization system according to claim 1, wherein when the operating voltage of the preceding or succeeding electrodeionization apparatus exceeds a predetermined range, the operating current of the preceding or succeeding electrodeionization apparatus or the electrodeionization apparatus immediately following the preceding electrodeionization apparatus is increased.

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