Water treatment system provided with electric deionized water production device and method for operating same

Abstract

This water treatment system provided with an EDI device comprises: a circulation pipe 36 that circulates treated water (deionized water) from an EDI device 10 to a preceding stage of the EDI device 10; a water supply pipe 35 that supplies the treated water toward a use point; a flow rate adjustment unit 34 that distributes the treated water to the circulation pipe 36 and the water supply pipe 35; a circulation pipe 44 that circulates concentrated water from the EDI device 10 to the preceding stage of the EDI device 10; a discharge pipe 43 that discharges the concentrated water to the outside of the system; a flow rate adjustment unit 42 that distributes the concentrated water to the circulation pipe 44 and the discharge pipe 43; and water quality monitoring devices 51 to 54 that monitor the water quality within the water treatment system from the EDI device 10. At least one of the flow rate adjustment units 34, 42 is controlled on the basis of a monitoring result from at least one of the water quality monitoring devices 51 to 54.

Description

Water treatment system equipped with electrodeionized water production device and its operating method

[0001] The present invention relates to a water treatment system including an electrodeionization water production apparatus and a method for operating the same.

[0002] One example of a deionized water production apparatus that can demineralize water while regenerating ion exchangers is the electrodeionization (EDI) apparatus, which produces deionized water from water by combining electrophoresis and electrodialysis. The EDI apparatus is equipped with a deionization compartment between an anode and a cathode, and a concentration compartment disposed on either side of the deionization compartment via an ion exchange membrane. The ion exchange membrane separating the concentration compartment located on the anode side of the deionization compartment from the deionization compartment is an anion exchange membrane, while the ion exchange membrane separating the concentration compartment located on the cathode side of the deionization compartment from the deionization compartment is a cation exchange membrane. The deionization compartment is filled with an ion exchanger such as an ion exchange resin. It is preferable to also fill the concentration compartment with an ion exchanger. In practice, EDI apparatuses often have deionization compartments and concentration compartments alternately arranged between the anode and cathode in the direction from the anode to the cathode. The anode and cathode may be disposed inside the concentrating compartments located at both ends when the deionization compartments and the concentrating compartments are alternately arranged, or may be disposed inside the anode and cathode compartments separated from the concentrating compartments at both ends by ion exchange membranes. The anode and cathode compartments are collectively referred to as electrode compartments. When electrode compartments are provided, it is preferable to fill the interiors of the electrode compartments with ion exchangers as well.

[0003] In the EDI device, when the water to be treated is passed through the desalination chamber while a direct current is applied between the anode and cathode, the ionic components in the water to be treated are adsorbed onto the ion exchanger, and at the same time, hydrogen ions (H ＋ ) and hydroxide ions (OH －) to regenerate the ion exchanger. Water is also supplied to the concentrating compartment. As a result, deionized water is discharged from the deionizing compartment as treated water. Anions desorbed from the ion exchanger in the deionizing compartment due to regeneration of the ion exchanger migrate through the anion exchange membrane to the concentrating compartment located on the anode side of the deionizing compartment and are discharged from the concentrating compartment. Similarly, cations desorbed from the ion exchanger migrate through the cation exchange membrane to the concentrating compartment located on the cathode side of the deionizing compartment and are discharged from the concentrating compartment. Therefore, water with concentrated ionic impurities, i.e., concentrated water, is discharged from the concentrating compartment. EDI devices have the advantage of being able to continuously demineralize the water being treated without the need for chemical regeneration of the ion exchange resin. EDI devices are used, for example, in semiconductor device manufacturing plants that require large amounts of pure water or ultrapure water.

[0004] When operating an EDI device, concentrated water with a high concentration of ionic impurities has traditionally been discharged outside the EDI device. However, in recent years, from the perspective of water resource conservation, there has been a demand to reduce the amount of concentrated water discharged outside and improve the water recovery rate of the EDI device. The water recovery rate of an EDI device is the sum of the amount of deionized water obtained from the EDI device and supplied to a point-of-use facility, etc., and the amount of water recovered from the EDI device for reuse in the EDI device, divided by the amount of water supplied to the EDI device. Frequent operation and shutdown of an EDI device is undesirable, and large changes in the amount of deionized water discharged are also undesirable. Therefore, some concentrated water is returned to the upstream stage of the EDI device, or excess deionized water is returned to the upstream stage of the EDI device when demand for deionized water is low. For example, Patent Document 1 discloses that in a water treatment system configured such that permeate from a first reverse osmosis membrane device is supplied to an EDI device, the concentrated water from the EDI device is mixed with the concentrated water from the first reverse osmosis membrane device and supplied to a second reverse osmosis membrane device, and the permeate from the second reverse osmosis membrane device is returned to the inlet of the first reverse osmosis membrane device, thereby improving the water recovery rate in the EDI device. Patent Document 1 further discloses that the concentrated water from the EDI device may be returned to the inlet of the first reverse osmosis membrane device in the water treatment system without providing a second reverse osmosis membrane device.

[0005] In water treatment systems equipped with EDI devices, it has also been proposed to monitor the flow rate and water quality to adjust the flow rate of concentrated water or to control the value of the applied current flowing between the anode and cathode. For example, Patent Document 2 discloses that in a primary pure water system incorporating an EDI device to produce primary pure water, the flow rate of concentrated water in the EDI device or the value of the applied current to the EDI device is adjusted based on the water quality of the water supplied to the primary pure water system, the water quality of the treated water (i.e., deionized water) discharged from the EDI device, and flow rates measured at various locations in the primary pure water system. Patent Document 3 discloses providing a pressure fluctuation responsive constant flow valve in the path through which concentrated water is discharged from the concentration chamber in an EDI device to control the flow rate balance between the treated water flow rate and the concentrated water flow rate.

[0006] JP 2021-102200 A JP 2021-126624 A JP 2007-222724 A

[0007] In a water treatment system including an EDI device, returning concentrated water to the upstream of the EDI device is an effective means for improving the water recovery rate of the EDI device. For example, if 100% of the concentrated water is returned to the upstream of the EDI device, the water recovery rate can be achieved by ignoring the electrode water discharged from the electrode chamber or by combining the electrode water with the concentrated water. However, this increases the ionic load in the water treatment system. If the water quality of the water supplied to the water treatment system from outside fluctuates significantly, returning some or all of the concentrated water to the upstream of the EDI device can significantly fluctuate the ionic load in the water treatment system, making stable operation of the EDI device difficult. Furthermore, if a water treatment system is configured to circulate treated water in addition to concentrated water to the inlet side of the EDI device, for example, fluctuations in the circulation rate of treated water can significantly fluctuate the water quality of the water supplied to the EDI device, making it difficult to properly control the operation of the EDI device and obtain treated water (deionized water) with stable water quality.

[0008] An object of the present invention is to provide a water treatment system including an EDI device that increases the water recovery rate of the EDI device and can produce treated water of stable quality even when the water quality throughout the water treatment system fluctuates greatly, and a method for operating such a water treatment system.

[0009] One embodiment of the water treatment system of the present invention is a water treatment system equipped with an electrodeionized water production apparatus (EDI apparatus), and includes a first circulation line that circulates treated water, which is deionized water discharged from the EDI apparatus, to a stage upstream of the EDI apparatus, a water supply line that delivers the treated water toward a point of use, a first adjustment means connected to the treated water outlet of the EDI apparatus and distributing the treated water to the first circulation line and the water supply line, a second circulation line that circulates concentrated water discharged from the EDI apparatus to a stage upstream of the EDI apparatus, a discharge line that discharges the concentrated water to the outside of the water treatment system, a second adjustment means connected to the concentrated water outlet of the EDI apparatus and distributing the concentrated water to the second circulation line and the discharge line, and a water quality monitoring device that monitors the water quality within the water treatment system, and at least one of the first adjustment means and the second adjustment means is controlled based on the monitoring results of the water quality monitoring device.

[0010] One embodiment of the present invention provides a method for operating a water treatment system having an EDI device, a first circulation line for circulating treated water, which is deionized water discharged from the EDI device, upstream of the EDI device, a water supply line for transporting the treated water toward a point of use, a second circulation line for circulating concentrated water discharged from the EDI device upstream of the EDI device, and a discharge line for discharging the concentrated water to the outside, wherein the method monitors the water quality within the water treatment system and, depending on the monitoring results, controls at least one of the distribution ratio of the treated water between the first circulation line and the water supply line and the distribution ratio of the concentrated water between the second circulation line and the discharge line.

[0011] According to the present invention, in a water treatment system including an EDI device, it is possible to increase the water recovery rate in the EDI device and produce treated water of stable quality even when there are large fluctuations in water quality throughout the water treatment system.

[0012] FIG. 1 is a diagram showing an example of the configuration of a water treatment system. FIG. 2 is a graph explaining the principle of an operation method of an embodiment. FIG. 3 is a flowchart explaining an operation method of an embodiment. FIG. 4 is a diagram showing another example of the configuration of a water treatment system. FIG. 5 is a diagram showing a water treatment system used in Operation Examples 1-1 and 1-2. FIG. 6 is a graph showing the results of Operation Example 1-1. FIG. 7 is a graph showing the results of Operation Example 1-2. FIG. 6 is a diagram showing a water treatment system used in Operation Examples 2-1 and 2-2. FIG. 7 is a graph showing the results of Operation Example 2-1. FIG. 8 is a graph showing the results of Operation Example 2-2. FIG. 7 is a diagram showing a water treatment system used in Operation Examples 3-1 and 3-2. FIG. 8 is a graph showing the results of Operation Example 3-1. FIG. 9 is a graph showing the results of Operation Example 3-2.

[0013] Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows an example of the configuration of a water treatment system. The water treatment system shown in FIG. 1 receives water to be treated, such as city water, well water, river water, or recycled water, via a pipe 20, treats the water, and supplies the treated water, which is deionized water, to a point of use (a use location). The point of use may be a subsystem (also called a secondary pure water system) that further purifies the treated water to produce ultrapure water. This water treatment system includes at least an EDI device (electrodeionized water production device) 10, and an operating method based on the present invention is applied to the water treatment system.

[0014] The water to be treated, supplied to the water treatment system via piping 20, is first supplied to a reverse osmosis membrane device (RO) 21 equipped with a reverse osmosis membrane 22. In the reverse osmosis membrane device 21, water that does not permeate the reverse osmosis membrane 22, i.e., RO concentrated water, is discharged to the outside of the system via piping 23. Meanwhile, water that permeates the reverse osmosis membrane 22 in the reverse osmosis membrane device 21, i.e., RO permeate, is sent via permeate piping 24 to a feed water tank 15 provided upstream of the EDI device 10. The feed water tank 15 temporarily stores the water to be supplied to the EDI device 10, i.e., EDI feed water. The capacity of the feed water tank 15 will be described later. A feed water pipe 31 is connected to the outlet of the feed water tank 15 to supply the EDI feed water to the concentration chamber (C) and deionization chamber (D) of the EDI device 10, and a pump 32 is provided in the feed water pipe 31 to feed the EDI feed water. By providing pump 32, EDI feed water is supplied at a constant flow rate to the deionization compartment (D) in EDI device 10, and EDI feed water is also supplied at a constant flow rate to the concentration compartment (C). EDI device 10 may be provided with an electrode compartment separate from the concentration compartment (C), in which case a portion of the EDI feed water supplied to the concentration compartment (C) is branched off and supplied to the electrode compartment.

[0015] Treated water, which is deionized water, is discharged at a constant flow rate from the deionization chamber (D) of the EDI device 10. This treated water is supplied to a point of use, but demand for treated water is not always present at the point of use. Furthermore, points of use are generally equipped with a receiving tank to temporarily store treated water from the water treatment system. However, once the receiving tank is full, no more treated water can be supplied to the point of use. Therefore, the supply of treated water from the EDI device 10 to the point of use may be unavoidable. On the other hand, the EDI device 10 is preferably operated continuously without interruption. In the water treatment system shown in FIG. 1 , one end of a treated water pipe 33 is connected to the outlet of the deionization chamber (D) of the EDI device 10, and the other end of the treated water pipe 33 is equipped with a flow rate regulator 34, e.g., a three-way valve. The flow rate regulator 34 is connected to a water supply pipe 35 that supplies the treated water to the point of use and a circulation pipe 36 that returns the treated water to the supply water tank 15. The flow rate adjuster 34 corresponds to the first adjustment means, the water supply pipe 35 constitutes a water supply line, and the circulation pipe 36 constitutes a first circulation line. By operating the flow rate adjuster 34, it is possible to switch between a state in which treated water from the desalination chamber (D) is supplied to the point of use via the water supply pipe 35 and a state in which treated water is circulated to the supply water tank 15 via the circulation pipe 36.

[0016] The flow rate adjuster 34 can be configured using components other than a three-way valve, as long as it can switch the treated water from the treated water pipe 33 between the water supply pipe 35 and the circulation pipe 36 and supply it thereto. For example, the flow rate adjuster 34 can be configured using, either alone or in combination, an automatic valve, a flow control valve, a constant flow valve, a pressure reducing valve, an adjustment valve, a manual valve, a solenoid valve, an on / off valve, etc. Even when the EDI device 10 is provided with multiple deionization compartments (D), a single flow rate adjuster 34 is provided in common for these multiple deionization compartments (D). In this case, the outlet water (deionized water) from the multiple deionization compartments (D) joins together, flows into the treated water pipe 33, and is supplied to the flow rate adjuster 34.

[0017] Concentrated water is discharged at a constant flow rate from the concentrating chamber (C) of the EDI device 10. Because the concentration of ionic impurities in concentrated water is high, it is preferable to discharge the concentrated water outside the system in order to remove impurities from the system. However, discharging the concentrated water outside the system reduces the water recovery rate in the EDI device 10, which is undesirable from the perspective of effective use of water resources. Therefore, the water treatment system shown in FIG. 1 is configured to not only discharge the concentrated water discharged from the concentrating chamber (C) outside but also circulate the concentrated water to a stage upstream of the EDI device 10, specifically, the feed water tank 15. To this end, a concentrated water pipe 41 is provided through which concentrated water flowing from each outlet of the multiple concentrating chambers (C) provided in the EDI device 10 is joined and supplied. The concentrated water pipe 41 extends to a flow rate adjustment unit 42. The flow rate adjustment unit 42 has three ports a to c, and the concentrated water pipe 41 is connected to port a. A discharge pipe 43 for discharging the concentrated water outside the system is connected to port b, and a circulation pipe 44 for circulating the concentrated water to the supply water tank 15 is connected to port c. The flow rate adjustment unit 42 corresponds to the second adjustment means, the discharge pipe 43 forms a discharge line, and the circulation pipe 44 forms a second circulation line. In the EDI device 10, the flow rate of the concentrated water is preferably 1 / 20 or more and 1 / 5 or less of the flow rate of the treated water.

[0018] The flow rate adjuster 42 is configured to change the ratio of the flow rate of the concentrated water sent to the discharge pipe 43 to the flow rate of the concentrated water sent to the circulation pipe 44, in a range of 0:100 to 100:0, either stepwise or continuously, with respect to the concentrated water sent through the concentrated water pipe 41. If the ratio of the concentrated water discharged from the EDI device 10 that is circulated to the upstream stage of the EDI device 10 is referred to as the concentrated water recovery rate r, the flow rate adjuster 42 is configured to change the recovery rate r between 0 and 100%. The flow rate adjuster 42, which functions in this manner, is configured, for example, by a three-way valve that can continuously change the distribution ratio. In addition to a three-way valve, the flow rate adjuster 42 can also be configured using, for example, a flow control valve, a constant flow valve, a pressure reducing valve, a regulating valve, a manual valve, a solenoid valve, an on-off valve, or the like, either alone or in combination. When an electrode chamber is provided in the EDI device 10 independently of the concentration chamber (C), the electrode water discharged from the electrode chamber is sent to the concentrated water pipe 41, for example, to merge with the concentrated water. In this case, it is preferable to remove gas components contained in the electrode water before the electrode water is combined with the concentrated water. If the amount of electrode water is negligible compared to the amount of concentrated water, the electrode water may be discharged directly to the outside of the system.

[0019] The water treatment system is equipped with one or more water quality monitoring devices that measure water quality at various locations within the system. In the illustrated example, a water quality monitoring device 51 that monitors the quality of the RO permeate from the reverse osmosis membrane device 21 is attached to the permeate piping 24, and a water quality monitoring device 52 that monitors the quality of the EDI feed water stored in the feed water tank 15 and supplied to the EDI device 10 is attached to the feed water piping 31. A water quality monitoring device 53 is attached to the treated water piping 33 to monitor the quality of the treated water (deionized water) discharged from the desalination chamber (D) of the EDI device 10, and a water quality monitoring device 54 is attached to the concentrated water piping 41 to monitor the quality of the concentrated water. Depending on the type of water quality to be monitored, the water quality monitoring devices 51-54 may be configured using one of the following devices, or a combination of two or more of them: a conductivity meter, a resistivity meter, a pH meter, a TOC meter that measures the concentration of total organic carbon (TOC) in water, a silica meter that measures the concentration of silica components in water, a boron meter that measures the concentration of boron in water, etc. Some of the water quality monitoring devices 51-54 shown in Figure 1 may not be provided, or a water quality monitoring device may be located at a location other than the water quality monitoring location shown in Figure 1.

[0020] The water treatment system further includes a control unit 50 that controls the operation of the water treatment system. In particular, the control unit 50 controls the flow rate adjustment unit 34 in response to factors such as the usage status of treated water at the point of use, and controls the flow rate adjustment unit 42 in response to water quality measurement results measured by at least one of the water quality monitoring devices 51-54. If a receiving tank for receiving and temporarily storing treated water is provided at the point of use, the control unit 50 controls the flow rate adjustment unit 42 based on a signal from a water level meter (level sensor) or the like provided in the receiving tank so that, when the water volume in the receiving tank is equal to or greater than a threshold, the treated water is circulated to the supply water tank 15 via the circulation pipe 36, and when the water volume in the receiving tank is less than the threshold, the treated water is supplied to the receiving tank via the water supply pipe 35. The control unit 50 may control the treated water flow rate adjustment unit 34 based on the water quality measured by the water quality monitoring devices 51-54, in addition to the control based on the water volume in the receiving tank.

[0021] Assume that the entire amount of treated water discharged from the deionization compartment (D) of the EDI device 10 is supplied to a point-of-use. If the entire amount of concentrated water discharged from the concentration compartment (C) were circulated to the supply water tank 15, the water recovery rate in the EDI device 10 would be 100%, but the impurity concentration in the system would increase. In the worst case, scale would form in the EDI device 10, or the EDI device 10 would be unable to produce deionized water as treated water. Therefore, the control unit 50 controls the flow rate adjustment unit 42 so that most of the concentrated water is circulated to the supply water tank 15, i.e., so that the recovery rate r for the concentrated water is increased, while monitoring the water quality values ​​in the water treatment system using the water quality monitoring devices 51-54. In this case, the recovery rate r for the concentrated water is set to, for example, 100%. Since the recovery rate r for the concentrated water is high and the water quality value gradually deteriorates, when the water quality value deteriorates below an acceptable value, the control unit 50 controls the flow rate adjustment unit 42 to reduce the recovery rate r and increase the proportion of the concentrated water discharged outside the system via the discharge pipe 43. This control is referred to as the first control. In order to improve the water recovery rate throughout the entire water treatment system, the concentrated water recovery rate r is set within a range in which the water recovery rate in the EDI device 10 is, for example, 90% or higher, preferably 95% or higher. In the standard case where the flow rate of the concentrated water is 1 / 10 of the flow rate of the treated water, if the concentrated water recovery rate r is 35%, the water recovery rate in the EDI device 10 will be 95% or higher. By increasing the proportion of concentrated water discharged outside the system, the water quality value gradually improves. Once the water quality value has improved to a certain extent, the control unit 50 again controls the flow rate adjustment unit 42 so that more of the concentrated water is circulated to the supply water tank 15. This control is called the second control.

[0022] When such control is performed, assuming that the concentration of a specific impurity component is used as the water quality value, the water quality value changes depending on the elapsed time from the time the water treatment system is started and the EDI device 10 begins operation, as shown in Figure 2. Figure 2 is a diagram illustrating the principle of the operating method according to the present invention. The impurity concentration reaches a maximum at P1, P2, ... and a minimum at Q1, Q2, .... During the period from the start of operation to P1, the period from Q1 to P2, and the period from Q2 to P3, almost the entire amount of concentrated water is circulated to the supply water tank 15. During the period from P1 to Q1, the period from P2 to Q2, and the period from P3 to Q3, a large proportion of the concentrated water is discharged outside the system. The length of time from one maximum value of the impurity concentration to the next maximum value is called a cycle. By appropriately setting the timing for switching the concentrated water recovery rate r according to the impurity concentration and the concentrated water recovery rate r before and after the switch, the water recovery rate of the EDI device 10 can be increased while gradually reducing the fluctuation range of the impurity concentration. In practice, frequent switching of the valves included in the flow rate adjustment unit 42 can cause malfunctions. Furthermore, switching of the valves in the flow rate adjustment unit 42 can cause pressure fluctuations within the water treatment system, which can result in an increase in the applied voltage to the EDI device 10. Therefore, it is preferable to limit the cycle to 10 cycles per hour or less. To limit the cycle to 10 cycles per hour, the sum of the number of times the first control is performed and the number of times the second control is performed can be set to 20 or less per hour. To set the cycle in this manner, the concentrated water recovery rate r can be appropriately set during periods in which a large proportion of concentrated water is discharged outside the system, such as the period from P1 to Q1, the period from P2 to Q2, or the period from P3 to Q3.

[0023] The above description assumes that water quality measurements are performed continuously and that the flow rate adjuster 42 is also continuously controlled based on the water quality measurement results. However, in an actual water treatment system, it is not necessary to continuously measure water quality values ​​and control the flow rate adjuster 42. Instead, water quality measurements and changes to the concentrated water recovery rate r in the flow rate adjuster 42 can be performed every control period, which can be several minutes to several hours. If it is not necessary to gradually reduce the fluctuation range of water quality values, the water treatment system can be operated according to the flowchart shown in FIG. 3. Here, a first threshold T1 and a second threshold T2 are predetermined for water quality values. The water quality value indicated by the first threshold T1 is assumed to be worse than the water quality value indicated by the second threshold T2. That is, if the water quality value is an impurity concentration or conductivity, T1 > T2, and if the water quality value is resistivity, T1 < T2.

[0024] In the processing procedure shown in FIG. 3 , first, in step 101, the flow rate adjustment unit 42 is set so that the concentrated water recovery rate r is R1, and the water treatment system is operated. Then, in step 102, it is determined whether the water quality value obtained by the water quality monitoring device has deteriorated and reached the first threshold value T1. If the water quality is better than the water quality indicated by the first threshold value T1, step 102 is repeated. If the water quality has reached or deteriorated beyond the water quality indicated by the first threshold value T1, in step 103, the flow rate adjustment unit 42 is set so that R1 > R2, and the concentrated water recovery rate r is R2, and operation of the water treatment system is continued. Then, in step 104, it is determined whether the water quality value obtained by the water quality monitoring device has improved and reached the second threshold value T2. If the water quality is worse than the water quality indicated by the second threshold value T2, step 104 is repeated. If the water quality has reached or improved beyond the water quality indicated by the second threshold value T2, the process returns to step 101, and the processing from step 101 onwards is repeated.

[0025] In order to increase the recovery rate in the EDI device 10, it is sufficient to return at least a portion of the concentrated water to an arbitrary position upstream of the EDI device 10. In the water treatment system shown in FIG. 1 , the circulation pipe 44 is connected to the supply water tank 15, and the concentrated water is returned to the supply water tank 15 via the circulation pipe 44. However, in the water treatment system shown in FIG. 4 , the circulation pipe 44 is connected to the pipe 20 for the water to be treated, and the concentrated water is merged with the water to be treated and circulated to the inlet of the reverse osmosis membrane device 21. Since the reverse osmosis membrane device 21 is provided upstream of the supply water tank 15, at least a portion of the concentrated water from the EDI device 10 is also returned to the upstream of the EDI device 10 in the water treatment system shown in FIG. 4 . The water treatment system shown in FIG. 4 has the same configuration as the water treatment system shown in FIG. 1 , except that the destination of the concentrated water circulation pipe 44 is different, and operates in the same manner as the water treatment system shown in FIG. 1 .

[0026] The operation method according to the present invention will be described in more detail below by describing an example in which the operation of a water treatment system was simulated as an operation example.

[0027] [Operational Example 1-1] In the water treatment system shown in Figure 1, a simulation was performed assuming that all treated water from the desalination compartment (D) of the EDI device 10 was supplied directly to the point of use via the treated water piping 33, i.e., that the flow rate regulator 34, water supply piping 35, and circulation piping 36 were not provided. Figure 5 shows the configuration of the main parts of the water treatment system in Operational Example 1-1. An overflow piping 16 for discharging tank overflow was attached to the supply water tank 15. RO permeate water with a boron concentration of 20 μg / L was supplied to the supply water tank 15, which had a capacity of 1,000 L, at a flow rate of 1,000 L / h. EDI feed water was supplied from supply water tank 15 to EDI device 10 at a flow rate of 550 L / h, and treated water with a boron concentration of 20 ng / L was discharged from EDI device 10 at a flow rate of 500 L / h, and concentrated water with a boron concentration of 400 μg / L was discharged at a flow rate of 50 L / h. The boron concentration of the EDI feed water measured by water quality monitoring device 52 was set at an upper limit (first threshold T1) of 100 μg / L and a lower limit (second threshold T2) of 80 μg / L, and the water treatment system was operated according to the flowchart shown in FIG.

[0028] The flow rate adjustment unit 42 installed in the concentrated water pipe 41 was configured to switch the concentrated water recovery rate r between six levels: 100%, 75%, 50%, 35%, 25%, and 0%. Calculations based on the flow rates of the EDI feed water, treated water, and concentrated water indicated that the water recovery rate in the EDI device 10 was 95% or higher when the concentrated water recovery rate r was 100%, 75%, and 50%. The relationship between the elapsed time from the start of operation and the boron concentration in the EDI feed water measured by the water quality monitoring device 52 was investigated. The results are shown in Figure 6. In Figure 6, P1, P2, ... indicate the maximum boron concentration, and Q1, Q2, ... indicate the minimum boron concentration. The boron concentration in the EDI feed water at the start of operation was 20 μg / L, the same as the boron concentration in the RO permeate. At the start of operation, the concentrate water recovery rate r is set to 100% (step 101 in FIG. 3 ). When the water treatment system is operated under this condition, the boron concentration in the EDI feed water gradually increases and reaches 100 μg / L, i.e., the upper limit (first threshold T1), after 7 hours (step 102 in FIG. 3 ). Once the upper limit is reached, the flow rate regulator 42 is controlled to reduce the concentrate water recovery rate r (step 103 in FIG. 3 ). To maintain the water recovery rate of the EDI device 10 at 95% or higher, the concentrate water recovery rate r can be selected to be either 75% or 50%. Therefore, the time change in the boron concentration in the EDI feed water was calculated for the cases where the concentrate water recovery rate r was 75% and 50%. As shown in FIG. 6 , when the concentrate water recovery rate r was 50%, the rate of decrease in boron concentration was high. Because a longer period, i.e., a longer time between the maximum boron concentration and the next maximum, is preferred, the concentrate water recovery rate r was selected to be 75%. As a result, 13 hours after the start of operation, the boron concentration reaches 79 μg / L, falling below the lower limit (second threshold T2) (step 104 in FIG. 3 ). Therefore, the concentrated water recovery rate r is set to 100%, the same as in the initial state, and operation of the water treatment system is continued. As a result, 15 hours after the start of operation, the boron concentration again reaches 100 μg / L, so the concentrated water recovery rate r is switched to 75%. Similar control is then repeated.

[0029] [Operational Example 1-2] The change in boron concentration in the EDI feed water in the water treatment system shown in Figure 5 was investigated in the same manner as in Operational Example 1-1, except that the capacity of the supply water tank 15 was 500 L, EDI feed water was supplied to the EDI device 10 from the supply water tank 15 at a flow rate of 945 L / h, treated water with a boron concentration of 200 ng / L was discharged from the EDI device 10 at a flow rate of 900 L / h, and concentrated water with a boron concentration of 420 μg / L was discharged at a flow rate of 45 L / h. The results are shown in Figure 7. In this case, the water recovery rate of the EDI device 10 exceeded 95% when the concentrated water recovery rate r was 100%, 75%, 50%, 35%, 25%, or 0%. In this case, too, 75% was selected as the concentrated water recovery rate r after the boron concentration reached the upper limit in order to lengthen the time, i.e., the period, between the maximum and the next maximum in boron concentration.

[0030] [Operational Example 2-1] In the water treatment system shown in Figure 1, a simulation was conducted to examine the change in boron concentration in the EDI feed water when alternately sending 100% of the treated water from the desalination chamber (D) of the EDI device 10 to the point of use and circulating and recovering 100% of the treated water in the feed water tank 15. Figure 8 shows the configuration of the main parts of the water treatment system in Operational Example 2-1, and Figure 9 shows the change in boron concentration in the EDI feed water. The feed water tank 15 is equipped with an overflow pipe 16 for discharging tank overflow. Here, as in Operational Example 1-1, RO permeate water with a boron concentration of 20 μg / L was supplied to the 1000 L capacity feed water tank 15 at a flow rate of 1000 L / h. EDI feed water was supplied from the supply water tank 15 to the EDI device 10 at a flow rate of 550 L / h, and treated water with a boron concentration of 20 ng / L was discharged from the EDI device 10 at a flow rate of 500 L / h, and concentrated water with a boron concentration of 400 μg / L was discharged at a flow rate of 50 L / h. The boron concentration measured by the water quality monitoring device 52 was set at an upper limit (first threshold T1) of 100 μg / L and a lower limit (second threshold T2) of 80 μg / L, and the water treatment system was operated according to the flowchart shown in Figure 3.

[0031] The flow rate adjustment unit 42 installed in the concentrated water pipe 41 was configured to switch the concentrated water recovery rate r between six levels: 100%, 75%, 50%, 35%, 25%, and 0%. Based on the flow rates of the EDI feed water, treated water, and concentrated water, calculations showed that the overall water recovery rate of the EDI device 10 was 95% or higher when the concentrated water recovery rate r was 100%, 75%, and 50%. At the start of operation, the entire treated water was sent to the point of use, and the concentrated water recovery rate r was set to 100%. Operating the water treatment system under this condition gradually increased the boron concentration in the EDI feed water. Seven hours after the start of operation, the boron concentration reached its upper limit of 102 μg / L, so the system was switched to circulating the entire treated water to the supply water tank 15. The concentrated water recovery rate r remained at 100%. The boron concentration then rapidly decreased and approached 40 μg / L. Then, 13 hours after the start of operation, when the system was switched to supplying the entire amount of treated water to the point-of-use, the boron concentration rose again. Because the boron concentration in the EDI feed water drops significantly when treated water is being recovered in the feed water tank 15, in the case of Operation Example 2-1, it is considered unnecessary to switch the concentrated water recovery rate r to a value less than 100%.

[0032] [Operational Example 2-2] The change in boron concentration in the EDI feed water in the water treatment system shown in Figure 8 was investigated in the same manner as in Operational Example 2-1, except that the flow rate of the EDI feed water was set to 945 L / h, treated water with a boron concentration of 200 ng / L was discharged from the EDI device 10 at a flow rate of 900 L / h, and concentrated water with a boron concentration of 420 μg / L was discharged at a rate of 45 L / h. The results are shown in Figure 10. In this case, the water recovery rate of the EDI device 10 as a whole exceeded 95% when the concentrated water recovery rate r was 100%, 75%, 50%, 35%, 25%, or 0%. However, since the boron concentration in the EDI feed water significantly decreased when treated water was being recovered in the feed water tank 15, it is believed that, in Operational Example 2-2, it is unnecessary to switch the concentrated water recovery rate r to a value less than 100%.

[0033] [Operational Example 3-1] In the water treatment system shown in Figure 1, a simulation was performed assuming that all treated water from the desalination compartment (D) of the EDI device 10 was supplied directly to the point of use via treated water piping 33 and that the supply of RO permeate to the supply water tank 15 was intermittent. Figure 11 shows the configuration of the main parts of the water treatment system in Operational Example 3-1. The water treatment system shown in Figure 11 is the same as the water treatment system in Operational Example 1-1 shown in Figure 5, except that an on-off valve 25 is provided on the permeate piping 24 that supplies the RO permeate to the supply water tank 15. Here, RO permeate having a boron concentration of 20 μg / L was supplied to the supply water tank 15 with a capacity of 1000 L at a flow rate of 1000 L / h. EDI feed water was supplied from the supply water tank 15 to the EDI device 10 at a flow rate of 945 L / h, and treated water with a boron concentration of 200 ng / L was discharged from the EDI device 10 at a flow rate of 900 L / h, and concentrated water with a boron concentration of 420 μg / L was discharged at a flow rate of 45 L / h. The boron concentration measured by the water quality monitoring device 52 was set at an upper limit (first threshold T1) of 100 μg / L and a lower limit (second threshold T2) of 80 μg / L, and the water treatment system was operated according to the flowchart shown in Figure 3.

[0034] The flow rate adjuster 42 provided in the concentrated water pipe 41 was configured to switch the concentrated water recovery rate r between six levels: 100%, 75%, 50%, 35%, 25%, and 0%. Calculations based on the flow rates of the EDI feed water, treated water, and concentrated water indicated that the water recovery rate of the EDI device 10 was 95% or higher for any of the concentrated water recovery rates r of 100%, 75%, 50%, 35%, 25%, and 0%. At the start of operation, the on-off valve 25 was opened to supply the RO permeate to the feed water tank 15, setting the concentrated water recovery rate r to 100%. By operating the water treatment system under this condition, the boron concentration in the EDI feed water gradually increased, and after 6 hours, the boron concentration reached 102 μg / L, exceeding the upper limit (first threshold T1). Therefore, at this point, the on-off valve 25f was closed to stop the supply of RO permeate water to the feed water tank 15, and the flow rate adjuster 42 was controlled to change the concentrate recovery rate r. Figure 12 shows the change in boron concentration in the EDI feed water, particularly the change in the boron concentration for each recovery rate r after the concentrate recovery rate r is changed after the boron concentration reaches its upper limit. As shown in Figure 12, when at least a portion of the concentrate water is circulated to the feed water tank 15 without the supply of new RO permeate water, the boron concentration in the EDI feed water exceeds the upper limit and further increases. Therefore, it is preferable to send the entire amount of treated water to the point of use and, when the supply of RO permeate water is interrupted, to discharge the entire amount of concentrate water outside the system.

[0035] Operational Example 3-2: Changes in the boron concentration of EDI feed water were investigated in the same manner as in Operational Example 3-1, except that the capacity of the feed water tank 15 was set to 500 L. The results are shown in Figure 13. When at least a portion of the concentrated water was circulated to the feed water tank 15 without the supply of new RO permeated water, the boron concentration of the EDI feed water exceeded the upper limit and further increased, and the rate of increase was greater than in Operational Example 3-1, in which the capacity of the feed water tank 15 was larger. In this case, too, it was found that it was preferable to send the entire amount of treated water to the point-of-use and, when the supply of RO permeated water was interrupted, to discharge the entire amount of concentrated water outside the system.

[0036] Comparing Operational Example 1-1 and Operational Example 1-2, the supply flow rate of the RO permeate water to the supply water tank 15 is the same at 1000 L / h, but the capacity of the supply water tank 15 is 1000 L in Operational Example 1-1 and 500 L in Operational Example 1-2. Similarly, comparing Operational Example 3-1 and Operational Example 3-2, the capacity of the supply water tank 15 is 1000 L in Operational Example 3-1 and 500 L in Operational Example 3-2. In order to reduce the impact of water quality fluctuations in the water supplied to the water treatment system, it is generally considered preferable to use a supply water tank 15 with a large capacity. However, as shown in Operational Example 1-2 or Operational Example 3-2, according to the operating method based on the present invention, it is possible to suppress water quality fluctuations in the treated water even when a supply water tank 15 with a small capacity is used. In a typical water treatment system, the capacity of the tank installed upstream of the EDI device to store the EDI feed water is approximately 1 to 2 times the amount of EDI feed water per hour. In contrast, according to the operating method based on the present invention, the capacity of the tank installed upstream of the EDI device to store the EDI feed water can be set to less than the amount of EDI feed water per hour. This reduces the footprint of the tank and contributes to the miniaturization of the water treatment system.

[0037] REFERENCE SIGNS LIST 10 EDI device 15 Feed water tank 21 Reverse osmosis membrane device 22 Reverse osmosis membrane 24 Permeate water piping 31 Feed water piping 32 Pump 33 Treated water piping 34, 42 Flow rate adjustment unit 35 Water supply piping 36, 44 Circulation piping 41 Concentrated water piping 43 Discharge piping 50 Control unit 51 to 54 Water quality monitoring device

Claims

1. A water treatment system equipped with an electrodeionized water production apparatus, comprising: a first circulation line that circulates treated water, which is deionized water discharged from the electrodeionized water production apparatus, to a stage upstream of the electrodeionized water production apparatus; a water supply line that delivers the treated water toward a point of use; a first adjustment means that is connected to the treated water outlet of the electrodeionized water production apparatus and distributes the treated water between the first circulation line and the water supply line; a second circulation line that circulates concentrated water discharged from the electrodeionized water production apparatus to a stage upstream of the electrodeionized water production apparatus; a discharge line that discharges the concentrated water to the outside of the water treatment system; a second adjustment means that is connected to the concentrated water outlet of the electrodeionized water production apparatus and distributes the concentrated water between the second circulation line and the discharge line; and a water quality monitoring device that monitors the water quality within the water treatment system, wherein at least one of the first adjustment means and the second adjustment means is controlled based on the monitoring results of the water quality monitoring device.

2. A water treatment system as described in claim 1, wherein when the water quality value obtained by the water quality monitoring device deteriorates and reaches a first threshold value for water quality, the second adjustment means controls to increase the amount of concentrated water sent to the discharge line, and when the water quality value improves and reaches a second threshold value for water quality, the second adjustment means controls to increase the amount of concentrated water sent to the second circulation line, and the water quality corresponding to the second threshold value is better than the water quality corresponding to the first threshold value.

3. The water treatment system according to claim 1 or 2, wherein the water quality monitoring device is provided in the water treatment system to monitor the quality of at least one of the feed water supplied to the electrodeionization water production apparatus, the concentrated water, and the treated water.

4. The water treatment system according to claim 3, wherein the water quality monitoring device is composed of one type of device selected from a conductivity meter, a resistivity meter, a pH meter, a TOC meter for measuring total organic carbon concentration, a silica meter for measuring silica concentration, and a boron meter for measuring boron concentration, or a combination of two or more types of devices.

5. A water treatment system as described in claim 1 or 2, wherein the first adjustment means is controlled in accordance with the demand for treated water at the use point so that the treated water flows into the water supply line when there is demand, and flows into the second circulation line when there is no demand.

6. A water treatment system as described in claim 5, wherein a receiving tank for receiving the treated water is provided at the use point, and when the amount of the treated water in the receiving tank is equal to or greater than a predetermined threshold, it is determined that there is no demand.

7. A water treatment system as described in claim 1 or 2, wherein the first adjustment means and the second adjustment means are each independently constituted by one type of valve selected from an automatic valve, a flow control valve, a constant flow valve, a pressure reducing valve, a regulating valve, a manual valve, and a three-way valve, or a combination of two or more types of valves.

8. A method for operating a water treatment system having an electrodeionized water production apparatus, a first circulation line for circulating treated water (deionized water) discharged from the electrodeionized water production apparatus to a stage upstream of the electrodeionized water production apparatus, a water supply line for transporting the treated water toward a point of use, a second circulation line for circulating concentrated water discharged from the electrodeionized water production apparatus to a stage upstream of the electrodeionized water production apparatus, and a discharge line for discharging the concentrated water to the outside, the method comprising: monitoring water quality within the water treatment system; and controlling, based on the monitoring results, at least one of the distribution ratio of the treated water between the first circulation line and the water supply line and the distribution ratio of the concentrated water between the second circulation line and the discharge line.

9. An operating method as described in claim 8, wherein when the water quality value obtained regarding the water quality deteriorates and reaches a first threshold value regarding the water quality, a first control is performed to increase the amount of the concentrated water sent to the discharge line, and when the water quality value improves and reaches a second threshold value regarding the water quality, a second control is performed to increase the amount of the concentrated water sent to the second circulation line, and the sum of the number of times the first control is performed and the number of times the second control is performed is 20 or less per hour.

10. An operating method as described in claim 8 or 9, in which the ratio of the amount of concentrated water sent to the second circulation line to the amount of concentrated water sent to the discharge line is controlled within a range that satisfies the condition that the water recovery rate in the electrodeionization water production apparatus is 95% or more.

11. An operating method as described in claim 8 or 9, wherein the flow rate of the concentrated water discharged from the electrodeionized water production apparatus is set to be 1 / 20 or more and 1 / 5 or less of the flow rate of the treated water discharged from the electrodeionized water production apparatus.

12. An operating method as described in claim 8 or 9, wherein the capacity of a tank provided upstream of the electrodeionized water production apparatus for temporarily storing the water to be supplied to the electrodeionized water production apparatus, including the concentrated water circulated via the second circulation line, is set to a capacity less than the amount of water supplied to the electrodeionized water production apparatus per hour.

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