Patent Publication Number: US-2020284742-A1

Title: Method and system for measuring conductivity of decationized water

Description:
TECHNICAL FIELD 
     The present application is based upon and claims priority from Japanese Patent Application No. 2017-183386 filed on Sep. 25, 2017, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     The present invention is related to a method and system for measuring conductivity of decationized water, particularly to a method of measuring the conductivity of condensate water in a thermal power plant. 
     BACKGROUND ART 
     A water circulation operation is conducted in a thermal power plant. That is, high temperature and high pressure steam is generated in a boiler, and is introduced into a steam turbine. The steam that is discharged from the steam turbine is condensed into condensate water in a condenser, and the condensate water is fed to the boiler as feed water. Since impurities, such as corrosion products, accumulate in the condensate water, a thermal power plant is provided with a condensate demineralizer that removes the impurities from the condensate water during normal operation. When the condenser is cooled by sea water, the condensate demineralizer also works to protect the condensate water system by removing sodium chloride and the like for a short period, which is contained in sea water that may mix in the condensate water. However, if more than a predetermined amount of sea water flows in the condensate demineralizer, then the condensate demineralizer will exceed its operational limit. For this reason, a thermal power plant has a conductivity meter for the purpose of detecting sea water component in the condensate water. 
     On the other hand, in a thermal power plant, an operation of adding a pH adjuster, such as ammonia, to the condensate water to alkalify the condensate water is conducted in order to prevent piping etc. of the condensate water system from corroding. For this reason, the condensate water has low specific resistance and high conductivity, and the specific resistance and the conductivity do not largely change when a small amount of sea water mixes in the condensate water system. Therefore, it is difficult to accurately detect the mixing of sea water by means of a conductivity meter. In order to solve this problem, cations, such as ammonia, may be removed by an electric decationizing apparatus in advance, and the condensate water, whose conductivity has been reduced, may be supplied to a conductivity meter (JP4671272, JP3704289). This method enhances the accuracy for detecting anions that come from sea water and makes it possible to detect the mixing of sea water more accurately. 
     SUMMARY OF INVENTION 
     In a thermal power plant, daily start and stop (DSS) operations and weekly start and stop (WSS) operations are conducted in order to cope with fluctuating load, and start and stop operations are frequently repeated in the thermal power plant. The electric decationizing apparatus operates while the thermal power plant is in operation, but when the operation stops, the operation of the electric decationizing apparatus also stops. Since no voltage is applied to the electric decationizing apparatus during stoppage, cations in the condensate water that remain in the electric decationizing apparatus diffuse in the electric decationizing apparatus. Accordingly, condensate water that contains a larger number of cations than the normal level may be discharged from the electric decationizing apparatus for a certain time after the electric decationizing apparatus restarts. In that case, highly conductive condensate water is supplied to the conductivity meter, and anions that come from sea water cannot be accurately detected. Although this state disappears after a while, the conductivity meter must wait until it is ready for measurement. 
     The present invention aims at providing a method and a system for measuring the conductivity of decationized water that can, after the electric decationizing apparatus starts, shorten the amount of time that is needed for the conductivity meter to begin measuring the conductivity. 
     According to an aspect of the present invention, a method of measuring conductivity of decationized water comprises the steps of: supplying water to be treated that contains cations and anions to an electric decationizing apparatus while applying a first voltage to the electric decationizing apparatus, and generating decationized water; supplying the decationized water that is generated by the electric decationizing apparatus to a conductivity meter in order to measure conductivity of the decationized water; and before the first voltage is applied to the electric decationizing apparatus and before the conductivity meter begins measuring the conductivity of the decationized water, applying a second voltage that is lower than the first voltage to the electric decationizing apparatus in a state in which the electric decationizing apparatus is charged with water to be treated. Furthermore, a system for measuring conductivity of decationized water comprises a control that controls the electric decationizing apparatus in this manner. 
     According to the present invention, it is possible to provide a method and a system for measuring conductivity of decationized water that can, after the electric decationizing apparatus starts, shorten the amount of time that is needed for the conductivity meter to begin measuring the conductivity. 
     The above-described and other objects, features, and advantages of this application will become apparent from the following detailed description with reference to the accompanying drawings that illustrate the present application. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a conceptual view of a system for measuring the conductivity of decationized water according to the first embodiment of the present invention; 
         FIG. 2A  is a conceptual view illustrating the distribution of cations in the deionization chamber before the operation of the plant stops according to the first embodiment and the comparative example; 
         FIG. 2B  is a conceptual view illustrating the distribution of cations in the deionization chamber after the operation of the plant stops according to the comparative example; 
         FIG. 2C  is a conceptual view illustrating the distribution of cations in the deionization chamber after the operation of the plant stops according to the first embodiment; 
         FIG. 3  is a conceptual view of a system for measuring the conductivity of decationized water according to the second embodiment of the present invention; and 
         FIG. 4  is a conceptual view of a system for measuring the conductivity of decationized water according to the third embodiment of the present invention. 
     
    
    
     LIST OF REFERENCE NUMERALS 
     
         
         
           
               1 ,  101 ,  201  System for measuring conductivity 
               2  Flow meter 
               3 ,  3   a ,  3   b  Electric decationizing apparatus 
               4  Conductivity meter 
               5 ,  105 ,  205  Control 
               6  Line 
               7  Valve 
               8  Condensate water line 
               31  Deionization chamber 
               32 ,  33  Condensation chamber 
               38 ,  38   a ,  38   b  Power supply 
               39  Cation exchanger 
           
         
       
    
     DESCRIPTION OF EMBODIMENTS 
     Some embodiments of the present invention will be described with reference to the drawings. In each embodiment, water to be treated is condensate water of a thermal power plant. In other words, the system for measuring conductivity in each embodiment of the present invention is a system that a thermal power plant has. However, the present invention is not limited to this, and can be applied to a method and a system for measuring the conductivity of decationized water that is generated by an electric decationizing apparatus. 
     First Embodiment 
       FIG. 1  shows a conceptual view of system for measuring the conductivity of decationized water  1  according to the first embodiment of the present invention (hereinafter referred to as system  1 ). System  1  has electric decationizing apparatus  3 , conductivity meter  4 , flow meter  2  and valve  7 . These apparatuses are arranged on line  6  that branches from condensate water line  8 . Electric decationizing apparatus  3  generates decationized water while applying first voltage V 1  to water to be treated that contains cations and anions. First voltage V 1  is, for example, 0.8V. Conductivity meter  4  measures the conductivity of decationized water that is generated by electric decationizing apparatus  3 . Flow meter  2  measures the flow rate of the condensate water (water to be treated) that flows into electric decationizing apparatus  3 . Valve  7  is normally opened, but is closed when system  1  has to be isolated from the condensate water system. Conductivity meter  4  is connected to a drain (not shown) at the downstream thereof, and the water is reused as condensate water or is disposed of. 
     Electric decationizing apparatus  3  has deionization chamber  31  and a pair of condensation chambers  32 ,  33  that are arranged on both sides of deionization chamber  31  via cation exchange membranes  34 ,  35 . Positive electrode  36  is arranged in condensation chamber  32 , and negative electrode  37  is arranged in condensation chamber  33 , and condensation chambers  32 ,  33  also function as electrode chambers. Positive electrode  36  and negative electrode  37  are connected to DC power supply  38 . Deionization chamber  31  is charged with cation exchanger  39 . The arrangement of cation exchanger  39  is not limited as long as it can capture and remove cation components, but a cation exchange resin, a monolith type porous cation exchanger (hereinafter referred to simply as “monolith”), a fibrous porous cation exchanger and a particle aggregation type porous ion exchanger are preferably used. 
     Examples of monoliths include a monolith having an open cell structure having mesopores with an average diameter of 1-1000 μm, preferably 10-100 μm, formed in the wall of interconnected macropores, wherein the monolith has a total pore volume of 1-50 ml/g, preferably 4-20 ml/g, ion exchange groups that are evenly distributed and an ion exchanging capacity of 0.5 mg equivalent/g dry porous structure or more. Other characteristics of the monolith and the method of manufacturing the same are disclosed, for example, in JP 2003-334560. 
     A monolith that is used as the cation exchanger can largely increase pore volume and specific surface area. This is very advantageous because the deionizing efficiency of the electro decationizing apparatus is significantly improved. A monolith having a total pore volume that is less than 1 ml/g is not advantageous because the amount of water that passes through a unit sectional area is decreased and the treatment capacity is lowered. On the other hand, a total pore volume that exceeds 50 ml/g is not advantageous because the percentage of the skeleton is decreased and the porous structure is significantly weakened. It is preferable in terms of ensuring both the strength of the porous structure and good deionizing efficiency to use a monolith having a total pore volume of 1-50 ml/g as the ion exchanger of the electro decationizing apparatus. Furthermore, a monolith having an ion exchanging capacity that is less than 0.5 mg equivalent/g dry porous structure is not advantageous because the ion absorbing capacity is insufficient. In addition, the state in which ion exchange groups are unevenly distributed is not advantageous because this state makes the movement of ions uneven in the porous cation exchanger and prevents the ions that have been absorbed from being promptly removed. 
     Examples of fibrous porous ion exchangers include a monofilament or a woven or nonwoven fabric that are disclosed in JP5-64726, wherein the fabric is formed by a group of monofilaments, as well as an ion exchanger in which exchange groups are introduced into these products by radiation graft polymerization and that are formed in a certain shape. Examples of particle aggregation type porous ion exchangers include a mixed polymer of a thermoplastic polymer and a thermosetting polymer, disclosed, for example, in JP10-192716 and JP10-192717, as well as an ion exchanger in which ion exchange resin particles are combined by a cross-linked polymer and that are formed in a certain shape. 
     Electric decationizing apparatus  3  of the present embodiment is an electro deionizing apparatus (EDI). In the EDI, cation components (NH 4   + , NA + , CA 2+ , Mg 2+  and the like) are captured by cation exchanger  39 , and at the same time, water dissociation reaction occurs in deionization chamber  31  such that hydrogen ions and hydroxide ions are generated. The cation components that have been captured by cation exchanger  39  are replaced with hydrogen ions and are released from cation exchanger  39 . The cation components that have been released move along cation exchanger  39  to cation exchange membrane  35  on the side of negative electrode  37  by electrophoresis, are electrodialyzed by cation exchange membrane  35 , and flow into condensation chamber  33 . The cation components that have moved to condensation chamber  33  are discharged together with condensed water that flows in condensation chamber  33 . Since the exchange groups of cation exchanger  39  release cation components after the exchange groups are combined with the cation components, and combine with hydrogen ions again, cation exchanger  39  is continuously regenerated. Since the removal of cation components and the regeneration of cation exchanger  39  occur automatically and continuously in the EDI in this manner, it is basically unnecessary to conduct the regeneration of cation exchanger  39  in a separate process. It should be noted that electric decationizing apparatus  3  is not limited to an EDI, and it may be an electrodialysis apparatus (ED) that is not charged with cation exchanger  39 . 
     System  1  further has control  5  that controls electric decationizing apparatus  3 . Control  5  is connected both to flow meter  2  and to power supply  38  of electric decationizing apparatus  3 . Now, the method of operating system  1  and the function of control  5  will be described. Condensate water flows through condensate water line  8  during operation of the plant. Valve  7  is opened, and a part of the condensate water is introduced into electric decationizing apparatus  3  through line  6 . The flow rate of the condensate water that flows through line  6 , that is, the flow rate of water to be treated that is introduced into electric decationizing apparatus  3 , is measured by flow meter  2  that is provided upstream of electric decationizing apparatus  3 . 
     Condensate water (water to be treated) contains ammonia or hydrazine in order to prevent the corrosion of the piping and facilities of the condensate water system. These are pH adjusters that are added to the condensate water in order to adjust the pH of the condensate water. The pH of the condensate water has been set at about 8.5-9.8 in conventional thermal power plants, but may recently be set at 10 or more. The pH adjuster is normally present in the condensate water in the form of cations (cation components). For example, ammonia is present in the form of NH 4   +  in the condensate water. Cations in the water to be treated are captured by cation exchanger  39  of electric decationizing apparatus  3 , and is discharged to condensation chamber  33  on the side of negative electrode  37  through cation exchange membrane  35 . The treated water from which cation components are removed by electric decationizing apparatus  3  becomes substantially pure water. Thus, the conductivity of the treated water that is measured by conductivity meter  4  is around 0.06 μS/cm. 
     There is possibility that sea water intrudes into the condensate water system that is connected to a sea water cooling type condenser. Specifically, since the pressure of the condenser is low on the steam side, if a pin hole or the like is generated on the condenser tube in which sea water flows, then sea water intrudes into the steam side from the tube, and thereby significantly increases salt concentration in the condensate water. Salts that are contained in sea water include NaCl, Na 2 SO 4  and the like. When the condensate water in which these salts are mixed is introduced into electric decationizing apparatus  3 , cation exchanger  39  captures NH 4   + , Na +  to allow treated water that contains HCl, H 2 SO 4  and the like to be discharged from electric decationizing apparatus  3 . Conductivity meter  4  detects anions that are present in the treated water in the form of Cl − , SO 4   −2  and the like. Therefore, the conductivity that is measured by conductivity meter  4  is larger than the normal level (for example, 0.1 μS/cm or more). In this manner, it is possible to detect the intrusion of sea water into the condensate water by using conductivity meter  4  that measures the conductivity of the treated water that is free of most cations and that contains anions that come from sea water, such as chlorine ions. Such conductivity meter  4  is also called an acid conductivity meter. 
     Next, the operation of system  1  at a time when a thermal power plant restarts after it stopped will be described. When a thermal power plant stops, the flow of condensate water also stops, and the flow rate of the condensate water that is introduced into electric decationizing apparatus  3  also becomes zero. The output signal of flow meter  2  is transmitted to control  5 , and control  5  judges that the flow rate is zero, that is, the thermal power plant has stopped. Control  5  controls power supply  38  in order to lower the voltage that is applied to positive electrode  36 . Second voltage V 2  that is lower than first voltage V 1  is applied to positive electrode  36 . 
       FIGS. 2A-2C  are conceptual views illustrating the distribution of cations C in deionization chamber  31  according to the present embodiment and a comparative example. The exchange groups of cation exchanger  39  in deionization chamber  31  holds cations, and the exchange groups are distributed on the side of negative electrode  37  before power supply  38  stops. Specifically, cations C are distributed on the side of negative electrode  37 , as shown in  FIG. 2A . This state is common both to the present embodiment and to the comparative example. No cation C is substantially present on the side of outlet of electric decationizing apparatus  3 , and treated water that is discharged from electric decationizing apparatus  3  is decationized water that is free of cations. Although not illustrated, cations C are also present in condensation chamber  33 . It should be noted that decationized water is not limited to water that is completely free of cations, but may contain a small number of cations. 
     In the comparative example, when control  5  judges that the flow rate of the condensate water that is introduced into electric decationizing apparatus  3  becomes zero, that is, the thermal power plant has stopped, control  5  deactivates power supply  38  in order to set the voltage between positive electrode  36  and negative electrode  37  of electric decationizing apparatus  3  at zero. Thereafter, cations C diffuse and are substantially evenly distributed in deionization chamber  31 , as shown in  FIG. 2B . A part of cations C in condensation chamber  33  also intrude into deionization chamber  31  through cation exchange membrane  35 . Thereafter, when the thermal power plant restarts and electric decationizing apparatus  3  is activated, cations C are gradually distributed on the side of negative electrode  37 , and the state shown in  FIG. 2A  is restored. Cations C that have intruded into deionization chamber  31  from condensation chamber  33  also flow back to condensation chamber  33  through cation exchange membrane  35 . 
     On the other hand, according to the present embodiment, second voltage V 2  that is smaller than first voltage V 1  is applied to electric decationizing apparatus  3  when the thermal power plant is not operating. The lever of second voltage V 2  may be set to prevent the cations that are held by cation exchangers  39  from diffusing in deionization chamber  31  and to prevent the cations that have flowed into condensation chamber  33  from intruding into (returning to) deionization chamber  31  through cation exchange membrane  35 , as shown in  FIG. 2C . Second voltage V 2  may be about the same level as first voltage V 1 , but no differences occur by setting the voltage too large (see Examples 1-1, 1-2). In addition, since water volume is reduced due to electrolysis of water and since thereby ion exchangers may be dried, it is preferable that second voltage V 2  be as low as possible as long as the above condition is met. Second voltage V 2  is, for example, 0.05V. By applying second voltage V 2  of this level, it is possible to substantially prevent cations C in condensation chamber  33  from intruding into deionization chamber  31  through cation exchange membrane  35 . 
     The timing when first voltage V 1  may be shift to second voltage V 2  is not limited, but, for example, it may be a time when the flow rate that is measured by flow meter  2  becomes zero. In this state, since there is no flow of water to be treated in deionization chamber  31 , cations C are distributed in a region having a substantially constant width (the dimension in a direction perpendicular both to positive electrode  36  and to negative electrode  37 ) on the side of negative electrode  37  between the upstream end (the inlet) and the downstream end (the outlet) of deionization chamber  31 . Specifically, the distribution of cations C at the outlet of deionization chamber  31  is not similar to  FIG. 2A , where no cation is present, but cations C are present in a certain region on the side of negative electrode  37 . However, as will be clear from the comparison between  FIG. 2C  and  FIG. 2B , cations C are only present in a limited region at the outlet of deionization chamber  31 . Therefore, when the thermal power plant restarts and the condensate water flows into deionization chamber  31  again, it takes only a short time to restore the state shown in  FIG. 2A . It should be noted that valve  7  may be opened because the condensate water substantially does not flow into line  6  when the thermal power plant is not operating, but the valve may be closed in order to prevent the condensate water from accidentally flowing into the line. 
     Second voltage V 2  may be continuously or intermittently applied when the thermal power plant is not operating, but it is not always necessary to do so. For example, it is possible to stop applying a voltage to electric decationizing apparatus  3  after the thermal power plant stops (this results in the state shown in  FIG. 2B ), and thereafter to apply second voltage V 2  immediately before the thermal power plant restarts (this results in the state shown in  FIG. 2C ). In addition, if the measurement is not conducted by conductivity meter  4 , then it is not necessary to apply second voltage V 2  even after the thermal power plant starts. Second voltage V 2  may be applied immediately before the measurement is conducted by conductivity meter  4 . In other words, according to the present embodiment, it is sufficient to apply second voltage V 2  that is lower than first voltage V 1  to electric decationizing apparatus  3  that is charged with water to be treated before conductivity meter  4  starts measuring the conductivity of the decationized water. 
     In addition, when the application of a voltage to electric decationizing apparatus  3  stops, ion components that have moved to condensation chambers  32 ,  33  diffuse in deionization chamber  31 . And, it is known that a weak reverse current flows between positive electrode  36  and negative electrode  37  in this state. Therefore, by limiting the reverse current, it is possible to prevent the ion components from diffusing from condensation chambers  32 ,  33  to deionization chamber  31 , and water quality can be expected to improve quickly when the operation restarts. Therefore, by inserting an electronic element that limits the reverse current into a circuit that connects the DC power supply to positive electrode  36  and negative electrode  37 , water quality can be improved quickly when the operation restarts. As an electronic element to limit the reverse current, for example, a semiconductor diode having a rectifying function may be used. 
     Second Embodiment 
       FIG. 3  shows a conceptual view of system for measuring the conductivity of decationized water  101  (hereinafter referred to as system  101 ) according to the second embodiment of the present invention. In the following description, differences from the first embodiment will be mainly described. Arrangements that are not explained are the same as those of the first embodiment. 
     System  101  of the present embodiment has water supply means  110  that supplies pure water to electric decationizing apparatus  3  before the thermal power plant stops or before electric decationizing apparatus  3  stops. Water supply means  110  has vessel  111  that stores pure water, water supply line  112  that connects vessel  111  to the inlet of electric decationizing apparatus  3  and transfer pump  113  that is provided on water supply line  112 . The outlet of conductivity meter  4  is connected to vessel  111  via line  114 , and the treated water that flows through line  114  is stored in vessel  111  without being discharged. Accordingly, the pure water is decationized water that is generated by electric decationizing apparatus  3 . Water supply means  110  further has pure water source  115  that is connected to the inlet of electric decationizing apparatus  3  via lines  116 ,  6 . A dedicated system may be provided as pure water source  115 , but a pure water supply system of the thermal power plant may be used instead. Supply line  116  of pure water source  115  merges with line  6 . Supply line  116  has valve  117 . In this way, water supply means  110  has two separate water supply sources, but one of them may be omitted. 
     When the flow rate of the water to be treated that is measured by flow meter  2  becomes zero, control  105  activates transfer pump  113  or opens valve  117  in order to supply the pure water to electric decationizing apparatus  3 . Valve  7  is closed in advance in order to prevent the pure water from flowing into condensate water line  8 . Power supply  38  of electric decationizing apparatus  3  continues to provide current. Accordingly, a certain level of voltage continues to be applied between positive electrode  36  and negative electrode  37 . Since no cation is newly supplied to electric decationizing apparatus  3 , the cations that are present in deionization chamber  31  are gradually removed before the pure water is supplied, and the number of cations that stay in deionization chamber  31  is sufficiently reduced after a certain time. The time duration for supplying the pure water is selected such that the cations in deionization chamber  31  are sufficiently discharged, and it is, for example, 10-20 minutes. The amount of water that is supplied is preferably about 20 times as large as the volume of deionization chamber  31 , and the effect is the same level even if pure water of 30 times or more is supplied. Control  105  has timer  120  that controls the time duration for supplying the pure water. Control  105  deactivates power supply  38  of electric decationizing apparatus  3  after a certain time for supplying the pure water passes. Most of the exchange groups of cation exchanger  39  are converted into H +  form (that is, substantially no exchange group, such as in NH 4   +  form, is present), and deionization chamber  31  stays substantially free of cations. 
     Next, when the thermal power plant starts, electric decationizing apparatus  3  is also activated. Valve  7  is opened in order to supply condensate water from condensate water line  8  to electric decationizing apparatus  3 . The pure water that is stored in deionization chamber  31  of electric decationizing apparatus  3  is discharged, and deionization chamber  31  is charged with condensate water. The pure water that filled deionization chamber  31  is not measured by conductivity meter  4  because it does not contain sea water components. Thereafter, condensate water is continuously supplied from condensate water line  8  to electric decationizing apparatus  3  and is treated by electric decationizing apparatus  3  before it is supplied to conductivity meter  4 . The deionization chamber is put in the state shown in  FIG. 2A  after a certain time and is ready to measure conductivity. Since the transfer to this state is done only in a short time, measuring the conductivity can be immediately started. 
     Vessel  111  only need to have a capacity that corresponds to the amount of treated water (pure water) that is supplied, and any capacity larger than this is not required. After treated water of a required volume has been stored in vessel  111 , vessel  111  may overflow with the water that is discharged from conductivity meter  4 , or vessel  111  may be bypassed by a line (not shown) that bypasses vessel  111  in order to prevent the water from flowing in vessel  111 . 
     According to the present embodiment, cations that stay in deionization chamber  31  of electric decationizing apparatus  3  are discharged while the pure water is supplied, and electric decationizing apparatus  3  stops thereafter. Thus, it is possible to limit cations that are contained in the water that is discharged from deionization chamber  31  after the thermal power plant restarts. As long as pure water is supplied before conductivity meter  4  begin measuring the conductivity of the decationized water and after the thermal power plant starts next time, the pure water may be supplied after the thermal power plant stops. It is preferable that the number of cations that are contained in water that is supplied to discharge the cations that have stayed in electric decationizing apparatus  3  be as small as possible, but cation concentration of the water may be lower than that of the water to be treated (low cation concentration water). 
     Third Embodiment 
       FIG. 4  shows a conceptual view of system for measuring the conductivity of decationized water  201  (hereinafter referred to as system  201 ) according to the third embodiment of the present invention. In the following description, differences from the first embodiment will be mainly described. Arrangements that are not explained are the same as those of the first embodiment. 
     System  201  of the present embodiment has first electric decationizing apparatus  3   a  and second electric decationizing apparatus  3   b  that are connected in series by a line. First electric decationizing apparatus  3   a  is located upstream of second electric decationizing apparatus  3   b , and the water that is discharged from first electric decationizing apparatus  3   a  is supplied to second electric decationizing apparatus  3   b  in order to achieve two stage cation removal. First electric decationizing apparatus  3   a  and second electric decationizing apparatus  3   b  may have the same arrangement and the same dimensions. Since most of the cations are removed by first electric decationizing apparatus  3   a , the distribution of cations C in deionization chamber  31  during operation is as shown in  FIG. 4 . 
     When the operation of the thermal power plant stops, power supply  38   a  of first electric decationizing apparatus  3   a  and power supply  38   b  of second electric decationizing apparatus  3   b  are also deactivated. Accordingly, as shown in  FIG. 2B , cations are distributed in the entire region of deionization chambers  31  of first and second electric decationizing apparatuses  3   a ,  3   b . However, since most of the cations are removed by first electric decationizing apparatus  3   a , the number of cations that flow into second electric decationizing apparatus  3   b  is limited. Therefore, when the thermal power plant restarts, the number of cations that are discharged from second electric decationizing apparatus  3   b  is limited to a level such that there is only a small number of cations to allow conductivity meter  4  to sufficiently accurately measure the conductivity based on anions. Immediately after first electric decationizing apparatus  3   a  restarts, a larger number of cations C than the normal level flow from first electric decationizing apparatus  3   a  to second electric decationizing apparatus  3   b , but since the cations are removed by second electric decationizing apparatus  3   b , the measurement of the conductivity is not largely affected. It should be noted that the number of electric decationizing apparatuses  3  that are connected in series is not limited to two, and any number of electric decationizing apparatuses  3  may be connected in series. Conductivity meter  4  is connected to the outlet of the most downstream electric decationizing apparatus, and the conductivity of the decationized water that is discharged from the most downstream electric decationizing apparatus is measured by conductivity meter  4 . 
     EXAMPLE 
     Water to be treated that contains ammonia at 1 mg/L was decationized by an EDI for ten hours, and the EDI was not operating for fifty hours. The EDI restarted, and water to be treated that contains ammonia at 1 mg/L was supplied at a flow rate of 10 L/h, and the time until the indication of the conductivity meter fell below 0.1 μS/cm was measured. The voltage that was applied to the EDI was 0.8V unless otherwise mentioned. The results were shown in Table 1. Examples 1-1, 1-2 correspond to the first embodiment, Example 2 correspond to the second embodiment, and Example 3 correspond to the third embodiment. In each Example, the time until the indication of the conductivity meter fell below 0.1 μS/cm was shortened, as compared to the comparative example. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Time until the 
               
               
                   
                   
                 conductivity 
               
               
                   
                   
                 fell below 
               
               
                   
                 Content 
                 0.1 μS/cm 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Comparative 
                 No countermeasure 
                 6 
                 min. 
               
               
                   
                 example 
               
               
                   
                 Example 1-1 
                 A voltage of 0.05 V was 
                 &lt;0.5 
                 min. 
               
               
                   
                   
                 continuously applied to 
               
               
                   
                   
                 the EDI during stoppage 
               
               
                   
                 Example 1-2 
                 A voltage of 0.1 V was 
                 &lt;0.5 
                 min. 
               
               
                   
                   
                 continuously applied to 
               
               
                   
                   
                 the EDI during stoppage 
               
               
                   
                 Example 2 
                 Pure water is supplied 
                 &lt;0.5 
                 min. 
               
               
                   
                   
                 to the EDI before stoppage 
               
               
                   
                 Example 3 
                 Two stage EDIs are employed 
                 1.5 
                 min. 
               
               
                   
                   
               
            
           
         
       
     
     Although some preferred embodiments of the present invention have been illustrated and described in detail, it should be appreciated that various changes and modifications can be made thereto without deviating from the spirit and the scope of the appended claims.