Patent Publication Number: US-2018036686-A1

Title: Water quality monitoring device, water treatment device, water treatment system, water quality monitoring method, and program

Description:
TECHNICAL FIELD 
     The present invention relates to a water quality monitoring device, a water treatment device, a water treatment system, a water quality monitoring method, and a program. 
     BACKGROUND ART 
     Reverse osmosis membranes used in seawater desalination plants are degraded by turbidity components, organic matter, and other fouling substances contained in seawater that is supplied to the reverse osmosis membranes. To prevent degradation of the reverse osmosis membrane, a sand filtration device, a dual media filter (DMF), a ceramic membrane filter (CMF), or other pretreatment device is usually provided upstream of the reverse osmosis membrane. It is also known for a method for preventing degradation of the reverse osmosis membrane, that monitoring the concentration of fouling substances contained in water supplied to the reverse osmosis membrane. 
     Patent Literature 1 discloses a technique of measuring the viscosity of water supplied to a membrane separation device by a torque meter and stopping the supply of water to the membrane separation device when a value detected by the torque meter is equal to or greater than a predetermined value. 
     CITATION LIST 
     Patent Literature 
     [Patent Literature 1] 
     Japanese Patent No. 3132044 
     SUMMARY OF INVENTION 
     Technical Problem 
     Most part of fouling substances contained in seawater is treated by the pretreatment device. Therefore, the amount of fouling substances contained in the water supplied to the reverse osmosis membrane is about several 100 parts per billion (ppb). A change in viscosity due to a change in the concentration of fouling substances at several 100 ppb is from about several tenths of one percent to several percent at most. However, a general torque meter does not have a resolution capable of detecting changes in viscosity of about several tenths of one percent to several percent by online measurement in plant environments. 
     Solution to Problem 
     According to a first aspect of the present invention, a water quality monitoring device which monitors water quality in a water treatment device that generates fresh water using a reverse osmosis membrane includes a speed determination unit which determines a speed of a wave passing through water present upstream of the reverse osmosis membrane to measure a parameter correlated with a concentration of organic matter in the water, and a concentration reduction processing unit which reduces the concentration of organic matter in water present upstream of the reverse osmosis membrane when the speed determined by the speed determination unit is greater than a predetermined threshold speed. 
     According to a second aspect of the present invention, the water quality monitoring device according to the first aspect further includes a density determination unit which determines a density of water present upstream of the reverse osmosis membrane, wherein, when the speed determined by the speed determination unit is greater than the predetermined threshold speed and the density determined by the density determination unit is less than a predetermined threshold density, the concentration reduction processing unit reduces the concentration of organic matter in water present upstream of the reverse osmosis membrane. 
     According to a third aspect of the present invention, in the water quality monitoring device according to the first or second aspect, the concentration reduction processing unit reduces the concentration of organic matter in water present upstream of the reverse osmosis membrane using a different method for each range of the speed determined by the speed determination unit. 
     According to a fourth aspect of the present invention, in the water quality monitoring device according to any one of the first to third aspects, the concentration reduction processing unit reduces the concentration of organic matter in water present upstream of the reverse osmosis membrane by outputting a command to add a flocculant to a chemical injection device which adds a flocculant to water that is supplied to a pretreatment device provided upstream of the reverse osmosis membrane. 
     According to a fifth aspect of the present invention, in the water quality monitoring device according to any one of the first to fourth aspects, the concentration reduction processing unit reduces the concentration of organic matter in water present upstream of the reverse osmosis membrane by activating a backwash device which backwashes a pretreatment device provided upstream of the reverse osmosis membrane. 
     According to a sixth aspect of the present invention, the water quality monitoring device according to the second aspect further includes a concentration determination unit which determines a parameter correlated with a concentration of organic matter and a parameter correlated with a concentration of inorganic microparticles on the basis of the speed determined by the speed determination unit and the density determined by the density determination unit, wherein the concentration reduction processing unit reduces the concentration of organic matter in water present upstream of the reverse osmosis membrane when the parameter correlated with the concentration of organic matter determined by the concentration determination unit is greater than the predetermined threshold speed and reduces the concentration of inorganic matter in water present upstream of the reverse osmosis membrane when the parameter correlated with the concentration of inorganic matter determined by the concentration determination unit is greater than the predetermined threshold speed. 
     According to a seventh aspect of the present invention, a water quality monitoring device which monitors water quality in a water treatment device that generates fresh water using a reverse osmosis membrane includes a speed determination unit which determines a speed of a wave passing through water present upstream of the reverse osmosis membrane to measure a parameter correlated with a concentration of organic matter in the water, and a presentation unit which presents the parameter correlated with the speed determined by the speed determination unit. 
     According to an eighth aspect of the present invention, the water quality monitoring device according to the seventh aspect further includes a density determination unit which determines a density of water present upstream of the reverse osmosis membrane, wherein the presentation unit presents parameters correlated with the speed and the density. 
     According to a ninth aspect of the present invention, the water quality monitoring device according to the eighth aspect further includes a concentration determination unit which determines a parameter correlated with a concentration of organic matter and a parameter correlated with a concentration of inorganic microparticles on the basis of the speed determined by the speed determination unit and the density determined by the density determination unit, wherein the presentation unit presents the parameter correlated with the concentration of organic matter and the parameter correlated with the concentration of inorganic microparticles. 
     According to a tenth aspect of the present invention, the water quality monitoring device according to any one of the first to ninth aspects further includes a storage processing unit which stores part of water present upstream of the reverse osmosis membrane in a predetermined container when the speed determined by the speed determination unit is greater than the predetermined threshold speed. 
     According to an eleventh aspect of the present invention, in the water quality monitoring device according to any one of the first to tenth aspects, the speed determination unit determines a speed of a wave passing through water before the water passes through a pretreatment device provided upstream of the reverse osmosis membrane and a speed of a wave passing through water after the water passes through the pretreatment device. 
     According to a twelfth aspect of the present invention, a water treatment device includes a reverse osmosis membrane, a wave transmitter which is provided upstream of the reverse osmosis membrane and which generates a wave in water present upstream of the reverse osmosis membrane, and a wave receiver which is provided upstream of the reverse osmosis membrane and which detects the wave generated by the wave transmitter. 
     According to a thirteenth aspect of the present invention, the water treatment device according to the twelfth aspect further includes a vibration tube through which water present upstream of the reverse osmosis membrane flows, an oscillator which vibrates the vibration tube, and a vibration detector which detects a vibrational amplitude of the vibration tube, wherein the wave transmitter and the wave receiver are provided on the vibration tube. 
     According to a fourteenth aspect of the present invention, a water treatment system includes the water treatment device according to the twelfth or thirteenth aspect, and the water quality monitoring device according to any one of the first to eleventh aspects. 
     According to a fifteenth aspect of the present invention, a water quality monitoring method includes a speed determining step including determining a speed of a wave passing through water present upstream of a reverse osmosis membrane, and a concentration reduction step including reducing a concentration of organic matter in water present upstream of the reverse osmosis membrane when the determined speed is greater than a predetermined threshold speed. 
     According to a sixteenth aspect of the present invention, a program causes a computer for a water quality monitoring device, which monitors water quality in a water treatment device that generates fresh water using a reverse osmosis membrane, to function as a speed determination unit which determines a speed of a wave passing through water present upstream of the reverse osmosis membrane to measure a parameter correlated with a concentration of organic matter in the water, and a concentration reduction processing unit which reduces the concentration of organic matter in water present upstream of the reverse osmosis membrane when the speed determined by the speed determination unit is greater than a predetermined threshold speed. 
     According to a seventeenth aspect of the present invention, a program causes a computer for a water quality monitoring device, which monitors water quality in a water treatment device that generates fresh water using a reverse osmosis membrane, to function as a speed determination unit which determines a speed of a wave passing through water present upstream of the reverse osmosis membrane to measure a parameter correlated with a concentration of organic matter in the water, and a presentation unit which presents the parameter correlated with the speed determined by the speed determination unit. 
     Advantageous Effects of Invention 
     According to at least one of the above aspects, the water quality monitoring device measures the speed of a wave generated in water present upstream of the reverse osmosis membrane. The speed of the wave propagating in the water has a correlation with the viscosity of the water. The speed of the wave is determined by a period of time during which the wave propagates. Therefore, since it is possible to improve the temporal resolution, it is possible to improve the detection accuracy of the wave speed. This allows the water quality monitoring device to detect changes in the concentration of fouling substances at several 100 ppb. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram showing a configuration of a seawater treatment system according to a first embodiment. 
         FIG. 2  is a cross-sectional view showing a structure of a measurement device according to the first embodiment. 
         FIG. 3  is a schematic block diagram showing a configuration of a water quality monitoring device according to the first embodiment. 
         FIG. 4  is a flowchart showing a sequence of a water quality monitoring process according to the first embodiment. 
         FIG. 5  is a schematic diagram showing a configuration of a seawater treatment system according to a second embodiment. 
         FIG. 6  is a flowchart showing a sequence of a water quality monitoring process according to the second embodiment. 
         FIG. 7  is a schematic diagram showing a configuration of a seawater treatment system according to a third embodiment. 
         FIG. 8  is a flowchart showing a sequence of a water quality monitoring process according to the third embodiment. 
         FIG. 9  is a schematic block diagram showing a configuration of a water quality monitoring device according to a fourth embodiment. 
         FIG. 10  is a flowchart showing a sequence of a water quality monitoring process according to the fourth embodiment. 
         FIG. 11  is a schematic block diagram showing a configuration of a water quality monitoring device according to a fifth embodiment. 
         FIG. 12  is a flowchart showing a sequence of a water quality monitoring process according to the fifth embodiment. 
         FIG. 13  is a schematic diagram showing a configuration of a seawater treatment system according to a sixth embodiment. 
         FIG. 14  is a schematic block diagram showing a configuration of a water quality monitoring device according to the sixth embodiment. 
         FIG. 15  is a flowchart showing a sequence of a water quality monitoring process according to the sixth embodiment. 
         FIG. 16  is a cross-sectional view showing a structure of a measurement device according to a modified example. 
         FIG. 17  is a schematic block diagram showing a configuration of a computer according to at least one of the embodiments. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     A first embodiment is described below. 
       FIG. 1  is a schematic diagram showing a configuration of a seawater treatment system according to the first embodiment. In  FIG. 1 , solid line arrows represent water distribution pipes and dashed line arrows represent communication lines. 
     A seawater treatment system  1  is a system for producing fresh water from seawater. The seawater treatment system  1  includes a water intake device  101 , a first water storage tank  102 , a first pump  103 , a DMF  104 , a chemical injection device  105 , a second water storage tank  106 , a second pump  107 , a measurement device  108 , a reverse osmosis membrane  109 , a third water storage tank  110 , and a water quality monitoring device  111 . 
     The water intake device  101  takes in seawater from a sea area which is a water intake target. The water intake device  101  allows the taken-in seawater to be stored in the first water storage tank  102 . 
     The first pump  103  delivers seawater stored in the first water storage tank  102  to the DMF  104 . 
     The DMF  104  internally has two types of filtration layers. Examples of the filtration layers include a sand layer and an anthracite layer. The DMF  104  passes seawater delivered by the first pump  103  through the internal filtration layers to filter the seawater. Seawater filtered by the DMF  104  is then stored in the second water storage tank  106 . 
     The chemical injection device  105  adds a flocculant to the seawater delivered by the first pump  103 . 
     The second pump  107  delivers seawater stored in the second water storage tank  106  to the reverse osmosis membrane  109 . The second pump  107  operates at a higher pressure than the first pump  103 . 
     The measurement device  108  measures the quality of seawater stored in the second water storage tank  106 . The seawater stored in the second water storage tank  106  is water present upstream of the reverse osmosis membrane  109 . 
     The reverse osmosis membrane  109  passes only water molecules of the seawater delivered by the second pump  107 . Fresh water obtained by filtration through the reverse osmosis membrane  109  is stored in the third water storage tank  110 . 
     The water quality monitoring device  111  controls the chemical injection device  105  on the basis of the quality of seawater supplied to the reverse osmosis membrane  109 . 
     Although the seawater treatment system  1  according to the present embodiment has a configuration shown in  FIG. 1 , the present invention is not limited to this and the seawater treatment system  1  may include at least the reverse osmosis membrane  109 , the measurement device  108 , and the water quality monitoring device  111 . For example, a seawater treatment system  1  according to another embodiment may include a sand filtration device, a CMF, or other pretreatment device instead of the DMF  104 . A seawater treatment system  1  according to another embodiment may include, for example, a plurality of reverse osmosis membranes  109  connected in parallel or in series. A seawater treatment system  1  according to another embodiment may include another treatment device which reduces the concentration of organic matter in water present upstream of the reverse osmosis membrane  109 , instead of the chemical injection device  105 . Examples of the treatment device which reduces the concentration of organic matter in water present upstream of the reverse osmosis membrane  109  include a backwash device for the DMF  104  and a pressure control device for the second pump  107 . A water treatment system according to another embodiment may generate fresh water from lake water, dam water, or other water instead of seawater. 
       FIG. 2  is a sectional view showing a structure of a measurement device according to the first embodiment. 
     The measurement device  108  includes a housing  201 , a partition plate  202 , a U-shaped tube  203 , an ultrasonic wave transmitter  204 , an ultrasonic wave receiver  205 , an oscillator  206 , a vibration detector  207 , and a calculator  208 . 
     The housing  201  forms an outer shell of the measurement device  108 . 
     The partition plate  202  divides an inner space of the housing  201  into a first compartment and a second compartment. 
     The U-shaped tube  203  is provided straddling both the first and second compartments of the housing  201 . Two ends of the U-shaped tube  203  protrude outward from a wall of the first compartment of the housing  201 . That is, the U-shaped tube  203  is provided so as to penetrate the partition plate  202  and the wall of the first compartment of the housing  201 . The two ends of the U-shaped tube  203  are attached to a pipe that connects the second pump  107  and the reverse osmosis membrane  109 . This structure allows seawater, which is to be supplied to the reverse osmosis membrane  109 , to flow into the U-shaped tube  203 . The U-shaped tube  203  is fixed to the partition plate  202  and the wall of the first compartment of the housing  201  such that the U-shaped tube  203  does not contact top and bottom surfaces of the housing  201 . The U-shaped tube  203  is formed of a highly corrosion-resistant material such as Hastelloy (registered trademark). This can increase the durability of the measurement device  108 . 
     The ultrasonic wave transmitter  204  is fixed to the U-shaped tube  203  in the first compartment of the housing  201 . The ultrasonic wave transmitter  204  emits an ultrasonic wave toward the U-shaped tube  203 . 
     The ultrasonic wave receiver  205  is provided opposite the ultrasonic wave transmitter  204 , across the U-shaped tube  203 . The ultrasonic wave receiver  205  receives the ultrasonic wave generated by the ultrasonic wave transmitter  204  through the U-shaped tube  203 . 
     The oscillator  206  is fixed to the U-shaped tube  203  in the second compartment of the housing  201 . The oscillator  206  applies vibrations of a predetermined frequency to the U-shaped tube  203 . The oscillator  206  vibrates in a direction perpendicular to a plane defined by the top and the two ends of the U-shaped tube  203 . 
     The vibration detector  207  is fixed to the U-shaped tube  203  in the second compartment of the housing  201 . The vibration detector  207  detects a vibrational amplitude of the U-shaped tube  203 . 
     The calculator  208  measures a period of time from when the ultrasonic wave transmitter  204  generates an ultrasonic wave to when the ultrasonic wave receiver  205  receives the ultrasonic wave. The calculator  208  according to the present embodiment measures the period of time with six or more significant figures accuracy. The calculator  208  calculates the sonic speed of the ultrasonic wave on the basis of the period of time from when the ultrasonic wave transmitter  204  generates the ultrasonic wave to when the ultrasonic wave receiver  205  receives the ultrasonic wave. The calculator  208  calculates a resonant frequency of the U-shaped tube  203  on the basis of a relationship between the frequency of vibration by the oscillator  206  and the vibrational amplitude detected by the vibration detector  207 . The calculator  208  calculates the density of water filling the U-shaped tube  203  on the basis of the resonant frequency of the U-shaped tube  203 . 
     Although the measurement device  108  according to the present embodiment has a structure shown in  FIG. 2 , the present invention is not limited to this and the measurement device  108  may include at least a transmitter that generates a wave and a receiver that receives the wave. For example, a measurement device  108  according to another embodiment may not include the oscillator  206  and the vibration detector  207 . A measurement device  108  according to another embodiment may include, for example, an ultrasonic wave transmitter  204  and an ultrasonic wave receiver  205  which are directly attached to a pipe which connects a second pump  107  and a reverse osmosis membrane  109 . A transmitter according to another embodiment may be configured to generate sound waves, light, or other waves instead of the ultrasonic waves. Although the ultrasonic wave receiver  205  according to the present embodiment is provided opposite the ultrasonic wave transmitter  204 , the present invention is not limited to this. For example, an ultrasonic wave receiver  205  according to another embodiment may be provided alongside the ultrasonic wave transmitter  204 . In this case, the ultrasonic wave receiver  205  receives the reflection of an ultrasonic wave generated by the ultrasonic wave transmitter  204 . 
       FIG. 3  is a schematic block diagram showing a configuration of a water quality monitoring device according to the first embodiment. 
     The water quality monitoring device  111  includes a speed determination unit  301 , a viscosity calculation unit  302 , a presentation unit  303 , an evaluation unit  304 , and a concentration reduction processing unit  305 . 
     The speed determination unit  301  acquires information indicating the speed of the ultrasonic wave from the measurement device  108 . 
     The viscosity calculation unit  302  calculates the viscosity of water which is supplied to the reverse osmosis membrane  109  on the basis of the information acquired by the speed determination unit  301 . 
     The presentation unit  303  allows a not-shown display device to display the viscosity calculated by the viscosity calculation unit  302 . The presentation unit  303  is an example of a process execution unit that performs a process based on the speed of the ultrasonic wave determined by the speed determination unit  301 . 
     The evaluation unit  304  evaluates whether or not the viscosity of water which is supplied to the reverse osmosis membrane  109  is greater than a predetermined threshold viscosity on the basis of the viscosity calculated by the viscosity calculation unit  302 . The evaluation unit  304  can detect changes of about several percent in the viscosity. This is because the measurement device  108  measures the period of time from when an ultrasonic wave is transmitted to when the ultrasonic wave is received with six or more significant figure accuracy. 
     The concentration reduction processing unit  305  outputs a command to add a flocculant to the chemical injection device  105  when the viscosity of water which is supplied to the reverse osmosis membrane  109  is greater than the predetermined threshold viscosity. Output of the command to add a flocculant is an example of a process for reducing the concentration of organic matter in water present upstream of the reverse osmosis membrane  109 . A concentration reduction processing unit  305  according to another embodiment may perform other processes for reducing the concentration of organic matter in water present upstream of the reverse osmosis membrane  109 . Other processes for reducing the concentration of organic matter in water include a command to increase the amount of a flocculant added, a command to change the type of the flocculant, a command to backwash the reverse osmosis membrane  109 , and the like. The concentration reduction processing unit  305  is an example of a process execution unit which performs a process based on the speed of an ultrasonic wave determined by the speed determination unit  301 . 
     It is known that the higher the concentration of fouling substances, the greater the influence upon fouling of the reverse osmosis membrane  109 . Even when the types of fouling substances in water are the same, the higher the concentration of fouling substances in water is, the greater the viscosity of water is. It is also known that the greater the molecular mass of fouling substances, the greater the influence upon fouling of the reverse osmosis membrane  109 . Even when the concentrations of fouling substances in water are the same, the higher the molecular mass of fouling substances in water, the greater the viscosity of water. That is, the level of viscosity of water corresponds to the level of risk of fouling. 
     The viscosity of water is an example of a parameter correlated with the concentration of organic matter. Other examples of a parameter correlated with the concentration of organic matter include the speed of an ultrasonic wave, the estimated concentration of organic matter, and the volume fraction of organic matter. 
     Although the water quality monitoring device  111  according to the present embodiment has a structure shown in  FIG. 3 , the present invention is not limited to this. For example, a presentation unit  303  according to another embodiment may allow the display device to display the speed of the ultrasonic wave instead of the viscosity. In this case, the water quality monitoring device  111  may not include the viscosity calculation unit  302 . A presentation unit  303  according to another embodiment may present information using a different presentation method instead of displaying the information on the display device. Examples of a different presentation method include audio output. A water quality monitoring device  111  according to another embodiment may not include the presentation unit  303 . Although the evaluation unit  304  according to the present embodiment evaluates whether or not the viscosity calculated by the viscosity calculation unit  302  is greater than the threshold viscosity, the present invention is not limited to this. For example, an evaluation unit  304  according to another embodiment may evaluate whether or not the speed of an ultrasonic wave determined by the speed determination unit  301  is greater than a predetermined threshold speed. Since the speed of an ultrasonic wave is positively correlated with the viscosity of water, evaluating whether or not the viscosity of water is greater than the threshold viscosity is equivalent to evaluating whether or not the speed of an ultrasonic wave is greater than the threshold speed. 
     A sequence of a water quality monitoring process according to the present embodiment will now be described. 
       FIG. 4  is a flowchart showing the sequence of the water quality monitoring process according to the first embodiment. 
     The water quality monitoring device  111  performs the following water quality monitoring process at regular intervals. When the water quality monitoring device  111  starts the water quality monitoring process, the speed determination unit  301  acquires information indicating the speed of an ultrasonic wave from the measurement device  108  (step S 401 ). The viscosity calculation unit  302  then calculates the viscosity of water that is supplied to the reverse osmosis membrane  109  on the basis of the information acquired by the speed determination unit  301  (step S 402 ). A relationship between the speed of an ultrasonic wave and the viscosity of water is previously obtained through experiments or simulation. The presentation unit  303  then allows the display device to display the viscosity calculated by the viscosity calculation unit  302  (step S 403 ). 
     The evaluation unit  304  evaluates whether or not the viscosity calculated by the viscosity calculation unit  302  is greater than the predetermined threshold viscosity (step S 404 ). The threshold viscosity according to the present embodiment is obtained by multiplying an average viscosity of water that is supplied to the reverse osmosis membrane  109  by a factor of 1 or more (for example, 1.1). The threshold viscosity according to another embodiment may be the viscosity of water which contains organic matter at 100 ppb higher than that of an average quality of water that is supplied to the reverse osmosis membrane  109 . In this case, the threshold viscosity can be determined previously by obtaining the viscosity of water obtained by dissolving a water-soluble polymer (for example, polyethylene oxide, xanthan gum, or guar gum) at 100 ppb in water having an average viscosity. 
     When the viscosity of water is equal to or less than the predetermined threshold viscosity (step S 404 : NO), the water quality monitoring device  111  terminates the water quality monitoring process and waits until the time of execution of a next water quality monitoring process. On the other hand, when the viscosity of water is greater than the predetermined threshold viscosity (step S 404 : YES), the concentration reduction processing unit  305  outputs a command to add a flocculant to the chemical injection device  105  (step S 405 ). The water quality monitoring device  111  then terminates the water quality monitoring process and waits until the time of execution of a next water quality monitoring process. 
     Upon receiving the addition command, the chemical injection device  105  adds a flocculant to water that is supplied to the DMF  104 . Adding the flocculant agglomerates organic substances dissolved in water that is supplied to the DMF  104 . The concentration of organic substances stored in the second water storage tank  106  is reduced since the agglomerated organic substances are easily filtered out by the DMF  104 . This allows the water quality monitoring device  111  to regulate the quality of water that is supplied to the reverse osmosis membrane  109 , thereby preventing degradation of the reverse osmosis membrane  109 . 
     As described above, according to the present embodiment, the water quality monitoring device  111  detects changes in the viscosity of water that is supplied to the reverse osmosis membrane  109  at a resolution of about several tenths of one percent to several percent. This is because the measurement device  108  measures the period of time from when an ultrasonic wave is transmitted to when the ultrasonic wave is received with six or more significant figure accuracy. In the calculator  208 , improving the resolution of measurement of time is easier than improving the resolution of measurement of rotational torque. Accordingly, by measuring the speed of an ultrasonic wave as in the present embodiment, it is possible to obtain the viscosity of water easily and with a high degree of accuracy. 
     In addition, according to the present embodiment, the measurement device  108  measures a parameter correlated with the viscosity of water without a moving part being included. This allows the water quality monitoring device  111  to monitor seawater using the measurement device  108  with high durability. Further, according to the present embodiment, the ultrasonic wave transmitter  204  and the ultrasonic wave receiver  205  are provided on an outer wall of the U-shaped tube  203 . That is, according to the present embodiment, the water quality monitoring device  111  measures a parameter correlated with the viscosity of water with the ultrasonic wave transmitter  204  and the ultrasonic wave receiver  205  not directly contacting water. This allows the water quality monitoring device  111  to monitor seawater using the measurement device  108  with high durability. 
     Furthermore, according to the present embodiment, the U-shaped tube  203  of the measurement device  108  serves as a bypass for the pipe that connects the second pump  107  and the reverse osmosis membrane  109 . This allows the measurement device  108  to measure the speed of an ultrasonic wave and the density of water without manual sampling of water that is supplied to the reverse osmosis membrane  109 . This enables the water quality monitoring device  111  to monitor the viscosity of water that is supplied to the reverse osmosis membrane  109  in an online manner. 
     Second Embodiment 
     A second embodiment is described below. 
       FIG. 5  is a schematic diagram showing a configuration of a seawater treatment system according to the second embodiment. 
     The water quality monitoring device  111  of the seawater treatment system  1  according to the first embodiment evaluates whether or not it is necessary to add flocculant on the basis of the result of measurement by the measurement device  108 . On the other hand, the water quality monitoring device  111  of the seawater treatment system  1  according to the second embodiment evaluates whether or not it is necessary to add a flocculant and whether or not it is necessary to backwash the DMF  104  on the basis of the result of measurement by the measurement device  108 . 
     The seawater treatment system  1  according to the second embodiment includes a backwash water tank  501 , a backwash pump  502 , a first valve  503 , and a second valve  504  in addition to the elements of the first embodiment. In addition, the seawater treatment system  1  according to the second embodiment includes not only the measurement device  108  on the pipe between the second pump  107  and the reverse osmosis membrane  109  but also a measurement device  108  on a pipe between the first pump  103  and the DMF  104 . 
     Concentrated water discharged from the reverse osmosis membrane  109  or seawater is stored in the backwash water tank  501 . 
     The backwash pump  502  backwashes the DMF  104  by delivering water stored in the backwash water tank  501  to the DMF  104  through a water outlet of the DMF  104 . Water delivered to the DMF  104  by the back-wash pump  502  is discharged to the sea or effluent treatment facilities. 
     The first valve  503  is provided between the water outlet of the DMF  104  and a water outlet of the backwash pump  502 . The first valve  503  is closed during a normal operation of the seawater treatment system  1  and is opened during backwash treatment. 
     The second valve  504  is provided between the water outlet of the DMF  104  and a water inlet of the second water storage tank  106 . The second valve  504  is opened during a normal operation of the seawater treatment system  1  and is closed during backwash treatment. 
     A sequence of a water quality monitoring process according to the present embodiment will now be described. 
       FIG. 6  is a flowchart showing the sequence of the water quality monitoring process according to the second embodiment. 
     The water quality monitoring device  111  performs the following water quality monitoring process at regular intervals. When the water quality monitoring device  111  starts the water quality monitoring process, the speed determination unit  301  acquires information indicating the speed of an ultrasonic wave from each of the measurement device  108  provided on a pipe between the first pump  103  and the DMF  104  and the measurement device  108  provided on a pipe between the second pump  107  and the reverse osmosis membrane  109  (step S 601 ). 
     The viscosity calculation unit  302  then calculates the viscosity of water before passing through the DMF  104  and the viscosity of water after passing through the DMF  104  on the basis of the information acquired by the speed determination unit  301  (step S 602 ). Specifically, the viscosity calculation unit  302  calculates the viscosity of water before passing through the DMF  104  on the basis of the speed of an ultrasonic wave measured by the measurement device  108  provided on the pipe between the first pump  103  and the DMF  104 . The viscosity calculation unit  302  also calculates the viscosity of water after passing through the DMF  104  on the basis of the speed of an ultrasonic wave measured by the measurement device  108  provided on the pipe between the second pump  107  and the reverse osmosis membrane  109 . The presentation unit  303  then allows the display device to display the viscosities calculated by the viscosity calculation unit  302  (step S 603 ). 
     The evaluation unit  304  calculates the difference between the viscosity of water before passing through the DMF  104  and the viscosity of water after passing through the DMF  104  (step S 604 ). The evaluation unit  304  then evaluates whether or not the calculated viscosity difference is less than a predetermined threshold viscosity difference (step S 605 ). When the difference in viscosity between water before passing through the DMF  104  and water after passing through the DMF  104  is small, this indicates a reduction in the organic matter filtration ability of the DMF  104 . 
     When the viscosity difference is less than the threshold viscosity difference (step S 605 : YES), the concentration reduction processing unit  305  activates the backwash pump  502  after opening the first valve  503  and closing the second valve  504  (step S 606 ). Backwashing the DMF  104  can restore the organic matter filtration ability of the DMF  104 . The concentration reduction processing unit  305  closes the first valve  503  and opens the second valve  504  after activating the backwash pump  502  for a predetermined period of time. The water quality monitoring device  111  then terminates the water quality monitoring process and waits until the time of execution of a next water quality monitoring process. Restoration of the organic matter filtration ability of the DMF  104  reduces the concentration of organic matter in water stored in the second water storage tank  106  during a normal operation after backwashing. This allows the water quality monitoring device  111  to regulate the quality of water that is supplied to the reverse osmosis membrane  109 , thereby preventing degradation of the reverse osmosis membrane  109 . 
     On the other hand, when the viscosity difference is equal to or greater than the threshold viscosity difference (step S 605 : NO), the evaluation unit  304  evaluates whether or not the viscosity of water after passing through the DMF  104  is greater than a predetermined threshold viscosity (step S 607 ). When the viscosity of water after passing through the DMF  104  is equal to or less than the predetermined threshold viscosity (step S 607 : NO), the water quality monitoring device  111  terminates the water quality monitoring process and waits until the time of execution of a next water quality monitoring process. 
     On the other hand, when the viscosity of water after passing through the DMF  104  is greater than the predetermined threshold viscosity (step S 607 : YES), the concentration reduction processing unit  305  outputs a command to add flocculant to the chemical injection device  105  (step S 608 ). The water quality monitoring device  111  then terminates the water quality monitoring process and waits until the time of execution of a next water quality monitoring process. 
     As described above, according to the present embodiment, the water quality monitoring device  111  detects a reduction in the filtration ability of the DMF  104  on the basis of the difference in viscosity between water before passing through the DMF  104  and water after passing through the DMF  104 . This allows the water quality monitoring device  111  to backwash the DMF  104  upon detecting a reduction in the filtration ability of the DMF  104 , thereby regulating the filtration ability of the DMF  104 . That is, the water quality monitoring device  111  can regulate the quality of water that is supplied to the reverse osmosis membrane  109 , thereby preventing degradation of the reverse osmosis membrane  109 , not only when the quality of seawater taken in by the water intake device  101  has been reduced but also when the filtration ability of the DMF  104  has been reduced. 
     Third Embodiment 
     A third embodiment is described below. 
       FIG. 7  is a schematic diagram showing a configuration of a seawater treatment system according to the third embodiment. 
     A water quality monitoring device  111  of a seawater treatment system  1  according to the third embodiment evaluates whether or not it is necessary to add a flocculant, the type of the flocculant added, whether or not it is necessary to backwash the DMF  104 , and whether or not it is necessary to stop operation of the seawater treatment system  1  on the basis of the result of measurement by the measurement device  108 . Types of the flocculant added by the chemical injection device  105  include an inorganic flocculant and a polymer flocculant. Examples of the inorganic flocculant include ferric chloride. Examples of the polymer flocculant include a cationic polymer flocculant such as a polyacrylate ester compound. The polymer flocculant is used to additionally agglomerate the organic matter agglomerated by the inorganic flocculant. 
     The seawater treatment system  1  according to the third embodiment does not include the measurement device  108  between the first pump  103  and the DMF  104  among the elements of the second embodiment. That is, the seawater treatment system  1  according to the third embodiment includes the backwash water tank  501 , the backwash pump  502 , the first valve  503 , and the second valve  504  in addition to the elements of the first embodiment. 
     A sequence of a water quality monitoring process according to the present embodiment will now be described. 
       FIG. 8  is a flowchart showing the sequence of the water quality monitoring process according to the third embodiment. 
     The water quality monitoring device  111  performs the following water quality monitoring process at regular intervals. When the water quality monitoring device  111  starts the water quality monitoring process, the speed determination unit  301  acquires information indicating the speed of an ultrasonic wave from the measurement device  108  (step S 801 ). The viscosity calculation unit  302  then calculates the viscosity of water that is supplied to the reverse osmosis membrane  109  on the basis of the information acquired by the speed determination unit  301  (step S 802 ). The presentation unit  303  then allows the display device to display the viscosity calculated by the viscosity calculation unit  302  (step S 803 ). 
     The evaluation unit  304  evaluates whether or not the viscosity calculated by the viscosity calculation unit  302  is greater than a first threshold viscosity (step S 804 ). The first threshold viscosity is obtained by multiplying an average viscosity of water that is supplied to the reverse osmosis membrane  109  by a factor of 1 or more (for example, 1.1). 
     When the viscosity of water is equal to or less than the first threshold viscosity (step S 804 : NO), the water quality monitoring device  111  terminates the water quality monitoring process and waits until the time of execution of a next water quality monitoring process. 
     On the other hand, when the viscosity of water is greater than the first threshold viscosity (step S 804 : YES), the evaluation unit  304  evaluates whether or not the viscosity calculated by the viscosity calculation unit  302  is greater than a second threshold viscosity (step S 805 ). The second threshold viscosity is greater than the first threshold viscosity. The second threshold viscosity is obtained by multiplying an average viscosity of water that is supplied to the reverse osmosis membrane  109  by a factor of 1 or more (for example, 1.2). 
     When the viscosity of water is equal to or less than the second threshold viscosity (step S 805 : NO), the concentration reduction processing unit  305  outputs a command to add an inorganic flocculant to the chemical injection device  105  (step S 806 ). Upon receiving the addition command, the chemical injection device  105  adds an inorganic flocculant to water that is supplied to the DMF  104 . The water quality monitoring device  111  then terminates the water quality monitoring process and waits until the time of execution of a next water quality monitoring process. 
     On the other hand, when the viscosity of water is greater than the second threshold viscosity (step S 805 : YES), the evaluation unit  304  evaluates whether or not the viscosity calculated by the viscosity calculation unit  302  is greater than a third threshold viscosity (step S 807 ). The third threshold viscosity is greater than the second threshold viscosity. The third threshold viscosity is obtained by multiplying an average viscosity of water that is supplied to the reverse osmosis membrane  109  by a factor of 1 or more (for example, 1.3). 
     When the viscosity of water is equal to or less than the third threshold viscosity (step S 807 : NO), the concentration reduction processing unit  305  outputs a command to add a polymer flocculant to the chemical injection device  105  (step S 808 ). Upon receiving the addition command, the chemical injection device  105  adds a polymer flocculant to water that is supplied to the DMF  104 . 
     When the viscosity of water is greater than the second threshold viscosity, this indicates that filtration by the DMF  104  is insufficient with only an inorganic flocculant added. Therefore, the water quality monitoring device  111  according to the present embodiment additionally adds a polymer flocculant when the viscosity of water is greater than the second threshold viscosity. This allows organic matter agglomerated by the inorganic flocculant to be additionally agglomerated by the polymer flocculant such that the organic matter is easily filtered out by the DMF  104 . 
     On the other hand, when the viscosity of water is greater than the third threshold viscosity (step S 807 : YES), the evaluation unit  304  evaluates whether or not the viscosity calculated by the viscosity calculation unit  302  is greater than a fourth threshold viscosity (step S 809 ). The fourth threshold viscosity is greater than the third threshold viscosity. The fourth threshold viscosity is obtained by multiplying an average viscosity of water that is supplied to the reverse osmosis membrane  109  by a factor of 1 or more (for example, 1.5). 
     When the viscosity of water is equal to or less than the fourth threshold viscosity (step S 809 : NO), the concentration reduction processing unit  305  activates the backwash pump  502  after opening the first valve  503  and closing the second valve  504  (step S 810 ). The concentration reduction processing unit  305  closes the first valve  503  and opens the second valve  504  after activating the backwash pump  502  for a predetermined period of time. The water quality monitoring device  111  then terminates the water quality monitoring process and waits until the time of execution of a next water quality monitoring process. 
     When the viscosity of water is greater than the third threshold viscosity, this indicates that filtration by the DMF  104  is insufficient with the flocculants added. That is, when the viscosity of water is greater than the third threshold viscosity, there is a possibility that the filtration ability of the DMF  104  has been reduced. Therefore, the water quality monitoring device  111  according to the present embodiment backwashes the DMF  104  when the viscosity of water is greater than the third threshold viscosity. 
     On the other hand, when the viscosity of water is greater than the fourth threshold viscosity (step S 809 : YES), the concentration reduction processing unit  305  stops operation of the second pump  107  (step S 811 ). This allows the concentration reduction processing unit  305  to stop operation of the seawater treatment system  1 . The water quality monitoring device  111  then terminates the water quality monitoring process. 
     When the viscosity of water is greater than the fourth threshold viscosity, this indicates that backwashing cannot restore the filtration ability of the DMF  104 . That is, when the viscosity of water is greater than the fourth threshold viscosity, there is a possibility that an abnormality has occurred in the seawater treatment system  1 . Therefore, when the viscosity of water is greater than the fourth threshold viscosity, the water quality monitoring device  111  stops operation of the seawater treatment system  1  to prevent contaminated raw water from entering the reverse osmosis membrane  109 . Although the water quality monitoring device  111  stops operation of the seawater treatment system  1  when the viscosity of water is greater than the fourth threshold viscosity in the present embodiment, the present invention is not limited to this. For example, in another embodiment, the water quality monitoring device  111  may reduce the amount of water treated by the seawater treatment system  1  instead of stopping operation of the seawater treatment system  1 . In this case, the concentration reduction processing unit  305  reduces the pressure of the second pump  107  instead of stopping operation of the second pump  107 . 
     As described above, according to the present embodiment, the water quality monitoring device  111  reduces the concentration of organic matter in water present upstream of the reverse osmosis membrane  109  using a different method for each range of the viscosity of water that is supplied to the reverse osmosis membrane  109 . This allows the water quality monitoring device  111  to regulate the quality of water that is supplied to the reverse osmosis membrane  109  using a method suitable for the amount of organic matter included in water, thereby preventing degradation of the reverse osmosis membrane  109 . 
     Although, in the present embodiment, the water quality monitoring device  111  reduces the concentration of organic matter in water present upstream of the reverse osmosis membrane  109  using methods according to four ranges, i.e., a range greater than the first threshold viscosity and equal to or less than the second threshold viscosity, a range greater than the second threshold viscosity and equal to or less than the third threshold viscosity, a range greater than the third threshold viscosity and equal to or less than the fourth threshold viscosity, and a range greater than the fourth threshold viscosity, the present invention is not limited to this. For example, in another embodiment, the water quality monitoring device  111  may reduce the concentration of organic matter in water present upstream of the reverse osmosis membrane  109  using methods according to at least two of the four ranges. In another embodiment, the water quality monitoring device  111  may reduce the concentration of organic matter in water present upstream of the reverse osmosis membrane  109  using methods according to five or more ranges. 
     Fourth Embodiment 
     A fourth embodiment is described below. 
     The water quality monitoring device  111  of the seawater treatment system  1  according to the first to third embodiments takes measures for reducing the concentration of organic matter when the viscosity of water is high. On the other hand, there is a possibility that inorganic salts, inorganic colloids, and other inorganic microparticles may be suspended in water that is supplied to the reverse osmosis membrane  109 . Therefore, when the increase in the viscosity of water is due to the suspension of inorganic microparticles, there is a possibility that the quality of water is not sufficiently improved by measures for reducing the concentration of organic matter. 
     The water quality monitoring device  111  of the seawater treatment system  1  according to the fourth embodiment determines whether to take measures against an increase in organic matter or to take measures against an increase in the number of inorganic microparticles when the viscosity of water is high. 
     The configuration of the seawater treatment system  1  according to the fourth embodiment is the same as that of the seawater treatment system  1  according to the first embodiment. The chemical injection device  105  according to the present embodiment adds a flocculant for agglomerating inorganic microparticles in addition to the flocculant for agglomerating organic matter. 
       FIG. 9  is a schematic block diagram showing a configuration of a water quality monitoring device according to the fourth embodiment. 
     The water quality monitoring device  111  according to the fourth embodiment includes a density determination unit  901  in addition to the elements of the first embodiment. 
     The density determination unit  901  acquires information indicating the density of water from the measurement device  108 . 
     The water quality monitoring device  111  according to the fourth embodiment is different from that of the first embodiment in the operations of the presentation unit  303 , the evaluation unit  304 , and the concentration reduction processing unit  305 . 
     The presentation unit  303  allows the display device to display the viscosity calculated by the viscosity calculation unit  302  and the density acquired by the density determination unit  901 . 
     The evaluation unit  304  evaluates whether or not it is necessary to add a flocculant on the basis of the viscosity calculated by the viscosity calculation unit  302 . The evaluation unit  304  determines the type of a flocculant to be added on the basis of the density acquired by the density determination unit  901 . 
     The concentration reduction processing unit  305  outputs a command to add a flocculant of the type determined by the determination unit to the chemical injection device  105 . 
     A sequence of a water quality monitoring process according to the present embodiment will now be described. 
       FIG. 10  is a flowchart showing the sequence of the water quality monitoring process according to the fourth embodiment. 
     The water quality monitoring device  111  performs the following water quality monitoring process at regular intervals. When the water quality monitoring device  111  starts the water quality monitoring process, the speed determination unit  301  acquires information indicating the speed of an ultrasonic wave from the measurement device  108  (step S 1001 ). The density determination unit  901  acquires information indicating the density of water from the measurement device  108  (step S 1002 ). The viscosity calculation unit  302  then calculates the viscosity of water that is supplied to the reverse osmosis membrane  109  on the basis of the information acquired by the speed determination unit  301  (step S 1003 ). The presentation unit  303  then allows the display device to display the viscosity calculated by the viscosity calculation unit  302  and the density acquired by the density determination unit  901  (step S 1004 ). 
     The evaluation unit  304  evaluates whether or not the viscosity calculated by the viscosity calculation unit  302  is greater than the predetermined threshold viscosity (step S 1005 ). When the viscosity of water is equal to or less than the predetermined threshold viscosity (step S 1005 : NO), the water quality monitoring device  111  terminates the water quality monitoring process and waits until the time of execution of a next water quality monitoring process. On the other hand, when the viscosity of water is greater than the predetermined threshold viscosity (step S 1005 : YES), the evaluation unit  304  evaluates whether or not the density acquired by the density determination unit  901  is greater than a predetermined threshold density (step S 1006 ). The threshold density according to the present embodiment is an average density of water that is supplied to the reverse osmosis membrane  109 . 
     The density of inorganic microparticles is greater than the density of water. On the other hand, the density of organic matter is less than the density of water. Therefore, when a large amount of inorganic microparticles are suspended in water that is supplied to the reverse osmosis membrane  109 , the density of the water is greater than the average density of water. When a large amount of organic matter is dissolved in water that is supplied to the reverse osmosis membrane  109 , the density of the water is equal to or less than the average density of water. 
     When the density acquired by the density determination unit  901  is greater than the predetermined threshold density (step S 1006 : YES), the concentration reduction processing unit  305  outputs a command to add a flocculant for agglomerating inorganic microparticles to the chemical injection device  105  (step S 1007 ). On the other hand, when the density acquired by the density determination unit  901  is equal to or less than the predetermined threshold density (step S 1006 : NO), the concentration reduction processing unit  305  outputs a command to add a flocculant for agglomerating organic matter to the chemical injection device  105  (step S 1008 ). 
     As described above, the water quality monitoring device  111  according to the present embodiment determines to take measures against an increase of inorganic microparticles when the density of water is greater than the threshold density. In addition, the water quality monitoring device  111  determines to take measures against an increase of organic matter when the density of water is equal to or less than the threshold density. This allows the water quality monitoring device  111  to take appropriate measures against fouling according to the type of matter contained in the water. 
     In the present embodiment, when the density acquired by the density determination unit  901  is greater than the predetermined threshold density, the chemical injection device  105  adds the flocculant for agglomerating inorganic microparticles, but the present invention is not limited to this. For example, in another embodiment, if it can be determined that the suspension of inorganic microparticles does not significantly affect the fouling of the reverse osmosis membrane  109 , the chemical injection device  105  may be configured to add no flocculant when the density acquired by the density determination unit  901  is greater than the predetermined threshold density. 
     In the present embodiment, the density determination unit  901  acquires information indicating the density calculated based on the resonant frequency from the measurement device  108  having the structure shown in  FIG. 2 , but the present invention is not limited to this. For example, the measurement device  108  according to another embodiment may calculate the density by measuring the weight of a specific amount of sampled water. 
     Fifth Embodiment 
     A fifth embodiment is described below. 
     The water quality monitoring device  111  of the seawater treatment system  1  according to the fourth embodiment determines whether to take measures against an increase in organic matter or to take measures against an increase of inorganic microparticles according to the density of water. On the other hand, a water quality monitoring device  111  of a seawater treatment system  1  according to the fifth embodiment determines the proportions of organic matter and inorganic microparticles present in water on the basis of the density of the water and determines whether to take measures against an increase of organic matter or to take measures against an increase of inorganic microparticles. 
       FIG. 11  is a schematic block diagram showing a configuration of the water quality monitoring device according to the fifth embodiment. 
     The water quality monitoring device  111  according to the fifth embodiment includes a volume fraction calculation unit  1101  in addition to the elements of the fourth embodiment. The volume fraction calculation unit  1101  calculates the volume fractions of organic matter and inorganic microparticles in water on the basis of the viscosity calculated by the viscosity calculation unit  302  and the density determined by the density determination unit  901 . 
     Here, a method of calculating the volume fractions of organic matter and inorganic microparticles in water will be described. The density ρ of water that is supplied to the reverse osmosis membrane  109  is expressed by the following equation (1). 
       [Math. 1] 
       ρ=ρ SW (1−φ O −φ I )+ρ O φ O +ρ I φ I   (1)
 
     ρ sw  is the standard density of seawater. ρ O  is the density of organic matter. ρ I  is the density of inorganic microparticles. φ O  is the volume fraction of organic matter. φ I  is the volume fraction of inorganic microparticles. 
     The relative viscosity η r  of water that is supplied to the reverse osmosis membrane  109  is expressed by the following equation (2). The relative viscosity is a value obtained by dividing the viscosity measured by the measurement device  108  by the average viscosity of water that is supplied to the reverse osmosis membrane  109 . 
       [Math. 2] 
       η r =1+ k   O φ O   +k   I φ I   (2)
 
     k O  is a coefficient of the viscosity of organic matter. k I  is a coefficient of the viscosity of inorganic microparticles. 
     For example, the coefficient k O  of the viscosity of organic substances can be determined previously by dissolving a water-soluble polymer (for example, polyethylene oxide, xanthan gum, or guar gum) in water having an average viscosity while varying the concentration of the water-soluble polymer and obtaining a linear equation indicating the relationship between the volume fraction and the viscosity. The intercept of the linear equation is 1. 
     For example, the coefficient k I  of the viscosity of inorganic microparticles can be determined previously by suspending inorganic microparticles (for example, silica fine particles or calcium carbonate fine particles) in water having an average viscosity while varying the concentration of the inorganic microparticles and obtaining a linear equation indicating the relationship between the volume fraction and the viscosity. The intercept of the linear equation is 1. 
     From the above equations (1) and (2), the volume fraction φ O  of the organic matter and the volume fraction φ I  of the inorganic microparticles can be expressed by the following equations (3). The volume fraction calculation unit  1101  calculates the volume fraction φ O  of the organic matter and the volume fraction φ I  of the inorganic microparticles on the basis of equations (3). 
     
       
         
           
             
               
                 
                   [ 
                   
                     Math 
                     . 
                     
                         
                     
                      
                     3 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   { 
                   
                     
                       
                         
                           
                             φ 
                             O 
                           
                           = 
                           
                             
                               
                                 
                                   k 
                                   I 
                                 
                                  
                                 
                                   ( 
                                   
                                     ρ 
                                     - 
                                     
                                       ρ 
                                       SW 
                                     
                                   
                                   ) 
                                 
                               
                               - 
                               
                                 
                                   ( 
                                   
                                     
                                       η 
                                       r 
                                     
                                     - 
                                     1 
                                   
                                   ) 
                                 
                                  
                                 
                                   ( 
                                   
                                     
                                       ρ 
                                       I 
                                     
                                     - 
                                     
                                       ρ 
                                       SW 
                                     
                                   
                                   ) 
                                 
                               
                             
                             
                               
                                 
                                   k 
                                   I 
                                 
                                  
                                 
                                   ( 
                                   
                                     
                                       ρ 
                                       O 
                                     
                                     - 
                                     
                                       ρ 
                                       SW 
                                     
                                   
                                   ) 
                                 
                               
                               - 
                               
                                 
                                   k 
                                   O 
                                 
                                  
                                 
                                   ( 
                                   
                                     
                                       ρ 
                                       I 
                                     
                                     - 
                                     
                                       ρ 
                                       SW 
                                     
                                   
                                   ) 
                                 
                               
                             
                           
                         
                       
                     
                     
                       
                         
                           
                             φ 
                             I 
                           
                           = 
                           
                             
                               
                                 
                                   k 
                                   O 
                                 
                                  
                                 
                                   ( 
                                   
                                     ρ 
                                     - 
                                     
                                       ρ 
                                       SW 
                                     
                                   
                                   ) 
                                 
                               
                               - 
                               
                                 
                                   ( 
                                   
                                     
                                       η 
                                       r 
                                     
                                     - 
                                     1 
                                   
                                   ) 
                                 
                                  
                                 
                                   ( 
                                   
                                     
                                       ρ 
                                       O 
                                     
                                     - 
                                     
                                       ρ 
                                       SW 
                                     
                                   
                                   ) 
                                 
                               
                             
                             
                               
                                 
                                   k 
                                   O 
                                 
                                  
                                 
                                   ( 
                                   
                                     
                                       ρ 
                                       I 
                                     
                                     - 
                                     
                                       ρ 
                                       SW 
                                     
                                   
                                   ) 
                                 
                               
                               - 
                               
                                 
                                   k 
                                   I 
                                 
                                  
                                 
                                   ( 
                                   
                                     
                                       ρ 
                                       O 
                                     
                                     - 
                                     
                                       ρ 
                                       SW 
                                     
                                   
                                   ) 
                                 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     The water quality monitoring device  111  according to the fifth embodiment is different from that of the fourth embodiment in the operations of the presentation unit  303  and the evaluation unit  304 . 
     The presentation unit  303  allows the display device to display the viscosity calculated by the viscosity calculation unit  302 , the density acquired by the density determination unit  901 , and the volume fraction calculated by the volume fraction calculation unit  1101 . 
     Based on the volume fraction calculated by the volume fraction calculation unit  1101 , the evaluation unit  304  evaluates whether or not it is necessary to add a flocculant used to agglomerate organic matter and a flocculant used to agglomerate inorganic microparticles. 
     A sequence of a water quality monitoring process according to the present embodiment will now be described. 
       FIG. 12  is a flowchart showing the sequence of the water quality monitoring process according to the fifth embodiment. 
     The water quality monitoring device  111  performs the following water quality monitoring process at regular intervals. When the water quality monitoring device  111  starts the water quality monitoring process, the speed determination unit  301  acquires information indicating the speed of an ultrasonic wave from the measurement device  108  (step S 1201 ). The density determination unit  901  acquires information indicating the density of water from the measurement device  108  (step S 1202 ). The viscosity calculation unit  302  then calculates the viscosity of water that is supplied to the reverse osmosis membrane  109  on the basis of the information acquired by the speed determination unit  301  (step S 1203 ). 
     The volume fraction calculation unit  1101  then calculates the volume fractions of organic matter and inorganic microparticles in water on the basis of the viscosity calculated by the viscosity calculation unit  302  and the density determined by the density determination unit  901  (step S 1204 ). The presentation unit  303  then allows the display device to display the viscosity calculated by the viscosity calculation unit  302 , the density acquired by the density determination unit  901 , and the volume fractions calculated by the volume fraction calculation unit  1101  (step S 1205 ). 
     The evaluation unit  304  evaluates whether or not the volume fraction of organic matter calculated by the volume fraction calculation unit  1101  is greater than a first threshold volume fraction (step S 1206 ). The first threshold volume fraction according to the present embodiment is a volume fraction corresponding to 100 ppb of organic matter. When the volume fraction of the organic matter is greater than the first threshold volume fraction (step S 1206 : YES), the concentration reduction processing unit  305  outputs a command to add a flocculant for agglomerating organic matter to the chemical injection device  105  (step S 1207 ). 
     When the volume fraction of the organic matter is equal to or less than the first threshold volume fraction (step S 1206 : NO) or when the concentration reduction processing unit  305  has output a command to add a flocculant for agglomerating organic matter, the evaluation unit  304  evaluates whether or not the volume fraction of the inorganic microparticles calculated by the volume fraction calculation unit  1101  is greater than a second threshold volume fraction (step S 1208 ). The second threshold volume fraction according to the present embodiment is a volume fraction of inorganic microparticles corresponding to a silt density index (SDI)  3 . When the volume fraction of the inorganic microparticles is greater than the second threshold volume fraction (step S 1208 : YES), the concentration reduction processing unit  305  outputs a command to add a flocculant for agglomerating inorganic microparticles to the chemical injection device  105  (step S 1209 ). 
     When the volume fraction of the organic matter is equal to or less than the first threshold volume fraction (step S 1208 : NO) or when the concentration reduction processing unit  305  has output a command to add a flocculant for agglomerating inorganic microparticles, the water quality monitoring device  111  terminates the water quality monitoring process and waits until the time of execution of a next water quality monitoring process. 
     As described above, the water quality monitoring device  111  according to the present embodiment determines whether or not to take measures against an increase of organic matter and whether or not to take measures against an increase of inorganic microparticles on the basis of the volume fractions of organic matter and inorganic microparticles. This allows the water quality monitoring device  111  to take appropriate measures against fouling according to the type of matter contained in the water. 
     Sixth Embodiment 
     A sixth embodiment is described below. 
       FIG. 13  is a schematic diagram showing a configuration of a seawater treatment system according to the sixth embodiment. 
     When the quality of water that is supplied to the reverse osmosis membrane  109  has been reduced, a seawater treatment system  1  according to the sixth embodiment samples the water. 
     The seawater treatment system  1  according to the sixth embodiment includes a sample tank  1301  and a three-way valve  1302  in addition to the elements of the first embodiment. 
     The three-way valve  1302  is provided at a branch point between a pipe, which connects a pipe connected to the second pump  107  and the reverse osmosis membrane  109 , and a pipe connected to the sample tank  1301 . The three-way valve  1302  switches the destination of water jumped by the second pump  107  between the reverse osmosis membrane  109  and the sample tank  1301 . 
       FIG. 14  is a schematic block diagram showing a configuration of a water quality monitoring device according to the sixth embodiment. 
     A water quality monitoring device  111  according to the sixth embodiment includes a sampling processing unit  1401  in addition to the elements of the first embodiment. 
     The sampling processing unit  1401  controls the opening and closing of the three-way valve  1302  on the basis of the result of evaluation by the evaluation unit  304 . The sampling processing unit  1401  is an example of a process execution unit which performs a process on the basis of the speed of the ultrasonic wave determined by the speed determination unit  301 . 
       FIG. 15  is a flowchart showing a sequence of a water quality monitoring process according to the sixth embodiment. 
     The water quality monitoring device  111  performs the following water quality monitoring process at regular intervals. When the water quality monitoring device  111  starts the water quality monitoring process, the speed determination unit  301  acquires information indicating the speed of an ultrasonic wave from the measurement device  108  (step S 1501 ). The viscosity calculation unit  302  then calculates the viscosity of water that is supplied to the reverse osmosis membrane  109  on the basis of the information acquired by the speed determination unit  301  (step S 1502 ). The presentation unit  303  then allows the display device to display the viscosity calculated by the viscosity calculation unit  302  (step S 1503 ). 
     The evaluation unit  304  evaluates whether or not the viscosity calculated by the viscosity calculation unit  302  is greater than the predetermined threshold viscosity (step S 1504 ). When the viscosity of water is equal to or less than the predetermined threshold viscosity (step S 1504 : NO), the water quality monitoring device  111  terminates the water quality monitoring process and waits until the time of execution of a next water quality monitoring process. On the other hand, when the viscosity of water is greater than the predetermined threshold viscosity (step S 1504 : YES), the sampling processing unit  1401  switches the opening and closing of the three-way valve  1302  such that the water pumped by the second pump  107  is delivered to the sample tank  1301  (step S 1505 ). The sampling processing unit  1401  waits until a predetermined amount of water is stored in the sample tank  1301  (step S 1506 ). Upon finishing the waiting, the sampling processing unit  1401  switches the opening and closing of the three-way valve  1302  such that the water pumped by the second pump  107  is delivered to the reverse osmosis membrane  109  (step S 1507 ). 
     The concentration reduction processing unit  305  then outputs a command to add a flocculant to the chemical injection device  105  (step S 1508 ). The water quality monitoring device  111  then terminates the water quality monitoring process and waits until the time of execution of a next water quality monitoring process. 
     According to the present embodiment, when the quality of the water supplied to the reverse osmosis membrane  109  has been reduced, the water quality monitoring device  111  can sample the water as described above. This allows the manager of the seawater treatment system  1  to analyze the quality of the sampled water. That is, the water quality monitoring device  111  according to the present embodiment can contribute to determining the causative substances of fouling through water quality analysis. 
     Although the embodiments have been described in detail with reference to the drawings, the specific configurations are not limited to those described above and various design changes or the like can be made. 
     In the embodiments described above, the water quality monitoring device  111  evaluates whether or not to perform a process of reducing the concentration of organic matter in water present upstream of the reverse osmosis membrane  109 , but the present invention is not limited to this. For example, in another embodiment, the manager of the seawater treatment system  1  may perform the same processes as that of the above-described embodiments by viewing parameters correlated with the concentration of organic matter presented by the presentation unit  303 . Examples of parameters correlated with the concentration of organic matter are warnings indicating that the viscosity of water, the speed of an ultrasound wave, the estimated concentration of organic matter, and the volume fraction of organic matter are high. In this case, the water quality monitoring device  111  may include at least the speed determination unit  301  and the presentation unit  303 . On the other hand, in another embodiment, the water quality monitoring device  111  may not include the presentation unit  303  when the water quality monitoring device  111  performs a process of reducing the concentration of organic matter in water present upstream of the reverse osmosis membrane  109 . 
     Although the speed determination unit  301  according to the above embodiments acquires information indicating the speed from the measurement device  108 , the present invention is not limited to this and the speed determination unit  301  may acquire another physical quantity related to the speed. For example, in another embodiment, the speed determination unit  301  may acquire information indicating the viscosity from the measurement device  108  when the viscosity is calculated based on the speed of an ultrasonic wave measured by the measurement device  108 . For example, the speed determination unit  301  according to another embodiment may acquire, from the measurement device  108 , information indicating a period of time from when an ultrasonic wave is transmitted to when the ultrasonic wave is received. 
     Although the density determination unit  901  according to the above embodiments acquires information indicating the density from the measurement device  108 , the present invention is not limited to this and the density determination unit  901  may acquire another physical quantity related to the density. For example, the density determination unit  901  according to another embodiment may acquire the resonant frequency of the U-shaped tube  203  from the measurement device  108 . 
       FIG. 16  is a cross-sectional view showing a structure of a measurement device according to a modified example. 
     Both ends of the U-shaped tube  203  of the measurement device  108  according to the above-described embodiments are attached directly to the pipe connecting the second pump  107  and the reverse osmosis membrane  109 , but the present invention is not limited to this. For example, in other embodiments, one or both ends of the U-shaped tube  203  may be attached to the pipe via a valve  1601  as shown in  FIG. 16 . Thus, while the calculator  208  measures the period of time from when an ultrasonic wave is transmitted to when the ultrasonic wave is received, it is possible to stop the flow of water in the U-shaped tube  203  during measurement by closing the valve  1601 . 
       FIG. 17  is a schematic block diagram showing a configuration of a computer according to at least one of the embodiments. 
     The computer  1700  includes a CPU  1701 , a main storage device  1702 , an auxiliary storage device  1703 , and an interface  1704 . 
     The water quality monitoring device  111  described above is mounted on the computer  1700 . The operations of the processing units described above are stored in the auxiliary storage device  1703  in the form of a program. The CPU  1701  reads the program from the auxiliary storage device  1703 , develops the program in the main storage device  1702 , and executes the above processes according to the program. 
     In at least one of the embodiments, the auxiliary storage device  1703  is an example of a non-transitory tangible medium. Other examples of a non-transitory tangible medium include a magnetic disk, a magneto-optical disk, a CD-ROM, a DVD-ROM, or a semiconductor memory connected via the interface  1704 . Further, when this program is delivered to the computer  1700  via a communication line, the computer  1700  may develop the program in the main storage device  1702  upon receiving the program and execute the above processes. 
     The program may be one for realizing some of the functions described above. The program may also be a so-called differential file (differential program) which realizes the functions described above in combination with a program which has already been recorded m the auxiliary storage device  1703 . 
     INDUSTRIAL APPLICABILITY 
     The water quality monitoring device  111  measures the speed of a wave generated in water present upstream of the reverse osmosis membrane  109 . Therefore, using a computer capable of processing at an appropriate temporal resolution, the water quality monitoring device  111  can detect changes in concentration of fouling substances at several hundred ppb. 
     REFERENCE SIGNS LIST 
     
         
           1  Seawater treatment system 
           108  Measurement device 
           109  Reverse osmosis membrane 
           111  Water quality monitoring device 
           204  Ultrasonic wave transmitter 
           205  Ultrasonic wave receiver 
           206  Oscillator 
           207  Vibration detector 
           301  Speed determination unit 
           302  Viscosity calculation unit 
           303  Presentation unit 
           304  Evaluation unit 
           305  Concentration reduction processing unit 
           901  Density determination unit 
           1101  Volume fraction calculation unit 
           1401  Sampling processing unit