Patent Publication Number: US-2017348618-A1

Title: Filter medium, process for producing filter medium, filtration device, method for operating filtration device, and filtration system

Description:
FIELD 
     The present invention relates to a filter medium, a process for producing the filter medium, a filtration device, a method for operating the filtration device, and a filtration system, and more specifically, relates to a filter medium, a process for producing the filter medium, a filtration device, a method for operating the filtration device, and a filtration system, which are capable of efficiently removing suspended substances in water to be treated. 
     BACKGROUND 
     Hitherto, fillers for water purifiers using fibrous activated carbon have been proposed (see, for example, Patent Literature 1). In this filler for water purifier, a fibrous activated carbon is used having a specific surface area of 1,300 m 2 /g or more as well as setting the cumulative pore volume occupied by pores having a pore radius of 0.9 nm or more and 1.6 nm or less to be in a predetermined range. Accordingly, the filler for water purifier is possible to efficiently remove trihalomethane present in tap water at an extremely low concentration of about several ppb. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP 3122205 B1 
     SUMMARY 
     Technical Problem 
     Incidentally, in a desalination apparatus to desalinate seawater by a reverse osmosis membrane filtration device, a filtration device (pretreatment device) such as a dual media filter (DMF) that filters seawater to be supplied to the reverse osmosis membrane filtration device to decrease the concentration of suspended substances is used in order to prevent contamination of the reverse osmosis membrane. In such a filtration device, filtration of seawater causes clogging of filter medium such as activated carbon and the pressure loss increases, and it is thus required to periodically conduct backwashing after operation for a predetermined period to remove dirt from the filter medium. 
     However, general activated carbon to be used as a filter medium in a conventional filtration device has a specific surface area of 1000 m 2 /g or more and also a great number of fine pores having a diameter of about several nm. Thus there is no significant difference between the pore size of activated carbon and the molecular size of suspended substances such as organic substances contained in seawater. For this reason, it is difficult to promptly remove the suspended substances adsorbed to the activated carbon in the conventional filter medium even if backwashing is conducted, and the filtration device cannot be necessarily efficiently operated in some cases. 
     The present invention has been achieved in view of such circumstances, and an object thereof is to provide a filter medium, a process for producing the filter medium, a filtration device, a method for operating the filtration device, and a filtration system, which are capable of promptly regenerating the adsorption power by backwashing and realizing efficient operation of the filtration device. 
     Solution to Problem 
     In a filter medium of this invention, a cumulative pore volume of pores having a pore radius of 2 nm or less is 25% or less with respect to a cumulative pore volume of pores having a pore radios of 50 nm or less. 
     According to this configuration, the micropores having a pore radius of 0.8 nm or more and 2 nm or less and the submicropores having a pore radius of 0.8 nm or less, which make it difficult to desorb the adsorbed suspended substances from the filter medium at the time of backwashing, are decreased. It is thus possible to promptly desorb the suspended substances from the filter medium at the time of backwashing. This makes it possible to realize a filter medium capable of more promptly regenerating the adsorption power by backwashing and realizing more efficient operation of a filtration device. 
     The filter medium of this invention is preferable to comprises a carbon-based material By this configuration, the filter medium has a lower specific gravity than the filter sand that is generally used in the sand filter layer of the filtration device and it is thus easy to provide a filter layer on the sand filter layer. In addition, the affinity of the filter medium for organic substances is improved, and it is thus possible to efficiently remove the suspended substances due to the organic substances in the water to be treated. 
     In the filter medium of this invention, the carbon-based material is preferable to contain activated carbon. By this configuration, the filter medium has a lower specific gravity than the filter sand that is generally used in the sand filter layer of the filtration device and it is thus easier to provide a filter layer on the sand filter layer. In addition, the affinity of the filter medium for organic substances is improved, and it is thus possible to efficiently remove the suspended substances due to the organic substances in the water to be treated. 
     A process for producing filter medium of this invention is a process for producing the filter medium and a carbon-based material is activated by water vapor. 
     A process for producing filter medium of this invention is a process for producing the filter medium and a carbon-based material is activated by a carbonic acid gas. 
     According to these processes, micropores having a pore radius of 0.8 nm or more and 2 nm or less in the carbon-based material are appropriately destroyed by an activation treatment with water vapor or a carbonic acid gas. It is thus possible to set the cumulative pore volume of pores having a pore radius of 2 nm or less to be 25% or less with respect to the cumulative pore volume of pores having a pore radius of 50 nm or less. This makes it possible for the method of manufacturing a filter medium to realize a filter medium capable of promptly desorbing the suspended substances from the filter medium at the time of backwashing as well as efficiently adsorbing the suspended substances in the water to be treated. 
     In the process for producing filter medium of this invention, it is preferable that the activation treatment is conducted with water vapor under a condition having a surface temperature of the carbon-based material of 750° C. or higher. By this method, the method of manufacturing a filter medium can appropriately destroy micropores having a pore radius of 2 nm or less in the carbon-based material, and it is thus easy to set the cumulative pore volume of pores having a pore radius of 2 nm or less to be 25% or less with respect to the cumulative pore volume of pores having a pore radius of 50 nm or less of the filter medium. 
     In the process for producing filter medium of this invention, it is preferable that the activation treatment is conducted with the carbonic acid gas under a condition having a surface temperature of the carbon-based material of 850° C. or higher. By this method, the method of manufacturing a filter medium can appropriately destroy micropores having a pore radius of 2 nm or less in the carbon-based material, and it is thus easy to set the cumulative pore volume of pores having a pore radius of 2 nm or less to be 25% or less with respect to the cumulative pore volume of pores having a pore radius of 50 nm or less of the filter medium. 
     In the process for producing filter medium of this invention, it is preferable that the activation treatment is conducted until a mass decrease of the carbon-based material reaches 50% or more. By this method, the destruction of micropores having a pore radius of 2 nm or less in the carbon-based material is conducted in an appropriate range, and it is thus easy to set the cumulative pore volume of pores having a pore radius of 2 nm or less to be 25% or less with respect to the cumulative pore volume of pores having a pore radius of 50 nm or less of the filter medium. 
     A filtration device of this invention comprises the filter medium or the filter medium obtained by the process for producing the filter medium. 
     According to this filtration device, a filter medium in which the cumulative pore volume of pores having a pore radius of 2 nm or less is 25% or less with respect to the cumulative pore volume of pores having a pore radius of 50 nm or less is equipped, and it is thus possible to efficiently desorb the suspended substances retained in the filter medium from the filter medium at the time of backwashing as well as to efficiently remove the suspended substances contained in the water to be treated. Hence, the filtration device can realize a filtration device capable of promptly regenerating the adsorption power by backwashing and realizing efficient operation. 
     A method for operating filtration device of this invention is a method for operating the filtration device, and the method comprises: a filtering step of filtering water to be treated through the filter medium to decrease suspended substances in the water to be treated; and a washing step of washing the filter medium by backwashing when an amount of suspended substances in water to be treated filtered through the filter medium reaches one third of a total amount adsorbed to the filter medium. 
     According to this method for operating the filtration device, it is possible to conduct backwashing under a condition having a sufficient margin with respect to the adsorption capacity of the filter medium and thus to prevent the adsorption of the suspended substances in the water to be treated to micropores having a pore radius of 2 nm or less from which it is difficult to desorb the suspended substances in the filter medium. This makes it possible to promptly desorb the suspended substances adsorbed to the filter medium from the filter medium at the time of backwashing, and it is thus possible to realize a method of operating a filtration device capable of promptly regenerating the adsorption power by backwashing and realizing efficient operation. 
     A method for operating filtration device of this invention is a method for operating the filtration device, and the method comprises: a concentration of suspended substances measuring step of measuring a first concentration of suspended substances in the water to be treated and measuring a second concentration of suspended substances in filtered water, the filtered water is obtained by filtering the water to be treated through the filter medium; and a filter medium washing step of conducting a calculation of a difference value between a first time integrated value of the first concentration of suspended substances measured and a second time integrated value of the second concentration of suspended substances measured and conducting a backwashing of the filter medium when the difference value calculated is equal to or less than a predetermined value. 
     According to this method for operating the filtration device, the filter medium is backwashed when the performance of the filter medium is deteriorated and the second concentration of suspended substances in the filtered water with respect to the first concentration of suspended substances in the water to be treated is equal to or higher than a predetermined value, and it is thus possible to appropriately wash the filter medium according to a change in the second concentration of suspended substances in the filtered water. This makes it possible to promptly desorb the suspended substances adsorbed to the filter medium from the filter medium at the time of backwashing, and it is thus possible to realize a method of operating a filtration device capable of promptly regenerating the adsorption power by backwashing and realizing efficient operation. 
     A filtration system of this invention comprises: a water to be treated filtering unit equipped with the filtration device according to claim  9  that filters water to be treated supplied through a water to be treated line to obtain filtered water; and a salt enriching unit that filters the filtered water through a separation membrane to obtain permeated water and enriched water. 
     According to this filtration system, a filtration device in which the micropores having a pore radius of 0.8 nm or more and 2 nm or less and the submicropores having a pore radius of 0.8 nm or less, which make it difficult to desorb the adsorbed suspended substances from the filter medium at the time of backwashing are decreased is equipped and it is thus possible to promptly desorb the suspended substances from the filter medium at the time of backwashing. This makes it possible to more promptly regenerate the adsorption power of the filter medium of the filtration device by backwashing and to realize more efficient operation of the filtration system. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to realize a filter medium, a process for producing filter medium, a filtration device, a method for operating the filtration device, and a filtration system, which are capable of promptly regenerating the adsorption power by backwashing and realizing efficient operation of the filtration device. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an outline diagram of a water treatment apparatus according to an embodiment of the present invention. 
         FIG. 2  is a schematic cross-sectional diagram illustrating an example of a dual media filtration device according to an embodiment of the present invention. 
         FIG. 3  is a schematic diagram of a general filter medium using activated carbon. 
         FIG. 4  is a schematic diagram of a filter medium according to an embodiment of the present invention. 
         FIG. 5A  is a diagram illustrating the relationship between the filtration time through a general filter medium using activated carbon and the concentration of suspended substances in filtered water. 
         FIG. 5B  is a diagram illustrating the relationship between the filtration time through a filter medium according to an embodiment of the present invention and the concentration of suspended substances in filtered water. 
         FIG. 6  is a diagram illustrating the relationship between the cumulative pore volume and the pore radius of a filter medium according to an embodiment of the present invention. 
         FIG. 7  is a schematic cross-sectional diagram illustrating another example of a dual media filtration device according to an embodiment of the present invention. 
         FIG. 8  is a flow diagram of a method of operating a filtration device according to an embodiment of the present invention. 
         FIG. 9  is a schematic diagram of a filtration system according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Incidentally, the present invention is not limited to the following embodiments and can be implemented with appropriate modifications. In addition, the respective following embodiments can be implemented in appropriate combination. 
     First, the outline of a water treatment apparatus equipped with a filtration device according to an embodiment of the present invention will be briefly described.  FIG. 1  is an outline diagram of a water treatment apparatus according to an embodiment of the present invention. As illustrated in  FIG. 1 , a water treatment apparatus  1  according to the present embodiment is a water treatment apparatus in which filtered water W 2  obtained by filtering water to be treated W 1  through a water to be treated filtering unit  11  as a dual media filtration device. In the water treatment apparatus  1 , the filtered water W 2  obtained is filtrated through a reverse osmosis membrane  12   a  of a reverse osmosis membrane filtering unit  12  to obtain permeated water W 3  and enriched water W 4 . The water to be treated W 1  is not particularly limited, and for example, it is possible to use seawater, river water, lake water, groundwater, municipal sewage, brackish water, industrial water, industrial wastewater, and water obtained by subjecting these water to treatments such as aggregation, precipitation, filtration, adsorption, and a biological treatment. 
     The water treatment apparatus  1  according to the present embodiment is equipped with: the water to be treated filtering unit  11  to which the water to be treated W 1  is supplied through a water to be treated line L 1 ; the reverse osmosis membrane filtering unit  12  provided to a filtered water line L 2  of the subsequent stage of the water to be treated filtering unit  11 ; and an energy recovery unit  13  provided to an enriched water line L 3  of the subsequent stage of the reverse osmosis membrane filtering unit  12 . 
     The water to be treated W 1  is supplied through the water to be treated line L 1  to the water to be treated filtering unit  11  by a liquid sending pump  14 . The water to be treated filtering unit  11  filters the water to be treated W 1  to obtain the filtered water W 2  from which the suspended substances in the water to be treated W 1  is removed. As the water to be treated filtering unit  11 , for example, it is possible to use a dual media filter (DMF) in which a first filter layer  24  (not illustrated in  FIG. 1 , see  FIG. 2 ) and a second filter layer  25  (not illustrated in  FIG. 1 , see  FIG. 2 ) are layered. The first filter layer  24  contains a filter medium according to the present embodiment, and the second filter layer  25  contains silica sand having a relatively smaller particle size with respective to the filter medium according to the present embodiment. 
     In the water to be treated filtering unit  11 , the pH of the filtered water W 2  is adjusted with an acid such as H 2 SO 4  or HCl so as to have a predetermined value (for example, pH 7.2 or lower) if necessary. By adjusting the pH as described above, it is possible to decrease the malfunction such as contamination (fouling) of the reverse osmosis membrane  12   a  of the reverse osmosis membrane filtering unit  12  due to the suspended substances in the water to be treated W 1 . 
     The pressurized filtered water W 2  is supplied to the reverse osmosis membrane filtering unit  12  through the filtered water line L 2  by a high pressure pump  15 . The reverse osmosis membrane filtering unit  12  is equipped with the reverse osmosis membrane  12   a  which permeates the filtered water W 2  supplied from the water to be treated filtering unit  11  to obtain the permeated water W 3  and the enriched water W 4  in which salts and the like in the filtered water W 2  are enriched. The reverse osmosis membrane filtering unit  12  discharges the permeated water W 3  through a permeated water line L 4  as well as discharges the enriched water W 4  through the enriched water line L 3 . 
     A filter member such as a micro cartridge filter may be further provided between the water to be treated filtering unit  11  and the reverse osmosis membrane filtering unit  12 . By allowing the filtered water W 2  to pass through the filter member, it is possible to remove fine particles which affect contamination of the reverse osmosis membrane  12   a  of the reverse osmosis membrane filtering unit  12 . 
     The energy recovery unit  13  recovers the energy of the high-pressure enriched water W 4  pressurized by the high pressure pump  15 . The energy recovered by the energy recovery unit  13  is used, for example, as the energy for driving the high pressure pump  15  and the energy for transducing the pressure of the filtered water W 2  to a high pressure. This makes it possible for the water treatment apparatus  1  to improve the energy efficiency of the entire water treatment apparatus  1 . 
     As the energy recovery unit  13 , for example, it is possible to use a Pelton Wheel type energy recovery device, a Turbocharger type energy recovery device, a PX (Pressure Exchanger) type energy recovery device, and a DWEER (DualWorkEnergy Exchanger) type energy recovery device. 
     Next, the water to be treated filtering unit  11  according to the present embodiment will be described in detail with reference to  FIG. 2 .  FIG. 2  is a schematic cross-sectional diagram of the water to be treated filtering unit  11  according to the present embodiment. As illustrated in  FIG. 2 , this water to be treated filtering unit  11  is a gravity filtration device. This water to be treated filtering unit  11  includes, for example, a rectangular parallelepiped-shaped filter tank  21 . A perforated block  22  as a filter bed is provided at the lower portion of the filter tank  21 . A ground layer  23  is provided on the perforated block  22  by gravel covered in layers. A first filter layer  24  is provided on the ground layer  23  by sand covered in layers. A second filter layer  25  is provided on the first filter layer  24  by a filter media covered in layers. The second filter layer  25  is configured to include the filter medium according to the present embodiment. This filter medium has a relatively lower specific gravity than the sand constituting the first filter layer  24 , and the particle size of the filter medium is relatively larger than the sand. 
     At the upper portion of the filter tank  21 , a water to be treated supply pipe  26  is provided above the second filter layer  25 . Seawater as the water to be treated W 1  is supplied into the filter tank  21  through the water to be treated supply pipe  26 . Incidentally, a flocculant such as ferric chloride is added to this seawater. This water to be treated supply pipe  26  is provided with a flow regulating valve V 1  which opens and closes the water to be treated supply pipe  26  to adjust the flow rate of the water to be treated W 1 . At the lower portion of the filter tank  21 , a filtered water effluence pipe  27  is provided below the perforated block  22 . This filtered water effluence pipe  27  is provided with a flow regulating valve V 2  capable of adjusting the flow rate of the filtered water W 2  by opening and closing the filtered water effluence pipe  27 . In addition, a washing water supply pipe  28  is provided below the perforated block  22  at the lower portion of the filter tank  21 . This washing water supply pipe  28  is provided with a liquid sending pump  29  which sends washing water W 5  into the filter tank  21 . 
     In addition, a drainage gutter  30  that is substantially U-shaped in cross-sectional view and extends in a substantially horizontal direction is provided above the second filter layer  25 . This drainage gutter  30  is supported by the filter tank  21  via a beam member (not illustrated). Both ends having an aperture of the drainage gutter  30  are connected to a drainage port (not illustrated) formed on the wall of the filter tank  21 . In addition, the position of the upper surface in the vertical direction of the drainage gutter  30  is positioned below the upper end of the filter tank  21 . A filter medium effluence preventing net  31  which prevents effluence of the filter medium is provided around the drainage gutter  30 . This filter medium effluence preventing net  31  is supported by the filter tank  21  by a support member (not illustrated). 
     In this water to be treated filtering unit  11 , the water to be treated W 1  supplied into the filter tank  21  through the water to be treated supply pipe  26  sequentially passes through the second filter layer  25 , the first filter layer  24 , the ground layer  23 , and the perforated block  22  as a downward flow, so that the suspended substances in the water to be treated W 1  are adsorbed and removed by the second filter layer  25  and the first filter layer  24  and the filtrated water W 2  is thus obtained. This filtered water W 2  is discharged out of the filter tank  21  through the filtered water effluence pipe  27 . 
     Moreover, after the operation for a predetermined period, the washing water W 5  is supplied into the filter tank  21  through the washing water supply pipe  28  provided at the lower portion of the filter tank  21  by the liquid sending pump  29 , and the first filter layer  24  and the second filter layer  25  are backwashed. This washing water W 5  sequentially passes through the perforated block  22 , the ground layer  23 , the first filter layer  24 , and the second filter layer  25  as an upward flow. By this, the suspended substances adsorbed to the first filter layer  24  and the second filter layer  25  are desorbed from the first filter layer  24  and the second filter layer  25  and removed as wastewater by the washing water W 5 , and the first filter layer  24  and the second filter layer  25  are thus regenerated. The wastewater containing the suspended substances is discharged via the drainage gutter  30  installed at the upper portion of the filter tank  21 . Incidentally, the first filter layer  24  and the second filter layer  25  are backwashed as it is judged that the first filter layer  24  and the second filter layer  25  have reached the adsorption equilibrium of the suspended substances in a case in which the concentration of suspended substances in the filtered water W 2  reaches a predetermined concentration or higher (for example, 2 mg/kg as TOC (Total Organic Carbon)). 
     Next, the filter medium according to the present embodiment will be described in detail. The filter medium according to the present embodiment is one which constitutes the second filter layer  25  described above, and in which the cumulative pore volume of pores having a pore radius of 2 nm or less is 25% or less with respect to the cumulative pore volume of pores having a pore radius of 50 nm or less. Incidentally, in the present invention, the pore radius and the pore volume are values measured by the nitrogen adsorption method in conformity with JIS Z 8831-2: 2010 and JIS Z 8831-2: 2010. 
     Here, the filter medium will be described in detail with reference to  FIGS. 3 and 4 .  FIG. 3  is a schematic diagram of a general filter medium using activated carbon, and  FIG. 4  is a schematic diagram of the filter medium according to the present embodiment. As illustrated in  FIG. 3 , a macropore  101  having a pore radius of 50 nm or more is formed on the surface of a general filter medium  100  using activated carbon. In this macropore  101 , a plurality of mesopores  102  having a pore radius of 2 nm or more and 50 nm or less are formed. In the plurality of these mesopores  102 , a large number of micropores  103  having a pore radius of 0.8 nm or more and 2 nm or less are formed. In the plurality of micropores  103 , submicropores (not illustrated) having a pore radius of 0.8 nm or less are formed. In the filter medium  100 , the micropores  103  accounts for 25% or more and 75% or less in the total pore volume, and the magnitude of the specific surface area becomes 1000 m 2 /g or more by this. Accordingly, the filter medium  100  has an excellent adsorption capacity to the suspended substances in the water to be treated W 1 . However, the molecular radius of the macromolecule such as a polysaccharide which is the suspended substances in the water to be treated W 1  is equivalent to the inner diameter of the micropore  103 . The suspended substances adsorbed to the inside of the micropore  103  maintain a state of being adsorbed to the filter medium  100  even if washing water passes through at the time of backwashing of the filtration device and are not easily desorbed from the filter medium  100  in some cases. Furthermore, the macromolecule adsorbed in the pores deteriorates the adsorptivity of the filter medium since it gradually elutes over a long period of time in some cases. 
     On the other hand, as illustrated in  FIG. 4 , in a filter medium  200  according to the present embodiment, a part of the micropores  103  of the filter medium  200  is destroyed by an activation treatment or the like. The cumulative pore volume of pores having a pore radius of 2 nm or less is thus 25% or less with respect to the cumulative pore volume of pores having a pore radius of 50 nm or less. This makes it possible to decrease the amount of suspended substances adsorbed to the deep inside of the micropore  103  while securing the adsorption capacity to the suspended substances by the plurality of mesopores  102  and the appropriate number of micropores  103 . It is thus possible to promptly desorb the suspended substances at the time of backwashing in the filter medium  200 . 
     Next, the relationship between the operation time of the filtration device and the concentration of suspended substances in the filtered water W 2  will be described with reference to  FIGS. 5A and 5B .  FIG. 5A  is a diagram illustrating the relationship between the filtration time through a general filter medium using activated carbon and the concentration of suspended substances in the filtered water W 2 .  FIG. 5B  is a diagram illustrating the relationship between the filtration time through the filter medium according to the present embodiment and the concentration of suspended substances in the filtered water W 2 . 
     As illustrated in  FIG. 5A , in the general filter medium using activated carbon having a specific surface area of 1000 m 2 /g or more, the cumulative pore volume of pores having a pore radius of 2 nm or less exceeds 25% with respect to the cumulative pore volume of pores having a pore radius of 50 nm or less. For this reason, in the case of using a general filter medium, the operation time t 1  until the first backwashing of the filter medium  100  after the start of operation is long, but the operation times t 2  to t 4  until the second backwashing to the fourth backwashing of the filter medium  100  are greatly shortened. It is considered that this result is because a relatively long operation time t 1  can be secured until the first backwashing since the suspended substances are adsorbed to a large number of micropores  103  inside the filter medium  100  until the first backwashing after the start of operation. On the other hand, it is considered that this result is because the suspended substances once adsorbed to the micropore  103  are desorbed from the filter medium  100  only to a certain extent even by backwashing and a large number of micropores  103  are clogged by the suspended substances and the operation times t 2  to t 4  until the second backwashing to the fourth backwashing are thus shortened. 
     On the contrary, as illustrated in  FIG. 5B , in the filter medium  200  according to the present embodiment, the operation time t 1  until the first backwashing of the filter medium  200  after the start of operation is shorter as compared to the general filter medium  100  using activated carbon. On the other hand, the operation times t 2  to t 4  until the second backwashing to the fourth backwashing of the filter medium  200  are substantially constant, to be the same as the operation time t 1 , the total operation time t 1  to t 4  until the first backwashing to the fourth backwashing is longer as compared to the case of the filter medium  100 . It is considered that this result is because the ratio of the micropores  103  from which it is difficult to desorb the adsorbed suspended substances is appropriately decreased and the cumulative pore volume of pores having a pore radius of 2 nm or less is 25% or less with respect to the cumulative pore volume of pores having a pore radius of 50 nm or less in the filter medium  200  according to the present embodiment. Thus the suspended substances adsorbed to the filter medium  200  are efficiently desorbed by backwashing and the filter medium  200  can be efficiently regenerated. 
     As described above, according to the filter medium  200  of the present embodiment, the cumulative pore volume of pores having a pore radius of 2 nm or less is 25% or less with respect to the cumulative pore volume of pores having a pore radius of 50 nm or less. Thus the pore radius of pores of the filter medium  200  is appropriately greater with respect to the molecular size of the suspended substances such as organic substances contained in the water to be treated W 1 . It is possible to appropriately prevent the adsorption of suspended substances to the fine pores of the filter medium  200  and to efficiently desorb the suspended substances retained in the filter medium  200  from the filter medium  200  at the time of backwashing. This makes it possible to promptly desorb the suspended substances adsorbed to this filter medium  200  at the time of backwashing without impairing the adsorption efficiency to the suspended substances in the water to be treated W 1 , and it is thus possible to regenerate the filter medium  200  in a short time and to efficiently operate the filtration device. 
     The specific surface area of the filter medium  200  according to the present embodiment is preferably 100 m 2 /g or more, since the adsorption capacity to the suspended substances contained in the water to be treated W 1  is improved. This increases the time until the filter medium  200  breaks through as compared to a case in which the specific surface area of the filter medium  200  is less than 100 m 2 /g, and thus it is possible to increase the operation time until the backwashing and the operation efficiency of the filtration device is improved. In addition, according to the filter medium  200 , the specific surface area is preferably 150 m 2 /g or more and more preferably 200 m 2 /g or more from the viewpoint of even further improving the adsorption capacity to the suspended substances described above. The specific surface area is preferably 800 m 2 /g or less, more preferably 500 m 2 /g or less, and still more preferably 400 m 2 /g or less from the viewpoint of even more efficiently desorbing the suspended substances at the time of backwashing. In consideration of the facts described above, the specific surface area of the filter medium  200  is preferably 100 m 2 /g or more and 800 m 2 /g or less, more preferably 150 m 2 /g or more and 500 m 2 /g or less, and still more preferably 200 m 2 /g or more and 400 m 2 /g or less. 
     Next, the relationship between the cumulative pore volume and the pore radius of the filter medium  200  measured by the method of JIS Z 8831-3: 2010 will be described with reference to  FIG. 6 . Incidentally, the cumulative pore volume is the cumulative volume of the entire pores belonging to the filter medium  200  in a measurable range.  FIG. 6  is a diagram illustrating the relationship between the cumulative pore volume and the pore radius of the filter medium  200  according to the present embodiment. Incidentally, in  FIG. 6 , the horizontal axis indicates the pore radius (nm) of the filter medium  200  and the vertical axis indicates the cumulative pore volume (%) of the filter medium  200 . As illustrated in  FIG. 6 , in the present embodiment, the specific surface area is set to 100 m 2 /g or more and 800 m 2 /g or less by appropriately removing the micropores  103  having a pore radius of 2 nm or less by an activation treatment or the like. Thus the cumulative pore volume of pores having a pore radius of 2 nm or less is smaller than that of general activated carbon and can be set to be 25% or less with respect to the cumulative pore volume of pores having a pore radius of 50 nm or less. This decreases the micropores  103  having a pore radius of 0.8 nm or more and 2 nm or less and the submicropores having a pore radius of 0.8 nm or less from which it is difficult to desorb the adsorbed suspended substances from the filter medium at the time of backwashing, and it is thus possible to promptly desorb the suspended substances from the filter medium at the time of backwashing. As a result, it is possible to realize a filter medium capable of more promptly regenerating the adsorption power by backwashing and realizing more efficient operation of the filtration device. 
     In the filter medium according to the present embodiment, the cumulative pore volume of pores having a pore radius of 2 nm or less is preferably 25% or less, more preferably 10% or less, and still more preferably 1% or less with respect to the cumulative pore volume of pores having a pore radius of 50 nm or less from the viewpoint of even further improving the effect described above. 
     Here, the cumulative pore volume will be described. In general, the saturated vapor pressure in the pores of the filter medium decreases depending on the curvature of the surface of the filter medium by the function of the surface tension, and thus the adsorption capacity increases as the specific surface area increases and the number of curved surfaces increases. In addition, the saturated vapor pressure P 0   r  in the pores having a pore radius r is represented by the Kelvin equation of the following Formula (1). In general, the adsorption capacity increases as p/p 0  increases, and thus the adsorption capacity increases as p 0  decreases. 
     
       
         
           
             
               
                 
                   
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                     ) 
                   
                 
               
             
           
         
       
     
     (In Formula (1), p 0  represents the saturated vapor pressure of the liquid plane surface. V m  represents the surface molar volume of the liquid. γ represents the surface tension of the liquid. R represents the gas constant. T represents the absolute temperature. θ represents the contact angle between the liquid and the pore wall.) 
     The cumulative pore volume is determined from the pore radius distribution. The pore radius distribution can be determined by comparing the adsorption isotherms by the t-plot method or the α s -plot method which conforms to JIS Z 8831-3: 2010. In these methods, the nitrogen adsorption isotherm representing the relationship between the adsorption capacity and the pressure is measured and compared with the standard sample to determine the pore radius distribution. Moreover, the adsorption capacity increases by the presence of pores, and thus the pore radius distribution is determined by determining the volume of the pores from an increase in adsorption capacity corresponding to the pore radius. 
     As the filter medium  200 , various kinds of filter media can be used in the range of achieving the effect of the present invention. As the filter medium  200 , for example, it is possible to use various kinds of ceramics such as alumina, various kinds of carbon-based materials such as coal, charcoal, coal coke, petroleum coke, pitch coke, activated carbon using these, and particulate activated carbon obtained by granulating carbon black with a thermosetting resin such as pitch/tar binder/phenol, activated alumina, activated clay, silica gel, zeolite, and the like. 
     In the present embodiment, the filter medium  200  is preferably formed of at least one kind of a carbon-based material or activated carbon and more preferably formed of activated carbon. By this, the filter medium  200  has a lower specific gravity than the filter sand used in the first filter layer  24  of the water to be treated filtering unit  11 , and it is thus easy to provide the second filter layer  25  containing the filter medium  200  on the first filter layer  24 . In addition, a carbon-based material and activated carbon exhibit high affinity for organic substances, and it is thus possible to efficiently remove the suspended substances due to the organic substances in the water to be treated W 1 . 
     Next, a process for producing the filter medium  200  according to the present embodiment will be described. In the process for producing the filter medium  200  according to the present embodiment, the filter medium  200  in which the cumulative pore volume of pores having a pore radius of 2 nm or less is 25% or less with respect to the cumulative pore volume of pores having a pore radius of 50 nm or less is manufactured by activating the various kinds of carbon-based materials described above with water vapor and/or a carbonic acid gas. According to this process, it is possible to decrease the specific surface area of the carbon-based material by appropriately destroying the micropores  103  having a pore radius of 2 nm or less in the carbon-based material by an activation treatment with at least either of water vapor or a carbonic acid gas. It is thus possible to set the cumulative pore volume of pores having a pore radius of 2 nm or less to be 25% or less with respect to the cumulative pore volume of pores having a pore radius of 50 nm or less. This makes it possible to manufacture the filter medium  200  capable of promptly desorbing the suspended substances from the filter medium at the time of backwashing as well as efficiently adsorbing suspended substances in the water to be treated W 1 . 
     As the condition for the activation treatment in a case in which the activation treatment is conducted with water vapor, a condition is preferable in which the surface temperature of the carbon-based material is 750° C. or higher and 850° C. or lower and the time is 12 hours or longer and 72 hours or shorter, that is longer than that for a general activation treatment of activated carbon. In addition, in a case in which the activation treatment is conducted with a carbonic acid gas, a condition is preferable in which the surface temperature of the carbon-based material is 850° C. or higher and 950° C. or lower and the time is 12 hours or longer and 72 hours or shorter, that is longer than that for a general activation treatment of activated carbon By appropriately destroying the micropores having a pore radius of 2 nm or less in the carbon-based material by activating the carbon-based material under such a condition, it is possible to obtain the filter medium  200  in which the cumulative pore volume of pores having a pore radius of 2 nm or less is 25% or less with respect to the cumulative pore volume of pores having a pore radius of 50 nm or less. 
     In addition as the condition for the activation treatment, it is preferable to conduct the activation treatment until the mass decrease of the carbon-based material caused by gasification and wear reaches 50% or more under the condition of temperature and time described above. By this, the destruction of the micropores  103  having a pore radius of 2 nm or less in the carbon-based material is conducted in an appropriate range. It is thus possible to easily obtain the filter medium  200  in which the cumulative pore volume of pores having a pore radius of 2 nm or less is 25% or less with respect to the cumulative pore volume of pores having a pore radius of 50 nm or less. In addition, the mass decrease by the activation treatment is more preferably 75% or more and still more preferably 80% or more from the viewpoint of even more easily manufacturing the filter medium  200  in which the cumulative pore volume of pores having a pore radius of 2 nm or less is 25% or less with respect to the cumulative pore volume of pores having a pore radius of 50 nm or less. 
     As described above, according to the filter medium  200  of the present embodiment, the cumulative pore volume of pores having a pore radius of 2 nm or less is 25% or less with respect to the cumulative pore volume of pores having a pore radius of 50 nm or less. It is thus possible to efficiently desorb the suspended substances retained in the filter medium from the filter medium at the time of backwashing as well as to efficiently remove the suspended substances contained in the water to be treated W 1 . Hence, it is possible to realize the filter medium  200  capable of promptly regenerating the adsorption power by backwashing and realizing efficient operation of the filtration device. Moreover, by using this filter medium  200 , it is possible to efficiently operate the filtration device by only applying the filter medium  200  without greatly changing the structure of the filtration device main body. 
     Incidentally, in the embodiment described above, an example in which the filter medium  200  according to the present embodiment is applied to the water to be treated filtering unit  11  has been described, but the present invention is not limited to the water to be treated filtering unit  11  and can be applied to various kinds of filtration devices. 
     Next, a method of operating a filtration device according to the present embodiment will be described. The method of operating a filtration device according to the present embodiment includes: a filtering step of filtering the water to be treated through the filter medium to decrease the suspended substances in the water to be treated; and a washing step of backwashing the filter medium when the amount of suspended substances in the water to be treated filtered through the filter medium reaches one third or more of the total amount adsorbed to the filter medium. In other words, in the method of operating a filtration device according to the present embodiment, the total amount adsorbed to the filter medium with respect to the concentration of suspended substances in the water to be treated is adjusted so as to be three-fold or more the integrated amount of suspended substances which pass through the filter medium during the operation period between the backwashing intervals of the filtration device. Incidentally, in the method of operating a filtration device according to the present embodiment, it is not necessarily required to use the filter medium  200  according to the embodiment described above as the filter medium, and various kinds of filter media can be used. 
     According to this method of operating a filtration device, it is possible to conduct backwashing under a condition having a sufficient margin with respect to the adsorption capacity of the filter medium and thus to prevent excessive adsorption of the suspended substances into pores of the filter medium. In addition, the filtration device is operated in a range having a sufficient margin in the adsorption capacity of the filter medium, and it is thus possible to prevent permeation of the suspended substances in the water to be treated through the filtration device even if there are suspended substances which are not desorbed from the inside of the pores of the filter medium by backwashing. Furthermore, there is a margin in the adsorption capacity of filter medium and it is thus possible to prevent permeation of the suspended substances in the water to be treated through the filtration device even in a case in which it is not possible to completely remove the suspended substances from the filter medium by one time of backwashing. This makes it possible to promptly desorb the suspended substances adsorbed to the filter medium from the filter medium at the time of backwashing, and it is thus possible to realize a method of operating a filtration device capable of promptly regenerating the adsorption power by backwashing and realizing efficient operation. 
     In the method of operating a filtration device according to the present embodiment, the total amount adsorbed to the filter medium with respect to the concentration of suspended substances in the water to be treated is set to be preferably five-fold or more and still more preferably 10-fold or more the integrated amount of suspended substances which pass through the filter medium during the operation period between the backwashing intervals of the filtration device from the viewpoint of even further improving the effect described above. 
     In addition, in the method of operating a dual media filtration device according to the present embodiment, for example, the kind and used amount of filter medium are determined so that the following relational expression is satisfied. 
     Amount adsorbed to filter medium per unit mass in concentration of suspended substances in water to be treated (mg/kg)×used amount (kg) of filter medium={concentration of suspended substances in water to be treated (mg/kg)−concentration of suspended substances in filtered water (mg/kg)}×operation time of filtration device between backwashing treatments (h)×flow rate of water to be treated permeating through filter medium (m 3 /h)×1000×3 (or 5 or 10) 
       FIG. 7  is a diagram illustrating another example of the filtration device according to the present embodiment. As illustrated in  FIG. 7 , this filtration device  210  is equipped with: a turbidity meter for water to be treated (device for measuring concentration of suspended substances in water to be treated)  211  that is provided to the water to be treated supply pipe  26 ; a turbidity meter for filtered water (device for measuring concentration of suspended substances in filtered water)  212  that is provided to the filtered water effluence pipe  27 ; and a control unit  213  which calculates the amount of suspended substances in the water to be treated W 1  measured by the turbidity meter for water to be treated  211  and the amount of suspended substances in the filtered water W 2  measured by the turbidity meter for filtered water  212  and controls the operation of the filtration device  210  based on the result of calculation in addition to the configuration of the filtration tank  21  illustrated in  FIG. 2 . 
     The turbidity meter for water to be treated  211  is provided on the upstream side to the filter medium  200  in the flow path of the water to be treated W 1 . The turbidity meter for water to be treated  211  measures the concentration of suspended substances in the water to be treated W 1  before being filtered through the filter medium  200 . The turbidity meter for filtered water  212  is provided on the downstream side to the filter medium  200  in the flow path of the water to be treated W 1 . The turbidity meter for filtered water  212  measures the concentration of the filtered water W 2  that is the water to be treated W 1  after being filtered through the filter medium. The turbidity meter for water to be treated  211  and the turbidity meter for filtered water  212  are not particularly limited as long as they can measure the concentration of the water to be treated W 1  or the filtered water W 2 . 
     The control unit  213  is realized, for example, by utilizing a general purpose or dedicated computer such as a CPU (Central Processing Unit), a ROM (Read Only Memory), or a RAM (Random Access Memory) and a program operating on the computer. The control unit  213  calculates the time integrated value (cumulative value per predetermined time) of the concentration of suspended substances in the water to be treated W 1  measured by the turbidity meter for water to be treated  211 . In addition, the control unit  213  calculates the time integrated value (cumulative value per predetermined time) per predetermined time of the concentration of suspended substances in the filtered water W 2  measured by the turbidity meter for filtered water  212 . The control unit  213  calculates a difference value between the time integrated value per predetermined time of the concentration of suspended substances in the water to be treated W 1  calculated and the time integrated value per predetermined time of the concentration of suspended substances in the filtered water W 2  calculated. The control unit  213  determines whether or not the difference value calculated is equal to or less than a predetermined threshold value. Furthermore, the control unit  213  continues the normal operation of the filtration device  210  in a case in which the difference value calculated is equal to or less than the predetermined threshold value. In addition, in a case in which the difference value calculated exceeds the predetermined threshold value, the control unit  213  closes the flow regulating valve V 1  of the water to be treated supply pipe  26  and the flow regulating valve V 2  of the filtered water effluence pipe  27  and then supplies the washing water W 5  into the filtration device  210  through the washing water supply pipe  28  by driving the liquid sending pump  29  to conduct backwashing of the filter medium  200 . In addition, after backwashing of the filtration device  210 , the control unit  213  erases the time integrated value per predetermined time of the concentration of suspended substances in the water to be treated W 1  calculated and the time integrated value per predetermined time of the concentration of suspended substances in the filtered water W 2  calculated. Thereafter, the control unit  213  calculates the time integrated value per predetermined time (integrated value) of the concentration of suspended substances in the water to be treated W 1  measured by the turbidity meter for water to be treated  211  and the time integrated value per predetermined time (integrated value) of the concentration of suspended substances in the filtered water W 2  measured by the turbidity meter for filtered water  212  again. 
     Next, the method for operating the filtration device  210  will be described in detail with reference to  FIG. 8 .  FIG. 8  is a flow diagram of the method for operating the filtration device  210  according to the present embodiment. As illustrated in  FIG. 8 , the method for operating the filtration device  210  according to the present embodiment includes: a concentration of suspended substances measuring step of respectively measuring the first concentration of suspended substances in the water to be treated W 1  and the second concentration of suspended substances in the filtered water W 2  obtained as the water to be treated W 1  is filtered through the filter medium; and a filter medium washing step of conducting backwashing of the filter medium based on the difference value between the first time integrated value (cumulative value) of the first concentration of suspended substances in the water to be treated W 1  measured and the second time integrated value (cumulative value) of the second concentration of suspended substances in the filtered water W 2  measured. 
     In the concentration of suspended substances measuring step, the turbidity meter for water to be treated  211  measures the first concentration of suspended substances in the water to be treated W 1  (step ST 111 ). In addition, in the concentration of suspended substances measuring step, the turbidity meter for filtered water  212  measures the second concentration of suspended substances in the filtered water W 2  (step ST 112 ). 
     After the start of operation of the filtration device  210 , in the amount of suspended substances evaluating step, the control unit  213  calculates the first time integrated value per predetermined time of the first concentration of suspended substances in the water to be treated W 1  measured by the turbidity meter for water to be treated  211  (step ST 121 ). Next, the control unit  213  calculates the second time integrated value per predetermined time of the second concentration of suspended substances in the filtered water W 2  measured by the turbidity meter for filtered water  212  (step ST 122 ). Subsequently, the control unit  213  calculates the difference value between the first time integrated value per predetermined time of the first concentration of suspended substances in the water to be treated W 1  calculated and the second time integrated value per predetermined time of the second concentration of suspended substances in the filtered water W 2  calculated (step ST 123 ) and determines whether or not the difference value calculated is equal to or less than a predetermined threshold value (step ST 124 ). Thereafter, the control unit  213  continues the normal operation of the filtration device  210  (step ST 125 ) in a case in which the difference value calculated is equal to or less than the predetermined threshold value (step ST 124 : Yes). In addition, in a case in which the difference value calculated exceeds the predetermined threshold value (step ST 124 : No), the control unit  213  closes the flow regulating valve V 1  of the water to be treated supply pipe  26  and the flow regulating valve V 2  of the filtered water effluence pipe  27  and then supplies the washing water W 5  into the filtration device  210  through the washing water supply pipe  28  by driving the liquid sending pump  29  to conduct backwashing of the filter medium  200  (step ST 126 ). Next, after backwashing of the filtration device  210 , the control unit  213  erases the first time integrated value per predetermined time of the first concentration of suspended substances in the water to be treated W 1  calculated and the second time integrated value per predetermined time of the second concentration of suspended substances in the filtered water W 2  calculated (step ST 127 ). Thereafter, the control unit  213  calculates the first time integrated value per predetermined time of the first concentration of suspended substances in the water to be treated W 1  measured by the turbidity meter for water to be treated  211  and the second time integrated value per predetermined time of the second concentration of suspended substances in the filtered water W 2  measured by the turbidity meter for filtered water  212  again (steps ST 111 , ST 112 , ST 121 , and ST 122 ). 
     In addition, in the method of operating a filtration device according to the present embodiment, the kind and used amount of the filter medium are determined so that the first concentration of suspended substances to be measured by the turbidity meter for water to be treated  211  provided on the upstream side of the filter medium and the second concentration of suspended substances to be measured by the turbidity meter for filtered water  212  provided on the downstream side of the filter medium satisfy the following relational expression. 
     Amount adsorbed to filter medium per unit mass in first concentration of suspended substances in water to be treated W 1  (mg/kg)×used amount (kg) of filter medium={first time integrated value of first concentration of suspended substances in water to be treated W 1  (mg/kg·h)−second time integrated value of second concentration of suspended substances in filtered water W 2  (mg/kg·h)}×flow rate of water to be treated W 1  permeating through filter medium (m 3 /h) 
     As described above, according to the method for operating the filtration device  210  according to the present embodiment, it is possible to accurately ascertain the amount of suspended substances adsorbed to the filter medium based on the difference value between the first time integrated value of the first concentration of suspended substances to be measured by the turbidity meter for water to be treated  211  provided on the upstream side of the filter medium and the second time integrated value of the second concentration of suspended substances to be measured by the turbidity meter for filtered water  212  provided on the downstream side of the filter medium. It is thus to sufficiently set the operation time of the filtration device  210  required until the backwashing. This makes it possible for the method for operating the filtration device  210  to prevent the deterioration in performance of the filtration device  210  due to poor washing of the suspended substances adsorbed to the filter medium at the time of backwashing caused in a case in which the operation time of the filtration device  210  is too long. In addition, the method for operating the filtration device  210  can also prevent a decrease in the rate of operation of the filtration device  210  due to an increase in the number of backwashing caused in a case in which the operation time of the filtration device  210  is too short. 
     Incidentally, in the embodiment described above, the turbidity meter for water to be treated  211  and the turbidity meter for filtered water  212  are used, but it is also possible to use a concentration meter for organic substances such as a TOG (total organic carbon) meter instead of a turbidity meter. It is possible to operate the filtration device  210  in the same manner as the above by taking the concentration of suspended substances as the concentration of organic substances in the case of using a concentration meter for organic substances. 
     Next, the filtration system according to the present embodiment will be described with reference to  FIG. 9 .  FIG. 9  is a schematic diagram of a filtration system  300  according to the present embodiment. As illustrated in  FIG. 9 , the filtration system  300  according to the present embodiment is equipped with: a filtration device  301  to which a water to be treated line L 10  is connected; and a salt enriching unit  302  provided at the subsequent stage of the filtration device  301 . A filtered water line L 11  is provided between the filtration device  301  and the salt enriching unit  302 . 
     The water to be treated line L 10  supplies the water to be treated W 1  of raw water such as seawater to the filtration device  301 . The filtration device  301  filters the water to be treated W 1  supplied through the water to be treated line L 10  to obtain the filtered water W 2 . In addition, the filtration device  301  supplies the filtered water W 2  to the salt enriching unit  302  via the filtered water line L 11 . The salt enriching unit  302  permeates the filtered water through a separation membrane  302   a  to obtain the permeated water W 3  in which the salts in the filtered water W 2  are removed and the enriched water W 4  in which the salts in the filtered water W 2  are enriched. In addition, the salt enriching unit  302  discharges the enriched water W 4  via the enriched water discharge line L 13  as well as supplies the permeated water W 3  to the various kinds of devices (not illustrated) at the subsequent stages via the permeated water line L 12 . The separation membrane  302   a  is not particularly limited as long as the permeated water W 3  and the enriched water W 4  can be obtained from the filtered water W 2 . 
     The filtration device  301  is equipped with a first filter layer  301   b  and a second filter layer  301   c  layered in a filtration device main body  301   a.  The first filter layer  301   b  is provided on a top portion  301   d  side of the filtration device main body  301   a  and is configured to include the filter medium  200  described above. The second filter layer  301   c  is provided on a bottom portion  301   e  side of the filtration device main body  301   a  and is configured to include a particulate filter medium such as silica sand. The inorganic impurities in the water to be treated W 1  are removed by these first filter layer  301   b  and second filter layer  301   c,  and it is thus possible to measure and ascertain the concentration of suspended substances in the water to be treated W 1  based on the organic substance-based impurities by a water quality evaluating unit  303  to be described later. 
     In addition, the filtration system  300  according to the present embodiment is equipped with: the water quality evaluating unit  303  which evaluates the quality of the filtered water W 2  flowing through the filtered water line L 11 ; a flocculant supply unit  304  which supplies a flocculant  304   a  to the water to be treated line L 10  via a flocculant supply line L 21 ; and a control unit  305  which controls the amount of flocculant to be supplied from the flocculant supply unit  304  based on the evaluation result of water quality of the filtered water W 2  by the water quality evaluating unit  303 . 
     The water quality evaluating unit  303  measures and monitors the concentration of suspended substances such as organic substances in the water to be treated W 1 . The control unit  305  determines whether or not the concentration of suspended substances in the water to be treated W 1  measured by the water quality evaluating unit  303  is equal to or higher than a predetermined threshold value. Thereafter, the control unit  305  supplies the flocculant  304   a  from the flocculant supply unit  304  to the water to be treated line L 10  by operating a chemical supply pump  306  provided to the flocculant supply line L 21  in a case in which the concentration of suspended substances in the water to be treated W 1  is equal to or higher than the predetermined threshold value. In addition, the control unit  305  stops supply of the flocculant  304   a  from the flocculant supply unit  304  to the water to be treated line L 10  by stopping the chemical supply pump  306  in a case in which the concentration of suspended substances in the water to be treated W 1  is less than the predetermined threshold value. 
     As described above, according to the filtration system  300  of the present embodiment, the filtration device  301  in which the micropores having a pore radius of 0.8 nm or more and 2 nm or less and the submicropores having a pore radius of 0.8 nm or less, which make it difficult to desorb the adsorbed suspended substances from the filter medium  200  at the time of backwashing are decreased is equipped and it is thus possible to promptly desorb the suspended substances from the filter medium  200  at the time of backwashing. This makes it possible to more promptly regenerate the adsorption power of the filter medium of the filtration device  310  by backwashing and to realize more efficient operation of the filtration system. 
     REFERENCE SIGNS LIST 
       1  Water Treatment Apparatus 
       11  Dual Media Filtration Device 
       12  Reverse Osmosis Membrane Filtering Unit 
       12   a  Reverse Osmosis Membrane 
       13  Energy Recovery Unit 
       14  Pump 
       15  High Pressure Pump 
       21 ,  210 , and  301  Filtration Device 
       22  Perforated Block 
       23  Ground Layer 
       24  First Filter Layer 
       25  Second Filter Layer 
       26  Water to be Treated Supply Pipe 
       27  Filtered Water Effluence Pipe 
       28  Washing Water Supply Pipe 
       29  Liquid Sending Pump 
       30  Drainage Gutter 
       31  Filter Medium Effluence Preventing Net 
       100  and  200  Filter Medium 
       101  Macropore 
       102  Mesopore 
       103  Micropore 
       211  Turbidity Meter for Water to be Treated (Device for Measuring Concentration of Suspended Substances in Water to be Treated) 
       212  Turbidity Meter for Filtered Water (Device for Measuring Concentration of Suspended Substances in Filtered Water) 
       213 ,  305  Control Unit 
       300  Filtration System 
       301   a  Filtration Device Main Body 
       301   b  First Filter Layer 
       301   c  Second Filter Layer 
       301   d  Top Portion 
       301   e  Bottom Portion 
       302  Salt Enriching Unit 
       302   a  Separation Membrane 
       303  Water Quality Evaluating Unit 
       304  Flocculant Supply Unit 
       304   a  Flocculant 
       306  Chemical Supply Pump 
     L 1  and L 10  Water to be Treated Line 
     L 2  and L 11  Filtered Water Line 
     L 3  Enriched Water Line 
     L 4  Permeated Water Line 
     L 12  Permeated Water Line 
     L 13  Enriched Water Discharge Line 
     L 21  Flocculant Supply Line 
     V 1  and V 2  Flow Regulating Valve 
     W 1  Water to be Treated 
     W 2  Filtered Water 
     W 3  Permeated Water 
     W 4  Enriched Water 
     W 5  Washing Water