Field The present invention relates to a water treatment system and a method therefor for effluent generated in a boiler plant or a chemical plant facility, for example. 5 Background For example, in a process plant of a power plant or a chemical plant, effluent is generated in, for example, a boiler, a reactor, a wet cooling tower of a condenser, a water treatment device, and so on. There are various kinds 10 of treatment devices proposed to treat such effluent, but there may be a problem in which any of these treatment devices requires high cost. To solve such a problem, there is a proposed boiler provided with a waste water treatment device in which alkaline blow water is neutralized by 15 spraying, into a flue gas duct, cooling water (blow water) of a cooling tower of a boiler as mists having a droplet diameter from 20 to 120 m (Patent Literature 1). Further, there is another proposed waste water treatment device in which an amount of waste water which 20 can be evaporated by spraying effluent into a flue gas duct is increased (Patent Literature 2). Citation List Patent Literature Patent Literature 1: Japanese Laid-open Patent 25 Publication No. 8-47693 Patent Literature 2: Japanese Laid-open Patent Publication No. 2001-29939 Summary Technical Problem 30 However, according to the invention of Patent Literature 1, the waste water can be treated easily and at 3 low cost, but there is problems in which treatment may not be performed in the case an amount of the waste water is increased relative to thermal energy (temperature, flow rate) of flue gas and an energy amount required to 5 evaporate the increased waste water becomes large. Further, according to the invention of Patent Literature 2, an amount of waste water can be reduced by a concentration device, but there is a problem in which output of a steam turbine may be decreased because a part 10 of steam generated in a boiler is bled in the concentration device. Furthermore, since the flue gas discharged from a boiler plant may contain trace amounts of toxic substances such as mercury in addition to nitrogen oxide and sulfur 15 oxide, a countermeasure against mercury is required in order to perform water treatment by mixing waste water in other plant facility. Therefore, providing an effective treatment technology is strongly demanded, according to which, the effluent 20 generated in a process plant of, for example, a power plant or a chemical plant, such as the effluent discharged from a boiler, a reactor, a wet cooling tower of a condenser, a water treatment device, and the like can be treated at low cost in consideration of a countermeasure against mercury 25 discharge without deteriorating efficiency of the boiler. In consideration of the above problems, the present invention is directed to providing a water treatment system and a method therefor for effluent generated in a plant facility. 30 Solution to Problem In order to solve the above-mentioned problem, the first aspect of the present invention isa water treatment 4 system including: a wet desulfurization device that removes sulfur oxide from boiler flue gas; a dewatering device that separates gypsum from desulfurization waste water which contains gypsum slurry and which is obtained from the wet desulfurization device; a mercury removing 5 unit to which is introduced separated water that is obtained from the dewatering device, and which immobilizes heavy metals present in the separated water by inputting a chelating agent; a solid-liquid separation unit that performs solid10 liquid separation with respect to solid content present in separated water obtained from the mercury removing unit; a mixing unit that mixes separated water obtained from the solid-liquid separation unit with effluent generated in a plant facility; a desalination device that removes salt 15 content from mixed water obtained by the mixing unit; and a spray drying device that includes a spraying unit, which sprays concentrated water in which salt content has been concentrated by the desalination device, and that performs spray drying using a part of the boiler flue gas. 20 The second aspect of the present invention is a water treatment system including: a wet desulfurization device that removes sulfur oxide from boiler flue gas; a dewatering device that separates gypsum from desulfurization waste water which contains gypsum slurry 25 and which is obtained from the wet desulfurization device; a mixing unit that mixes separated water obtained from the dewatering device with effluent generated in a plant facility; a reaction tank to which mixed water obtained by the mixing unit is introduced and which immobilizes heavy 30 metals present in separated water by inputting a chelating agent; a solid-liquid separation unit that performs solidliquid separation with respect to solid content present in mixed water obtained from the reaction tank; a desalination 5 device that removes salt content from mixed water subjected to solid-liquid separation; and a spray drying device that includes a spraying unit which sprays concentrated water in which salt content has been concentrated by the desalination device, and that performs 5 orms spray drying using a part of the boiler flue gas. The third aspect of the present invention is the water treatment system according to the first or the second aspect further including a membrane treatment unit that 10 performs treatment using a membrane having univalent selectivity with respect to separated water obtained after separation performed by the solid-liquid separation unit. The fourth aspect of the present invention is the water treatment system according to any one of the first to 15 the third aspects, wherein the desalination device removes bivalent salt content present in the effluent. The fifth aspect of the present invention is the water treatment system according to any one of the first to the fourth aspects, wherein the desalination device 20 includes a scale prevention agent supplying unit that supplies a scale prevention agent to the mixed water containing bivalent ions such as Ca ions; a first desalination device that is disposed at a downstream side of the scale prevention agent supplying unit and that 25 separates the mixed water into reclaimed water and concentrated water in which the Ca ions have been concentrated; a crystallization tank that is disposed at a downstream side of the first desalination device and that is used in crystalizing gypsum from the concentrated water; 30 a separating unit that separates the gypsum crystalized and concentrated water obtained from the first desalination device; and a second desalination device that is disposed at a downstream side of the separating unit and separates 6 the concentrated water into reclaimed water and concentrated water in which the Ca ions have been concentrated. The sixth aspect of the present invention is the water treatment system according to the fifth 5 aspect, wherein the desalination device further includes a separating unit that separates concentrated water obtained from the second desalination device; and a third desalination device that is disposed at a downstream side 10 of the separating unit and that separates the concentrated water into reclaimed water and concentrated water in which the Ca ions have been concentrated. The seventh aspect of the present invention is the water treatment system according to the fifth aspect of the 15 invention, further including an oxidation-reduction potentiometer that measures the oxidation-reduction potential of mixed water to be introduced into the first desalination device. The eighth aspect of the present invention is the 20 water treatment system according to the seventh aspect of the invention, wherein a value (X) of the oxidationreduction potential of the mixed water, which is measured by the oxidation-reduction potentiometer, satisfies - 0.69V First, metal ions present in the effluent 22 are roughly removed as metal hydroxide at the sedimentation tank 53 and the filtering device 54. 29 In the case where the effluent 22 is highly acidic, an alkaline agent (e.g., Ca(OH)2) 71 and polymer (e.g., anionic polymer (trade name: Hishifloc H3O5 manufactured by Mitsubishi Heavy Industries Mechatronics Systems Ltd.) 72 are input to the effluent 22 at an adding tank 52 5 2 adjacent to the upstream side of the sedimentation tank 53, and pH inside the sedimentation tank 53 is controlled be in an alkaline pH area (e.g., pH: 8.5 to 11). In this pH area, solubility of calcium carbonate and 10 metal hydroxide is low, and when calcium carbonate and metal hydroxide is supersaturated, the calcium carbonate and metal hydroxide are precipitated and deposit on a bottom portion of the sedimentation tank 53. Further, solubility of metal hydroxide depends on pH. 15 The more acidic the metal ions are, the higher the solubility in water is. Since most of the metal hydroxide has low solubility in the above-mentioned pH area, the metals contained in the effluent 22 deposit on the bottom portion of the sedimentation tank 53 as the metal 20 hydroxide. Here, sediment 53a is separately discharged from the bottom portion for treatment. The effluent 22 which is supernatant liquid inside the sedimentation tank 53 is discharged from the sedimentation tank 53. Ferrous flocculant (e.g., FeCl3) 73 is added to 25 the discharged effluent 22, and the solid content of calcium carbonate, metal hydroxide, etc. present in the effluent 22 is flocculated together with Fe(OH)3. The mixed water 111 is fed to the filtering device 54. The solid content flocculated together with Fe(OH)3 is 30 removed by the filtering device 54. Among the metals, Fe is easily precipitated as a hydroxide in the case of an acidic state. When the mixed water 111 containing a large amount of Fe ions is flown 30 into the first desalination device 55A, scale containing Fe is generated in the first desalination device 55A, and further iron hydroxide, etc. deposit in the crystallization tank 61. Due to this, in the present embodiment, treatment conditions at the sedimentation tank 53, an adding 5 ng amount of FeCl3, etc. are suitably set such that the concentration of Fe ions in the mixed water 111 becomes 0.05 ppm or lower after pretreatment with alkali and before flowing into the first desalination device 55A, considering prevention of 10 scale generation in the first desalination device 55A. Note that the pretreatment can be omitted depending on water quality of the mixed water 111. In the supplying unit that supplies the scale 15 prevention agent 74, a predetermined amount of the scale prevention agent 74 is supplied to the mixed water 111 from a tank not illustrated. A control unit not illustrated controls the concentration of the scale prevention agent 74 to be a predetermined value set in accordance with 20 properties of the mixed water 111. The supplying unit of a pH adjuster 75 in a first pH adjustment process controls, by using the scale prevention agent 74, the pH of the mixed water 111 at an entrance side 25 of the first desalination device 55A so as to be a value (e.g., about pH 5.5) at which precipitation of scale containing Ca (gypsum, calcium carbonate) is suppressed. This control includes measuring the pH of the mixed water 111 at the entrance side of the first desalination device 30 55A. Meanwhile, this first pH adjustment process is omitted in a modified example in which the first pH adjusting unit is not provided. 31 The pH-adjusted mixed water 111 is treated in the first desalination device 55A. Permeated water which has permeated the reverse osmosis membrane 55a of the first desalination device 55A is recovered 5 ed as the reclaimed water 33a from which salt content has been removed. In this upstream side separation process, ions contained in the mixed water 111 and the scale prevention agent 74 cannot permeate the reverse osmosis membrane 55a. 10 Therefore, highly concentrated water 31a having the high ion concentration is present on a non-impermeable side of the reverse osmosis membrane 55a. The concentrated water 31a of the first desalination device 55A is fed to the crystallization tank 61. For example, in the case of using 15 a different desalination device such as a continuous deionization, the mixed water 111 is also separated into treated water and the concentrated water having the high ion concentration. Here, in the case where the pH is high in the first 20 desalination device 55A, silica exists on a surface of the reverse osmosis membrane 55a as ionic silica. More specifically, in the case where the concentration is 200 mg SiO2/L or higher, for example, silica can exist as the ionic silica when the pH is high. 25 In contrast, in the case where the pH is low in the first desalination device 55A, silica is precipitated as gelled silica. More specifically, in the case where the concentration is 200 mg SiO2/L or lower, for example, silica can exist as 30 the ionic silica when the pH is high. In the case where the concentration is 200 mg SiO2/L or lower, silica can be prevented from gelling (or gelling of silica can be delayed) by using the scale prevention 32 agent 74 when the pH is low. Further, in the case where the pH is high in the first desalination device 55A, calcium ions (Ca2+) may be precipitated on the surface of the reverse osmosis membrane 55a as a crystal CaCO3, but such precipitation can b5 e prevented by supplying the scale prevention agent 74 for Ca. Further, in the case where the pH is high in the first desalination device 55A, magnesium ions (Mg2+) may be 10 precipitated on the surface of the reverse osmosis membrane 55a as crystals Mg(OH)2, MgSiO3 in the high pH, but such precipitation can be prevented by supplying the scale prevention agent 74 for Mg. 15 In a control unit not illustrated, pH of the concentrated water 31a in the crystallization tank 61, obtained from the first desalination device 55A, is controlled to be a value (e.g., pH 4 or lower) at which the gypsum present in the concentrated water 31a can be 20 precipitated by decreasing the functions of the scale prevention agent 74. The pH-adjusted concentrated water 31a by the second pH adjuster 75B is stored in the crystallization tank 61. 25 In the case of providing a seed crystal supplying unit, the seed crystal supplying unit adds the seed crystal gypsum 32a of the seed crystal to the concentrated water 31a in the crystallization tank 61. By adding the second pH adjuster 75B, the functions of 30 the scale prevention agent 74 are deactivated in the crystallization tank 61. Due to this, the gypsum 32 supersaturated in the crystallization tank 61 is crystallized. In the case of separately inputting the seed 33 crystal gypsum 32a as the seed crystal in this crystallization process, the gypsum 32 is grown as the crystal by having this input seed crystal gypsum 32a as a nucleus. Here, a part of the gypsums 32 separated 5 ed by the dewatering device 63 is used as the seed crystal gypsum 32a. In the case of supplying the seed crystal gypsum 32a, the first pH adjustment by adding the first pH adjuster 75A 10 is not performed, and the pH permeating the first desalination device 55A may be set as the alkaline side. In this case, purity of the gypsum 32 is slightly more deteriorated than the case where pH is adjusted by adding the acid which is the first pH adjuster 75A. When the pH 15 is set to the alkaline side, a crystal of calcium carbonate (CaCO3) is generated. Therefore, purity is deteriorated because calcium carbonate (CaCO3) is mixed in the gypsum (CaSO4). The gypsum 32 having low water content and high purity 20 can be precipitated by adjusting pH to the predetermined value in the second pH adjustment process in which the acid is added as the second pH adjuster 75B like the present embodiment and by adding the seed crystal gypsum 32a in the crystallization process. 25 Note that silica present in the concentrated water 31a gels in the low pH, and reacts with Ca2+ and Mg2+ present in the concentrated water 31a, thereby forming a reactant of for example CaSiO3 or MgSiO3 and precipitating the same. Here, FIGS. 10 and 11 are microscope photographs of 30 the gypsum obtained in crystallization. FIG. 10 is an observation result in the case of adding the seed crystal gypsum 32a which is the seed crystal as a condition. FIG. 11 is an observation result in the case of not adding the 34 seed crystal gypsum 32a which is the seed crystal as a condition. As illustrated in FIG. 10, in the case of adding the seed crystal gypsum 32a, large gypsum is precipitated. Generally, the larger the precipitated gypsum is, the 5 less the water content is. When an average particle diameter is 10 m or more, preferably, 20 m or more, the gypsum having the water content sufficiently reduced can be obtained. Here, the “average particle diameter” in the present 10 invention indicates a particle diameter measured by a method specified in JISZ8825 (laser diffraction method). Judging from the results of FIGS. 10 and 11, the highly-purified gypsum having the low water content can be precipitated by adjusting the pH in the second pH 15 adjustment process and adding the seed crystal in the crystallization process. The more the adding amount of the seed crystal is (the higher the concentration of the seed crystal in the crystallization tank 61 is), the faster a speed of precipitation of the gypsum 32 is. The adding 20 amount of the seed crystal gypsum 32a which is the seed crystal is suitably set based on retention time in the crystallization tank 61, the concentration of the scale prevention agent 74, and the pH. Further, the gypsum 32 having the average particle 25 diameter of 10 m or more, preferably 20 m or more, is separated from the concentrated water 31a by the liquid cyclone 62 which is the separating unit. A part of the gypsum 32 recovered at the dewatering device 63 disposed adjacent to the liquid cyclone 62 is stored in a seed 30 crystal tank (not illustrated) via a seed crystal circulation unit not illustrated. A part of the recovered gypsum 32 is supplied to the crystallization tank 61 as the seed crystal gypsum 32a. 35 Here, acid treatment may be applied to the stored gypsum 32 in the seed crystal tank. In the case where the scale prevention agent 74 adheres to the gypsum 32 separated by the dewatering device 63, the function of the adhering scale prevention agent is decreased by the 5 acid treatment. A kind of the acid used here is not limited, but sulfuric acid is optimal in consideration of power reduction at the second desalination device 55B. The gypsum 32 crystallized in the crystallization tank 10 61 has wide particle diameter distribution, but since the gypsum 32 having the particle diameter 10 m or more is separated and recovered from the concentrated water 31a at the liquid cyclone 62, the large gypsum can be utilized as the seed crystal. When the large seed crystal is input, the 15 large gypsum can be crystallized. In other words, the gypsum having high quality can be obtained with a high recovery rate. Further, the large gypsum can be more easily separated at the liquid cyclone 62, thereby achieving to downsize the liquid cyclone 62 and further leading to power 20 reduction. The large gypsum is more easily dewatered by the dewatering device 63, thereby capable of downsizing the dewatering device 63 and further leading to power reduction. Here, since the water treatment system in FIG. 3 is an 25 open system except for the reverse osmosis membrane device, mixed water 111 and concentrated water 31a contact with the air, and carbonate ions are dissolved in the water. However, as described above, the mixed water 111 and concentrated water 31a are adjusted in the first pH 30 adjustment process and second pH adjustment process so as to have the pH area at which solubility of calcium carbonate is high. The carbonate ions in the concentrated water is reduced in a step prior to the crystallization 36 tank 61 or a step in the crystallization tank 61, and calcium carbonate has saturation solubility or lower. Further, since the pH is in the lower area by adding the acid as the pH adjuster 75, an environment having the low concentration of carbonate ions is made in accordance 5 with the following balanced equation (1). Due to this, the concentration of calcium carbonate is kept sufficiently lower than the saturating concentration and calcium carbonate is not crystallized in the crystallization tank 10 61. Therefore, calcium carbonate is little contained in the recovered gypsum 32. Thus, the gypsum 32 is highly purified. CO2+H2OH2CO3HCO3 -+H+CO3 2-+2H+ (1) Further, salt containing metals has high solubility in 15 the acidic area. In the case where the metals remain in the mixed water 111 even after passing the pretreatment (sedimentation tank 53), hydroxide containing the metals is not precipitated in the crystallization process if the pH of the concentrated water 31a at the first desalination 20 device 55A is lowered in the first pH adjustment process as described above. Further, in the case where the mixed water 111 has the property containing a large amount of Fe ions, Fe concentration is reduced through the above-described pretreatment. Therefore, hydroxide containing Fe(OH)3 25 hardly deposits in the crystallization tank 61. Thus, by using the water treatment method and the water treatment system of the present embodiment, it is possible to separate and recover the highly-purified gypsum 32, as valuables, which hardly contains impurities such as 30 calcium carbonate and metal hydroxide present in the mixed water 111 including the effluent 22 discharged from the cooling tower 21. Here, in the case of crystallizing the large gypsum 32 37 having the average particle diameter of 10 m or more, preferably, 20 m or more, a crystallization speed is generally slowed, thereby extending retention time in the crystallization tank 61. According to the present embodiment, the pH is adjusted so as to 5 decrease the functions of the scale prevention agent 74, and also the concentration of the seed crystal is increased so as to secure the appropriate crystallization speed. 10 The concentrated water 31a containing the gypsum 32 is discharged from the crystallization tank 61, and fed to the liquid cyclone 62 which is the separating unit, and the gypsum 32 is separated from the discharged concentrated water 31a. The gypsum 32 having the average particle 15 diameter of 10 m or more deposits on the bottom portion of the liquid cyclone 62, and the gypsum having the small particle diameter remain in the supernatant liquid. The gypsum 32 having deposited on the bottom portion of the liquid cyclone 62 is transferred to the dewatering device 20 63, and further dewatered and recovered. The highlypurified gypsum 32 having low water content and containing no impurity can be separated and recovered at the high recovery rate by the recovery process. According to the present embodiment, crystallization is performed by adding 25 the seed crystal. Therefore, the gypsum 32 having the average particle diameter of 10 m or more is mainly precipitated, and ratio of the gypsum having small diameter is reduced. Here, a separated liquid 64 separated at the dewatering device 63 is supplied to the spray drying device 30 23 so as to be subjected to spray drying. Further, instead of supplying the separated liquid to the spray drying device 23 to be subjected to spray drying, 38 the separated liquid 64 may be introduced to the discharged concentrated water 31a contained in the liquid cyclone 62, and may be treated at the second desalination device 55B together with the concentrated water 31a. In the case of omitting 5 itting the liquid cyclone 62 which is the separating unit as a modified example of the present embodiment, depositing-side concentrated water is discharged from the bottom portion of the crystallization tank 61. The large crystallized gypsum 32 deposits in the 10 concentrated water on the bottom portion of the crystallization tank 61. The highly-purified gypsum 32 can be recovered by dewatering the concentrated water mainly containing the large gypsum 32 at the dewatering device 63. Further, since the gypsum 32 has the low water content, it 15 is not necessary to enlarge the volume of the dewatering device 63. The concentrated water 31a on the supernatant side discharged from the liquid cyclone 62 is fed to the 20 sedimentation tank 53 and the filtering device 54. In the same process as the above-described sedimentation tank 53 and filtering device 54, the gypsum 32 and calcium carbonate remaining in the concentrated water after separation process and the metal hydroxide having remained 25 in the concentrated water are removed. The concentrated water 31a discharged from the filtering device 54 is fed to the second desalination device 55B. The scale prevention agent 74 may be further added to the concentrated water 31a before the concentrated 30 water is flown into the second desalination device 55B. Further, after adding the scale prevention agent 74, acid or alkali which is the pH adjuster 75 may be supplied to the concentrated water 31a. 39 In the second desalination device 55B, the concentrated water 31a obtained from the first desalination device 55A is treated. The water which has permeated through the reverse osmosis membrane 55b of the second desalination device 55B is recovered 5 ed as the permeated water and as the reclaimed water 33b. The concentrated water 31b of the second desalination device 55B is introduced to the spray drying device 23, and subjected to spray drying here. In the case of disposing the second desalination 10 device 55B, the reclaimed water 33b can be further recovered from the concentrated water 31a on the supernatant liquid side after crystallizing the gypsum 32, thereby more improving a water recovery rate. Since the gypsum 32 is removed from the concentrated 15 water 31a obtained from the first desalination device 55A by the treatment at the crystallization tank 61, the ion concentration is lowered. Therefore, the required power is reduced because osmotic pressure can be more reduced compared to the case where the gypsum 32 is not removed at 20 the second desalination device 55B. Further, an evaporator (not illustrated) may be provided as well. The water is evaporated from the concentrated water at the evaporator, and the ions contained in the concentrated water are precipitated as 25 solids and then recovered as solids. Since the water is recovered on the upstream side of the evaporator and an amount of the concentrated water is largely reduced, the evaporator can be formed compact and energy required for evaporation can be reduced. 30 According to the present embodiment, “a desalination/crystallization device” is used as the desalination device, which includes the first desalination device 55A that removes salt after introducing the scale 40 prevention agent 74 into the mixed water 111 obtained by mixing the separated water 43 with the effluent 22 from the cooling tower 21, the crystallization tank 61 that crystallizes the gypsum 32 after the first desalination device 55A, and the liquid cyclone 62 that separates 5 ates the crystallized gypsum 32. However, the present invention is not limited thereto. Here, the gypsum 32 crystallized in the crystallization tank 61 of the desalination device 30 is 10 discharged via a gypsum discharge line L22 as illustrated in FIG. 1, and the reclaimed water 33 (33a, 33b) joins to the return line L32 that returns the separated water 43 to the wet desulfurization device 16 via a reclaimed water supply line L23, and is utilized as the makeup water for 15 the gypsum slurry used in the wet desulfurization device 16. As another embodiment of the desalination device besides the desalination/crystallization device illustrated in FIG. 3, a separation device using the cold lime process 20 illustrated in FIG. 12 may be adopted. In FIG. 12, an example of the separation device based on the cold lime process is schematically illustrated. As illustrated in FIG. 12, the desalination device based on the cold lime process adds calcium hydroxide 25 (Ca(OH)2) 92 to the mixed water 111 at an adding tank 91, calcium carbonate (CaCO3) 94 is made to settle out in a settling tank 93 and then removed. Next, sodium carbonate (NaCO3) 96 is added at an adding tank 95 to make calcium carbonate (CaCO3) 94 settle 30 out in a settling tank 97 and be removed. After that, the ferrous flocculant (e.g., FeCl3) 73 is added to flocculate suspended solid content (e.g., buoyant solids such as gypsum, silica, calcium carbonate, and 41 magnesium hydroxide). Then, same as the operation illustrated in FIG. 3, membrane separation treatment is performed by introducing the scale prevention agent 74 and the pH adjuster 75 at the time of performing treatment in 5 the first desalination device 55A. There are other processes that can be exemplified, such as an optimized pretreatment and unique separation (OPUS) process (by Veolia Group) in which a chemical softening unit (chemical softening) is applied after 10 performing degassing and oil content removal with respect to water to be treated, and then treatment using a reverse osmosis membrane is performed after filtering suspended solid particles like metals; and a high efficiency reverse osmosis (HERO) process (by GE) in which, for example, Ca 15 and Mg are removed from the water to be treated using the chemical softening agent or ion-exchanger resin, subsequently acid is added to adjust the pH to the acidic side, CO2 gas is separated, and treatment by a reverse osmosis membrane is performed while preventing 20 precipitation by adjusting the pH to the alkaline side for ionizing. Further, as a membrane separation unit, an “RO membrane” is used in the first and second desalination devices 55A, 55B of the present embodiment, but an “NF 25 membrane” may also be used. In the case of using the NF membrane, a bivalent ion can be removed but a univalent ion cannot be completely removed same as the RO membrane. Therefore, for example, the reclaimed water cannot be supplied to desulfurization 30 makeup water at the desulfurization device, and preferably, is supplied to the cooling tower and utilized as feed water therein. The reason is that the scale prevention agent 74 cannot be removed by the NF membrane. 42 According to the water treatment system of the present embodiment, bivalent metals (e.g., calcium salt, magnesium salt, etc.) contained in the mixed water 111, and sulfate ions and carbonate ions can be efficiently separated. Further, in the case of using the RO membrane, barium 5 arium salt and strontium salt can be removed in addition to calcium salt and magnesium salt. According to the present embodiment, an amount of waste water (before concentration) which can be subjected 10 to spray drying can be remarkably increased by concentrating the mixed water 111 by using a desalination device 30A as illustrated in FIG. 3. For example, 100/(100 - 95) = 20 times of the waste water can be eliminated by using the desalination crystallization device when the 15 recovery rate of the reclaimed water thereof is 95%. FIG. 4 is a configuration diagram illustrating an example of a different desalination device according to the present embodiment. In the desalination device 30A illustrated in FIG. 3, the sedimentation tank 53 and the 20 filtering device 54 are disposed in the upstream side of the first desalination device 55A, and the metal content present in the mixed water 111 are made to deposit and removed as metal hydroxide, and calcium content are made to deposit and removed as calcium carbonate. However, this 25 pretreatment may be omitted in the present invention. As illustrated in FIG. 4, in the desalination device 30B of the present embodiment, the first desalination device 55A, crystallization tank 61, liquid cyclone 62, and second desalination device 55B are disposed, and scale 30 adhesion to the reverse osmosis membranes 55a, 55b of the first and second desalination devices 55A, 55B is prevented by adding the scale prevention agents 74 on the upstream sides of the first and second desalination devices 55A, 55B 43 respectively. Note that acid (e.g., sulfuric acid, etc.) and an alkaline agent (e.g. sodium hydroxide, calcium hydroxide, etc.) are added as the pH adjuster 75. The pretreatment is not required depending on the type of mixed water 111, such that a configuration 5 of the desalination device is simplified. As the mixed water 111 to be treated in such a simplified desalination device 30B, effluent having low concentration of carbonate ion may be exemplified. 10 Further, effluent having low concentration of scale content such as Ca2+ and Mg2+ may be applied, too. Here, there are acid and alkaline agents as the pH adjuster 75. As the acid used to lower the pH, general pH adjuster such as hydrochloric acid, sulfuric acid, and 15 citric acid may be exemplified. Further, as the alkaline agents used to raise the pH, the general pH adjuster such as sodium hydroxide and calcium hydroxide may be exemplified. FIG. 5 is a configuration diagram illustrating an 20 example of a different desalination device according to the present embodiment. Further, as a desalination device 30C illustrated in FIG. 5, a third desalination device 55C may be additionally disposed on the downstream of the concentrated water side 25 of the second desalination device 55B so as to apply threestep desalination. In the case where the third desalination device 55C including a reverse osmosis membrane 55c is disposed, reclaimed water 33c can be further recovered from the 30 concentrated water 31b, thereby improving a water recovery rate to 97%. Meanwhile, pretreatment units of the sedimentation tank 53 and the filtering device 54 illustrated in FIG. 3, and addition of the scale prevention 44 agent 74 and the pH adjuster 75 are provided between the second desalination device 55B and the third desalination device 55C, but in FIG. 5, these components are omitted. FIG. 6 is a configuration diagram illustrating an example of a different 5 erent desalination device according to the present embodiment. Further, a deaeration unit 50 which is a carbonic acid gas separation unit that separates carbonic acid gas is provided on the upstream side of the adding tank 52 like a 10 desalination device 30D illustrated in FIG. 6. More specifically, the deaeration unit 50 is a deaeration tower including filler to disperse carbon dioxide, or a separation membrane. In the desalination device 30D of FIG. 6, the mixed 15 water 111 before flowing into to the deaeration unit 50 is adjusted to the low pH. Carbonic acid present in the mixed water 111 is held in equilibrium in accordance with its pH. In the case where the pH is low, like pH 6.5 or lower, carbonic acid in the mixed water 111 is present mainly in states of HCO3 20 - and CO2. The mixed water 111 containing CO2 flows into the deaeration unit 50. CO2 is removed from the mixed water 111 at the deaeration unit 50. According to the present embodiment, same as FIG. 5, three-step desalination is performed by additionally 25 disposing the third desalination device 55C on the downstream of the concentrated water side of the second desalination device 55B. Second Embodiment Next, a water treatment system according to a second 30 embodiment will be described. FIG. 7 is a schematic diagram illustrating the water treatment system according to the present embodiment. As for components same as a water treatment system of a first embodiment, repetition of the 45 same description will be omitted by denoting the components by the same reference signs. As illustrated in FIG. 7, the water treatment system according to the present embodiment is provided with an oxidation-reduction potential (ORP) 5 ) meter 130 that measures oxidation-reduction potential of mixed water 111 to be introduced into a first desalination device 55A. Further, in the case where a value of the oxidationreduction potential of the mixed water 111 to be introduced 10 into the first desalination device 55A is outside a range of a predetermined value, oxidizer 132 is supplied from an oxidizer supplying unit 131. Mercury may remain inside the separated water 43 although mercury is removed from separated water 43 15 separated from desulfurization waste water in a previous step before being mixed at a mixing unit 110. Since the remaining mercury exists in various forms, removal by an RO membrane can be performed in the case where the mercury is in an ionic state. In the case of 20 metallic mercury, the mercury is non-polar and liquid form and permeates through the RO membrane although not permeating the membrane to a permeated water side. Therefore, a form of mercury is transformed to the ionic state by controlling the value of the oxidation25 reduction potential in the mixed water 111 within the predetermined range so as to remove the mercury by the RO membrane. Here, preferably, the value (X) of the oxidationreduction potential in the mixed water 111 measured by the 30 oxidation-reduction potential meter 130 is in the range of -0.69 V < ORP value (X) < 1.358 V. The reason is that it is not preferable that chloride Cl- is transformed to chlorine gas (Cl2) in the case where 46 the ORP value (X) exceeds “1.358 V”. Further, a lower limit value of the ORP value (X) is varied depending on a coexisting substance, and the lower limit of the ORP is -0.69 V or higher, preferably 0.2680 V or higher, more preferably 0.6125 V or higher, even 5 more preferably 0.796 V or higher. Most of salts contained in waste water are Cl- and SO4 2-. Hg(l) is oxidized to Hg2SO7 (solid) in the case of + 0.2680 V or higher, and oxidized to Hg2Cl2 (solid) in the 10 case of + 0.6125 V or higher. Meanwhile, S2- and I-, Br- are also contained in the waste water, but amounts thereof are smaller compared to Cl- and SO4 2-. Further, on the other hand, in the case where no impurity is contained, + 0.798 V is required to oxidize Hg(l) to Hg2 15 2+. The reason for setting an upper limit value at 1.358 V is that standard electrode potential for oxidation from Clto Cl2 is + 1.3583 V. Meanwhile, the upper limit value is at least + 1.3583 V or lower, preferably, + 0.89 V or lower 20 because reaction from Cl- to ClO- requires + 0.89 V. Therefore, the oxidizer 132 is input by ORP control so as to make a situation in which mercury is not reduced. As a result, mercury is kept not in metallic mercury but in mercury oxide, and chlorine gas which is repellent 25 to the RO membrane is kept in a form of chloride ions. Therefore, operation without damaging the RO membrane can be performed while removing mercury. Here, preferably, air is used as the oxidizer 132. By using the air as the oxidizer 132, oxidation can be 30 performed under mild conditions while preventing the RO membrane from being damaged. Further, after supplying the oxidizer 132, a solidliquid separation device 133 is disposed on the upstream 47 side of the first desalination device 55A so as to separate solidified mercury (e.g., mercury chloride (HgCl2), etc.) present in the mixed water 111. By this, the solidified mercury (e.g., mercury chloride (HgCl2), etc.) is prevented from adhering to a surface of the RO membrane, 5 e, thereby capable of suppressing deterioration of desalination capacity. Third Embodiment Next, a water treatment system according to a third 10 embodiment will be described. FIG. 8 is a schematic diagram illustrating the water treatment system according to the present embodiment. As for components same as a water treatment system of the first embodiment, repetition of the same description will be omitted by denoting the components 15 by the same reference signs. As illustrated in FIG. 8, the water treatment system according to the present embodiment is provided with an ultrafiltration membrane (UF) membrane 122 and an NF membrane device 121 including a nano-filtration membrane 20 (NF membrane) 121a on a downstream side of a solid-liquid separation unit 103 which separates heavy metal sludge 104 and on an upstream side of a mixing unit 110. According to the present embodiment, desalination is performed by the membrane after removing/separating mercury 25 at the solid-liquid separation unit 103 by adding a chelating agent 102. Preferably, the membrane for this membrane treatment is the NF membrane 121a. A part of permeated liquid 123 from the NF membrane 121a is returned as returned water 123a to a wet 30 desulfurization device 16 via a return line L36 and also concentrated liquid 124 from the NF membrane 121a is fed to an introduction line L24 side via a concentrated liquid line L25, and then mixed at a mixing unit 110 with effluent 48 22 from a cooling tower 21 to obtain mixed water 111. By performing desalination at the NF membrane 121a, multivalent ions (e.g., Ca2+, Mg2+, SO4 2-) can be concentrated on the concentrated liquid 124 side, and univalent ions (e.g., Na+, Cl-) can be concentrated on 5 the permeated liquid 123 side. Here, salts contained in waste water from the wet desulfurization device 16 is mainly calcium chloride (CaCl2). Therefore, in the concentrated liquid 124, material of gypsum (e.g., Ca2+, SO4 10 2-) is concentrated by gypsum crystallization in the later stage by performing membrane separation by the NF membrane 121a having univalent selectivity while Cl- which is a load to an RO membrane at a first desalination device 55A in a later 15 stage can be reduced. This can reduce concentration of soluble evaporation residues (Total dissolved solid(s); TDS) supplied to the first desalination device 55A in the later stage, and also can improve a concentration rate. 20 By this, concentrated water 31a from the first desalination device 55A can be reduced. As a result, a spray drying device 23 to spray-dry the concentrated water can be downsized. Meanwhile, a scale inhibitor may be added in order to 25 prevent clogging at the desalination device caused by the NF membrane 121a. According to the present embodiment, the multivalent ions can be selectively concentrated by additionally desalinating separated water 43 by the NF membrane 121a 30 after mercury is treated with the chelating agent 102. This improves the concentration rate of the RO membrane at the first desalination device 55A and leads to downsizing of the spray drying device 23. 49 Main substances having deliquescence are CaCl2 and MgCl2, and divalent cations (Ca2+, Mg2+) thereof and chloride ions Cl- can be separated by the NF membrane 121a. Therefore, a problem related to deliquescence of dried salt 5 at the spray drying device 23 can be reduced. In the concentrated water 124 at the NF membrane 121a, the multivalent ions becomes rich, and the concentrated water is concentrated by the RO after recovering and crystallizing gypsum at a crystallizer, and then the RO 10 concentrated water can be subjected to spray drying at the spray drying device 23. Fourth Embodiment Next, a water treatment system according to a fourth embodiment will be described. FIG. 9 is a schematic diagram 15 illustrating the water treatment system according to the present embodiment. As for components same as a water treatment system of the first embodiment, repetition of the same description will be omitted by denoting the components by the same reference signs. 20 As illustrated in FIG. 9, the water treatment system according to the present embodiment includes: a wet desulfurization device 16 that removes sulfur oxide present in the boiler flue gas 12 obtained from the boiler 11; a dewatering device 42 that separates gypsum 32 from 25 desulfurization waste water 41 containing gypsum slurry obtained from the wet desulfurization device 16; a mixing unit 110 that mixes separated water 43 obtained from the dewatering device 42 with effluent 22 generated in a plant facility; a reaction tank 101 in which the mixed water 111 30 obtained by the mixing unit 110 is introduced and in which a chelating agent 102 is input so as to immobilize heavy metals present in the separated water 43; a solid-liquid separation unit 103 that performs solid-liquid separation 50 with respect to solid content (heavy metal sludge) 104 present in the mixed water 111 obtained from the reaction tank 101; a desalination device 30 that removes salt content from the mixed water 111 subjected to solid-liquid separation; and a spray drying device 23 that includes 5 cludes a spray unit, which sprays the concentrated water 31 in which salt content has been concentrated by the desalination device 30, and that performs spray drying using a part 12a of the boiler flue gas 12. 10 By mixing the effluent 22 from the cooling tower 21 with the separated water 43, impurity contained in the effluent 22 from the cooling tower 21 can be removed by using the chelating agent 102. Different from the treatment system of the first 15 embodiment, concentration of salt is decreased and a load to an RO membrane can be reduced because an amount of water introduced into a first desalination device 55A is increased. Fifth Embodiment 20 Next, a water treatment system according to a fifth embodiment will be described. FIG. 13 is a schematic diagram illustrating the water treatment system according to the present embodiment. As for components same as a water treatment system of the first embodiment, repetition 25 of the same description will be omitted by denoting the components by the same reference signs. As illustrated in FIG. 13, in the water treatment system according to the present embodiment, a chelating agent 102 is added into concentrated water 31 supplied to a 30 spray drying device 23. According to the present embodiment, the chelating agent 102 to immobilize heavy metals remaining in the concentrated water 31 is added from a chelating agent 51 adding unit not illustrated to a line L21 that supplies the concentrated water 30 obtained from the desalination device 30 to the spray drying device 23. According to the present embodiment, dried salt present in flue gas 12b generated at the spray 5 y drying device 23 is sufficiently mixed with the chelating agent 102 by mixing the chelating agent 102 into the concentrated water 30 before the concentrated water is supplied to the spray drying device 23. 10 After that, solid content 141 separated at a solid content separator 140 is landfilled as it is. Here, in the case of adding the chelating agent 102, the chelating agent is needed to be handled at a heatproof temperature thereof or lower. Therefore, preferably, a 15 temperature (T1) of the flue gas 12b at the time of the spray drying device 23 finishing drying is kept 200C or lower, preferably 150C or lower. Note that a temperature of sprayed droplets while drying is about 80C at the spray drying device 23, and 20 does not rise any higher. Therefore, a temperature at the time of start drying is not limited. Meanwhile, a temperature (T2) of branched flue gas 18a at an entrance to the spray drying device 23 is, for example, about 350C. Therefore, the chelating agent 102 is prevented from being 25 deteriorated by changing one or both of an evaporation amount of dehydration filtrate and an introducing flow rate of the branched discharge flue gas 12a and keeping the temperature (T1) of the flue gas 12b at 200C or lower, preferably 150C or lower. As a result, heavy metals are 30 prevented from being eluted at the time of landfilling the solid content 141. According to the present embodiment, the heavy metals 52 such as mercury etc. from the solid content 141 can be prevented from being eluted by performing drying at the spray drying device 23 after adding the chelating agent 102 which immobilizes the heavy metals present in the 5 concentrated water 30. Also, flocculant may be further added together with the chelating agent 102. As the flocculant, coagulant which forms a nucleus of a solid, and polymer flocculant which increases flocs of 10 solids can be used. Here, aluminum sulfate, iron chloride, PAC, etc. may be exemplified as the coagulant. Further, as the polymer flocculant, “Taki floc (trade name; manufactured by Taki Chemical Co., Ltd.) anionic, nonionic, cationic, 15 amphoteric)”, “Epo floc L-1(trade name); manufactured by JIKCO Ltd.”, etc. may be exemplified. Reference Signs List 11 Boiler 12 Boiler flue gas (flue gas) 20 18 Flue gas treatment system 21 Cooling tower 22 Effluent 23 Spray drying device 30 Desalination device 25 31 (31a to 31c) Concentrated water 33 (33a to 33c) Reclaimed water 55A to 55C First to third desalination devices 61 Crystallization tank 62 Liquid cyclone 30 74 Scale prevention agent 75 pH adjuster 101 Reaction tank 53 102 Chelating agent 103 Solid-liquid separation unit 104 Heavy metal sludge 110 Mixing unit 111 5 Mixed water We Claim: 1. A water treatment system comprising: a wet desulfurization device that removes sulfur oxide from boiler flue gas; a dewatering device that separates gypsum fro5 m desulfurization waste water which contains gypsum slurry and which is obtained from the wet desulfurization device; a mercury removing unit to which is introduced 10 separated water that is obtained from the dewatering device, and which immobilizes heavy metals present in the separated water by inputting a chelating agent; a solid-liquid separation unit that performs solidliquid separation with respect to solid content present 15 in separated water obtained from the mercury removing unit; a mixing unit that mixes separated water obtained from the solid-liquid separation unit with effluent generated in a plant facility; 20 a desalination device that removes salt content from mixed water obtained by the mixing unit; and a spray drying device that includes a spraying unit, which sprays concentrated water in which salt content has been concentrated by the desalination 25 device, and that performs spray drying using a part of the boiler flue gas. 2. A water treatment system comprising: a wet desulfurization device that removes sulfur oxide from boiler flue gas; 30 a dewatering device that separates gypsum from desulfurization waste water which contains gypsum slurry and which is obtained from the wet 55 desulfurization device; a mixing unit that mixes separated water obtained from the dewatering device with effluent generated in a plant facility; a reaction tank to which mixed water obtained 5 by the mixing unit is introduced and which immobilizes heavy metals present in separated water by inputting a chelating agent; a solid-liquid separation unit that performs solid10 liquid separation with respect to solid content present in mixed water obtained from the reaction tank; a desalination device that removes salt content from mixed water subjected to solid-liquid separation; and 15 a spray drying device that includes a spraying unit which sprays concentrated water in which salt content has been concentrated by the desalination device, and that performs spray drying using a part of the boiler flue gas. 20 3. The water treatment system according to claim 1 or 2, further comprising a membrane treatment unit that performs treatment using a membrane having univalent selectivity with respect to separated water obtained after separation performed by the solid-liquid 25 separation unit. 4. The water treatment system according to any one of claims 1 to 3, wherein the desalination device removes bivalent salt content present in the effluent. 5. The water treatment system according to any one of 30 claims 1 to 3, wherein the desalination device includes a scale prevention agent supplying unit that 56 supplies a scale prevention agent to the mixed water containing bivalent ions such as Ca ions; a first desalination device that is disposed at a downstream side of the scale prevention agent supplying unit and that separates the mixed water into 5 reclaimed water and concentrated water in which the Ca ions have been concentrated; a crystallization tank that is disposed at a downstream side of the first desalination device and 10 that is used in crystalizing gypsum from the concentrated water; a separating unit that separates the gypsum crystalized and concentrated water obtained from the first desalination device; and 15 a second desalination device that is disposed at a downstream side of the separating unit and separates the concentrated water into reclaimed water and concentrated water in which the Ca ions have been concentrated. 20 6. The water treatment system according to claim 5, wherein the desalination device further includes a separating unit that separates concentrated water obtained from the second desalination device; and a third desalination device that is disposed at a 25 downstream side of the separating unit and that separates the concentrated water into reclaimed water and concentrated water in which the Ca ions have been concentrated. 7. The water treatment system according to claim 5, 30 further comprising an oxidation-reduction potentiometer that measures the oxidation-reduction potential of mixed water to be introduced into the first 57 desalination device. 8. The water treatment system according to claim 7, wherein a value (X) of the oxidation-reduction potential of the mixed water, which is measured by the oxidation-reduction potentiometer, 5 satisfies - 0.69V