Water treatment system and water treatment

Abstract

WATER TREATMENT SYSTEM AND WATER TREATMENT ABSTRACT According to one embodiment, a water treatment system includes, a desalting separation unit that separates water to be treated delivered from a introduction unit into concentrated water and deionized water, a plurality of concentrating membrane separation units disposed in series, wherein the concentrated water is concentrated by a membrane to be separated into concentrated water having a higher degree of concentration and permeated water, a first measuring device for measuring a first flow rate of concentrated water, a second measuring device for measuring a second flow rate of the circulating water, and a control device that adjusts at least the delivered flow rate of the water to be treated based on the first flow rate and the second flow rate.

Description

FIELDEmbodiment described herein relate generally to a water treatment system and awater treatment method.BACKGROUNDIn recent years, legal regulations for realizing sound water circulation have beenstrengthened. Zero Liquid Discharge (ZLD) means that water is regenerated and used in afactory from the viewpoint of reducing the risk of water quality pollution, regeneratingwaste water, and reusing the waste water, and the discharged water discharged from thefactory to the outside is further reduced to zero to preserve the water environment. In orderto reduce the discharged water to zero, it is necessary to finally separate the dischargedwater into a solid content and deionized water by an evaporation method.The evaporation method is a method in which waste water is heated to generatewater vapor, the water vapor is cooled to obtain deionized water, and the water vapor isseparated into a solid content and deionized water. This method has an advantage that atwo-stage flash evaporation method, a multi-stage flash evaporation method, or the like isput into practical use, and deionized water having very high purity can be obtained.However, there is a disadvantage that the energy efficiency is low due to the need for a heatsource. Therefore, from the viewpoint of reducing energy consumption, it is required toreduce the amount of waste water treated by the evaporation method as much as possible byincreasing the degree of concentration of waste water as much as possible.In view of such a demand, a reverse osmosis (RO) membrane (hereinafter, may bereferred to as a "RO membrane") for separating fresh water from concentrated waste watercontaining (or dissolving) a solid content is used in a previous stage of the evaporationmethod. The desalination and concentration system using the RO membrane includes abasic process of pressure-introducing water to be treated into the RO membrane to obtaindeionized water which is water which has permeated through the RO membrane andconcentrated water which is concentrated without permeating through the RO membrane.Since deionized water can be reused, preferably the amount of deionized water is larger andthe amount of concentrated water is smaller.RO membranes have an effect of removing almost all ionic substances, fineparticles, organic substances, some dissolved gases, and the like, and are widely usedbecause it is not necessary to perform a discontinuous process such as regeneration unlessclogging or trouble occurs. In addition, in order to reduce the amount of concentrated water,a multi-stage RO membrane is used by connecting RO membranes.However, in the multi-stage RO membrane method, it is difficult to stably reducethe volume of waste water and recycle the water due to variation in the water quality of thewater to be treated, membrane fouling due to a hardness scale or the like, and clogging of apipe, a flow rate adjusting valve, and the like. This results in evaporating the concentratedwater that is not concentrated to the expected degree of concentration, resulting in anincrease in the thermal energy required for evaporation and an increase in the overalltreatment cost. In addition, the amount of deionized water is reduced, and the amount ofwater to be reused is reduced.SUMMARYAn object of the present invention is to provide a water treatment system and awater treatment method capable of stably obtaining concentrated water and deionized waterof expected amounts and qualities.A water treatment system according to an embodiment includes an introduction unit,a desalting separation unit, a plurality of continuation of concentrating membraneseparation units disposed in series, a first measuring device, a second measuring device, anda control device. The introduction unit delivers the water to be treated. The desaltingseparation unit separates the water to be treated delivered from the introduction unit intoconcentrated water and deionized water. In a plurality of continuation of concentratingmembrane separation units disposed in series, the concentrated water is concentrated by amembrane to be separated into concentrated water having a higher degree of concentrationthan the concentrated water and permeated water that permeated through the membrane, aconcentrating membrane separation unit disposed in first continuation among the pluralityof continuation of concentrating membrane separation units concentrates the concentratedwater separated by the desalting separation unit, concentrating membrane separation unitsother than the concentrating membrane separation unit disposed in first continuation amongthe plurality of continuation of concentrating membrane separation units concentrate theconcentrated water concentrated by the concentrating membrane separation unit disposed inthe immediately previous continuation and supply the permeated water separated from theconcentrated water to the concentrating membrane separation units disposed in theimmediately previous continuation or on the first continuation side rather than theimmediately previous continuation so as to merge with the permeated water separated bythe concentrating membrane separation unit disposed in the immediately previouscontinuation or on the first continuation side rather than the immediately previouscontinuation, and the concentrating membrane separation unit disposed in first continuationsupplies permeated water that is merged with the permeated water introduced from theconcentrating membrane separation unit disposed in the immediately after continuation oron final continuation side rather than the immediately after continuation and permeatedthrough a membrane of the concentrating membrane separation unit disposed in firstcontinuation to the introduction unit as circulating water. The first measuring devicemeasures the flow rate of the concentrated water introduced from the desalting separationunit to the concentrating membrane separation unit disposed in first continuation. Thesecond measuring device measures the flow rate of the circulating water introduced into theintroduction unit from the concentrating membrane separation unit disposed in firstcontinuation. The control device adjusts at least one selected from the group consisting of adelivered flow rate, a discharged flow rate, and a first merged flow rate, based on the flowrate measured by the first measuring device and the flow rate measured by the secondmeasuring device, wherein the delivered flow rate is a flow rate of the water to be treatedwhich is delivered from the introduction unit to the desalting separation unit, the dischargedflow rate is a flow rate of the concentrated water which is concentrated by the concentratingmembrane separation unit disposed in final continuation among the plurality of continuationof concentrating membrane separation units, and the first merged flow rate is a flow rate of,among the concentrated water concentrated by the concentrating membrane separation unitdisposed in final continuation, the concentrated water to be merged with the permeatedwater separated by the concentrating membrane separation unit disposed in finalcontinuation.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method of a first embodiment is applied;FIG. 2 is a conceptual diagram illustrating a detailed configuration example of theintroduction unit;FIG. 3 is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method according to a modificationexample 1 of the first embodiment is applied;FIG. 4 is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method according to a modificationexample 2 of the first embodiment is applied;FIG. 5 is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method of a second embodiment isapplied;FIG. 5A is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method of a modification example of thesecond embodiment is applied;FIG. 6 is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method of a third embodiment isapplied;FIG. 7 is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method of a fourth embodiment isapplied;FIG. 8 is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method of a fifth embodiment is applied;FIG. 9 is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method of a sixth embodiment isapplied;FIG. 10 is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method of a modification example 1 ofthe sixth embodiment is applied;FIG. 11 is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method of a seventh embodiment isapplied;FIG. 12 is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method of an eighth embodiment isapplied; andFIG. 13 is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method of a ninth embodiment isapplied.DETAILED DESCRIPTIONHereinafter, a water treatment system to which the water treatment method of eachembodiment of the present invention is applied is described with reference to the drawings.Note that, in the following description of all the embodiments, the same parts asthose already described are denoted by the same reference numerals, and redundantdescription is avoided.(First embodiment)A first embodiment will be described.FIG. 1 is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method of a first embodiment is applied.A water treatment system 1 includes an introduction unit 10, a desalting membraneseparation unit 20, a pressure reducing unit 30, a concentrating membrane separation unit1003, a concentrating membrane separation unit introduction flow rate measuring device1004, a concentrated water flow rate measuring device 1006, a circulating water flow ratemeasuring device 1008, a concentrated water flow rate adjusting device 1010, and a controldevice 1012.First, the introduction unit 10 is described.FIG. 2 is a conceptual diagram illustrating a detailed configuration example of theintroduction unit 10.The introduction unit 10 includes a liquid mixing tank 11, a pump 14 that introducesa water to be treated a introduced from a flow path 211 into the liquid mixing tank 11, and apressure increasing pump 15 that increases a pressure of the liquid (mixed water of thewater to be treated a described above and a circulating water d described later) stored in theliquid mixing tank 11 and delivers the liquid to the desalting membrane separation unit 20via a flow path 201.The liquid mixing tank 11 includes a water quality meter 16 for measuring the waterquality of a stored liquid, a stirrer 17 for stirring the stored liquid, and a stirring line 1001for circulating the stored liquid. In addition, in the stirring line 1001, a liquid mixing tankstirring pump 1002 for taking the liquid stored in the liquid mixing tank 11 into the stirringline 1001 is disposed.A flow path 204, which is a return line of the circulating water d from theconcentrating membrane separation unit 1003, is also connected to the liquid mixing tank11. As a result, the mixed water of the water to be treated a introduced from the flow path211 and the circulating water d introduced from the flow path 204 is stored in the liquidmixing tank 11. The liquid mixing tank 11 is further connected with a chemical solutionsupply line 12 into which a chemical solution is introduced and a gas supply line 13 intowhich a gas is introduced.The water quality meter 16 includes a pH meter that measures the pH of the mixedwater stored in the liquid mixing tank 11. In addition, the water quality meter 16 mayinclude a water level meter for measuring the water level of the mixed water stored in theliquid mixing tank 11, a conductivity meter for measuring conductivity, an ORP meter formeasuring oxidation-reduction potential, a zeta potential meter for measuring zeta potential,and a dissolved oxygen meter for measuring the amount of dissolved oxygen.The chemical solution introduced into the liquid mixing tank 11 via the chemicalsolution supply line 12 includes, for example, a pH adjusting agent, a disinfectant, an antiscaleagent, an anti-biofouling agent, and a membrane cleaner. The chemical solution isused depending on, for example, the water quality measured by the water quality meter 16,and the quality of material and performance of the RO membrane used in the desaltingmembrane separation unit 20 and the concentrating membrane separation unit 1003. Thechemical solution can also be used properly according to the time of steady operation andthe time of maintenance of the water treatment system 1.The pH adjusting agent includes, for example, alkali agents such as caustic soda andpotassium hydroxide. As a result, the introduction unit 10 adjusts the pH of the mixed waterin the liquid mixing tank 11 so that the pH of the circulating water d circulated from theconcentrating membrane separation unit 1003 to the liquid mixing tank 11 via the flow path204 and the pH of a concentrated water cn discharged from the concentrating membraneseparation unit 1003 to the outside of the concentrating membrane separation unit 1003 viaa flow path 1013 are both 2 or more and 6 or less.The gas introduced from the gas supply line 13 into the liquid mixing tank 11 is, forexample, air or carbon dioxide.The amount of dissolved oxygen in the water to be treated a is preferably increased,for example, to 2 mg / L or more. Therefore, the mixed water in the liquid mixing tank 11 isstirred by stirring the mixed water in the liquid mixing tank 11 with the stirrer 17 and / or byrepeatedly taking a part of the mixed water in the liquid mixing tank 11 into the stirring line1001 and returning to the liquid mixing tank 11 by the liquid mixing tank stirring pump1002.It is desirable that fine bubbles having an average diameter of 1000 (nm) or less becontained in a concentration of 1.0 x 106 (pieces / mL) or more in the concentrated water cndischarged from the concentrating membrane separation unit 1003 through the water to betreated a, the circulating water d, and the flow path 1013. Therefore, as the liquid mixingtank stirring pump 1002, for example, a fine bubble generation pump can be used.The pressure increasing pump 15 is started when it is detected by the water qualitymeter 16 that the water quality such as the pH of the mixed water in the liquid mixing tank11 has an appropriate value to be delivered to the desalting membrane separation unit 20,increases a pressure of the mixed water in the liquid mixing tank 11 so as to have theintroduction pressure and flow rate of the concentrating membrane separation unitcontrolled by the control device 1012, and delivers the mixed water to the desaltingmembrane separation unit 20.Note that the pressure increasing pump 15 is provided with an inverter 15a. Thecontrol device 1012 sends an input signal to the inverter 15a, and the inverter 15a outputs adriving frequency corresponding to the input signal to the pressure increasing pump 15. Thepressure increasing pump 15 rotates at a rotational speed corresponding to the drivingfrequency, and discharges the mixed water to the desalting membrane separation unit 20.The pressure for pressurizing is, for example, at least a pressure higher than theosmotic pressure of the mixed water and a pressure higher than the pressure at which thecirculating water d from the concentrating membrane separation unit 1003 can be returned,and the lower limit value is about 4 MPa. The upper limit value is 12 MPa, which is themaximum resisting pressure of an RO membrane module 21 of the desalting membraneseparation unit 20 described later.Next, the desalting membrane separation unit 20 will be described.Returning to FIG. 1, the desalting membrane separation unit 20 is disposed on thedownstream side of the introduction unit 10, that is, on the flow direction side of the mixedwater, and includes the RO membrane module 21.Although only one RO membrane module 21 is shown in FIG. 1 as an example, thedesalting membrane separation unit 20 can also include one or more RO membranemodules 21.The RO membrane module 21 includes a pressure-resistant sealed pressure vessel(hereinafter, simply referred to as a "pressure vessel") 22 of, for example, 5 MPa or more,and an RO membrane element 23 that partitions the inside of the pressure vessel 22 into afirst chamber 24 and a second chamber 25 is disposed in the pressure vessel 22.As the RO membrane element 23, for example, an RO membrane having a spiralshape or a hollow fiber shape can be used.The flow path 201 extending from the introduction unit 10 is connected to the firstchamber 24. Mixed water is introduced into the first chamber 24 from the introduction unit10 via the flow path 201. As described above, the mixed water is increased in pressure toabout 4 MPa to 12 MPa by the pressure increasing pump 15.Since this pressure is higher than the osmotic pressure of the mixed waterintroduced from the introduction unit 10, in the RO membrane module 21, a deionizedwater b of the mixed water introduced from the introduction unit 10 efficiently permeatesthrough the RO membrane element 23 and reaches the second chamber 25 side, and theconcentrated dissolved solid content (salt content) remains on the first chamber 24 side. Inthis way, the RO membrane module 21 separates the mixed water into the deionized water bon the second chamber 25 side and a concentrated water c0 on the first chamber 24 side.A flow path 210 leading to the outside of the desalting membrane separation unit 20is connected to the second chamber 25, and the deionized water b flows in the flow path210 from the second chamber 25 and is discharged to the outside of the desalting membraneseparation unit 20.A flow path 202 is connected to the first chamber 24, and the concentrated water c0generated in the first chamber 24 flows in the flow path 202 and is discharged from thedesalting membrane separation unit 20. The flow path 202 is provided with theconcentrating membrane separation unit introduction flow rate measuring device 1004.The concentrating membrane separation unit introduction flow rate measuringdevice 1004 includes a flow meter 1005 that measures the flow rate of the concentratedwater c0 flowing in the flow path 202. The flow meter 1005 can be, for example, anultrasonic flow meter or an electromagnetic flow meter regardless of the type as long as theflow rate of the solution can be measured. The concentrating membrane separation unitintroduction flow rate measuring device 1004 also has a communication function such as awireless LAN for transmitting a measurement result measured by the flow meter 1005 tothe control device 1012.The concentrating membrane separation unit introduction flow rate measuringdevice 1004 is connected to a flow path 203 leading to the concentrating membraneseparation unit 1003, and the concentrated water c0 whose flow rate is measured by the flowmeter 1005 flows in the flow path 203 toward the concentrating membrane separation unit1003.Next, the pressure reducing unit 30 will be described.The flow path 203 is provided with a pressure reducing unit 30. The pressurereducing unit 30 is preferably disposed on the concentrating membrane separation unit 1003side rather than the concentrating membrane separation unit introduction flow ratemeasuring device 1004, but may be provided on the desalting membrane separation unit 20side rather than the concentrating membrane separation unit introduction flow ratemeasuring device 1004, that is, in the flow path 202.The pressure reducing unit 30 includes a pressure reducing device 31 such as apressure reducing valve, and rapidly reduces the pressure of the concentrated water c0 whenthe high-pressure concentrated water c0 from the first chamber 24 flowing in the flow path203 passes. The amount of reduced pressure is preferably 0.5 MPa or more. The pressure ofthe concentrated water c0 from the first chamber 24 flowing in the flow path 203 is reducedby 0.5 MPa or more to increase the amount of fine bubbles generated in the concentratedwater c0, and the effect of preventing scale and biofouling in the concentrating membraneseparation unit 1003 on the downstream side can be enhanced.Next, the concentrating membrane separation unit 1003 will be described.The concentrating membrane separation unit 1003 is configured by disposing aplurality of continuation of concentrating RO membrane separation units 40 to 50 havingthe same configuration in series. Note that, in the present specification, the term"continuation" is used to distinguish the disposition order of the plurality of concentratingRO membrane separation units disposed in series. Therefore, in the concentratingmembrane separation unit 1003, the concentrating RO membrane separation unit disposedon the most upstream side along the direction in which a concentrated water c flows isreferred to as a "first continuation" concentrating RO membrane separation unit 40, and theconcentrating RO membrane separation unit disposed on the most downstream side isreferred to as a "final continuation" concentrating RO membrane separation unit 50. Notethat, in FIG. 1, as an example, only two concentrating RO membrane separation unit, thatis, the first continuation concentrating RO membrane separation unit 40 and the finalcontinuation concentrating RO membrane separation unit 50 are illustrated. However, anarbitrary number of the intermediate continuous concentrating RO membrane separationunit (not illustrated) may be disposed in series between the first continuation concentratingRO membrane separation unit 40 and the final continuation concentrating RO membraneseparation unit 50.The first continuation concentrating RO membrane separation unit 40 includes anRO membrane module 41 having a configuration similar to that of the RO membranemodule 21.The RO membrane module 41 includes a pressure vessel 42, and an RO membraneelement 43 that partitions the inside of the pressure vessel 42 into a first chamber 44 and asecond chamber 45 is disposed in the pressure vessel 42. As the RO membrane element 43,for example, an RO membrane having a spiral shape or a hollow fiber shape can be used.The flow path 203 is connected to the first chamber 44. Therefore, the concentratedwater c0 flowing in the flow path 203 is introduced into the first chamber 44. The sum ofthe pressure and the osmotic pressure of the concentrated water c0 introduced into the firstchamber 44 is higher than the sum of the pressure and the osmotic pressure of a mergedwater gi introduced into the second chamber 45. Note that the definition of the mergedwater will be described later. Therefore, in the first chamber 44, the water componentcontained in the concentrated water c0 permeates through the RO membrane element 43 andreaches the second chamber 45. This water is referred to as a permeated water t1.Accordingly, in the first chamber 44, a concentrated water c1 in which the concentratedwater c0 is further concentrated is obtained.A flow path 205 is further connected to the first chamber 44, and the concentratedwater c1 obtained in the first chamber 44 flows in the flow path 205 and is discharged fromthe first continuation concentrating RO membrane separation unit 40.The flow path 205 is further connected to the first chamber of the concentrating ROmembrane separation unit disposed in the immediately after continuation. Therefore, theconcentrated water c1 obtained in the first chamber 44 flows in the flow path 205 and isintroduced into the first chamber of the concentrating RO membrane separation unitdisposed in the immediately after continuation. FIG. 1 particularly shows an example inwhich the concentrating RO membrane separation unit disposed in the immediately aftercontinuation is the final continuation concentrating RO membrane separation unit 50.The final continuation concentrating RO membrane separation unit 50 also includesan RO membrane module 51 similar to the RO membrane module 41 of the firstcontinuation concentrating RO membrane separation unit 40. The RO membrane module 51includes a pressure vessel 52 having a first chamber 54 and a second chamber 55 whoseinside is partitioned by an RO membrane element 53.Therefore, as shown in the example of FIG. 1, when the concentrating ROmembrane separation unit disposed in the immediately after continuation of the firstcontinuation concentrating RO membrane separation unit 40 is the final continuationconcentrating RO membrane separation unit 50, the concentrated water c1 obtained in thefirst chamber 44 flows in the flow path 205 and is introduced into the first chamber 54. Atthis time, the concentrated water c1 is introduced in a state where the sum of the pressureand the osmotic pressure is higher than the sum of the pressure and the osmotic pressure ofthe second chamber 55.Note that FIG. 1 is an example in which the concentrating membrane separation unit1003 includes only two concentrating RO membrane separation unit, first continuationconcentrating RO membrane separation unit 40 and final continuation concentrating ROmembrane separation unit 50. As described above, the concentrating membrane separationunit 1003 may include any number of intermediate continuous concentrating RO membraneseparation units disposed in series between the first continuation concentrating ROmembrane separation unit 40 and the final continuation concentrating RO membraneseparation unit 50. In such a case, since the concentrated water introduced into the firstchamber 54 is not the concentrated water c1 obtained in the first chamber 44, in FIG. 1, theconcentrated water introduced into the first chamber 54 is generally expressed as aconcentrated water ci obtained by the first chamber of an i-th continuous concentrating ROmembrane separation unit.In the first chamber 54, the water contained in the concentrated water ci permeatesthrough the RO membrane element 53, so that a permeated water tn is obtained in thesecond chamber 55, and accordingly, in the first chamber 54, the concentrated water cn inwhich the concentrated water ci is further concentrated is obtained.A flow path 206 is further connected to the first chamber 54, and the concentratedwater cn obtained in the first chamber 54 flows in the flow path 206 and is discharged fromthe final continuation concentrating RO membrane separation unit 50.The flow path 206 is provided with the concentrated water flow rate measuringdevice 1006. The concentrated water flow rate measuring device 1006 includes a flowmeter 1007 that measures a flow rate of concentrated water cn discharged from the firstchamber 54 and flows in the flow path 206.As the flow meter 1007, an ultrasonic flow meter and an electromagnetic flow metercan be applied regardless of the type as long as the flow rate of a solution can be measured.In addition, the concentrated water flow rate measuring device 1006 transmits themeasurement result measured by the flow meter 1007 to the control device 1012 using acommunication function such as a wireless LAN.The flow path 206 is also provided with a concentrated water flow rate adjustingdevice 1010. The concentrated water flow rate adjusting device 1010 is preferably providedon the downstream side of the concentrated water flow rate measuring device 1006 in theflow path 206, and includes a flow rate adjusting device 1011 that adjusts the flow rate ofthe concentrated water cn flowing in the flow path 206. The flow rate adjusting device 1011is preferably a proportional control valve whose opening degree is adjusted by the controldevice 1012.The flow path 1013 is also connected to the concentrated water flow rate adjustingdevice 1010, and the concentrated water cn whose flow rate is measured by the flow rateadjusting device 1011 flows in the flow path 1013 and is discharged to the outside of thewater treatment system 1.A flow path 208 for introducing a part of the concentrated water cn into the secondchamber 55 is also branched and connected to the flow path 206 on the upstream side of theconcentrated water flow rate measuring device 1006. The flow path 208 is provided with acirculating water pressure reducing unit 60.The circulating water pressure reducing unit 60 includes a pressure reducing device61. The pressure reducing device 61 is preferably a proportional control valve whoseopening degree is controlled by the control device 1012. A suction outlet of the pressurereducing device 61 is connected to the flow path 208, and a discharge outlet of the pressurereducing device 61 is connected to a flow path 209 connected to the second chamber 55.When the control device 1012 outputs a signal for increasing the opening degree ofthe valve (not illustrated) of the pressure reducing device 61, the amount of theconcentrated water cn taken into the flow path 208 from the flow path 206 increases, andthus the amount of the concentrated water cn introduced into the second chamber 55increases.Conversely, when the control device 1012 outputs a signal for reducing the openingdegree of the valve (not illustrated) of the pressure reducing device 61, the amount of theconcentrated water cn taken into the flow path 208 from the flow path 206 decreases, andthus the amount of the concentrated water cn introduced into the second chamber 55decreases.In this manner, the amount of the concentrated water cn introduced into the secondchamber 55 can be adjusted according to the opening degree of the valve of the pressurereducing device 61.As described above, most of the concentrated water cn discharged from the firstchamber 54 flows in the flow path 206 and the flow path 1013 and is discharged from thefinal continuation concentrating RO membrane separation unit 50, but a part thereof istaken into the flow path 208 by the pressure reducing device 61, further flows in the flowpath 209, and is returned to the second chamber 55.Note that, when introducing the concentrated water cn into the second chamber 55by the pressure reducing device 61, the circulating water pressure reducing unit 60 reducesthe pressure of the concentrated water cn introduced into the second chamber 55 to be lowerthan the pressure of the concentrated water ci in the first chamber 54. Specifically, thepressure reducing device 61 preferably reduces the pressure of the concentrated water cnintroduced into the second chamber 55 by 1 MPa or more, and in particular, morepreferably by 3 MPa or more.By reducing the pressure by 3 MPa or more, the pressure of the concentrated watercn introduced into the second chamber 55 can be made sufficiently lower than the pressureof the concentrated water ci introduced into the first chamber 54, and the water componentcontained in the concentrated water ci in the first chamber 54 permeates through the ROmembrane element 53 and smoothly moves to the second chamber 55 as the permeatedwater tn.As described above, since the concentrated water cn is introduced into the secondchamber 55 from the flow path 209, in the second chamber 55, the permeated water tn thathas permeated through the RO membrane element 53 and the concentrated water cnintroduced from the flow path 209 merge to become a merged water g1.A flow path 207 is further connected to the second chamber 55, and the mergedwater g1 flows in the flow path 207 and is introduced into the second chamber of theconcentrating RO membrane separation unit disposed in the immediately previouscontinuation. FIG. 1 particularly shows an example in which the concentrating ROmembrane separation unit disposed in the immediately previous continuation is the firstcontinuation concentrating RO membrane separation unit 40.Therefore, when the concentrating RO membrane separation unit disposed in theimmediately previous continuation is the first continuation concentrating RO membraneseparation unit 40, the merged water g1 from the second chamber 55 flows in the flow path207 and is introduced into the second chamber 45 of the first continuation concentrating ROmembrane separation unit 40.Note that, as described above, one or more intermediate continuous concentratingRO membrane separation unit (not illustrated) may be disposed in series between the finalcontinuation concentrating RO membrane separation unit 50 and the first continuationconcentrating RO membrane separation unit 40. Therefore, in general expression, themerged water gi discharged from the second chamber of the second continuationconcentrating RO membrane separation unit is introduced into the second chamber 45 ofthe first continuation concentrating RO membrane separation unit 40.Accordingly, in the second chamber 45, the permeated water t1 that has permeatedthrough the RO membrane element 43 and the merged water gi introduced from the flowpath 207 are merged to become a merged water gn.Note that, in FIG. 1, the flow path 205 is connected from the right side surface of thepressure vessel 42 of the first continuation concentrating RO membrane separation unit 40to the first chamber 44, and from the left side surface of the pressure vessel 52 of the finalcontinuation concentrating RO membrane separation unit 50 to the first chamber 54. Inaddition, the flow path 207 is connected from the right side surface of the pressure vessel 42of the first continuation concentrating RO membrane separation unit 40 to the secondchamber 45, and from the left side surface of the pressure vessel 52 of the final continuationconcentrating RO membrane separation unit 50 to the second chamber 55.That is, both the flow paths 205 and 207 are connected to the first and secondchambers from the right side surface of the pressure vessel of the concentrating ROmembrane separation unit on the upstream side, and are connected to the first and secondchambers from the left side surface of the pressure vessel of the concentrating ROmembrane separation unit on the downstream side, respectively. However, this is anexample, and a place to which the flow paths 205 and 207 are connected in the pressurevessel can be appropriately adjusted, and such an example is also shown in anotherembodiment described later.The flow path 204 for circulating the merged water gn as the circulating water d tothe liquid mixing tank 11 is further connected to the second chamber 45.The flow path 204 is provided with the circulating water flow rate measuring device1008. The circulating water flow rate measuring device 1008 includes a flow meter 1009that measures a flow rate of the circulating water d flowing in the flow path 204. As theflow meter 1009, an ultrasonic flow meter and an electromagnetic flow meter can beapplied regardless of the type as long as the flow rate of a solution can be measured. Inaddition, the circulating water flow rate measuring device 1008 also transmits themeasurement result measured by the flow meter 1009 to the control device 1012 using acommunication function such as a wireless LAN.Next, the control device 1012 will be described.The control device 1012 is communicably connected to the pressure increasingpump 15, the circulating water pressure reducing unit 60, the concentrating membraneseparation unit introduction flow rate measuring device 1004, the concentrated water flowrate measuring device 1006, the circulating water flow rate measuring device 1008, and theconcentrated water flow rate adjusting device 1010 by a communication function such as awireless LAN, an electrical connection, or the like.Based on the measurement result transmitted from the concentrating membraneseparation unit introduction flow rate measuring device 1004 and the measurement resulttransmitted from the circulating water flow rate measuring device 1008, the control device1012 adjusts at least one of the flow rate of the mixed water delivered from the introductionunit 10 to the desalting membrane separation unit 20 (delivered flow rate), the flow rate atwhich the concentrated water cn concentrated by the final continuation concentrating ROmembrane separation unit 50 is discharged from the first chamber 54 (discharged flow rate),and the flow rate of the permeated water tn that has permeated through the RO membraneelement 53 and the concentrated water cn merged in the second chamber 55 among theconcentrated water cn concentrated in the first chamber 54 (merged flow rate).Specifically, the control device 1012 controls at least one of the pressure increasingpump 15, the pressure reducing device 61, and the flow rate adjusting device 1011 so that aratio (hereinafter, referred to as a "flow rate ratio") of the flow rate (that is, the flow ratemeasured by the flow meter 1009) transmitted from the circulating water flow ratemeasuring device 1008 to the flow rate (that is, the flow rate measured by the flow meter1005) transmitted from the concentrating membrane separation unit introduction flow ratemeasuring device 1004 is an expected predetermined value.When controlling the pressure increasing pump 15, the control device 1012transmits an input signal corresponding to the discharge amount of the pressure increasingpump 15 to the inverter 15a. Upon receiving the input signal, the inverter 15a outputs adriving frequency corresponding to the input signal to the pressure increasing pump 15. Thepressure increasing pump 15 rotates at a rotational speed corresponding to the drivingfrequency. In this way, the control device 1012 controls the discharge amount of thepressure increasing pump 15, and adjusts the flow rate (discharge flow rate) of the water tobe treated a discharged from the introduction unit 10 to the desalting membrane separationunit 20.Similarly, when controlling the flow rate adjusting device 1011 such as aproportional control valve, for example, the control device 1012 sends a signalcorresponding to the valve opening degree to the flow rate adjusting device 1011 to changethe valve opening degree, thereby adjusting the flow rate (discharged flow rate) of theconcentrated water cn discharged from the first chamber 54.When controlling the pressure reducing device 61 such as a proportional controlvalve, for example, the control device 1012 sends a signal corresponding to the valveopening degree to the pressure reducing device 61, and changes the valve opening degree toadjust the flow rate (merged flow rate) of the concentrated water cn flowing from the firstchamber 54 through the flow paths 206, 208, and 209 and merging into the second chamber55.Next, a treatment method of waste water having a total dissolved solids (TDS)concentration of, for example, about 0, 1% to several% such as the water to be treated a bythe water treatment system 1 of the first embodiment shown in FIGS. 1 and 2, will bedescribed.First, as shown in FIG. 2, the water to be treated a such as waste water, for example,is introduced from the flow path 211 into the liquid mixing tank 11 by driving the pump 14.The circulating water d is also introduced into the liquid mixing tank 11 from theflow path 204.When the pH of the mixed water of the water to be treated a and the circulatingwater d exceeds 6, membrane fouling occurs in the RO membrane elements 23, 43, and 53of the RO membrane modules 21, 41, and 51 on the downstream side due to inorganiccomponents such as calcium and magnesium, and clogging easily occurs in a pipe, a valve,and the like.Therefore, in the liquid mixing tank 11, a pH adjusting agent is also introduced fromthe chemical solution supply line 12 into the mixed water of the water to be treated a andthe circulating water d, whereby the pH of the mixed water is maintained at 2 or more and 6or less.In addition, it is known that when fine bubbles having an average diameter of 1000(nm) or less are contained at a concentration of 1 x 106 (pieces / mL) or more, the effect ofpreventing occurrence of the membrane fouling and clogging of a pipe, a valve, and the likeis improved.Therefore, a gas such as air is also introduced into the mixed water in the liquidmixing tank 11 from the gas supply line 13, and furthermore, fine bubbles having anaverage diameter of 1000 (nm) or less are contained in the mixed water at a concentrationof 1 x 106 (pieces / mL) or more by stirring by the stirrer 17 and the liquid mixing tankstirring pump 1002.The mixed water in the liquid mixing tank 11 is increased in pressure within therange of 4 MPa to 12 MPa by the pressure increasing pump 15, flows in the flow path 201,and is delivered to the first chamber 24 of the RO membrane module 21 of the desaltingmembrane separation unit 20. Note that the upper limit of the increased pressure amount, 12MPa, is determined from the maximum resisting pressure of the pressure vessel 22 of thedesalting membrane separation unit 20.The discharge amount by the pressure increasing pump 15 is controlled by thecontrol device 1012 such that the above-described flow rate ratio is an expectedpredetermined value (for example, 40% or more and 90% or less).In the RO membrane module 21, due to a large pressure difference between the firstchamber 24 whose pressure was increased by the delivered mixed water and the secondchamber 25, the water component of the mixed water in the first chamber 24 permeatesthrough the RO membrane element 23 to become deionized water b and reach the secondchamber 25. The deionized water b flows in the flow path 210 from the second chamber 25,is discharged from the RO membrane module 21, and is finally discharged to the outside ofthe water treatment system 1. On the other hand, the mixed water remaining in the firstchamber 24 is concentrated to become the concentrated water c0.The concentrated water c0 obtained in the first chamber 24 flows in the flow path202 and is discharged from the RO membrane module 21. The flow path 202 is providedwith the concentrating membrane separation unit introduction flow rate measuring device1004 including the flow meter 1005, the flow rate of the concentrated water c0 flowing inthe flow path 202 is measured by the flow meter 1005, and the measurement result istransmitted to the control device 1012 by the concentrating membrane separation unitintroduction flow rate measuring device 1004.The concentrated water c0 whose flow rate is measured by the flow meter 1005flows toward the first continuation concentrating RO membrane separation unit 40 in theflow path 203.The flow path 203 is provided with a pressure reducing unit 30 including a pressurereducing device 31 such as a pressure reducing valve, and the concentrated water c0 flowingin the flow path 203 is reduced in pressure by 0.5 MPa or more by the pressure reducingdevice 31.The gas dissolved in the concentrated water c0 expands and bubbles are generated bythe reduced pressure. The bubbles are finely pulverized by rapid pressure recovery tobecome fine bubbles having an average diameter of 1000 (nm) or less. The reducedpressure amount by the pressure reducing device 31 is set to a value at which such finebubbles are generated in the concentrated water c0 at a concentration of 1 x 106 (pieces / mL)or more.The fine bubbles may disappear while flowing in the water treatment system 1.Therefore, in the concentrating membrane separation unit 1003 on the downstream side ofthe pressure reducing unit 30, the amount of fine bubbles contained in the concentratedwater is generally smaller than the amount generated in the pressure reducing unit 30.Even in such a case, when the concentrated water cn discharged from the flow path1013 which is the most downstream of the water treatment system 1 contains fine bubblesat a concentration of at least 1 x 106 (pieces / mL) or more, it is possible to prevent scaleoccurrence and biofouling in the membrane elements 43 and 53 of the RO membranemodules 41 and 51 disposed on the upstream side of it.The concentrated water c0 containing fine bubbles having a sufficient concentrationas described above flows in the flow path 203 and is introduced into the first chamber 44 ofthe RO membrane module 41.In the first chamber 44, the water component of the concentrated water c0 permeatesthrough the RO membrane element 43 to become the permeated water t1, and reaches thesecond chamber 45. As a result, in the first chamber 44, the concentrated water c1 in whichthe concentrated water c0 is further concentrated is obtained.The first chamber 44 is connected with a flow path 205 leading to the first chamberof the concentrating RO membrane separation unit disposed in the immediately aftercontinuation. Therefore, the concentrated water c1 flows in the flow path 205 from the firstchamber 44, and is introduced into the first chamber of the concentrating RO membraneseparation unit disposed in the immediately after continuation. Then, in the same manner,the water component of the concentrated water c1 permeates through the RO membraneelement to become the permeated water t2, and reaches the second chamber 45, so that theconcentrated water c2 in which the concentrated water c1 is further concentrated is obtainedin the first chamber, and the concentrated water c2 similarly flows in the flow path 205connected to the first chamber, and is introduced into the first chamber of the concentratingRO membrane separation unit disposed in the immediately after continuation and is alsoconcentrated there by the RO membrane element.Such a concentration treatment is repeated, finally, the concentrated water ci flows inthe flow path 205 from the first chamber of the concentrating RO membrane separation unitdisposed in the second continuation from the end, and is introduced into the first chamber54 of the RO membrane module 51 of the final continuation concentrating RO membraneseparation unit 50 at a pressure higher than the osmotic pressure of the RO membraneelement 53.Accordingly, in the first chamber 54, the water component of the concentrated waterci permeates through the RO membrane element 53, so that the permeated water tn isobtained in the second chamber 55. As a result, in the first chamber 54, the concentratedwater cn in which the concentrated water ci is further concentrated is obtained.The concentrated water cn in the first chamber 54 flows in the flow path 206 and isdischarged to the outside of the first chamber 54. However, a part of the concentrated watercn flowing in the flow path 206 flows through the flow path 208 branched from the flowpath 206 to reach the pressure reducing device 61 of the circulating water pressure reducingunit 60, and is reduced in pressure in the pressure reducing device 61. The concentratedwater cn with reduced pressure has a pressure lower than the pressure of a concentratedwater cI delivered to the first chamber 54 of the final continuation concentrating ROmembrane separation unit 50, flows in the flow path 209, and is introduced into the secondchamber 55.The proportional control valve can be used as the pressure reducing device 61, andthe valve opening degree thereof is controlled according to an input signal from the controldevice 1012.The pressure decrease amount of the concentrated water cn introduced into thesecond chamber 55 is preferably 1 MPa or more, and more preferably 3 MPa or more.The concentrated water cn introduced into the second chamber 55 merges with thepermeated water tn that has permeated through the RO membrane element 53 in the secondchamber 55 to become the merged water g1.The second chamber 55 is connected with the flow path 207 connected to the secondchamber of the concentrating membrane separation unit disposed in the immediatelyprevious continuation. The merged water g1 flows in the flow path 207 and is introducedinto the second chamber of the concentrating membrane separation unit disposed in theimmediately previous continuation. In the same manner, the merged water g1 merges with apermeated water tn-1 that has permeated through the RO membrane element to become amerged water g2. The merged water g2 flows in the flow path 207 and is introduced into thesecond chamber of the concentrating membrane separation unit disposed in the immediatelyprevious continuation.When such an operation is repeated, finally, the merged water gi having a lower sumof pressure and osmotic pressure than the sum of pressure and osmotic pressure of the firstchamber 44 flows in the flow path 207 from the second chamber of the concentrating ROmembrane separation unit disposed in the second continuation from the first, and isintroduced into the second chamber 45 of the first continuation concentrating ROmembrane separation unit 40. Then, in the second chamber 45, the merged water gi ismerged with the permeated water t1 to become the merged water gn.The flow path 204 leading to the liquid mixing tank 11 is also connected to thesecond chamber 45, and the merged water gn flows in the flow path 204 and is returned tothe liquid mixing tank 11 as the circulating water d.As described above, according to the water treatment system 1 of the firstembodiment, the concentrated water c0 discharged from the desalting membrane separationunit 20 is concentrated in stages in the first chamber of each continuation of theconcentrating RO membrane separation unit in the concentrating membrane separation unit1003, and finally the concentrated water cn can be obtained. Meanwhile, a part of theconcentrated water cn obtained in the first chamber 54 of the final continuationconcentrating RO membrane separation unit 50 can be reduced in pressure by the pressurereducing device 61 of the circulating water pressure reducing unit 60 and then introducedinto the second chamber 55.By the operation to reduce pressure, the pressure of the concentrated water cn in thesecond chamber 55 becomes lower than the pressure of the concentrated water ci in the firstchamber 54, so that the water component of the concentrated water ci in the first chamber54 can be permeated from the RO membrane element 53 to the second chamber 55 as thepermeated water tn by using the pressure difference between the concentrated water ci in thefirst chamber 54 and the concentrated water cn introduced into the second chamber 55(pressure of concentrated water ci in first chamber 54 > pressure of concentrated water cnintroduced into second chamber 55).As a result, the concentrated water cn further concentrated can be obtained from theconcentrated water ci introduced into the first chamber 54.In addition, the concentrated water cn introduced into the second chamber 55 can bediluted with the permeated water tn from the first chamber 54.Further, the circulating water d can be returned from the second chamber 45 of theRO membrane module 41 of the first continuation concentrating RO membrane separationunit 40 to the liquid mixing tank 11.The circulating water d is generated by diluting a part of the concentrated water cnobtained in the first chamber 54 of the RO membrane module 51 of the final continuationconcentrating RO membrane separation unit 50 in multiple stages by the permeated water inthe second chamber of each continuation of concentrating RO membrane separation unit.As a result, the TDS concentration of the circulating water d returned to the liquidmixing tank 11 is diluted to, for example, about 30% of the TDS concentration of theconcentrated water cn discharged by flowing in the flow path 1013 from the concentratingmembrane separation unit 1003, and the TDS concentration of the circulating water d canbe made equal to or less than the TDS concentration of the water to be treated a.Further, the flow rate of the circulating water d flowing in the flow path 204 towardthe liquid mixing tank 11 can be measured by the flow meter 1009, and the measurementresult can be transmitted from the circulating water flow rate measuring device 1008 to thecontrol device 1012.Further, the flow rate of the concentrated water cn discharged from the first chamber54 and flowing in the flow path 206 can be measured by the flow meter 1007, and themeasurement result can be transmitted from the concentrated water flow rate measuringdevice 1006 to the control device 1012.The control device 1012 can control the pressure increasing pump 15, the circulatingwater pressure reducing unit 60, and the concentrated water flow rate adjusting device 1010so that the above-described flow rate ratio becomes an expected predetermined value.Note that the above-described flow rate ratio is desirably 40% or more and 90% orless. If the flow rate ratio is less than 40%, the amount of the concentrated water increases,the volume of the waste water which is the concentrated water cn discharged from the flowpath 1013 is not reduced, and the treatment cost increases. On the other hand, when theflow rate ratio is 90% or more, the amount of the concentrated water cn decreases, and thepossibility of scale occurrence increases.For example, when the water quality of the water to be treated a fluctuates and theconcentration of the concentrated water c0 increases, in the concentrating membraneseparation unit 1003, the amount of the permeated water t1 from the first chamber 44 to thesecond chamber 45 of the RO membrane module 41 of the first continuation concentratingRO membrane separation unit 40 and the amount of the permeated water tn from the firstchamber 54 to the second chamber 55 of the RO membrane module 51 of the finalcontinuation concentrating RO membrane separation unit 50 decrease, so that the abovedescribedflow rate ratio decreases, the amount of concentrated water increases, and thetreatment cost increases.In such a case, the control device 1012 outputs a signal for increasing the valveopening degree of the pressure reducing device 61 to the circulating water pressurereducing unit 60 in order to set the above-described flow rate ratio to an expectedpredetermined value by increasing the amount of the circulating water d.On the other hand, when the water quality of the water to be treated a fluctuates andthe concentration of the concentrated water c0 decreases, in the concentrating membraneseparation unit 1003, the amount of the permeated water t1 from the first chamber 44 to thesecond chamber 45 of the RO membrane module 41 of the first continuation concentratingRO membrane separation unit 40 and the amount of the permeated water tn from the firstchamber 54 to the second chamber 55 of the RO membrane module 51 of the finalcontinuation concentrating RO membrane separation unit 50 increase, so that the above20described flow rate ratio increases, the amount of concentrated water decreases, and thepossibility of scale occurrence increases.In such a case, the control device 1012 outputs a signal for reducing the valveopening degree of the pressure reducing device 61 to the circulating water pressurereducing unit 60 in order to set the above-described flow rate ratio to an expectedpredetermined value by reducing the amount of the circulating water d.Therefore, in the water treatment system 1 of the first embodiment, the water to betreated a is not limited to waste water having a TDS concentration of about 0.1% toseveral%, and can be waste water such as food factory discharged water containing organicsubstances, for example.In addition, by realizing a stable treatment as expected, the amount of theconcentrated water cn discharged from the concentrating membrane separation unit 1003and the amount of the deionized water b discharged from the desalting membraneseparation unit 20 can be stabilized.Furthermore, since the concentrated water cn having a high degree of concentrationas expected can be recovered from the concentrating membrane separation unit 1003, areduced amount of the concentrated water cn can be efficiently obtained from the water tobe treated a. As a result, when the discharged concentrated water cn is recovered andsubjected to evaporation treatment, the heat energy required for evaporation can be reduced,so that the overall treatment cost can be reduced. Note that for details of the evaporationtreatment of the concentrated water cn, refer to the fourth embodiment described later.In addition, in the liquid mixing tank 11, by supplying a pH adjusting agent from thechemical solution supply line 12, the pH of the mixed water of the water to be treated a andthe circulating water d stored can be adjusted to 2 or more and 6 or less. As a result, the pHof all the water handled by the water treatment system 1 including the concentrated water cndischarged from the flow path 1013 can be adjusted to 2 or more and 6 or less.As described above, by setting the pH of all the water handled by the watertreatment system 1 to 2 or more and 6 or less, the bicarbonate ion concentration in theconcentrated water c is lowered, the generation of carbonates of calcium and magnesium issuppressed, and membrane fouling and clogging of pipes, flow rate adjusting valves, andthe like can be suppressed.In the liquid mixing tank 11, the mixed liquid of the water to be treated a and thecirculating water d is stirred by the stirrer 17, the stirring line 1001, and the liquid mixingtank stirring pump 1002. As a result, fine bubbles having an average diameter of 1000 (nm)or less can be generated in the mixed liquid at a concentration of 1.0 x 106 (pieces / mL) ormore.In addition, the pressure reducing unit 30 reduces the pressure of the concentratedwater c0 by, for example, 0.5 MPa or more, and expands the gas dissolved in theconcentrated water c0 by the pressure reducing action. Further, the bubbles are finelypulverized by the subsequent rapid pressure recovery, and fine bubbles having an averagediameter of 1000 (nm) or less can be further generated.As described above, by the stirring action in the liquid mixing tank 11 and thepressure reducing action by the pressure reducing unit 30, fine bubbles having an averagediameter of 1000 (nm) or less can be generated at a concentration of 1.0 x 106 (pieces / mL)or more in all water handled by the water treatment system 1.Such fine bubbles have high stability in mixed water and high permeability to theRO membrane elements 23, 43, and 53. As a result, scale occurrence and biofouling in theRO membrane elements 23, 43, and 53 can be prevented.Furthermore, since the surface of the fine bubbles is hydrophobic and charged, whenthe anti-scale agent, the anti-biofouling agent, the disinfectant, and the membrane cleanerare supplied from the chemical solution supply line 12 to the liquid mixing tank 11 by theintroduction unit 10, these chemicals can be easily attached to the RO membrane elements23, 43, and 53. As a result, scale occurrence and biofouling in the RO membrane elements23, 43, and 53 can be more effectively prevented.As a result, the occurrence of scale and clogging due to biofouling of the ROmembrane elements 23, 43, and 53 can be suppressed or prevented, thereby reducing thelabor and frequency of cleaning or the like of the RO membrane elements 23, 43, and 53, sothat the operation rate of the water treatment system 1 can be improved.Note that, in the above description, as an example, it has been described that themerged water g is introduced into the second chamber of the RO membrane module in theimmediately previous continuation. However, the merged water g is not necessarily limitedto being introduced into the second chamber of the RO membrane module in theimmediately previous continuation. For example, instead of introducing the merged water ginto the second chamber of the RO membrane module in the immediately previouscontinuation, the merged water g may be introduced into the second chamber of the ROmembrane module on the upstream side of the immediately previous continuation, that is,on the first continuation side.(Modification example 1 of first embodiment)Next, a modification example 1 of the first embodiment will be described.FIG. 3 is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method according to the modificationexample 1 of the first embodiment is applied.A water treatment system 1A exemplified in FIG. 3 is different from the watertreatment system 1 exemplified in FIG. 1 in connection sections of the flow paths 204, 207,and 209.First, the flow path 204 is connected to the left side surface of the pressure vessel 42in which the second chamber 45 of an RO membrane module 41 is located as shown inFIG. 1 in the water treatment system 1, whereas it is connected to the right side surface ofthe pressure vessel 42 in which the second chamber 45 is located as shown in FIG. 3 in thewater treatment system 1A.Further, the flow path 207 is connected to the left side surface of the pressure vessel52 in which the second chamber 55 of an RO membrane module 51 is located as shown inFIG. 1 in the water treatment system 1, whereas it is connected to the right side surface ofthe pressure vessel 52 in which the second chamber 55 of the RO membrane module 51 islocated as shown in FIG. 3 in the water treatment system 1A.The flow path 209 is connected to the right side surface of the pressure vessel 52 inwhich the second chamber 55 of the RO membrane module 51 is located as shown in FIG. 1in the water treatment system 1, whereas it is connected to the left side surface of thepressure vessel 52 in which the second chamber 55 of the RO membrane module 51 islocated as shown in FIG. 3 in the water treatment system 1A.According to the water treatment system 1A of modification example 1 of the firstembodiment having such a configuration, the flow direction of the concentrated water c0flowing in the first chamber 44 of the RO membrane module 41 and the flow direction ofthe merged water gn flowing in the second chamber 45 are the same, and the flow directionof the concentrated water ci flowing in the first chamber 54 of the RO membrane module 51and the flow direction of the merged water g1 flowing in the second chamber 55 are thesame.Even in such a configuration, it is possible to achieve the same functions and effectsas those of the water treatment system 1 of the first embodiment.(Modification example 2 of first embodiment)Next, a modification example 2 of the first embodiment will be described.FIG. 4 is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method according to the modificationexample 2 of the first embodiment is applied.A water treatment system 1B exemplified in FIG. 4 has a configuration in which anew flow path 211 is added to the water treatment system 1 exemplified in FIG. 1.The flow path 211 is branched from the flow path 209 and merges to the flow path207.According to the water treatment system 1B having such a configuration, themerged water g1 in the second chamber 55 of the RO membrane module 51 flows in theflow path 207 and is merged with a part of the concentrated water cn reduced in pressure bythe pressure reducing device 61 before being introduced into the second chamber of the ROmembrane module of the concentrating RO membrane separation unit in the upstreamcontinuation (in FIG. 4, the first continuation concentrating RO membrane separation unit40).Thereby, the merged water g1 flowing in the flow path 207 is reduced in pressureand then introduced into the second chamber of the RO membrane module of theconcentrating membrane separation unit in the upstream continuation. In general, themerged water gi is introduced into the second chamber 45 of the RO membrane module 41of the first continuation concentrating RO membrane separation unit 40.As a result, in the RO membrane module 41 of the first continuation concentratingRO membrane separation unit 40, since the difference in the sum of the pressure and theosmotic pressure between the concentrated water c0 introduced into the first chamber 44and the merged water gi introduced into the second chamber 45 becomes sufficiently large(sum of the pressure and the osmotic pressure of the concentrated water c0 > sum of thepressure and the osmotic pressure of the merged water gi), the water component in theconcentrated water c0 in the first chamber 44 efficiently permeates from the RO membraneelement 43 to the second chamber 45 to become the permeated water t1, and theconcentration effect on the concentrated water c0 in the RO membrane module 41 can beenhanced.As described above, according to the water treatment systems 1, 1A, and 1B towhich the water treatment method of the first embodiment is applied, it is possible to stablyobtain a concentrated water and a deionized water of an expected amount and quality in thewater concentration treatment using the multi-stage RO membrane.(Second embodiment)A second embodiment will be described.FIG. 5 is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method of the second embodiment isapplied.A water treatment system 1C exemplified in FIG. 5 is a modification example of thewater treatment system 1 exemplified in FIG. 1, and is different from the water treatmentsystem 1 particularly in that an intermediate continuous concentrating RO membraneseparation unit 1020 is added between a first continuation concentrating RO membraneseparation unit 40 and a final continuation concentrating RO membrane separation unit 50in a concentrating membrane separation unit 1003.The intermediate continuous concentrating RO membrane separation unit 1020 hasthe same configuration as the first continuation concentrating RO membrane separation unit40 and the final continuation concentrating RO membrane separation unit 50, and includesan RO membrane module 1021 having the same configuration as the RO membranemodules 41 and 51.That is, the RO membrane module 1021 includes a pressure-resistant sealed pressurevessel 1022, and an RO membrane element 1023 that partitions the inside of the pressurevessel 1022 into a first chamber 1024 and a second chamber 1025 is disposed in thepressure vessel 1022. As the RO membrane element 1023, for example, an RO membranehaving a spiral shape or a hollow fiber shape can be used.A flow path 212 connected from the first chamber 44 of the RO membrane module41 is connected to the first chamber 1024. Therefore, a concentrated water c1 flows in theflow path 212 and is introduced into the first chamber 1024 from the first chamber 44.The sum of the pressure and the osmotic pressure of the concentrated water c1introduced into the first chamber 1024 is higher than the sum of the pressure and theosmotic pressure of a merged water gi-1 introduced into the second chamber 1025.Therefore, the water component contained in the concentrated water c1 permeates throughthe RO membrane element 1023 from the first chamber 1024 and reaches the secondchamber 1025 as a permeated water ti. As a result, in the first chamber 1024, a concentratedwater ci in which the concentrated water c1 is further concentrated is obtained.The first chamber 1024 is further connected with a flow path 205 connected to afirst chamber 54 of an RO membrane module 51. Therefore, the concentrated water ciobtained in the first chamber 1024 flows in the flow path 205 and is introduced into the firstchamber 54 of the RO membrane module 51.On the other hand, a flow path 207 connected from a second chamber 55 of the ROmembrane module 51 is connected to the second chamber 1025. Therefore, a merged waterg1 flows in the flow path 207 and is introduced into the second chamber 1025 from thesecond chamber 55. The merged water g1 merges with the permeated water ti in thechamber 1025 to become a merged water gi.The second chamber 1025 is further connected with a flow path 213 connected tothe second chamber 45 of the RO membrane module 41. Therefore, the merged water gifrom the second chamber 1025 flows in the flow path 213 and is introduced into the secondchamber 45 of the RO membrane module 41.According to the water treatment system 1C of the second embodiment having sucha configuration, since the intermediate continuous concentrating RO membrane separationunit 1020 is added between the first continuation concentrating RO membrane separationunit 40 and the final continuation concentrating RO membrane separation unit 50, the ROmembrane modules 41, 1021, and 51 in the three continuation are disposed in series, so thatit is possible to realize higher concentration of the concentrated water as compared with thewater treatment system 1 of the first embodiment shown in FIG. 1.(Modification example of second embodiment)Next, a modification example of the second embodiment will be described.FIG. 5A is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method of a modification example of thesecond embodiment is applied.A water treatment system 1D exemplified in FIG. 5A is a modification example ofthe water treatment system 1C exemplified in FIG. 5, and is different from the watertreatment system 1C in that another RO membrane module 1031 is disposed in series withthe RO membrane module 1021 in the intermediate continuous concentrating ROmembrane separation unit 1020.Similarly to the RO membrane module 1021, the RO membrane module 1031 alsoincludes a pressure-resistant sealed pressure vessel 1032, and an RO membrane element1033 that partitions the inside of the pressure vessel 1032 into a first chamber 1034 and asecond chamber 1035 is disposed in the pressure vessel 1032. As the RO membraneelement 1033, for example, an RO membrane having a spiral shape, a hollow fiber shape,and the like can be used.A flow path 1036 connected from the first chamber 1024 of the RO membranemodule 1021 is connected to the first chamber 1034. Therefore, the concentrated water ciflows in the flow path 1036 and is introduced into the first chamber 1034 from the firstchamber 1024.The sum of the pressure and the osmotic pressure of the concentrated water ciintroduced into the first chamber 1034 is higher than the sum of the pressure and theosmotic pressure of a merged water gi-2 introduced into the second chamber 1035.Therefore, the water component contained in the concentrated water ci permeates throughthe RO membrane element 1033 from the first chamber 1034 and reaches the secondchamber 1035 as a permeated water ti+1. As a result, in the first chamber 1034, aconcentrated water ci+1 in which the concentrated water ci is further concentrated isobtained.The first chamber 1034 is further connected with the flow path 205 connected to thefirst chamber 54 of the RO membrane module 51. Therefore, the concentrated water ci+1obtained in the first chamber 1034 flows in the flow path 205 and is introduced into the firstchamber 54 of the RO membrane module 51.On the other hand, the flow path 207 connected from the second chamber 55 of theRO membrane module 51 is connected to the second chamber 1035. Therefore, the mergedwater g1 flows in the flow path 207 and is introduced into the second chamber 1035 fromthe second chamber 55. The merged water g1 merges with the permeated water ti+1 in thechamber 1025 to become the merged water gi-1.The second chamber 1035 is further connected with a flow path 1037 connected tothe second chamber 1025 of the RO membrane module 1021. Therefore, the merged watergi-1 from the second chamber 1035 flows in the flow path 1037 and is introduced into thesecond chamber 1025 of the RO membrane module 1021.According to the water treatment system 1D of the modification example of thesecond embodiment having such a configuration, by disposing the other RO membranemodule 1031 in series with the RO membrane module 1021 in the intermediate continuousconcentrating RO membrane separation unit 1020, a total of four RO membrane modules41, 1021, 1031, and 51 are disposed in series, so that it is possible to realize higherconcentration of the concentrated water than the water treatment system 1C of the secondembodiment shown in FIG. 5.Note that, although not illustrated, an arbitrary number of RO membrane modulesmay be further disposed in series on the downstream side of the RO membrane modules1021 and 1031 in the intermediate continuous concentrating RO membrane separation unit1020. As described above, by increasing the number of RO membrane modules of theintermediate continuous concentrating RO membrane separation unit 1020, it is possible torealize further higher concentration of the concentrated water.(Third embodiment)A third embodiment will be described.FIG. 6 is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method of the third embodiment isapplied.A water treatment system 1E exemplified in FIG. 6 is a modification example of thewater treatment system 1C exemplified in FIG. 5, and is different from the water treatmentsystem 1C particularly in the following points.That is, the water treatment system 1E includes three RO membrane modules 41a,41b, and 41c disposed in parallel to each other in a first continuation concentrating ROmembrane separation unit 40, and includes two RO membrane modules 1021a and 1021bdisposed in parallel to each other in an intermediate continuous concentrating ROmembrane separation unit 1020. The configurations of the RO membrane modules 41a, 41b,and 41c are the same as those of an RO membrane module 41. The configurations of theRO membrane modules 1021a and 1021b are the same as those of an RO membrane module1021.First chambers 44a, 44b, and 44c of the RO membrane modules 41a, 41b, and 41care respectively connected to flow paths 214, 215, and 216 provided to be branched from aflow path 203. As a result, a concentrated water c0 from the flow path 203 flows through theflow paths 214, 215, and 216, respectively, and is introduced into the first chambers 44a,44b, and 44c.Flow rate control devices 410a, 410b, and 410c including, for example, a flow meterand a valve are provided in the flow paths 214, 215, and 216, respectively. The flow ratecontrol devices 410a, 410b, and 410c measure the flow rate of each of the concentratedwater c0 flowing in the flow paths 214, 215, and 216 with a flow meter, and output themeasurement results to a control device 1012.Then, the flow rate control devices 410a, 410b, and 410c can adjust the flow rate ofeach concentrated water c0 flowing in the flow paths 214, 215, and 216 by changing theopening degree of the valve in response to the control signal output from the control device1012.For example, the control device 1012 can control the valve opening degree of thevalve of each of the flow rate control devices 410a, 410b, and 410c according to themeasurement results of the flow rate output from each of the flow rate control devices 410a,410b, and 410c such that the flow rates of the respective concentrated waters c0 flowing inthe flow paths 214, 215, and 216 become the same.Accordingly, the concentrated water c0 is introduced into the first chambers 44a,44b, and 44c at the same flow rate. Then, in the first chambers 44a, 44b, and 44c, asdescribed above, the concentrated water c0 is concentrated to obtain a concentrated water c1.A flow path 218 for introducing the concentrated water c1 obtained in each of thefirst chambers 44a, 44b, and 44c to the RO membrane modules 1021a and 1021b side isconnected to each of the first chambers 44a, 44b, and 44c. A total of three flow paths 218connected to each of the first chambers 44a, 44b, and 44c are merged to one flow path 218and then connected to a flow path 219. As a result, the concentrated water c1 obtained ineach of the first chambers 44a, 44b, and 44c flows in the flow path 218 connected to eachof the first chambers 44a, 44b, and 44c and merges in the flow path 219.The flow path 219 is then branched into two and connected to a first chamber 1024aof the RO membrane module 1021a and a first chamber 1024b of the RO membranemodule 1021b, respectively.Flow rate control devices 10210a and 10210b each including, for example, a flowmeter and a valve are also provided in each of the branched flow paths 219. The flow ratecontrol devices 10210a and 10210b measure the flow rate of the concentrated water c1flowing in each flow path 219 by the flow meter, and output the measurement results to thecontrol device 1012.Then, the flow rate control devices 10210a and 10210b can adjust the flow rate ofthe concentrated water c1 flowing in each flow path 219 by changing the opening degree ofthe valve in response to the control signal output from the control device 1012.For example, the control device 1012 can control the valve opening degree of thevalve of each of the flow rate control devices 10210a and 10210b according to themeasurement results of the flow rate output from each of the flow rate control devices10210a and 10210b so that the flow rate of the concentrated water c1 flowing in each of theflow paths 219 becomes the same.Accordingly, the concentrated water c1 is introduced into the first chambers 1024aand 1024b at the same flow rate. Also, in the first chambers 1024a and 1024b, as describedabove, the concentrated water c1 is concentrated to obtain a concentrated water ci.A flow path 205 connected from a first chamber 54 of an RO membrane module 51is connected to the first chamber 1024a. In addition, a flow path 207 merged to the flowpath 207 is connected to the first chamber 1024b.As a result, the concentrated water ci obtained in the first chamber 1024a flowsthrough the flow path 205 and is introduced into the first chamber 54. Further, theconcentrated water ci obtained in the first chamber 1024b flows in the flow path 207,further flows in the flow path 205 after merging with the flow path 205, and is introducedinto the first chamber 54.In the first chamber 54, as described with reference to FIG. 5, the concentratedwater cn in which the concentrated water ci is further concentrated is obtained, and in asecond chamber 55, a merged water g1 is obtained.A flow path 220 connected to a second chamber 1025a is connected to the secondchamber 55. Further, a flow path 222 connected to a second chamber 1025b is branchedfrom the flow path 220.As a result, a part of the merged water g1 obtained in the second chamber 55 flowsin the flow path 220 and is introduced into the second chamber 1025a, and the remainingpart branches from the flow path 220 to the flow path 222, flows in the flow path 222, andis introduced into the second chamber 1025b.In the second chambers 1025a and 1025b, a merged water gi is obtained as describedwith reference to FIG. 5.Each of the RO membrane modules 1021a and 1021b is connected with a flow path224 for introducing the merged water gi to the RO membrane modules 41a, 41b, and 41c.The flow path 224 is once merged and then connected to a flow path 225. The flow path225 is branched into three, and is connected to each of second chambers 45a, 45b, and 45cof the RO membrane modules 41a, 41b, and 41c.As a result, the merged water gi from the second chambers 1025a and 1025b flowsin the flow path 224, and once merged, then flows in the flow path 225. Since the flow path225 is branched into three and connected to the second chambers 45a, 45b, and 45c,respectively, the merged water gi is introduced from the respective flow paths 225 into thesecond chambers 45a, 45b, and 45c.Each of the three branched flow paths 225 is provided with flow rate control devices412a, 412b, and 412c each including, for example, a flow meter and a valve. The flow ratecontrol devices 412a, 412b, and 412c measure the flow rate of the merged water gi flowingin each flow path 225 by the flow meter, and output the measurement results to the controldevice 1012.Then, in response to the control signal output from the control device 1012, the flowrate control devices 412a, 412b, and 412c can adjust the flow rate of the merged water giflowing in each flow path 225 by changing the opening degree of the valve.For example, the control device 1012 can control the valve opening degree of thevalve of each of the flow rate control devices 412a, 412b, and 412c according to themeasurement results of the flow rate output from each of the flow rate control devices 412a,412b, and 412c such that the flow rate of the merged water gi flowing in each of the flowpaths 225 becomes the same.As a result, the merged water gi is introduced into the second chambers 45a, 45b,and 45c at the same flow rate. Then, in the second chambers 45a, 45b, and 45c, a mergedwater gn is obtained as described above.Flow paths 226, 227, and 228 merged to a flow path 204 are connected to the secondchambers 45a, 45b, and 45c, respectively. As a result, the merged water gn from each of thesecond chambers 45a, 45b, and 45c flows in the flow paths 226, 227, and 228, merges in theflow path 204, and is returned to a liquid mixing tank 11 as a circulating water d.Flow rate control devices 411a, 411b, and 411c including, for example, a flow meterand a valve are also provided in the flow paths 226, 227, and 228, respectively. The flowrate control devices 411a, 411b, and 411c measure the flow rate of each of the mergedwaters gn flowing in the flow paths 226, 227, and 228 with a flow meter, and output themeasurement results to the control device 1012.Then, the flow rate control devices 411a, 411b, and 411c can adjust the flow rate ofeach merged water gn flowing in the flow paths 214, 215, and 216 by changing the openingdegree of the valve in response to the control signal output from the control device 1012.Each of the flow rate control devices 410a, 410b, 410c, 411a, 411b, 411c, 412a,412b, 412c, 10210a, and 10210b includes a flow meter and a valve, the flow meter outputsa measurement result to the control device 1012 when measuring the flow rate, and thecontrol device 1012 outputs a control signal corresponding to the opening degree of thevalve according to the measurement result from each flow meter.Therefore, according to the measurement results from the flow rate control devices410a, 410b, and 410c, the control device 1012 can control the flow rates of the concentratedwater c0 introduced into the first chambers 44a, 44b, and 44c of the RO membrane modules41a, 41b, and 41c to be the same, or can control a specific valve to be closed so that theconcentrated water c0 is not introduced into the first chamber 44 of the RO membranemodules 41a, 41b, and 41c that cannot be operated due to a failure or maintenance. Inaddition, according to the measurement results from the flow rate control devices 411a,411b, and 411c, it can control the flow rates of the merged water gn from the secondchambers 45a, 45b, and 45c of the RO membrane modules 41a, 41b, and 41c to be thesame, or can control a specific valve to be closed so that the merged water gn is notdischarged from the second chamber 45 of the RO membrane modules 41a, 41b, and 41cthat cannot be operated due to failure or maintenance.In this manner, the flow rate of the concentrated water c0 introduced into the firstchambers 44a, 44b, and 44c, the flow rate of the concentrated water c1 discharged from thefirst chambers 44a, 44b, and 44c, the flow rate of the merged water gi introduced into thesecond chambers 45a, 45b, and 45c, and the flow rate of the merged water gn from thesecond chambers 45a, 45b, and 45c can be controlled.Similarly, the flow rate of the concentrated water c1 introduced into the firstchambers 1024a and 1024b, the flow rate of the concentrated water ci discharged from thefirst chambers 1024a and 1024b, the flow rate of the mixed water g1 introduced into thesecond chambers 1025a and 1025b, and the flow rate of the merged water gi dischargedfrom the second chambers 1025a and 1025b can be controlled.Note that, as in the water treatment system 1E, in a concentrating membraneseparation unit 1003, the configuration in which the three RO membrane modules 41a, 41b,and 41c disposed in parallel in the first continuation concentrating RO membraneseparation unit 40 are disposed in series in the two RO membrane modules 1021a and1021b disposed in parallel in the intermediate continuous concentrating RO membraneseparation unit 1020, and the two RO membrane modules 1021a and 1021b disposed inparallel in the intermediate continuous concentrating RO membrane separation unit 1020are disposed in series in the RO membrane module 51 of a final continuation concentratingRO membrane separation unit 50 is merely a typical example of the present embodiment,and if the configuration is not limited to the configuration of the water treatment system 1E,and the number of RO membrane modules included in the concentrating membraneseparation unit 1003 is not larger than the number of RO membrane modules included inthe concentrating RO membrane separation unit disposed in the immediately previouscontinuation, it is included in the present embodiment.That is, in the example of FIG. 6, "(Number of RO membrane modules 41 of firstcontinuation concentrating RO membrane separation unit 40) : (Number of RO membranemodules 1021 of intermediate continuous concentrating RO membrane separation unit1020) : (Number of final continuation concentrating RO membrane separation unit 50)" is"3 : 2 : 1 ", but other than such a configuration, for example, a configuration such as" 3 : 3 :1 ", "5 : 4 : 3 ", or" 10 : 5 : 1 " may be used. Further, the concentrating RO membraneseparation unit disposed in series are not limited to three continuation, and for example,when the concentrating RO membrane separation unit disposed in series are set as fourcontinuation, the ratio of the number of RO membrane modules included in eachconcentrating RO membrane separation unit may be used to "4 : 3 : 2 : 1".As described above, according to the water treatment system of the thirdembodiment, as in the three RO membrane modules 41a, 41b, and 41c in the firstcontinuation concentrating RO membrane separation unit 40 and the two RO membranemodules 1021a and 1021b in the intermediate continuous concentrating RO membraneseparation unit 1020, the concentrating RO membrane separation unit includes the pluralityof RO membrane modules disposed in parallel under the conditions that the number ofconcentrating RO membrane separation unit is not larger than the number of RO membranemodules of the concentrating RO membrane separation unit on the upstream side, so that itis possible to increase the treatment capacity.Thus, for example, even when one RO membrane module 41 cannot be operated dueto a failure or maintenance as in the RO membrane module 41a, the operation can becontinued by another RO membrane module 41 such as the RO membrane modules 41b and41c, so that the operation redundancy can be increased.(Fourth embodiment)A fourth embodiment will be described.FIG. 7 is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method of the fourth embodiment isapplied.A water treatment system 1F exemplified in FIG. 7 is a modification example of thewater treatment system 1 of the first embodiment exemplified in FIG. 1, and is differentfrom the water treatment system 1 particularly in including a heat treatment unit 70.The heat treatment unit 70 is connected to a flow path 236 connected to a flow path1013 in the water treatment system 1F.The heat treatment unit 70 can include, for example, an evaporation concentrator, anevaporation dryer, and the like.The heat treatment unit 70 receives a concentrated water cn flowing in the flow path1013 from the flow path 236, heats the concentrated water cn to evaporate moisture, therebyfurther concentrating the concentrated water cn, and further heats and dries the concentratedconcentrated water cn to form a solid salt.A flow path 237 and a flow path 238 are connected to the heat treatment unit 70, andthe solid salt is discharged from the flow path 237 and the evaporated moisture isdischarged from the flow path 238.According to the water treatment system 1F of the fourth embodiment having such aconfiguration, the concentrated water cn having a high degree of concentration dischargedfrom the first chamber 54 of the RO membrane module 51 of the final continuationconcentrating RO membrane separation unit 50 can be further subjected to concentration,evaporation, drying treatment, and the like in the heat treatment unit 70.By the treatment, the solid salt can be recovered from the concentrated water cn. Forexample, when a main component of an ion content in the concentrated water cn is a salt ioncontent, the salt ion content can be recovered as a solid salt. Such a solid salt is dischargedfrom the flow path 237 to the outside of the system. In addition, moisture is dischargedfrom the flow path 238 to the outside of the system.The heat treatment unit 70 may further include a crystallizer, a centrifugal separator,and the like. By further providing a crystallizer and a centrifugal separator, recovery of thesolid salt can be performed more smoothly.The heat treatment unit 70 is not limited to being added to the water treatmentsystem 1 of the first embodiment, and can be similarly added to water treatment systems ofother embodiments.For example, as in the water treatment systems 1C, 1D, and 1E described above, theheat treatment unit 70 can be added to a configuration capable of realizing further higherconcentration of the concentrated water cn.The heat treatment unit 70 requires thermal energy for concentration, evaporation,and drying treatment as described above. However, as the degree of concentration of theconcentrated water cn is higher, the amount of moisture contained in the concentrated watercn is smaller. Therefore, when the heat treatment unit 70 is added to the configurationcapable of further higher concentration of the concentrated water cn as in the watertreatment systems 1C, 1D, and 1E, for example, the thermal energy consumed at the time ofconcentration, evaporation, and drying treatment in the heat treatment unit 70 can bereduced.(Fifth embodiment)A fifth embodiment will be described.FIG. 8 is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method of the fifth embodiment isapplied.A water treatment system 1G exemplified in FIG. 8 is a modification example of thewater treatment system 1F of the fourth embodiment exemplified in FIG. 7, and is differentfrom the water treatment system 1F particularly in that a pretreatment unit 80 is provided onthe upstream side of an introduction unit 10 of the water treatment system 1F.The pretreatment unit 80 includes a solid content / organic component removingapparatus 81, a water softening treatment apparatus 82, and a degassing apparatus 83.FIG. 8 illustrates, as an example, a configuration in which the solid content / organiccomponent removing apparatus 81, the water softening treatment apparatus 82, and thedegassing apparatus 83 are disposed in series along the flow direction of a water to betreated a, but the disposition order is not limited thereto, and can be appropriately changed.In the water treatment system 1G, with the addition of the pretreatment unit 80, aflow path 211 for receiving the water to be treated a is not connected to a liquid mixing tank11 but is connected to the solid content / organic component removing apparatus 81.A pump (not illustrated) is disposed in a flow path 239 between the solidcontent / organic component removing apparatus 81 and the water softening treatmentapparatus 82 and a flow path 240 between the water softening treatment apparatus 82 andthe degassing apparatus 83, and the water to be treated a is delivered to the apparatus on thedownstream side by the pump.Note that the pretreatment unit 80 does not necessarily need to include all of thesolid content / organic component removing apparatus 81, the water softening treatmentapparatus 82, and the degassing apparatus 83, and may include any one of them.As the solid content / organic component removing apparatus 81, for example, amembrane separation apparatus including a microfiltration (MF) membrane, anultrafiltration (UF) membrane, and a membrane bioreactor (MBR) method can be applied.Such a solid content / organic component removing apparatus 81 filters the water to betreated a introduced from the flow path 211 to remove the solid content / organic componentfrom the water to be treated a. The water to be treated a from which the solid content andthe organic component have been removed flows in the flow path 239 by driving of a pump(not illustrated) disposed in the flow path 239 and is delivered to the water softeningtreatment apparatus 82.As the water softening treatment apparatus 82, for example, an ion exchange resinor a flocculation precipitation apparatus can be applied. The water softening treatmentapparatus 82 removes hardness components such as calcium and magnesium in thedelivered water to be treated a to soften the water to be treated a. The softened water to betreated a flows in the flow path 240 and is delivered to the degassing apparatus 83 bydriving of a pump (not illustrated) disposed in the flow path 240.As the degassing apparatus 83, for example, an existing degassing apparatus usedfor pure water production or the like, such as a filled decarbonation tower, an aerator, adegassing apparatus, or a vacuum degassing apparatus, can be applied. For the purpose ofimproving the degassing efficiency, it is desirable that the degassing apparatus 83 add anacid such as hydrochloric acid or sulfuric acid to the water to be treated a and lower the pHof the water to be treated a to 7.0 or less, preferably 5.5 or less to perform the treatment.The degassed water to be treated a flows in a flow path 241. A pump 14 of the introductionunit 10 is disposed in the flow path 241, and the water to be treated a from the degassingapparatus 83 is delivered to the liquid mixing tank 11 by the pump 14.According to the water treatment system 1G of the fifth embodiment having such aconfiguration, by the pretreatment unit 80 added to the upstream side of the introductionunit 10, it is possible to remove a solid content and an organic component from the water tobe treated a such as factory waste water in addition to the salt content-containing wastewater, and further perform softening of water and degassing.In each of RO membrane modules 21, 41, and 51 on the downstream side, the waterto be treated a thus pretreated is concentrated, so that the water treatment system 1G canenhance the effect of preventing scale and biofouling in RO membrane elements 23, 43, and53.Note that the pretreatment unit 80 is not limited to being added to the watertreatment system 1F of the fourth embodiment, and can be similarly added to watertreatment systems of other embodiments.(Sixth embodiment)A sixth embodiment will be described.FIG. 9 is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method of the sixth embodiment isapplied.A water treatment system 1H exemplified in FIG. 9 is a modification example of awater treatment system 1G of the fifth embodiment exemplified in FIG. 8, and is differentfrom the water treatment system 1G particularly in that a membrane separation unit 90 isadded between a pretreatment unit 80 and an introduction unit 10 of the water treatmentsystem 1G.The membrane separation unit 90 includes an RO membrane module 91. Similarlyto other RO membrane modules 21, 41, and 51, the RO membrane module 91 includes apressure-resistant sealed pressure vessel 92, and in the pressure vessel 92, an RO membraneelement 93 that partitions the inside of the pressure vessel 92 into a first chamber 94 and asecond chamber 95 is disposed.The pretreated water to be treated a is introduced into the first chamber 94 from thepretreatment unit 80. For the purpose, a flow path 241 connected to a degassing apparatus83 disposed on the most downstream side of the pretreatment unit 80 is connected to thepressure vessel 92 in which the first chamber 94 is located. In the flow path 241, a pressureincreasing pump 101 that delivers the pretreated water to be treated a from the degassingapparatus 83 to the first chamber 94 is disposed.A flow path 242 leading to a liquid mixing tank 11 of the introduction unit 10 is alsoconnected to the pressure vessel 92 in which the first chamber 94 is located.Further, a flow path 243 is connected to the pressure vessel 92 in which the secondchamber 95 is located. The flow path 243 merges to a flow path 210.According to the water treatment system 1H of the sixth embodiment having such aconfiguration, the water to be treated a from the degassing apparatus 83 increased inpressure by the pressure increasing pump 101 and is delivered from the flow path 241 to thefirst chamber 94 of the RO membrane module 91.In the RO membrane module 91, the water component in the water to be treated apermeates through the RO membrane element 93 from the first chamber 94 to become apermeated water t0, and moves to the second chamber 95. As a result, the water to be treateda in which the solid content (salt content) is concentrated is obtained on the first chamber94 side.The concentrated water to be treated a obtained in the first chamber 94 flows in theflow path 242 and is introduced into the liquid mixing tank 11.On the other hand, the permeated water t0 that has permeated the second chamber 95flows in the flow path 243 and is discharged to the outside of the RO membrane module 91.Since the flow path 243 merges to the flow path 210, when the permeated water t0 flowingin the flow path 243 reaches the flow path 210, the permeated water t0 merges with thedeionized water b discharged from a second chamber 25, flows in the flow path 210, and isdischarged to the outside of the system.As described above, the water treatment system 1H of the sixth embodiment canintroduce the water to be treated a in which the solid content (salt content) is concentratedin the membrane separation unit 90 into the liquid mixing tank 11.As a result, since the load on RO membrane elements on the downstream side suchas RO membrane elements 23, 43, and 53, for example, is reduced, it is possible to providegood conditions for the deionization operation in a desalting membrane separation unit 20and the concentration operation in a first continuation concentrating RO membraneseparation unit 40 and a final continuation concentrating RO membrane separation unit 50.Note that the membrane separation unit 90 is not limited to being added to the watertreatment system 1G of the fifth embodiment, and can be similarly added to water treatmentsystems of other embodiments.(Modification example 1 of sixth embodiment)Next, a modification example 1 of the sixth embodiment will be described.FIG. 10 is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method according to the modificationexample 1 of the sixth embodiment is applied.A water treatment system 1I exemplified in FIG. 10 is a modification example 1 ofthe water treatment system 1H exemplified in FIG. 9, and in particular, a water storage tank102 is added between the membrane separation unit 90 and the introduction unit 10. Inaddition, not the flow path 242 but a flow path 244 is connected from the first chamber 94,and the concentrated water to be treated a obtained in the first chamber 94 flows in the flowpath 244 and is sent to the water storage tank 102.A flow path 245 is connected to a lower side surface of the water storage tank 102,and a pump 103 is provided at the other end of the flow path 245. The discharge outlet ofthe pump 103 is connected to the flow path 242 leading to the liquid mixing tank 11.According to the water treatment system 1I of the modification example 1 of thesixth embodiment having such a configuration, the concentrated water to be treated a in thefirst chamber 94 of the membrane separation unit 90 flows through the flow path 244, issent to the water storage tank 102, and is stored in the water storage tank 102.The water to be treated a stored in the water storage tank 102 flows in the flow paths245 and 242 by the pump 103 driven at an arbitrary timing and is delivered to the liquidmixing tank 11 of the introduction unit 10. The arbitrary timing is, for example, a timingaccording to an operator's instruction, a timing for each designated cycle, a timing fordriving the heat treatment unit 70, and the like.As a result, since the mixed water stored in the liquid mixing tank 11 can be alwaysmaintained at an appropriate amount, stable liquid mixing performance in the liquid mixingtank 11 can be exhibited.(Modification example 2 of sixth embodiment)Next, a modification example 2 of the sixth embodiment will be described.In the water treatment system of the present modification example, a nanofiltrationmembrane module is used instead of the RO membrane module 91 in the water treatmentsystems 1H and 1I shown in FIGS. 9 and 10. In the nanofiltration membrane module, ananofiltration membrane element (not illustrated) is applied instead of the RO membraneelement 93 in order to partition the first chamber 94 and the second chamber 95 of themembrane separation unit 90.As the nanofiltration membrane element, for example, a membrane having a spiralshape or a hollow fiber shape can be used. Note that a plurality of nanofiltration membraneelements may be provided in the pressure vessel 92.It is possible to select whether to use the membrane separation unit 90 according tothe properties of the water to be treated a delivered from the pretreatment unit 80, or to usea nanofiltration membrane element.As described above, an appropriate membrane can be used instead of the ROmembrane module 91 in accordance with the properties of the water to be treated adelivered from the pretreatment unit 80.(Seventh embodiment)A seventh embodiment will be described.FIG. 11 is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method of the seventh embodiment isapplied.A water treatment system 1J of a seventh embodiment illustrated in FIG. 7 is amodification example of the water treatment system 1 of the first embodiment exemplifiedin FIG. 1, and is different from the water treatment system 1 particularly in including apower recovery unit 110.The power recovery unit 110 is an apparatus that recovers the pressure energy of aconcentrated water cn having a high pressure delivered from a first chamber 54 of an ROmembrane module 51 of a final continuation concentrating RO membrane separation unit50.The power recovery unit 110 includes a power recovery mechanism 111 and a pump112. The power recovery mechanism 111 and the pump 112 are connected to each other.The power recovery mechanism 111 is connected to a flow path 1013, and receivesthe concentrated water cn discharged from the first chamber 54 and flowing in the flow path1013.Although not illustrated, the power recovery mechanism 111 includes, for example,a turbine or a piston processed to be adaptable to a high salt content, and converts pressureenergy of the concentrated water cn into kinetic energy by the turbine or the piston whenreceiving the concentrated water cn from the flow path 1013.A flow path 246 leading to the outside is connected to the power recoverymechanism 111. Therefore, the power recovery mechanism 111 discharges the concentratedwater cn from which the pressure energy has been recovered to the outside from the flowpath 246.The power recovery mechanism 111 is connected to the pump 112 so that kineticenergy can be transmitted to the pump 112. Such a connection can be realized, for example,but not limited to, by directly or indirectly connecting the rotation shaft of the turbine of thepower recovery mechanism 111 and the rotation shaft of the pump 112.Therefore, the power recovery mechanism 111 can transmit kinetic energy obtainedby converting the pressure energy of the concentrated water cn to the pump 112.The pump 112 is disposed in a flow path 211 leading to a liquid mixing tank 11. Asa result, the pump 112 can function as an introduction pump for increasing the pressure ofthe water to be treated a flowing in the flow path 211 and delivering a water to be treated ato the liquid mixing tank 11.According to the water treatment system 1J of the seventh embodiment having sucha configuration, the pressure energy of the concentrated water cn is converted into kineticenergy by the power recovery mechanism 111, the pump 112 is driven by the kinetic energy,and the water to be treated a can be delivered to the liquid mixing tank 11. Such effectiveuse of energy makes it possible to reduce the energy consumption of an introduction unit10. It is therefore also possible to eliminate a pump 14 in the introduction unit 10.Such a power recovery unit 110 is not limited to being added to the water treatmentsystem 1 of the first embodiment, and can be similarly added to water treatment systems ofother embodiments.In addition, the pump 112 is eliminated from the power recovery unit 110, and thepower recovery mechanism 111 is connected to any pump (for example, pumps 14, 15, 101,103, and 1002) used not only in the water treatment system 1 of the first embodiment butalso in the water treatment systems of other embodiments so as to transmit kinetic energyinstead of the pump 112, thus the drive energy of all the pumps used in the water treatmentsystem 1J can be further reduced.(Eighth embodiment)An eighth embodiment will be described.FIG. 12 is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method of the eighth embodiment isapplied.A water treatment system 1K of the eighth embodiment illustrated in FIG. 12 is amodification example of the water treatment system 1 of the first embodiment exemplifiedin FIG. 1, and in particular, a place where a circulating water pressure reducing unit 60including a pressure reducing device 61 is disposed and a flow direction of water in asecond chamber 55 are different from those of the water treatment system 1 of the firstembodiment illustrated in FIG. 1.In both the water treatment systems 1 and 1K, the pressure reducing device 61 has asuction outlet connected to a flow path 208 and a discharge outlet connected to a flow path209.However, in water treatment system 1, the flow path 208 is branched from a flowpath 206, whereas in water treatment system 1K, the flow path 208 is branched from a flowpath 205.Further, in each of the water treatment systems 1 and 1K, the flow path 209 isconnected to the second chamber 55. However, in the case of the water treatment system 1,the flow path 209 is connected to the second chamber 55 from the right side surface in thedrawing, whereas in the case of the water treatment system 1K, the flow path 209 isconnected to the second chamber 55 from the left side surface in the drawing.In addition, a flow path 207 is connected to the left side of the second chamber 55 inthe drawing in the water treatment system 1, but is connected to the right side of the secondchamber 55 in the drawing in the water treatment system 1K on the contrary.As a result, the flow of water in the second chamber 55 is from the right side to theleft side in the drawing in the water treatment system 1, whereas the flow of water is fromthe left side to the right side in the drawing in the water treatment system 1K.According to the water treatment system 1K of the eighth embodiment having sucha configuration, the circulating water pressure reducing unit 60 reduces in pressure of a partof the concentrated water ci from the first chamber (in the example of FIG. 12, a firstchamber 44) of the RO membrane module (in the example of FIG. 12, an RO membranemodule 41) of the final continuation and immediately previous continuation concentratingRO membrane separation unit (in the example of FIG. 12, a first continuation concentratingRO membrane separation unit 40), and delivers the pressure-reduced concentrated water cito a second chamber 55.As a result, the concentrated water ci having a pressure lower than that of theconcentrated water ci in a first chamber 54 is introduced into the second chamber 55.For example, the pressure decrease amount of the concentrated water ci introducedinto the second chamber 55 is preferably 1 MPa or more, and more preferably 3 MPa ormore.By setting the pressure decrease amount to 3 MPa or more, the pressure of theconcentrated water ci introduced into the first chamber 54 becomes sufficiently higher thanthe pressure of the concentrated water ci introduced into the second chamber 55, and thus,the water component contained in the concentrated water ci in the first chamber 54 easilypermeates through an RO membrane element 53 and reaches the second chamber 55, so thatit becomes easy to concentrate the concentrated water ci in the first chamber 54 to theconcentrated water cn having a higher concentration.Note that, as exemplified in FIG. 12, the feature of the eighth embodiment that thecirculating water pressure reducing unit 60 is disposed immediately before a finalcontinuation concentrating RO membrane separation unit 50 can be similarly applied notonly to the water treatment system 1 exemplified in FIG. 1 but also to other water treatmentsystems already described. In particular, a modification example in which the eighthembodiment is applied to water treatment system 1C illustrated in FIG. 5 is described as aninth embodiment described below.(Ninth embodiment)A ninth embodiment will be described.FIG. 13 is a schematic diagram illustrating a functional configuration example of awater treatment system to which a water treatment method of the ninth embodiment isapplied.A water treatment system 1L of the ninth embodiment illustrated in FIG. 13 is amodification example in which the concept of the water treatment system 1K of the eighthembodiment exemplified in FIG. 12 is applied to a water treatment system 1C exemplifiedin FIG. 5.Therefore, the water treatment system 1L is different from the water treatmentsystem 1K in that the water treatment system 1L includes an intermediate continuousconcentrating RO membrane separation unit 1020 between a first continuationconcentrating RO membrane separation unit 40 and a final continuation concentrating ROmembrane separation unit 50 in a concentrating membrane separation unit 1003.The intermediate continuous concentrating RO membrane separation unit 1020 is asdescribed with reference to FIG. 5 in the second embodiment, and thus redundantdescription is avoided here.According to the water treatment system 1L of the ninth embodiment having such aconfiguration, since the number of the concentrating RO membrane separation unit isincreased by one continuation as compared with the water treatment system 1K shown inFIG. 12, the concentration of a concentrated water cn discharged from the final continuationconcentrating RO membrane separation unit 50 can be further increased.Note that the concepts of other embodiments described above can be similarlyapplied to the water treatment system 1L of the ninth embodiment shown in FIG. 13.For example, on the downstream side of a flow path 1013, a heat treatment unit 70can be added as described in the fourth embodiment.In addition, on the upstream side of an introduction unit 10, a pretreatment unit 80can be added as described in the fifth embodiment.Furthermore, on the upstream side of the introduction unit 10, the pretreatment unit80 and a membrane separation unit 90 can be added as described in the sixth embodiment,or a water storage tank 102 can be further added as described in modification example 1 ofthe sixth embodiment.Furthermore, on the downstream side of the flow path 1013, a power recovery unit110 can be added as described i n the seventh embodiment.As described above, when various components are added to the water treatmentsystem 1L of the present embodiment, the functions and effects obtained by adding thesecomponents can be obtained in addition to the functions and effects of highly concentratingthe concentrated water cn obtained in the present embodiment.As described above, according to the water treatment system to which the watertreatment methods of the first to ninth embodiments are applied, it is possible to stablyobtain concentrated water and deionized water of expected amounts and qualities by usingthe multi-stage RO membrane.While certain embodiments have been described, these embodiments have beenpresented by way of example only, and are not intended to limit the scope of the inventions.Indeed, the novel embodiments described herein may be embodied in a variety of otherforms; furthermore, various omissions, substitutions and changes in the form of theembodiments described herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intended to cover suchforms or modifications as would fall within the scope and spirit of the inventions.For example, in the above embodiment, the RO membrane is applied forconcentration in a desalting membrane separation unit 20 and the concentrating membraneseparation unit 1003, but a nanofiltration membrane may be applied instead of the ROmembrane. In the above embodiment, it is understood that a water treatment system towhich a nanofiltration membrane is applied instead of the RO membrane is also included inthe scope of the invention of the present application.

Claims

1. A water treatment system comprising: an introduction unit that delivers water to be treated; a desalting separation unit that separates the water to be treated delivered from the introduction unit into concentrated water and deionized water; a plurality of continuation of concentrating membrane separation units disposed in series, wherein the concentrated water is concentrated by a membrane to be separated into concentrated water having a higher degree of concentration than the concentrated water and permeated water that permeated through the membrane, wherein a concentrating membrane separation unit disposed in first continuation among the plurality of continuation of concentrating membrane separation units concentrates the concentrated water separated by the desalting separation unit, concentrating membrane separation units other than the concentrating membrane separation unit disposed in first continuation among the plurality of continuation of concentrating membrane separation units concentrate the concentrated water concentrated by the concentrating membrane separation unit disposed in the immediately previous continuation and supply the permeated water separated from the concentrated water to the concentrating membrane separation unit disposed in the immediately previous continuation or on the first continuation side rather than the immediately previous continuation so as to merge with the permeated water separated by the concentrating membrane separation unit disposed in the immediately previous continuation or on the first continuation side rather than the immediately previous continuation, and the concentrating membrane separation unit disposed in first continuation supplies permeated water that is merged with the permeated water introduced from the concentrating membrane separation unit disposed in the immediately after continuation or on the final continuation side of the immediately after continuation and permeated through a membrane of the concentrating membrane separation unit disposed in first continuation to the introduction unit as circulating water; a first measuring device for measuring a flow rate of concentrated water introduced from the desalting separation unit to the concentrating membrane separation unit disposed in first continuation; a second measuring device for measuring a flow rate of the circulating water introduced into the introduction unit from the concentrating membrane separation unit disposed in first continuation; and a control device that adjusts at least one selected from the group consisting of a delivered flow rate, a discharged flow rate, and a first merged flow rate, based on the flow rate measured by the first measuring device and the flow rate measured by the second measuring device, wherein the delivered flow rate is a flow rate of the water to be treated which is delivered from the introduction unit to the desalting separation unit, the discharged flow rate is a flow rate of the concentrated water which is concentrated by and discharged from the concentrating membrane separation unit disposed in final continuation among the plurality of continuation of concentrating membrane separation units, and the first merged flow rate is a flow rate of, among the concentrated water concentrated by the concentrating membrane separation unit disposed in final continuation, the concentrated water to be merged with the permeated water separated by the concentrating membrane separation unit disposed in final continuation.

2. The water treatment system according to claim 1, wherein the introduction unit comprises a pump that increases the pressure of the water to be treated and delivers the water to be treated, and the control device controls the pump when the delivered flow rate is adjusted.

3. The water treatment system according to claim 1 or 2, further comprising a flow rate adjusting device that adjusts the discharged flow rate, wherein the control device controls the flow rate adjusting device when the discharged flow rate is adjusted.

4. The water treatment system according to any one of claims 1 to 3, further comprising a pressure reducing device disposed in a branched flow path in order to merge part of the concentrated water concentrated by the concentrating membrane separation unit disposed in final continuation with the permeated water separated by the concentrating membrane separation unit disposed in final continuation, wherein the control device controls the pressure reducing device when the first merged flow rate is adjusted.

5. The water treatment system according to any one of claims 1 to 4, wherein the adjusting by the control device based on the flow rate measured by the first measuring device and the flow rate measured by the second measuring device is to adjust so that the ratio of the flow rate measured by the second measuring device to the flow rate measured by the first measuring device is a predetermined value.

6. The water treatment system according to claim 5, wherein the predetermined value is 0.4 or more and 0.9 or less.

7. The water treatment system according to any one of claims 1 to 6, wherein each of the plurality of continuation of concentrating membrane separation units comprises one or more membrane modules disposed in parallel to each other, each of the membrane modules comprises the membrane, and the number of the membrane modules comprised in the concentrating membrane separation unit is not larger than the number of the membrane modules comprised in the concentrating membrane separation unit disposed in the immediately previous continuation.

8. The water treatment system according to any one of claims 1 to 7, further comprising a heat treatment unit that heats concentrated water concentrated by the concentrating membrane separation unit disposed in final continuation.

9. The water treatment system according to any one of claims 1 to 8, further comprising a pretreatment unit that performs a pretreatment including at least one selected from the group consisting of removal of a solid content, removal of an organic component, softening of water, and degassing with respect to the water to be treated before being introduced into the introduction unit.

10. The water treatment system according to claim 9, further comprising a concentrating part that concentrates the water to be treated pretreated by the pretreatment unit before being introduced into the introduction unit.

11. The water treatment system according to claim 10, further comprising a tank that sores the water to be treated concentrated by the concentrating part and a delivering part that delivers the water to be treated stored in the water storage tank to the introduction unit.

12. The water treatment system according to claim 1, wherein the control device adjusts, instead of the first merged flow rate, a second merged flow rate which is a flow rate of, among the concentrated water concentrated by concentrating membrane separation unit disposed in the immediately previous continuation of the final continuation, the concentrated water to be merged with permeated water separated by the concentrating membrane separation unit disposed in final continuation.

13. The water treatment system according to claims 12, further comprising a pressure reducing device disposed in a branched flow path in order to merge part of the concentrated water concentrated by the concentrating membrane separation unit disposed in the immediately previous continuation of the final continuation with the permeated water separated by the concentrating membrane separation unit disposed in final continuation, wherein the control device controls the pressure reducing device when the second merged flow rate is adjusted.

14. The water treatment system according to any one of claims 1 to 13, wherein a fine bubble having an average diameter of 1000 (nm) or less are contained in the circulating water at a concentration of 1 x 106 (pieces / mL) or more.

15. The water treatment system according to any one of claims 1 to 14, wherein the pH of the concentrated water concentrated by the concentrating membrane separation unit disposed in final continuation and the pH of the circulating water are 6 or less.

16. A water treatment method comprising: delivering water to be treated to an introduction unit; separating the water to be treated delivered from the introduction unit into concentrated water and deionized water by a desalting separation unit; separating the concentrated water into concentrated water having a higher degree of concentration than the concentrated water and permeated water that has permeated through the membrane by concentrating the concentrated water with a membrane in each of the plurality of continuation of concentrating membrane separation units disposed in series, the separating the concentrated water comprising; concentrating, by a concentrating membrane separation unit disposed in first continuation among the plurality of continuation of concentrating membrane separation units, the concentrated water separated by the desalting separation unit, concentrating, by the concentrating membrane separation units other than the concentrating membrane separation unit disposed in first continuation among the plurality of continuation of concentrating membrane separation units, the concentrated water concentrated by the concentrating membrane separation unit disposed in the immediately previous continuation, and supplying the permeated water separated from the concentrated water to the concentrating membrane separation unit disposed in the immediately previous continuation or on the first continuation side rather than the immediately previous continuation so as to merge with the permeated water separated by the concentrating membrane separation unit disposed in the immediately previous continuation or on the first continuation side rather than the immediately previous continuation, and supplying, by the concentrating membrane separation unit disposed in first continuation, the permeated water merged with the permeated water sent from the concentrating membrane separation unit disposed in the immediately after continuation or on the final continuation side of the immediately after continuation and obtained by separation using the membrane of the concentrating membrane separation unit disposed in first continuation to the introduction unit as circulating water; measuring a flow rate of concentrated water introduced from the desalting separation unit to the concentrating membrane separation unit disposed in first continuation; measuring a flow rate of the circulating water introduced from the concentrating membrane separation unit disposed in first continuation to the introduction unit; and adjusting at least one selected from the group consisting of a delivered flow rate, a discharged flow rate, and a merged flow rate, based on the flow rate measured by the measuring flow rate of concentrated water and the flow rate measured by the measuring flow rate of the circulating water, wherein the delivered flow rate is a flow rate of the water to be treated which is delivered from the introduction unit to the desalting separation unit, the discharged flow rate is a flow rate of the concentrated water which is concentrated by and discharged from the concentrating membrane separation unit disposed in final continuation among the plurality of continuation of concentrating membrane separation units, and the merged flow rate is a flow rate of, among the concentrated water concentrated by the concentrating membrane separation unit disposed in final continuation, the concentrated water to be merged with the permeated water separated by the concentrating membrane separation unit disposed in final continuation.