1. A desalination system, comprising: a water intake device that takes therein water to be treated; a pretreatment device that performs a pretreatment to the water to be treated taken by the water intake device for removing impurities therein; a first pump that pressure-feeds the water to be treated to which the pretreatment has been subjected by the pretreatment device; a plurality of first reverse osmosis treatment devices each of which membrane- filters the water to be treated which is discharged from the first pump and flows in a primary side of the first reverse osmosis treatment devices itself via branch piping; a second pump that pressurizes the water to be treated which flows out from respective primary sides of the first reverse osmosis treatment devices and is merged via junction piping; a second reverse osmosis treatment device that membrane-filters the water to be treated which is discharged from the second pump and flows in a primary side of the second reverse osmosis treatment device itself; a first energy recovery device that pressurizes the water to be treated which is branched off from more upstream of the first pump using a residual pressure of a concentrated water flowing out from the primary side of the second reverse osmosis treatment device, and that pressure-feeds the pressurized water to be treated to the respective primary sides of the first reverse osmosis treatment devices; a water supply device that supplies permeated water flowing out from respective secondary sides of the first reverse osmosis treatment devices as well as permeated water flowing out from a secondary side of the second reverse osmosis treatment device, to a demand side; and a discharge device that discharges the concentrated water flowing out from the primary side of the second reverse osmosis treatment device through the first energy recovery device.

2. A desalination system, comprising: a water intake device that takes therein water to be treated; a pretreatment device that performs a pretreatment to the water to be treated taken by the water intake device for removing impurities therein; a first pump that pressure-feeds the water to be treated to which the pretreatment has been subjected by the pretreatment device; a plurality of first reverse osmosis treatment devices each of which membrane-filters the water to be treated which is discharged from the first pump and flows in a primary side of the first reverse osmosis treatment devices itself via branch piping; a second pump that pressurizes the water to be treated whichflows out from respective primary sides of the first reverse osmosis treatment devices and is merged via junction piping; a second reverse osmosis treatment device that membrane-filters the water to be treated which is discharged from the second pump and flows in a primary side of the second reverse osmosis treatment device itself; a first energy recovery device that pressurizes the water to be treated which is branched off from more upstream of the first pump using a residual pressure of a concentrated water flowing out from the primary side of the second reverse osmosis treatment device, and that pressure-feeds the pressurized water to be treated to the respective primary sides of the first reverse osmosis treatment devices; a flow rate control unit that controls a flow rate of the water to be treated which is pressurized by the first energy recovery device; a water supply device that supplies permeated water flowing out from respective secondary sides of the first reverse osmosis treatment devices as well as permeated water flowing out from a secondary side of the second reverse osmosis treatment device, to a demand side; and a discharge device that discharges the concentrated water flowing out from the primary side of the second reverse osmosis treatment device through the first energy recovery device.

3. The desalination system as claimed in Claims 1 or 2, further comprising a second energy recovery device of pressure recovery type that pressurizes the water to be treated which flows out from the respective primary sides of the first reverse osmosis treatment devices and is merged via the junction piping, using a residual pressure of the concentrated water flowing out from the primary side of the second reverse osmosis treatment device, and pressure-feeds the pressurized water to be treated to an intake side of the second pump, and that is disposed such that the concentrated water whose residual pressure has been used flows in the first energy recovery device, wherein the first energy recovery device pressurizes the water to be treated which is branched off from more upstream of the first pump, using a residual pressure of the concentrated water flowing from the second energy recovery device, and pressure-feeds the pressurized water to be treated to the primary side of the first reverse osmosis treatment device.

4. The desalination system as claimed in any one of Claims 1 to 3, wherein a ratio of a pressure applying force of the second pump to a pressure applying force of the first pump is set to not less than 0.2and not more than 0.5.

TECHNICAL FIELD

[0001] The present invention relates to a desalination system equipped with a reverse osmosis membrane.

BACKGROUND ART

[0002] Water has been increasingly required in these years because of influence of global climate change or shortage of water, which leads to expectations of expanding a size of water market. Particular interest is focused on a desalination system in which seawater is flowed in a primary side of a reverse osmosis treatment device at a high pressure in excess of the osmotic pressure and is passed through a reverse osmosis membrane, to thereby obtain permeated water (freshwater) on a secondary side thereof. In such a system, concentrated water flowed out from the primary side of the reverse osmosis treatment device still has a prescribed residual pressure. There is thus a need for recovering energy possessed by the concentrated water and improving efficiency of the entire system.

[0003] For example, Patent Document 1 describes such a seawater desalinating method that "A portion of seawater is supplied to the reverse osmosis membrane module under pressure and is desalted. The pressure energy of concentrated water discharged from the reverse osmosis membrane module is recovered by the pressure transmission type energy recovering device".

[RELATED ART DOCUMENT]

[PATENT DOCUMENT]

[0004]

[Patent Document 1] Japanese Laid-Open Patent Application, Publication No. 2004-081913

SUMMARY OF THE INVENTION [PROBLEM TO BE SOLVED BY THE INVENTION]

[0005] In the technique disclosed in Patent Document 1, the seawater desalinating method has a configuration in which the remaining portions of seawater is pressurized using energy recovered by the pressure transmission type energy recovering device and is supplied to a second reverse osmosis membrane module. There is a need, however, for further enhancing a recovery ratio of the freshwater to the water to be treated in the entire desalination system and equalizing fluxes (permeation fluxes) of the reverse osmosis membrane module.

[0006] In light of the described above, the present invention has been made in an attempt to provide a desalination system having a high recovery ratio. [MEANS

FOR SOLVING THE PROBLEM]

[0007] To solve the above-described problem, one aspect of the present invention features a desalination system including: a water intake device that takes therein water to be treated; a pretreatment device that performs a pretreatment to the water to be treated taken by the water intake device for removing impurities therein; a first pump that pressure-feeds the water to be treated to which the pretreatment has been subjected by the pretreatment device; a plurality of first reverse osmosis treatment devices each of which membrane-filters the water to be treated which is discharged from the first pump and flows in a primary side of the first reverse osmosis treatment devices itself via branch piping; a second pump that pressurizes the water to be treated which flows out from respective primary sides of the first reverse osmosis treatment devices and is merged via junction piping; a second reverse osmosis treatment device that membrane-filters the water to be treated which is discharged from the second pump and flows in a primary side of the second reverse osmosis treatment device itself; a first energy recovery device that pressurizes the water to be treated which is branched off from more upstream of the first pump using a residual pressure of a concentrated water flowing out from the primary side of the second reverse osmosis treatment device, and that pressure-feeds the pressurized water to be treated to the respective primary sides of the first reverse osmosis treatment devices; a water supply device that supplies permeated water flowing out from respective secondary sides of the first reverse osmosis treatment devices as well as permeated water flowing out from a secondary side of the second reverse osmosis treatment device, to a demand side; and a discharge device that discharges the concentrated water flowing out from the primary side of the second reverse osmosis treatment device through the first energy recovery device.

[0008] Another aspect of the present invention features a desalination system including: a water intake device that takes therein water to be treated; a pretreatment device that performs a pretreatment to the water to be treated taken by the water intake device for removing impurities therein; a first pump that pressure-feeds the water to be treated to which the pretreatment has been subjected by the pretreatment device; a plurality of first reverse osmosis treatment devices each of which membrane-filters the water to be treated which is discharged from the first pump and flows in a primary side of the first reverse osmosis treatment devices itself via branch piping; a second pump that pressurizes the water to be treated which flows out from respective primary sides of the first reverse osmosis treatment devices and is merged via junction piping; a second reverse osmosis treatment device that membrane-filters the water to be treated which is discharged from the second pump and flows in a primary side of the second reverse osmosis treatment device itself; a first energy recovery device that pressurizes the water to be treated which is branched off from more upstream of the first pump using a residual pressure of a concentrated water flowing out from the primary side of the second reverse osmosis treatment device, and that pressure-feeds the pressurized water to be treated to the respective primary sides of the first reverse osmosis treatment devices; a flow rate control unit that controls a flow rate of the water to be treated which is pressurized by the first energy recovery device; a water supply device that supplies permeated water flowing out from respective secondary sides of the first reverse osmosis treatment devices as well as permeated water flowing out from a secondary side of the second reverse osmosis treatment device, to a demand side; and a discharge device that discharges the concentrated water flowing out from the primary side of the second reverse osmosis treatment device through the first energy recovery device.

[ADVANTAGEOUS EFFECTS OF THE INVENTION]

[0009] The present invention can provide a desalination system having a high recovery ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]

[FIG. 1] A block diagram illustrating a desalination system according to a first embodiment of the present invention.

[FIG. 2] A graph illustrating respective permeation fluxes (which may be simply referred to as fluxes) of RO elements which desalination systems according to the first embodiment and a comparative example are equipped with.

[FIG. 3] A block diagram illustrating a desalination system according to a second embodiment of the present invention.

[FIG. 4] A block diagram illustrating a desalination system according to a third embodiment of the present invention.

[FIG. 5] A block diagram illustrating a desalination system according to the comparative example.

EMBODIMENTS

[0011] Next are described in detail embodiments of the present invention with reference to related drawings. Note that the same reference numbers are given to components used in common in the drawings, and duplicate description of those components is omitted herefrom.

[0012] First Embodiment Configuration of Desalination System FIG. 1 is a block diagram illustrating a desalination system according to a first embodiment. Description below is made assuming that, as an example, a desalination system SI is used for desalinating seawater (water to be treated) to produce freshwater.
As shown in FIG. 1, the desalination system SI includes, as its principal components, an intake pump 1, a pretreatment device 3, a high pressure pump 5, first reverse osmosis treatment devices 6p, 6q, a booster pump 7, a second reverse osmosis treatment device 8, and an energy recovery device 11.

[0013] Intake pump The intake pump 1 (which may also be referred to as a water intake device) is a pump for taking therein water to be treated (for example, seawater around the desalination system SI), has an intake side connected to an inlet A via a pipe al, and also has a discharge side connected to an intake tank 2 (which may also be referred to as a water intake device) via a pipe a2. When the intake pump 1 is driven, the water to be treated flowing from the inlet A is taken in the intake pump 1 via the pipe al and is then discharged to the intake tank 2 via the pipe a2. The intake tank 2 is a tank for temporarily storing the water to be treated flowing thereinto via the pipe a2.

[0014] Pretreatment device The pretreatment device 3 is a device which performs a prescribed pretreatment to the water to be treated flowing thereinto via a pipe a3 so as to remove impurities. The pretreatment described above includes, for example, a chemical addition treatment, a flocculation magnetic separation treatment, a sand filter treatment, and an ultrafiltration treatment. Those treatments can be appropriately combined according to types of impurities contained in the water to be treated or the like. Next is explained each of those treatments.

[0015] The chemical addition treatment is a treatment of adding any one of chemicals such as, for example, sodium hypochlorite, sodium sulfite, ozone, chlorine dioxide, acetyl hydroperoxide, and hydrogen peroxide, or a combination thereof, to the water to be treated, and then, of stirring the mixture. Note that a treatment of neutralizing the water to be treated is also included in the chemical addition treatment. The flocculation magnetic separation treatment is a treatment of separating or removing magnetic floe from the water to be treated. In the flocculation magnetic separation treatment, magnetic powder, flocculant, and polymer flocculant is added to the water to be treated; the mixture is sequentially stirred at a high speed and at a low speed, to thereby grow magnetic floe; and the magnetic floe is collected by adsorption using a magnetic drum (not shown).

[0016] The sand filter treatment is a treatment of removing suspended matter from the water to be treated by, when the water to be treated descends with gravity, making the water to be treated pass through a filtering medium (such as silica sand and proppant sand and gravel). The ultrafiltration treatment is a treatment of filtering minute impurities using an ultrafilter (such as an UF membrane). After subjected to the pretreatment by the pretreatment device 3, the water to be treated is flowed into an RO (Reverse Osmosis) raw water tank 4 via a pipe a4 . The RO raw water tank 4 is a tank for temporarily storing the water to be treated having been subjected to the pretreatment.

[0017] High pressure pump The high pressure pump 5 (which may also be referred to as a first pump) is a pump which pressure-feeds the water to be treated having been subjected to the pretreatment by the pretreatment device 3. The high pressure pump 5 has an inlet connected to the RO raw water tank 4 via a pipe a5 and an outlet connected to respective primary sides of the first reverse osmosis treatment devices 6p, 6q via branch piping. The "branch piping" described above is configured to split, in two, the water to be treated flowing from the high pressure pump 5 toward the first reverse osmosis treatment devices 6p, 6q and includes pipes a6, bl, cl.

[0018] A pressure applying force of the high pressure pump 5 is a prescribed pressure which allows the water to be treated to pass through respective primary sides to respective secondary sides of the first reverse osmosis treatment devices 6p, 6q across reverse osmosis membranes (not shown) and can be set arbitrarily. In this embodiment, the pressure applying force of the high pressure pump 5 is set to 4. 8 MPa (<5 MPa) . The pressure applying force of the high pressure pump 5 set as low as herein (compared to that of the comparative example to be described later) makes it possible to equalize permeation fluxes of the first reverse osmosis treatment devices 6p, 6q and a permeation flux of the second reverse osmosis treatment device 8.

[0019] First reverse osmosis treatment device The first reverse osmosis treatment device 6p is a device which membrane-filtrates the water to be treated discharged flowing from the high pressure pump 5 and to the primary side thereof via the pipes a6, bl, to thereby obtain permeated water. The permeated water used herein means water obtained by making the water to be treated permeate through a RO (Reverse Osmosis) element and contains very little impurities such as salt. The first reverse osmosis treatment device 6p includes a cylindrical vessel (not shown), one or more RO elements (not shown) disposed in the vessel, and a water collecting pipe (not shown) disposed in such a way as to penetrate the center of each of the RO elements.

[0020] The RO element is prepared by winding up a reverse osmosis membrane (not shown) such that a plurality of pores (not shown) formed in the water collecting pipe are overlaid. Note that in a case where the first reverse osmosis treatment device 6p includes a plurality of RO elements, the RO elements are arranged in series with prescribed spaces apart from each other in an axial direction of the vessel. The reverse osmosis membrane described above has a property of allowing water contained in the water to be treated which is supplied with a pressure in excess of the osmotic pressure to pass therethrough, and not allowing impurities such as ions and salts other than the water to pass there through. Note that an outer circumferential surface of the RO element is in close contact with an inner circumferential surface of the above-described vessel.

[0021] The primary side of the first reverse osmosis treatment device 6p means an area more upstream of the RO element in a space within the vessel (if a plurality of RO elements are arranged in series, an area more upstream of the RO element positioned on the most downstream side) . The secondary side of the first reverse osmosis treatment device 6p means an area inside the above-described water collecting pipe. The same definition is applied to the second reverse osmosis treatment device 8 to be described hereinafter. The vessel of the first reverse osmosis treatment device 6p is connected to a pipe b3 for flowing out a water having a high saline concentration which would not pass through the RO element (to be referred to as a concentrated water) . On the other hand, a water which is allowed to pass through the RO element of the first reverse osmosis treatment device 6p as well as the above-described pores (that is, the permeated water) flows in the water collecting pipe and then flows into the permeated water tank 9 via the pipe b2.

[0022] In this embodiment, two RO elements are disposed in series in the vessel of the first reverse osmosis treatment device 6p. If the water to be treated is pumped in the vessel via the pipe bl at a prescribed pressure in excess of the osmotic pressure (for example, 4.8 MPa) , part of the water to be treated passes through the RO element on the upstream side and flows into the water collecting pipe via the above-described pores. The other part of the water to be treated which has not passes through the RO element on the upstream side is flowed to the RO element on the downstream side. Further, part of the other part of the water to be treated flowing to the RO element on the downstream side passes through the RO element, flows into the water collecting pipe, and is supplied to the permeated water tank 9 via the pipe b2. On the other hand, a resultant concentrated water (the water to be treated) which has passed through the RO element neither on the upstream side nor the RO element on the downstream side flows into an intake side of the booster pump 7 via pipes b3, dl.

[0023] The permeated water tank 9 is a tank which temporarily stores the permeated water supplied from the respective secondary sides of the first reverse osmosis treatment devices 6p, 6q and the permeated water supplied from the secondary side of the second reverse osmosis treatment device 8 . The permeated water stored in the permeated water tank 9 flows into a water supply pump 10 (which may also be referred to as a water supply device) via a pipe el and is then supplied to a demand side via a pipe e2. The demand side used herein corresponds to industrial areas, agricultural areas, cities, and the like in a vicinity of the desalination system SI.

[0024] The first reverse osmosis treatment device 6q has a configuration same as that of the first reverse osmosis treatment device 6p described above. That is, two RO elements (not shown) are also arranged in series in a vessel (not shown) of the first reverse osmosis treatment device 6q. The permeated water passes through either of the RO elements, flows into the water collecting pipe (not shown) , and then flows in the permeated water tank 9 via a pipe c2 shown in FIG. 1. On the other hand, concentrated water (the water to be treated) which has passed through neither of the RO elements flows into the intake side of the booster pump 7 via junction piping. The "junction piping" described above is configured such that the water to be treated (the concentrated water) flowing from the primary sides of the first reverse osmosis treatment devices 6p, 6q is made to merge and flow to the intake side of the booster pump 7. The junction piping includes pipes b3, c3, dl.

[0025] Booster pump The booster pump 7 (which may also be referred to as a second pump) is a pump for boosting a pressure of the water to be treated merged via the above-described junction piping. That is, the booster pump 7 raises the pressure of the water to be treated flowing via the pipe dl and pressure-feeds the pressurized water to be treated to the primary side of the second reverse osmosis treatment device 8. The booster pump 7 has an outlet connected to a primary side of the second reverse osmosis treatment device 8 via the pipe d2. The water to be treated flowing into an inlet of the booster pump 7 via a pipe dl (concentrated water) has a prescribed residual pressure which can be calculated by subtracting a pressure loss at the first reverse osmosis treatment device 6p or 6q from a discharge pressure of the high pressure pump 5. The residual pressure of the water to be treated is boosted to a prescribed pressure by the booster pump 7, which allows more permeated water to be extracted.

[0026] As described above, the pressure applying force of the high pressure pump 5 is set to a rather low level of 4.8 MPa (which is lower than the pressure set in the comparative example to be described later). This results in a relatively low saline concentration of the water to be treated flowing from the first reverse osmosis treatment devices 6p, 6q (concentrated water). In this embodiment, a pressure applying force of the booster pump 7 is set to 1.8 MPa (>1.0 MPa). The pressure applying force of the booster pump 7 is thus set to a rather high level (compared to that set in the comparative example to be described later). This makes it possible to extract more permeated water from the second reverse osmosis treatment device 8 and also to equalize permeation fluxes in the first reverse osmosis treatment devices 6p, 6q and the second reverse osmosis treatment device 8. The flux (m3/day) used herein means a volume of water permeating a RO element per unit area (1 m2) in a prescribed time period.

[0027] For example, a ratio of the pressure applying force of the booster pump 7 to the pressure applying force of the high pressure pump 5 is preferably not less than 0.2 and not more than 0.5. If the high pressure pump 5 and the booster pump 7 are driven within the above-described range (that is, while the pressure applying force of the high pressure pump 5 is kept relatively low, the pressure applying force of the booster pump 7 is maintained relatively high), the permeation fluxes in the first reverse osmosis treatment devices 6p, 6q positioned on an upstream side is equalized to the permeation flux in the second reverse osmosis treatment device 8 positioned on a downstream side.

[0028] A case is assumed where the first reverse osmosis treatment devices 6p, 6q each have two RO elements, and the second reverse osmosis treatment device 8 to be described later has three RO elements. It is then preferable that the pressure applying force of the high pressure pump 5 is not less than 4 MPa; and not more than 5 MPa and that the pressure applying force of the booster pump 7 is not less than 1 MPa and not more than 2 MPa. This makes it possible to make the water to be treated flow into the RO elements of respective reverse osmosis treatment devices at prescribed pressures in excess of the osmotic pressure, thus allowing the permeated water to be extracted efficiently. Note that in this embodiment, the pressure applying force of the high pressure pump 5 is set to 4 . 8 MPa and the pressure applying force of the booster pump 7 is set to 1.8 MPa. In this case, the ratio of the pressure applying force of the high pressure pump 7 to the pressure applying force of the booster pump 5 is 0.375.

[0029] Second reverse osmosis treatment device The second reverse osmosis treatment device 8 is a device which membrane-filtrates the water to be treated discharged from the booster pump 7 and flowing into the primary side via the pipe d2. The second reverse osmosis treatment device 8 has a primary side connected to an outlet of the booster pump 7 via a pipe d2 and has a secondary side connected to the permeated water tank 9 via a pipe d3. The primary side of the second reverse osmosis treatment device 8 is also connected to a pipe d4 in which the concentrated water not having been passed through the RO elements (not shown) of the second reverse osmosis treatment device 8 is flowed. In this embodiment, the second reverse osmosis treatment device 8 is configured to have three RO elements arranged in series. The second reverse osmosis treatment device 8 has the configuration similar to those of the first reverse osmosis treatment devices 6p, 6q except the described above, detailed description of which is thus omitted herefrom.

[0030] As described above, the water to be treated (the concentrated water) is discharged from the booster pump 7 with the relatively high pressure applying force. This makes it possible to effectively utilize the three RO elements of the second reverse osmosis treatment device 8 and to efficiently extract the permeated water.

[0031] In this embodiment, a ratio of the number of the RO elements of the second reverse osmosis treatment device 8 to the number of the RO elements of each of the first reverse osmosis treatment devices 6p, 6q is set to 1.5 (=3/2) . That is, the second reverse osmosis treatment device 8 has a larger number of RO elements than each of the first reverse osmosis treatment devices 6p, 6q. This makes it possible to equalize the permeation fluxes in the first reverse osmosis treatment devices 6p, 6q and that in the second reverse osmosis treatment device 8, thus allowing a recovery ratio of the entire desalination system Si to be improved. Note that the "recovery ratio" of the entire desalination system SI described above means a ratio [%] of a volume of the water to be treated supplied from the inlet A during a prescribed time period (a supplied quantity), to a volume of the permeated water (a desalinated water quantity). The permeated water flowing in the secondary side of the second reverse osmosis treatment device 8 (that is, the water collecting pipe; not shown) flows into the permeated water tank 9 via the pipe d3.

[0032] Power recovery device The energy recovery device 11 (ERD: which may also be referred to as a first energy recovery device) is a device in which a residual pressure of the concentrated water flowing out from the primary side of the second reverse osmosis treatment device 8 is used for pressurizing the water to be treated which is branched off from the pipe a5 positioned more upstream of the high pressure pump 5, and the pressurized water to be treated is pressure-fed to respective primary sides of the first reverse osmosis treatment devices 6p, 6q. The energy recovery device 11 has a primary side inlet (not shown) connected to the primary side of the second reverse osmosis treatment device 8 via a pipe d4 and a primary side outlet (not shown) connected to the concentrated water tank 12 via a pipe d5.

[0033] The concentrated water tank 12 is a tank for temporarily storing concentrated water flowing thereinto via the pipe d5. The concentrated water stored in the concentrated water tank 12 flows into a discharge pump 13 (which may also be referred to as a discharge device) via a pipe d6 and is then discharged via a pipe d7 from an outlet B to the sea or the like. That is, the concentrated water tank 12 and the discharge pump 13 are disposed so as to discharge the concentrated water flowing from the primary side of the second reverse osmosis treatment device 8 through the energy recovery device 11.

[0034] Description is returned to the energy recovery device 11. The energy recovery device 11 has a secondary side inlet (not shown) connected to the pipe a5 on an intake side of the high pressure pump 5 via a pipe f 1 and a secondary side inlet (not shown) connected to a pipe a6 on a discharge side of the high pressure pump 5 via a pipe f2.

[0035] As the energy recovery device 11, for example, an energy recovery device of positive displacement type or of pressure recovery type may be used.
The "energy recovery device of positive displacement type" used herein means a device which recovers power by moving a piston in a cylinder (not shown) making use of a residual pressure of concentrated water. The "energy recovery device of pressure recovery type" used herein means a device which recovers power by directly rotating a turbine using a residual pressure of concentrated water.

[0036] The concentrated water on the primary side which supplies energy to the water to be treated on the secondary side of the energy recovery device 11 flows into the concentrated water tank 12 via the pipe d5. On the other hand, the water to be treated on the secondary side thereof which has been pressurized by the residual pressure of the concentrated water flowing through the primary side thereof flows into the pipe a6 on the discharge side of the high pressure pump 5 via the pipe f 2. That is, the desalination system SI has a configuration in which the water to be treated pressurized by the high pressure pump 5 and the water to be treated pressurized by the energy recovery device 11 are merged in the pipe a6 and flow into the primary sides of the first reverse osmosis treatment devices 6p, 6q.

[0037] Advantageous Effects The desalination system SI according to this embodiment is a tree-type two-step recovery system in which a pair of the first reverse osmosis treatment devices 6p, 6q are disposed in parallel on a downstream side of the high pressure pump 5, and the second reverse osmosis treatment device 8 is disposed on a further downstream side thereof via the booster pump 7. A plurality of the first reverse osmosis treatment devices 6p, 6q are thus disposed on the upstream side in which a saline concentration of the water to be treated is relatively low. This makes it possible to efficiently extract permeated water even if the pressure applying force of the high pressure pump 5 is relatively low.

[0038] In this embodiment, the water to be treated (concentrated water) flowing from the primary sides of the first reverse osmosis treatment devices 6p, 6q is pressurized by the booster pump 7, and is pressure-fed to the primary side of the second reverse osmosis treatment device 8, to thereby further extract permeated water from the water to be treated. As described above, extraction of the permeated water using the tree structure in two steps enables an increase in a recovery ratio of the permeated water in the entire desalination system Si.

[0039] Further, in this embodiment, each of the first reverse osmosis treatment devices 6p, 6q has two RO elements, and the pressure applying force of the high pressure pump 5 positioned upstream thereof is set to a value lower than that of the comparative example to be described later. On the other hand, the second reverse osmosis treatment device 8 has three RO elements, and the pressure applying force of the booster pump 7 positioned upstream is set to a value higher than that of the comparative example to be described later. Thus each value of the permeation fluxes in the first reverse osmosis treatment devices 6p, 6q can be made small, and a value of the permeation flux in the second reverse osmosis treatment device 8 can be made large. This makes it possible to equalize the permeation fluxes in the first reverse osmosis treatment devices 6p, 6q and the second reverse osmosis treatment device 8 and to reduce fouling (clogging owing to adhesion of impurities) in the first reverse osmosis treatment devices 6p, 6q positioned upstream of the second reverse osmosis treatment device 8.

[0040] This embodiment has the configuration in which the energy recovery device 11 is used for pressurizing the water to be treated from the pipe fl by the residual pressure of the water to be treated (concentrated water) flowing out from the primary side of the second reverse osmosis treatment device 8, and the pressurized water to be treated is merged at the pipe a6 with the water to be treated flowing therein. This makes it possible to efficiently recover energy possessed by the concentrated water and enhance energy efficiency of the entire desalination system SI. As a result, power consumption for driving the high pressure pump 5 and the like can be reduced.

[0041] Comparison with Comparative Example Next are described results of simulation of the comparative example and a specific example according to this embodiment. FIG. 5 is a block diagram illustrating a desalination system S20 according to a comparative example. The desalination system S20 according to the comparative example has a configuration in which: water to be treated is pressurized by a high pressure pump 21 and is pressure-fed to a primary side of the reverse osmosis treatment device 22, to thereby obtain permeated water from a secondary side thereof, a residual pressure of the concentrated water flowing out from the primary side is used for driving an energy recovery device 23 of positive displacement type. Also, the water to be treated on a more upstream side of the high pressure pump 21 is made to flow in the secondary side of the energy recovery device 23, and the water to be treated flowing out from the secondary side is further pressurized by a booster pump 24 and is made to merge with a discharge side of the high pressure pump 21. Note that in FIG. 5, a water intake device, a pretreatment device, a water supply device, a discharge device, and the like are not shown.

[0042] In this embodiment (see FIG. 1) and the comparative example (see FIG. 5), reverse osmosis membranes included in the RO elements (not shown) each having properties shown below in Table 1 were used. The "Feed TDS (Total Dissolved Solid)" used herein means a quantity of impurities contained in the water to be treated.

[0043] Table 1

[0044] In this embodiment, as described above, each of the first reverse osmosis treatment devices 6p, 6q (see FIG. 1) was configured to have two RO elements arranged in series. The second reverse osmosis treatment device 8 (see FIG. 1) was configured to have three RO elements arranged in series (that is, the desalination system SI has seven RO elements in total). On the other hand, in the desalination system S20 according to the comparative example (see FIG. 5), the reverse osmosis treatment device 22 was configured to have seven RO element (not shown) arranged in series.

[0045] A simulation was then performed using a supplied quantity of the water to be treated and pressures of the high pressure pump 5 and the booster pump 7, as parameters in each of the desalination systems SI, S20, with an intention of obtaining respective desalinated water quantities (that is, permeated water quantities) of substantially same values (approximately 87 m3/d: see Table 2).
Note that conditions of the water to be treated (raw seawater) were set as the feed TDS to 39000 ppm and the temperature to 25C°. Table 2 below shows results of the simulation. The TDS in Table 2 represents a saline concentration of the finally-obtained permeated water.

[0046] Table 2

[0047] As shown in Table 2, in producing the substantially same desalinated water quantities (approximately 87 m3/d) , this embodiment was able to make a necessary supplied quantity of the water to be treated much less than that of the comparative example (approximately 25% less than the comparative example). It was also found that this embodiment was able to make the TDS (saline concentration) of the permeated water lower than the comparative example. It is contemplated that this is because the desalination system SI according to this embodiment, which is a tree-structured two-step recovery system, can acquire more permeated water having a less saline concentration from the RO elements at a primary step (on an upstream side) than that acquired by a commonly-used partial split method.

[0048] Thus, in this embodiment, the permeated water having a low saline concentration can be obtained, and a recovery ratio in producing the permeated water from the water to be treated can be improved. As a result, capital expenditure (CAPEX) required for the water intake device (the intake pump 1 or the like, see FIG. 1) or the pretreatment device 3 can be reduced.

[0049] FIG. 2 is a graph illustrating respective permeation fluxes of the RO elements which desalination systems according to the first embodiment and the comparative example are equipped with. The abscissa axis of the graph of FIG. 2 shows element numbers, and the ordinate axis, permeation fluxes. The element numbers in this embodiment are numbered: as No. 1 and No. 2 in order from upstream side of the RO elements of the first reverse osmosis treatment device 6p shown in FIG. 1; as No. 3 and No. 4 in order from upstream side of the RO elements of the first reverse osmosis treatment device 6q; and as No. 5 to No. 7 in order from upstream side of the RO elements of the second reverse osmosis treatment device 8. The element numbers according to the comparative example are numbered as No. 1 to No. 7 in order from upstream side of the seven RO elements of the reverse osmosis treatment device 22 shown in FIG. 5.

[0050] As shown in FIG. 2, in the desalination system S20 according to the comparative example (in broken line), the more downstream, the smaller the permeation fluxes of the RO elements. That is, in the comparative example, there is a large difference between values of the permeation fluxes on the upstream side and the downstream side, meaning that a permeation flux on the upstream side is relatively large. As a result, fouling tends to occur in the RO element on the upstream side, which reduces a recovery ratio of the entire desalination system S20 (see Table 2) .

[0051] On the other hand, in this embodiment, permeation fluxes of the RO elements (No. 1 to No. 4) included in the first reverse osmosis treatment devices 6p, 6q and of the RO elements (No. 5 to No. 7) included in the second reverse osmosis treatment device 8 are more equalized than those in the comparative example. In the respective reverse osmosis treatment devices of the desalination system SI, the more downstream side, the smaller the values of the permeation fluxes of the RO element. This is caused by pressure loss in the RO elements on the upstream side.

[0052] As described above, in this embodiment, the permeation fluxes of the RO elements can be equalized compared to those of the comparative example, and fouling can be prevented from occurring in the RO element at the primary step (on the upstream side). As a result, lifetime of the RO elements becomes longer, thus allowing the OPEX (operating expenditure) to be reduced.

[0053] Second Embodiment A desalination system S2 (see FIG. 3) according to the second embodiment is similar to the desalination system SI according to the first embodiment, except that the energy recovery device 11 of positive displacement type as explained in the first embodiment is used and also that a flow rate control pump 14 is additionally disposed at the pipe fl. Description herein is thus made focusing on the different configuration, and description duplicated with the first embodiment is omitted herefrom.

[0054] As described above, the energy recovery device 11 of positive displacement type is a device which recovers power by moving a piston in a cylinder (not shown) by a residual pressure of concentrated water. The energy recovery device 11 of positive displacement type can recover power with high efficiency regardless of operating conditions, thus allowing an efficient recovery of a large part of energy possessed by the concentrated water.

[0055] As the energy recovery device 11 of positive displacement type, for example, a DWEER (Dual Work Energy Exchanger) type energy recovery device or a PX (Pressure Exchanger) energy recovery device can be used. The DWEER type energy recovery device includes a plurality of cylindrical pressure vessels (not shown). Each of the pressure vessels includes a piston (not shown) which partitions the concentrated water flowing therein via the pipe d4 and the water to be treated flowing therein via the pipe fl. A switch means (not shown) the energy recovery device 11 of DWEER type alternately switches a direction in which the concentrated water on the primary side flows therein and a direction in which the water to be treated on the secondary side flows therein, to thereby reciprocate the piston. With this configuration, the residual pressure possessed by the concentrated water can be recovered as energy, and the water to be treated can be efficiently pressurized.

[0056] The PX type energy recovery device includes a plurality of revolver-like cylindrical rotating bodies (not shown). A switch means (not shown) of the energy recovery device 11 of PX type alternately switches a direction in which the concentrated water on the primary side flows therein and a direction in which the water to be treated on the secondary side flows therein, to thereby recover the residual pressure possessed by the concentrated water as energy and pressurize the water to be treated. Note that it is necessary for the DWEER type energy recovery device and the PX type energy recovery device to achieve a ratio of a flow rate in an energy supply source to a flow rate in an energy supply destination, to approximately 1:1.

[0057] Further, in this embodiment, the flow rate control pump 14 (which may also be referred to as a flow rate control unit) which controls a flow rate of the water to be treated flowing through the secondary side (an energy supply destination) of the energy recovery device 11 is configured to be disposed at the pipe fl which is connected to a secondary side inlet of the energy recovery device 11. The flow rate control pump 14 is set such that a flow rate of the water to be treated is gradually increased and finally reaches that for a rated operation of the energy recovery device 11 of positive displacement type for a quick activation thereof.

[0058] Note that also in this embodiment, the ratio of the pressure applying force of the booster pump 7 to the pressure applying force of the high pressure pump 5 is preferably not less than 0.2 and not more than 0.5, for the same reason as in the first embodiment.

[0059] Advantageous Effects In the desalination system S2 according to this embodiment, the energy recovery device 11 of positive displacement type is used. This makes it possible to recover energy of the concentrated water flowing in the primary side (an energy supply source) with high efficiency and pressurize the water to be treated flowing in the secondary side (an energy supply destination) . As a result, energy efficiency of the entire desalination system S2 can be improved, and power consumption required for operating the devices therein can be reduced. Further, because the flow rate control pump 14 is disposed at the pipe fl, the energy recovery device 11 of positive displacement type can be quickly activated to perform a rated operation and can be thus maximize a function thereof.

[0060] Third Embodiment A desalination system S3 (see FIG. 4) according to a third embodiment is similar to the desalination system S2 according to the second embodiment (see FIG. 3) except that the energy recovery device 15 of pressure recovery type is additionally disposed. Description herein is thus made focusing on the different configuration, and description duplicated with the second embodiment is omitted herefrom.

[0061] Configuration of Desalination System FIG. 4 is a diagram illustrating the desalination system S3 according to the third embodiment. The energy recovery device 15 (which may also be referred to as a second energy recovery device) is a device which pressurizes the water to be treated merged from the primary sides of the first reverse osmosis treatment devices 6p, 6q via the junction piping (pipes b3, c3, gl) and pressure-feeds the pressurized water to the intake side of the booster pump 7, using a residual pressure of the concentrated water flowing out from the primary side of the second reverse osmosis treatment device 8.
The energy recovery device 15 is disposed such that the concentrated water of which residual pressure has been used on the primary side of the energy recovery device 15 itself flows into the primary side of the energy recovery device 11 of positive displacement type.

[0062] As shown in FIG. 4, the energy recovery device 15 has a primary side inlet (not shown) connected to the primary side of the second reverse osmosis treatment device 8 via a pipe g5, and a primary side outlet (not shown) connected to the primary side inlet of the energy recovery device 11 of positive displacement type via a pipe g6. The energy recovery device 15 has a secondary side inlet (not shown) connected to the primary side of the first reverse osmosis treatment device 6p via the pipes gl, b3, and also to the primary side of the first reverse osmosis treatment device 6q via pipes gl, c3. Further, the energy recovery device 15 has a secondary side outlet (not shown) connected to an inlet (not shown) of the booster pump 7 via a pipe g2.

[0063] The energy recovery device 15 is embodied by, for example, a turbocharger or an energy recovery device of Pelton type. The turbocharger as the energy recovery device 15 rotates a turbine (not shown) using the residual pressure of the water to be treated (concentrated water) flowing from the primary sides of the first reverse osmosis treatment devices 6p, 6q. Torque of the turbine pressurizes the water to be treated flowing in the secondary side of the energy recovery device 15. The energy recovery device of Pelton type as the energy recovery device 15 is configured to connect a rotary shaft of a Pelton turbine (not shown) with a main shaft of a motor (not shown) for driving a pump (not shown). Torque of the Pelton turbine pressurizes the water to be treated flowing in the secondary side of the energy recovery device 15.

[0064] As described above, it is necessary for the energy recovery device 11 of the positive displacement type to achieve the ratio of a flow rate in an energy supply source to a flow rate in an energy supply destination, to approximately 1:1. It is not necessary, however, for the energy recovery device 15 of pressure recovery type, however, to take the above-described flow rate ratio into account because the energy recovery device 15 is structured to directly rotate the turbine using the residual pressure of the concentrated water. Hence, similarly to the first embodiment, what is necessary herein is that the flow rate control pump 14 is controlled such that balance in a flow rate in the energy recovery device 11 of positive displacement type be satisfied.

[0065] As shown in FIG. 4, the water to be treated (concentrated water) flowing in the secondary side of the energy recovery device 15 via the pipe gl is pressurized using the residual pressure of the concentrated water flowing in the primary side of the energy recovery device 15, and is then flowed in the booster pump 7 via the pipe g2. The water to be treated is further pressurized at the booster pump 7 and is flowed in the primary side of the second reverse osmosis treatment device 8 via a pipe g3. As described above, because the energy recovery device 15 is disposed in such a way as to assist operations of the booster pump 7, load of the booster pump 7 can be reduced.

[0066] The concentrated water flowing out from the primary side of the energy recovery device 15 has a prescribed residual pressure. The concentrated water flows in the primary side of the energy recovery device 11 via the pipe g6. The residual pressure of the concentrated water flowing in the primary side of the energy recovery device 11 is used for pressurizing the water to be treated flowing in the secondary side. As described above, the desalination system S3 is configured to recover the residual pressure of the concentrated water flowing from the primary side of the second reverse osmosis treatment device 8 in two steps by the energy recovery device 15 of pressure recovery type and the energy recovery device 11 of positive displacement type. This makes it possible to improve energy efficiency of the entire desalination system S3 and drastically reduce power consumption in producing a prescribed volume of permeated water.

[0067] Also in this embodiment, the ratio of the pressure applying force of the booster pump 7 to the pressure applying force of the high pressure pump 5 is preferably not less than 0.2 and not more than 0.5, for the same reason as in the first embodiment.

[0068] Advantageous Effects The desalination system S3 according to this embodiment is configured to sequentially recover energy of the concentrated water flowing out from the primary side of the second reverse osmosis treatment device 8, by the energy recovery device 15 of pressure recovery type and the energy recovery device 11 of positive displacement type. The residual pressure of the above-described concentrated water can be utilized further effectively, compared to the first embodiment. Moreover, with the assistance by the energy recovery device 15, the pressure applying force of the booster pump 7 can be made smaller. This makes it possible to reduce power consumption required for operations of the entire desalination system S3.

[0069] Next is described a calculation of a power reduction rate of the desalination system S3 compared to the desalination system SI according to the first embodiment. Let the desalination system S3 operate under conditions similar to those explained in the specific example according to the first embodiment (see Table 1) . A power reduction rate Ratio is calculated by Expression 1 shown below. In Expression 1, Qf represents a supplied volume of the water to be treated to the first reverse osmosis treatment devices 6p, 6q. Rt represents an energy recovery ratio of the energy recovery device 11 of positive displacement type. Ph represents a pressure applying force of the high pressure pump 5. Qc represents a concentrated water (water to be treated) supplied to the second reverse osmosis treatment device 8. Pb represents a pressure applying force of the booster pump 7. Rb represents an energy recovery ratio of the energy recovery device 15 of pressure recovery type.

[0070] ... (1)

[0071] The calculated result from Expression 1 described above demonstrates that the desalination system S3 according to this embodiment requires power as little as 83% of that of the desalination system SI according to the first embodiment, in producing a prescribed volume of permeated water (for example, approximately 87m3/d, see Table 2). Hence, this embodiment can drastically reduce operating expenditure (OPEX) required for the desalination system S3.

[0072] Variation The desalination systems SI to S3 of the present invention have been described above according to the embodiments. The present invention is not, however, limited to the description, and various modifications thereof are possible. For example, in the above-described embodiments, description has been made assuming that two first reverse osmosis treatment devices 6p, 6q are connected in parallel. The present invention is not, however, limited to this. Another configuration is also possible in which three or more first reverse osmosis treatment devices are connected in parallel, and the second reverse osmosis treatment device 8 is disposed on a downstream side thereof. [0073]
In the above-described embodiments, description has been made assuming that, as an example, seawater is desalinated to produce freshwater. The present invention is not, however, limited to this. For example, the present invention can be applied to a treatment of purifying industrial waste water. It is preferable in this case that the pretreatment device 3 performs a prescribed pretreatment of the industrial waste water according to a quality thereof, and then, an appropriate reverse osmosis treatment devices extracts permeated water.

[0074] In the above-described embodiments, description has been made assuming that each of the first reverse osmosis treatment devices 6p, 6q includes two RO elements and the second reverse osmosis treatment device 8 includes three RO elements. The present invention is not, however, limited to this. Instead, another configuration is possible in which the number of the RO elements included in each of the reverse osmosis treatment devices is appropriately set in designing phase. The pressure applying force of the high pressure pump 5 and the pressure applying force of the booster pump 7 are not limited to a range exemplified in the above-described embodiments. Those pressure applying forces can be appropriate set according to a saline concentration of the water to be treated, the number of the RO elements included in the first reverse osmosis treatment devices 6p, 6q and the second reverse osmosis treatment device 8, layout and configuration of the energy recovery devices 11, 14, or the like.

[0075] In the second and third embodiments, description has been made assuming that the flow rate control pump 14 is disposed at the pipe f 1 which is connected to the secondary side inlet of the energy recovery device 11. The present invention is not, however, limited to this. Another configuration is also possible in which a flow rate control pump (which may also be referred to as a flow rate control unit) is disposed at the pipe f2 which is connected to the secondary side outlet of the energy recovery device 11. Instead of the flow rate control pump 14, a flow rate control valve (which may also be referred to as a flow rate control unit) which controls a flow rate of the water to be treated flowing through the secondary side of the energy recovery device 11 may be disposed in the pipe fl or the pipe f2.

We Claim:

1. A desalination system, comprising: a water intake device that takes therein water to be treated; a pretreatment device that performs a pretreatment to the water to be treated taken by the water intake device for removing impurities therein; a first pump that pressure-feeds the water to be treated to which the pretreatment has been subjected by the pretreatment device; a plurality of first reverse osmosis treatment devices each of which membrane- filters the water to be treated which is discharged from the first pump and flows in a primary side of the first reverse osmosis treatment devices itself via branch piping; a second pump that pressurizes the water to be treated which flows out from respective primary sides of the first reverse osmosis treatment devices and is merged via junction piping; a second reverse osmosis treatment device that membrane-filters the water to be treated which is discharged from the second pump and flows in a primary side of the second reverse osmosis treatment device itself; a first energy recovery device that pressurizes the water to be treated which is branched off from more upstream of the first pump using a residual pressure of a concentrated water flowing out from the primary side of the second reverse osmosis treatment device, and that pressure-feeds the pressurized water to be treated to the respective primary sides of the first reverse osmosis treatment devices; a water supply device that supplies permeated water flowing out from respective secondary sides of the first reverse osmosis treatment devices as well as permeated water flowing out from a secondary side of the second reverse osmosis treatment device, to a demand side; and a discharge device that discharges the concentrated water flowing out from the primary side of the second reverse osmosis treatment device through the first energy recovery device.

2. A desalination system, comprising: a water intake device that takes therein water to be treated; a pretreatment device that performs a pretreatment to the water to be treated taken by the water intake device for removing impurities therein; a first pump that pressure-feeds the water to be treated to which the pretreatment has been subjected by the pretreatment device; a plurality of first reverse osmosis treatment devices each of which membrane-filters the water to be treated which is discharged from the first pump and flows in a primary side of the first reverse osmosis treatment devices itself via branch piping; a second pump that pressurizes the water to be treated whichflows out from respective primary sides of the first reverse osmosis treatment devices and is merged via junction piping; a second reverse osmosis treatment device that membrane-filters the water to be treated which is discharged from the second pump and flows in a primary side of the second reverse osmosis treatment device itself; a first energy recovery device that pressurizes the water to be treated which is branched off from more upstream of the first pump using a residual pressure of a concentrated water flowing out from the primary side of the second reverse osmosis treatment device, and that pressure-feeds the pressurized water to be treated to the respective primary sides of the first reverse osmosis treatment devices; a flow rate control unit that controls a flow rate of the water to be treated which is pressurized by the first energy recovery device; a water supply device that supplies permeated water flowing out from respective secondary sides of the first reverse osmosis treatment devices as well as permeated water flowing out from a secondary side of the second reverse osmosis treatment device, to a demand side; and a discharge device that discharges the concentrated water flowing out from the primary side of the second reverse osmosis treatment device through the first energy recovery device.

3. The desalination system as claimed in Claims 1 or 2, further comprising a second energy recovery device of pressure recovery type that pressurizes the water to be treated which flows out from the respective primary sides of the first reverse osmosis treatment devices and is merged via the junction piping, using a residual pressure of the concentrated water flowing out from the primary side of the second reverse osmosis treatment device, and pressure-feeds the pressurized water to be treated to an intake side of the second pump, and that is disposed such that the concentrated water whose residual pressure has been used flows in the first energy recovery device, wherein the first energy recovery device pressurizes the water to be treated which is branched off from more upstream of the first pump, using a residual pressure of the concentrated water flowing from the second energy recovery device, and pressure-feeds the pressurized water to be treated to the primary side of the first reverse osmosis treatment device.

4. The desalination system as claimed in any one of Claims 1 to 3, wherein a ratio of a pressure applying force of the second pump to a pressure applying force of the first pump is set to not less than 0.2and not more than 0.5.