JP7787384B2 - Seawater desalination system - Google Patents

Description

The present invention relates to a seawater desalination system.

Seawater desalination, which uses reverse osmosis (RO) to obtain freshwater from seawater, is now widely used because it consumes less energy than evaporation methods.

While RO seawater desalination consumes less energy than evaporation, the energy cost accounts for a large proportion of the total cost of producing water (seawater desalination), making reducing this energy cost a challenge. Furthermore, discharging the concentrated seawater produced by RO seawater desalination directly into the ocean raises environmental concerns, as it increases the salinity of the seawater.

In recent years, attention has been focused on technologies that recover the energy of concentrated seawater during seawater desalination to reduce the power consumption of seawater desalination systems, as the seawater that is concentrated without passing through the semipermeable membrane of the RO module (concentrated seawater) has high osmotic pressure and high hydrostatic pressure.

One known method of recovering the osmotic energy of concentrated seawater is to utilize the forward osmosis phenomenon caused by the difference in concentration (osmotic pressure difference) between concentrated seawater and a liquid with a significantly lower salt concentration (low osmotic pressure liquid).

For example, a forward osmosis (FO) module includes a first and second compartments separated by a semipermeable membrane. A low-osmotic pressure liquid is supplied to the first compartment, and concentrated seawater is supplied to the second compartment. When the concentrated seawater and the low-osmotic pressure liquid are brought into contact with each other through the semipermeable membrane (FO membrane), forward osmosis generates a flow of water that permeates the semipermeable membrane from the low-osmotic pressure liquid (first compartment) to the concentrated seawater (second compartment) in proportion to the osmotic pressure difference between the two liquids. As a result, the concentrated seawater is diluted, increasing its flow rate. The energy generated by this increased flow rate due to forward osmosis can be recovered using an energy recovery device (such as a generator or mechanical energy recovery device) to convert the osmotic energy of the concentrated seawater into power or electrical energy for use in the seawater desalination system.

For example, Patent Document 1 (JP 2014-200708 A) discloses a method of reducing the power consumption of a seawater desalination RO plant by supplying concentrated seawater produced during seawater desalination and treated sewage water with a low salt concentration to an FO module, and recovering energy (from osmotic pressure) from the concentrated seawater whose flow rate has increased due to forward osmosis.

Furthermore, Patent Document 2 (US Patent Application Publication No. 2013/0160435) discloses a method for recovering energy from concentrated seawater, similar to Patent Document 1, and then supplying the diluted concentrated seawater to an RO module for seawater desalination, thereby increasing the amount of permeate (product water) obtained.

In addition, according to the method of recovering the osmotic energy of concentrated seawater as described in Patent Documents 1 and 2, the concentrated seawater is diluted, thereby preventing the concentrated seawater from being discharged directly into the ocean.

On the other hand, the semipermeable membranes (RO membranes) used in the RO process for seawater desalination typically have a sodium chloride rejection rate of 99% or more, but a low boron rejection rate. For this reason, particularly when the resulting freshwater is to be used as drinking water (tap water), as shown in Figure 2, boron-containing water (first permeate) that has permeated the semipermeable membrane 10 of the first RO module 1 is often supplied to another RO module 3, 4 (second RO module 2) and permeated again through semipermeable membranes 30, 40 to obtain freshwater (second permeate) with an increased rejection rate of boron-containing solutes. By using such a two-stage RO module and passing seawater through the RO membrane two or more times, freshwater (product water) that meets drinking water quality standards can be obtained.

In a two-stage RO module (conventional seawater desalination system) such as that shown in Figure 2, the water (concentrated water) that does not pass through the semipermeable membranes 30, 40 of the second RO module 2 typically has a lower salinity than seawater. For this reason, this concentrated water is mixed with the seawater supplied to the first RO module 1, for example, to reduce the salinity of the seawater and increase the efficiency (energy efficiency) of freshwater recovery.

In recent years, it has been considered to use the concentrated water discharged from the second RO module 2 not simply as a mixture with the seawater supplied to the first RO module 1, but also as a low osmotic pressure liquid for a forward osmosis module (osmotic power generation: PRO) to recover the osmotic energy of the concentrated seawater (in place of the treated sewage water used in the above-mentioned Patent Documents 1 and 2).

For example, Patent Document 3 (JP 2019-72660 A) discloses a method for reducing the power consumption of a seawater desalination system using two-stage RO modules (a first RO module and a second RO module), in which concentrated seawater discharged from the first RO module is supplied to the second chamber of the FO module, and concentrated water (having a lower salinity than seawater) discharged from the second RO module is supplied to the first chamber of the FO module, and the osmotic energy of the concentrated seawater is recovered using forward osmosis, as in Patent Documents 1 and 2.

Another advantage of this method is that it does not require the supply of treated sewage water as a low osmotic pressure liquid from a source separate from the seawater desalination system, as in Patent Documents 1 and 2. Furthermore, while the permeate produced from seawater diluted with treated sewage water, as disclosed in Patent Document 2, has limitations on its use as drinking water, this method does not have such problems.

Meanwhile, methods for recovering the hydrostatic energy of concentrated seawater are also being considered. For example, as shown in Figure 3, it is being considered to reduce the energy consumption of the seawater desalination system by supplying the concentrated seawater to a mechanical energy recovery device 5 and converting the hydrostatic energy of the concentrated seawater into the pressure of seawater (supplied to the first RO module 1) for effective use.

Japanese Patent Application Laid-Open No. 2014-200708 US Patent Application Publication No. 2013/0160435 Japanese Patent Application Laid-Open No. 2019-72660

The present invention aims to reduce the amount of concentrated seawater discharged and improve the efficiency of producing freshwater from seawater in a seawater desalination system using the RO method, while further reducing energy consumption compared to conventional systems.

[1]
A seawater desalination system for producing fresh water from seawater,
the seawater desalination system includes a first reverse osmosis module, a second reverse osmosis module, a forward osmosis module, a first mechanical energy recovery device, and a second mechanical energy recovery device;
Each of the first reverse osmosis module, the second reverse osmosis module, and the forward osmosis module has a semipermeable membrane and a first chamber and a second chamber separated by the semipermeable membrane;
In the first reverse osmosis module, the seawater that has been pressurized in advance is supplied to the first chamber, a first permeate that has permeated the semipermeable membrane is discharged from the second chamber, and concentrated seawater, which is the seawater that has been concentrated without permeating the semipermeable membrane, is discharged from the first chamber.
In the second reverse osmosis module, the first permeated water, which has been pressurized in advance, is supplied to the first chamber, the second permeated water that has permeated the semipermeable membrane is discharged from the second chamber, and concentrated water, which is the first permeated water that has been concentrated without permeating the semipermeable membrane, is discharged from the first chamber.
In the first energy recovery device, a portion of the concentrated seawater and a portion of the seawater are supplied, and the seawater is pressurized using hydrostatic pressure energy of the concentrated seawater;
The seawater pressurized by the first energy recovery device is supplied to the first reverse osmosis module;
In the forward osmosis module, at least a portion of the concentrated seawater is supplied to the first chamber, a portion of the concentrated seawater is supplied to the second chamber, and water moves from the first chamber to the second chamber by permeating the semipermeable membrane, whereby forward osmosis diluted seawater, which is the diluted concentrated seawater, is discharged from the second chamber, and forward osmosis concentrated water, which is the concentrated water, is discharged from the first chamber,
The forward osmosis diluted seawater and the forward osmosis concentrated water are supplied to the first chamber of the first reverse osmosis module,
In the second energy recovery device, a portion of the concentrated seawater and a portion of the forward osmosis diluted seawater are supplied, and the forward osmosis diluted seawater is pressurized using hydrostatic pressure energy of the concentrated seawater;
The seawater desalination system, wherein the forward osmosis diluted seawater pressurized by the second energy recovery device is supplied to the first reverse osmosis module.

[2]
The seawater desalination system according to [1], wherein the concentrated water is supplied to the first chamber of the forward osmosis module by utilizing the hydrostatic pressure of the concentrated water.

[3]
The seawater desalination system according to [1] or [2], wherein the seawater pressurized by the first energy recovery device and the forward osmosis diluted seawater pressurized by the second energy recovery device are mixed and supplied to the first reverse osmosis module.

According to the present invention, in a seawater desalination system using the RO method, it is possible to reduce the amount of concentrated seawater discharged, improve the efficiency of producing freshwater from seawater, and further reduce energy consumption compared to conventional systems.

1 is a schematic diagram illustrating an example of a seawater desalination system according to an embodiment. FIG. 1 is a schematic diagram showing an example of a conventional seawater desalination system. FIG. 3 is a schematic diagram showing an example of a conventional improved version of the seawater desalination system of FIG. 2.

The seawater desalination system of this embodiment is a seawater desalination system that obtains fresh water from seawater using the reverse osmosis (RO) method. The seawater desalination system of this embodiment uses a two-stage RO module to pass seawater through an RO membrane at least twice to obtain fresh water (product water). An example of the seawater desalination system of this embodiment is described below with reference to Figure 1.

As shown in Figure 1, the seawater desalination system comprises a first RO module 1, a second RO module 2, an FO module 7, a first energy recovery device 5, and a second energy recovery device 6. Each of the first RO module 1, the second RO module 2, and the FO module 7 has a semipermeable membrane and a first chamber and a second chamber separated by the semipermeable membrane.

(First RO module)
The first RO module 1 has a semipermeable membrane 10 and a first chamber 11 and a second chamber 12 separated by the semipermeable membrane 10 .

In the first RO module 1, pre-pressurized seawater is supplied to the first chamber 11, the first permeate that has permeated the semipermeable membrane 10 is discharged from the second chamber 12, and concentrated seawater, which is seawater that has been concentrated without permeating the semipermeable membrane 10, is discharged from the first chamber 11.

As shown in Figure 1, seawater is pressurized, for example, by passing through a booster pump 81 and a high-pressure pump 82 in that order, and then supplied to the first chamber 11 of the first RO module 1. This results in water (first permeate) that has permeated the semipermeable membrane 10 of the first RO module 1.

The semipermeable membrane (RO membrane) of the first RO module is not particularly limited, and can be, for example, an RO membrane used for seawater desalination, which has high pressure resistance and a high salt rejection rate. Examples of materials for the semipermeable membrane include polyamide and cellulose triacetate.

The shape of the RO membrane is not particularly limited, but examples include flat membranes, spiral membranes, and hollow fiber membranes. Hollow fiber membranes (hollow fiber semipermeable membranes) are advantageous in that they can increase the membrane area per module compared to spiral semipermeable membranes, thereby improving membrane separation efficiency.

The pressure of the seawater supplied to the first RO module is not particularly limited and can be set depending on the salinity of the seawater and the target recovery rate of the RO membrane, but is generally 6 to 8 MPa when the salinity of the seawater is 3.0 to 4.5% by mass.

The seawater supplied to the first chamber 11 of the first RO module 1 may be pretreated. Pretreatment of seawater involves, for example, removing fine particles, organic matter, bacteria, and other contaminants contained in the seawater (raw seawater obtained from the ocean). This pretreatment prevents clogging and deterioration of the semipermeable membranes in the RO module, enabling stable operation of the system. Methods for pretreatment of seawater include, but are not limited to, filtration using an ultrafiltration membrane (UF membrane) or a microfiltration membrane (MF membrane), DAF (deaeration floatation), coagulation and sedimentation using iron chloride, and sterilization by adding sodium hypochlorite.

(Second RO module)
The second RO module 2 is composed of two RO modules (RO module 3 and RO module 4). The RO module 3 and RO module 4 each have a semipermeable membrane 30, 40, and a first chamber 31, 41 and a second chamber 32, 42 separated by the semipermeable membrane 30, 40.

In Figure 1, the second RO module 2 is composed of two RO modules 3 and 4, but it may also be composed of one RO module, or three or more RO modules. The first RO module 1 may also be composed of two or more RO modules, and the FO module 7 may also be composed of two or more FO modules.

The first permeate discharged from the second chamber 12 of the first RO module 1 still contains boron, making it unsuitable for drinking water. Therefore, the first permeate is pressurized by the pump 83 and supplied to the second RO module 2 (the first chamber 31 of the RO module 3). This allows water (second permeate: freshwater) to be obtained that has permeated the semipermeable membranes 30, 40 of the second RO module 2 (RO modules 3, 4). Because boron has been sufficiently removed from this second permeate, it can be used as drinking water, etc.

More specifically, in the second RO module 2 (RO modules 3 and 4), the first permeate, which has been pressurized in advance, is supplied sequentially to the first chambers 31 and 41, the second permeate that has permeated the semipermeable membranes 30 and 40 is discharged from the second chambers 32 and 42, and the concentrated first permeate that has not permeated the semipermeable membranes 30 and 40 is discharged sequentially from the first chambers 31 and 41. The concentrated water is ultimately discharged from the first chamber 41 of the RO module 4.

The pressure (hydrostatic pressure) of the first permeate supplied to the first chamber of the second RO module 2 (first chamber 31 of the RO module 3) is typically lower than that of the seawater supplied to the first RO module. This is because the second RO module 2 processes the water (first permeate) that has permeated the semipermeable membrane of the first RO module, so it does not need to apply as high a pressure as the first RO module.

The pressure (hydrostatic pressure) of the first permeate supplied to the first chamber of the second RO module 2 is not particularly limited, but in order to increase the recovery rate and water permeation flux, it is preferable that it be approximately 0.5 MPa to 3.0 MPa.

The semipermeable membranes 30, 40 of the second RO module 2 (RO modules 3, 4) are not particularly limited, but because there is no need to apply as high a pressure as in the first RO module, a highly permeable, low-pressure RO membrane can be suitably used. Furthermore, a semipermeable membrane having the same material, shape, etc. as the semipermeable membrane (RO membrane) of the first RO module can be used.

In addition, if the second RO module 2 is composed of multiple RO modules, the multiple RO modules can be arranged (connected) in series as shown in Figure 1 to increase the freshwater recovery rate.

(Energy Recovery Device: ERD)
In this embodiment, the first energy recovery device (first ERD) 5 and the second energy recovery device (second ERD) 6 are mechanical energy recovery devices (ERDs). A mechanical ERD is a device that recovers energy using a mechanical mechanism without converting it into electric power. Examples of mechanical ERDs include a pressure converter (PX), a Pelton wheel, and a turbocharger, and a PX can be preferably used.

In the first energy recovery device 5, a portion of the concentrated seawater and a portion of the seawater are supplied, and the seawater is pressurized using the hydrostatic energy of the concentrated seawater (by pressure conversion, etc.).

The seawater pressurized by the first energy recovery device 5 is further pressurized, for example, by a booster pump 84 and supplied to the first RO module.

As described above, in the seawater desalination system of this embodiment, a portion of the seawater is pressurized by the booster pump 81 and high-pressure pump 82 and supplied to the first RO module 1. The remainder of the seawater is pressurized by mechanically exchanging hydrostatic energy (pressure conversion, etc.) with concentrated seawater using the mechanical first energy recovery device 5, and then supplied to the first chamber 11 of the first RO module 1. This allows the hydrostatic energy of the concentrated seawater to be used to pressurize the liquid supplied to the first chamber 11 of the first RO module 1, thereby reducing the energy consumption of the seawater desalination system.

Furthermore, a portion of the concentrated seawater and a portion of the FO diluted seawater described below are supplied to the second energy recovery device 6, and the FO diluted seawater is pressurized using the hydrostatic energy of the concentrated seawater (by pressure exchange, etc.). The FO diluted seawater pressurized by this second energy recovery device 6 is supplied to the first chamber 11 of the first RO module 1.

In this way, in the seawater desalination system shown in Figure 1, the two energy recovery devices (first energy recovery device 5 and second energy recovery device 6) can utilize the hydrostatic pressure energy of the concentrated seawater to pressurize the liquid supplied to the first chamber 11 of the first RO module 1, thereby reducing the energy consumption of the seawater desalination system.

In addition, the seawater desalination system of this embodiment may be equipped with one or more other energy recovery devices (ERDs) in addition to the first energy recovery device 5 and the second energy recovery device 6.

In the seawater desalination system shown in FIG. 1, it is preferable that seawater pressurized by the first energy recovery device 5 and FO diluted seawater pressurized by the second energy recovery device 6 are mixed and supplied (in a pressurized state by the booster pump 84) to the first chamber 11 of the first RO module 1. The concentrated seawater supplied to the second energy recovery device 6 must be depressurized before being supplied to the FO module 7 in order to obtain osmotic energy. This is because the pressure of the concentrated seawater discharged from the first RO module is greater than the osmotic pressure difference between the concentrated seawater and the concentrate discharged from the second RO module 2. The pressure of the concentrated seawater discharged from the second energy recovery device 6 will be described later.

The remainder of the FO diluted seawater that is not supplied to the second energy recovery device 6 is mixed with seawater pressurized by the booster pump 81 and supplied to the high-pressure pump 82. At this time, the ratio of the pressure (discharge pressure) at the outlet of the booster pump 81 to the hydrostatic pressure of the FO diluted seawater is preferably 90 to 110%, and more preferably substantially the same. In this way, by adjusting the output of the booster pump 81, it is possible to prevent seawater from flowing back into the FO module 2.

In this case, it is preferable that seawater pressurized by the booster pump 81 and FO diluted seawater are mixed and supplied to the high-pressure pump 82.

(FO module)
The F0 module 7 has a semipermeable membrane (FO membrane) 70 and a first chamber 71 and a second chamber 72 separated by the semipermeable membrane 70 .

In the FO module 7, at least a portion of the concentrated water discharged from the second RO module 2 is supplied to the first chamber 71, and a portion of the concentrated seawater (brine) discharged from the first RO module 1 is supplied to the second chamber 72.

Because concentrated seawater has a higher osmotic pressure than concentrated water, water moves through the semipermeable membrane (FO membrane) from the first chamber 71 to the second chamber 72 due to forward osmosis (i.e., fresh water in the concentrated water moves to the concentrated seawater side via the semipermeable membrane). As a result, the concentrated seawater supplied to the second chamber 72 of the FO module 7 is diluted and discharged from the second chamber 72 of the FO module 7 at an increased flow rate. At the same time, the concentrated water supplied to the first chamber 71 of the FO module 7 is concentrated and discharged from the first chamber 71 of the FO module 7 at a decreased flow rate.

Here, the flow rate of the concentrated seawater being diluted in the second chamber 72 of the FO module 7 increases while maintaining the supply pressure (hydrostatic pressure). This means that energy has increased within the FO module 7 due to forward osmosis (without external energy supply). The amount of energy increase can be calculated as the product of the "increased flow rate" and the "discharge pressure" (hydrostatic pressure of the FO diluted seawater discharged from the second chamber 72 of the FO module 7).

By recovering the energy of the concentrated seawater thus increased in the second chamber 72 of the FO module 7 using an energy recovery device (not shown) (for example, a generator such as an osmotic pressure power generation (PRO) device, or a mechanical energy recovery device), the osmotic energy of the concentrated seawater can be converted into motive or electrical energy for use in the seawater desalination system. This reduces the energy consumption of the seawater desalination system.

In addition, the remaining concentrated seawater that is not supplied to the second chamber 72 of the FO module 7 is used to pressurize the seawater using the first energy recovery device 5, as described above.

The concentrated seawater discharged from the RO module during seawater desalination typically has high pressure (hydrostatic pressure). However, if the difference in osmotic pressure between the concentrated seawater and the low-osmotic pressure liquid is greater than the difference in hydrostatic pressure between the concentrated seawater and the low-osmotic pressure liquid (hydrostatic pressure of the concentrated seawater), a flow of water will occur through the semipermeable membrane from the low-osmotic pressure liquid to the concentrated seawater due to forward osmosis. The concentrated seawater produced during seawater desalination has an osmotic pressure of, for example, approximately 50 to 70 atmospheres, which is equivalent to a head of approximately 500 to 700 meters in a hydroelectric dam.

However, if the pressure (hydrostatic pressure) of the concentrated seawater supplied to the second chamber 72 of the FO module 7 is greater than the osmotic pressure difference between the two liquids (concentrated seawater and concentrated water) supplied to the FO module 7, forward osmosis will not occur, and instead fresh water may move from the second chamber 72 to the first chamber 71. For this reason, it is preferable that the hydrostatic pressure of the concentrated seawater (strictly speaking, the hydrostatic pressure difference between the concentrated seawater and concentrated water) be equal to or less than the osmotic pressure difference between the two liquids, and more preferably be less than the osmotic pressure difference between the two liquids.

In the seawater desalination system shown in Figure 1, the second energy recovery device 6 pressurizes the FO diluted seawater while simultaneously depressurizing the concentrated seawater. This reduces the hydrostatic pressure of the concentrated seawater supplied to the FO module 7, allowing the hydrostatic pressure of the concentrated seawater supplied to the second chamber 72 of the FO module 7 to be smaller than the osmotic pressure difference between the two liquids (concentrated seawater and concentrated water). This improves the membrane separation efficiency of the FO module 7 (the proportion of water that passes through the semipermeable membrane 70 from the first chamber 71 to the second chamber 72 due to forward osmosis), thereby improving the energy recovery efficiency of the FO module 7.

The pressure of the concentrated seawater supplied to the second chamber 72 of the FO module 7 can be set appropriately based on the concentration of the concentrated seawater, the flow rate, and the osmotic pressure that changes within the second chamber 72 of the FO module 7. However, if the pressure is too high, forward osmosis may not occur, and if the pressure is too low, the energy generated by forward osmosis may be reduced. For this reason, the pressure of the concentrated seawater supplied to the second chamber 72 of the FO module 7 is preferably 0.5 MPa or higher and 5.0 MPa or lower, and more preferably 1.0 MPa or higher and 4.0 MPa or lower.

The pressure (hydrostatic pressure) of the concentrated water supplied to the first chamber 71 of the FO module 7 is not particularly limited as long as it is a hydrostatic pressure equal to or greater than the flow pressure loss within the FO module 7, but if it is too high, the system may consume a lot of energy, so it is preferably 0.3 MPa or more and 2.0 MPa or less, and more preferably 0.5 MPa or more and 1.5 MPa or less.

It is preferable that the concentrated water be supplied to the first chamber 71 of the FO module 7 by utilizing the hydrostatic pressure of the concentrated water. A pump may be provided to supply the concentrated water to the first chamber 71 of the FO module 7, but since the concentrated water has approximately the same pressure as the first permeate supplied to the second RO module 2, this pressure can be used to supply the concentrated water to the first chamber 71 of the FO module 7, eliminating the need for an additional pump or energy for liquid transport.

Next, the FO diluted seawater (diluted concentrated seawater discharged from the second chamber 72) and the FO concentrated water (concentrated concentrated water discharged from the first chamber 71) discharged from the FO module 7 are supplied to the first chamber 11 of the first RO module 1.

The FO concentrated water is supplied to the first chamber 11 of the first RO module 1, for example, by being pressurized by the high-pressure pump 85 and supplied downstream of the high-pressure pump 82 (between the high-pressure pump 82 and the first RO module 1).

In the seawater desalination system of this embodiment, (not limited to the configuration shown in FIG. 1 ) the FO concentrate (concentrated water concentrated in the FO module 7) discharged from the first chamber 71 of the FO module 7 may be supplied upstream of the high-pressure pump 82 (and booster pump 81), pressurized by the high-pressure pump 82, and then supplied to the first chamber 11 of the first RO module 1. However, in this case, the hydrostatic energy of the FO concentrate discharged from the first chamber 71 of the FO module 7 (the hydrostatic energy of the concentrate discharged from the second RO module 2) is hardly recovered and is lost by mixing with seawater. In order to effectively utilize the hydrostatic energy of the FO concentrate (concentrated water) to supply liquid to the first chamber 11 of the first RO module 1, it is preferable that the FO concentrate be supplied downstream of the high-pressure pump 82 (between the high-pressure pump 82 and the first chamber 11 of the first RO module 1) (see FIG. 1 ). The concentrated water discharged from the second RO module 2 has a pressure (hydrostatic pressure) of, for example, 7 to 10 atmospheres.

FO diluted seawater is supplied to the first chamber 11 of the first RO module 1, for example, by being supplied between the booster pump 81 and the high-pressure pump 82. This allows it to be supplied to the first RO module 1 without losing the osmotic energy acquired by the FO module 7.

In the seawater desalination system shown in Figure 1, a portion of the FO diluted seawater is supplied to the second energy recovery device 6, where it is pressurized, and then supplied between the first energy recovery device 5 and the booster pump 84. As a result, a portion of the FO diluted seawater is further pressurized by the booster pump 84 and supplied downstream of the high-pressure pump 82 (between the high-pressure pump 82 and the first RO module 1), and is then supplied to the first chamber 11 of the first RO module 1. This allows the osmotic energy acquired by the FO module 7 to be supplied to the first RO module 1 without being lost.

The material constituting the semipermeable membrane 70 (FO membrane) used in the forward osmosis module 7 is not particularly limited, but examples include cellulose-based resins, polysulfone-based resins, polyamide-based resins, etc.

The cellulose-based resin is preferably a cellulose acetate-based resin. Cellulose acetate-based resins are resistant to chlorine, a disinfectant, and have the ability to inhibit the growth of microorganisms. The cellulose acetate-based resin is preferably cellulose acetate, and from the standpoint of durability, cellulose triacetate is more preferred.

The polysulfone-based resin is preferably a polyethersulfone-based resin. The polyethersulfone-based resin is preferably a sulfonated polyethersulfone.

The shape of the FO membrane is not particularly limited, but examples include flat membranes, spiral membranes, and hollow fiber membranes. Hollow fiber membranes (hollow fiber semipermeable membranes) are advantageous over spiral semipermeable membranes in that they can increase the membrane area per module and improve permeation efficiency.

One example of a specific hollow fiber semipermeable membrane is a single-layer membrane made entirely of a cellulose-based resin. However, the single-layer structure referred to here does not necessarily mean that the entire layer is uniform; for example, as disclosed in Patent Document 1, it is preferable that the membrane has a dense layer near the outer surface, and that this dense layer essentially serves as the separation active layer that determines the pore size of the hollow fiber semipermeable membrane.

Another specific example of a hollow fiber semipermeable membrane is a two-layer membrane having a dense layer of polyphenylene resin (e.g., sulfonated polyethersulfone) on the outer surface of a support layer (e.g., a layer made of polyphenylene oxide). Another example is a two-layer membrane having a dense layer of polyamide resin on the outer surface of a support layer (e.g., a layer made of polysulfone or polyethersulfone).

Usually, concentrated seawater flows on the outside of the hollow fiber semipermeable membrane, and concentrated seawater flows on the inside (inside the hollow portion). That is, the outside of the hollow fiber semipermeable membrane becomes the second chamber 72 of the FO module 7, and the inside becomes the first chamber 71 of the FO module 7. This is because when high-pressure concentrated seawater flows on the inside of the hollow fiber semipermeable membrane, the large pressure loss tends to result in an insufficient flow rate of concentrated seawater, resulting in low membrane separation efficiency.

In the seawater desalination system of the present embodiment described above, by using the above-mentioned energy recovery device and FO module, it is possible to further reduce energy consumption compared to conventional systems.
Furthermore, the efficiency of producing freshwater from seawater (recovery rate of freshwater from seawater: amount of freshwater recovered per unit amount of seawater) is improved by using the concentrated water, FO concentrated water, etc. to reduce the concentration of seawater that is finally supplied to the first RO module 1. As a result, the amount of seawater required to produce freshwater is reduced, which makes it possible to reduce the chemicals and equipment required for pretreatment of seawater.
Furthermore, since the concentrated seawater is diluted by the FO module 7 and supplied to the first RO module 1, the amount of concentrated seawater discarded can be reduced.

1 First RO (reverse osmosis) module, 10 Semipermeable membrane (RO membrane), 11 First chamber, 12 Second chamber, 2 Second RO module, 3, 4 RO module, 30, 40 Semipermeable membrane (RO membrane), 31, 41 First chamber, 32, 42 Second chamber, 5 (First) energy recovery device, 6 Second energy recovery device, 7 Forward osmosis (FO) module, 70 Semipermeable membrane (FO membrane), 71 First chamber, 72 Second chamber, 81, 84 Booster pump, 82, 85 High-pressure pump, 83 Pump.

Claims (3)

A seawater desalination system for producing fresh water from seawater,
the seawater desalination system includes a first reverse osmosis module, a second reverse osmosis module, a forward osmosis module, a first mechanical energy recovery device, and a second mechanical energy recovery device;
Each of the first reverse osmosis module, the second reverse osmosis module, and the forward osmosis module has a semipermeable membrane and a first chamber and a second chamber separated by the semipermeable membrane;
In the first reverse osmosis module, the seawater that has been pressurized in advance is supplied to the first chamber, a first permeate that has permeated the semipermeable membrane is discharged from the second chamber, and concentrated seawater, which is the seawater that has been concentrated without permeating the semipermeable membrane, is discharged from the first chamber.
In the second reverse osmosis module, the first permeated water, which has been pressurized in advance, is supplied to the first chamber, the second permeated water that has permeated the semipermeable membrane is discharged from the second chamber, and concentrated water, which is the first permeated water that has been concentrated without permeating the semipermeable membrane, is discharged from the first chamber.
In the first energy recovery device, a portion of the concentrated seawater and a portion of the seawater are supplied, and the seawater is pressurized using hydrostatic pressure energy of the concentrated seawater;
The seawater pressurized by the first energy recovery device is supplied to the first reverse osmosis module;
In the forward osmosis module, at least a portion of the concentrated seawater is supplied to the first chamber, a portion of the concentrated seawater is supplied to the second chamber, and water moves from the first chamber to the second chamber by permeating the semipermeable membrane, whereby forward osmosis diluted seawater, which is the diluted concentrated seawater, is discharged from the second chamber, and forward osmosis concentrated water, which is the concentrated water, is discharged from the first chamber,
The forward osmosis diluted seawater and the forward osmosis concentrated water are supplied to the first chamber of the first reverse osmosis module,
In the second energy recovery device, a portion of the concentrated seawater and a portion of the forward osmosis diluted seawater are supplied, and the forward osmosis diluted seawater is pressurized using hydrostatic pressure energy of the concentrated seawater;
The seawater desalination system, wherein the forward osmosis diluted seawater pressurized by the second energy recovery device is supplied to the first reverse osmosis module. A seawater desalination system as described in claim 1, wherein the concentrated water is supplied to the first chamber of the forward osmosis module by utilizing the hydrostatic pressure of the concentrated water. A seawater desalination system as described in claim 1 or 2, wherein the seawater pressurized by the first energy recovery device and the forward osmosis diluted seawater pressurized by the second energy recovery device are mixed and supplied to the first reverse osmosis module.