JP6965680B2 - Seawater desalination method and seawater desalination system - Google Patents

Description

The present invention relates to a seawater desalination method and a seawater desalination system.

A desalination system (seawater desalination equipment) that produces freshwater from seawater supplies seawater that has been boosted to a predetermined pressure by a booster pump to a reverse osmosis (RO) membrane module and allows it to pass through the RO membrane. , A system that removes salt in seawater and extracts fresh water. The remaining salt water is discharged from the RO membrane module as concentrated salt water (brine).

Water treated with RO membranes is also used for drinking water, but with increasing safety awareness, compliance with water quality standards is required. In order to achieve the WHO water quality standard value, a two-stage RO membrane treatment system that removes boron etc. by treating permeated water treated with RO membrane (high pressure RO membrane treatment) again with RO membrane (low pressure RO membrane treatment) is known. (Patent Document 1: Japanese Unexamined Patent Publication No. 2004-97911).

On the other hand, Patent Document 2 (US Patent Application Publication No. 2013/0160435) discloses an osmotic power generation system utilizing the osmotic energy of concentrated salt water generated in the RO treatment. In this osmotic power generation system, concentrated salt water (DS: draw solution) after taking out fresh water is applied to one side of the semitransparent membrane of a forward osmosis (FO: Forward Osmosis) membrane module (semi-transparent membrane permeator for power generation). By flowing low osmotic water (FS: feed solution), which has a lower osmotic pressure than seawater, on the other side of the semi-permeable membrane, the flow rate on the concentrated salt water side is increased by the forward osmosis phenomenon, and the water flow at the increased flow rate. The generator is driven to generate power. By efficiently recovering energy with such an energy recovery device, the energy consumption of the seawater desalination device can be reduced.

Japanese Unexamined Patent Publication No. 2004-97911 U.S. Patent Application Publication No. 2013/0160435

In the conventional two-stage RO membrane treatment system, the concentrated salt water (first concentrated water) generated by the first-stage RO membrane treatment is mixed with low-osmotic water such as river water and wastewater to dilute the salt concentration in the ocean, etc. Was released to. However, it is being studied to recover the energy generated by the osmotic pressure difference between concentrated salt water and low osmotic water to suppress the energy consumption of the seawater desalination apparatus. Here, low osmotic pressure water such as river water used for energy recovery is usually subjected to pretreatment such as coagulation sedimentation filtration method, ultrafiltration (UF) membrane microfiltration (MF) membrane filtration method or the like. There is a need. On the other hand, the concentrated water (second concentrated water) generated in the second stage RO membrane treatment was drained without being used.

In the present invention, in the two-stage RO membrane treatment system, the concentrated salt water (first concentrated water) produced in the first-stage RO membrane treatment is diluted with low osmosis water and then drained, and the concentrated salt water and low osmosis water are used. When recovering energy by utilizing the osmosis pressure difference of, reduce the pretreatment amount of low osmosis water and effectively utilize the concentrated water (second concentrated water) generated in the second stage RO membrane treatment. The purpose is to provide a method for desalination of seawater.

[1] Seawater is supplied to a first reverse osmosis membrane module provided with a first reverse osmosis membrane, and the first permeated water that has passed through the first reverse osmosis membrane and the first concentrated water that is the concentrated seawater are mixed. Obtain, the first reverse osmosis step,
The first permeated water is supplied to the second reverse osmosis membrane module provided with the second reverse osmosis membrane, and the second permeated water that has passed through the second reverse osmosis membrane and the concentrated first permeated water are the second. The second reverse osmosis step to obtain concentrated water,
An energy recovery step of recovering energy generated by an osmotic pressure difference between the first concentrated water and the low osmotic pressure water containing the second concentrated water.
Seawater desalination methods, including.

[2] In the energy recovery step, the first concentrated water and the low osmotic water are brought into contact with each other via a semipermeable membrane, and the amount of the first concentrated water is increased by a forward osmosis phenomenon, so that the amount of water is increased. The seawater desalination method according to [1], wherein the energy of the first concentrated water is recovered.

[3] The seawater desalination according to [1] or [2], wherein the pH of the first permeated water is adjusted to alkaline with a pH adjuster between the first reverse osmosis step and the second reverse osmosis step. Method.

[4] The seawater desalination method according to [2], wherein the semipermeable membrane is a hollow fiber membrane.

[5] The method for desalinating seawater according to any one of [1] to [4], wherein the low osmotic water includes river water or wastewater.

[6] A seawater desalination system used in the seawater desalination method according to any one of [1] to [5].
The first reverse osmosis membrane module having the first reverse osmosis membrane and
With the second reverse osmosis membrane module having the second reverse osmosis membrane,
An energy recovery device that recovers energy generated by the osmotic pressure difference between the first concentrated water and the low osmotic water containing the second concentrated water.
A seawater desalination system equipped with.

According to the present invention, in the two-stage RO membrane treatment system, the concentrated salt water (first concentrated water) produced in the first-stage RO membrane treatment is diluted with low osmosis water and then drained, and the concentrated salt water and low osmosis pressure are discharged. When recovering energy by utilizing the osmotic pressure difference with water, the pretreatment amount of low osmosis water is reduced, and the concentrated water (second concentrated water) generated in the second stage RO membrane treatment is effectively used. can do.

It is a schematic diagram which shows the structure of the seawater desalination method (seawater desalination system) which concerns on Embodiment 1. It is a schematic diagram which shows the structure of the seawater desalination method (seawater desalination system) which concerns on Embodiment 2.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings, the same reference numerals indicate the same parts or corresponding parts.

<Embodiment 1>
The seawater desalination method of the present embodiment includes the following first reverse osmosis step, second reverse osmosis step, and energy recovery step.

[First reverse osmosis step]
In this step, as shown in FIG. 1, seawater is supplied to the first reverse osmosis membrane module 11 provided with the first reverse osmosis membrane 11a, and the first permeated water that has passed through the first reverse osmosis membrane 11a is concentrated. The first concentrated water which is the seawater is obtained.

Specifically, first, seawater is pretreated and then supplied to the first booster pump 31.

Next, the seawater is boosted to a predetermined pressure by the first booster pump 31 and supplied to the first RO membrane module 11. Here, the predetermined pressure is higher than the osmotic pressure of seawater (about 2.5 to 3 MPa), for example, 6 to 8 MPa.

The first RO membrane module 11 separates the first permeated water from the seawater boosted to a predetermined pressure by the first booster pump 31 via the first RO membrane 11a. By passing through the first RO membrane 11a, it is possible to obtain first permeated water (for example, a salt content of less than 400 mg / L) from which 99% by mass or more, preferably 99.5% by mass or more of the salt content in seawater has been removed. can. The first permeated water is sent to the second booster pump 32 and supplied to the second RO membrane module 12.

The seawater concentrated without passing through the membrane (first concentrated water) is discharged from the first RO membrane module 11 while maintaining a high pressure, and the pressure is transferred into the membrane element from, for example, the inlet pressure of the supplied seawater. This is the pressure at which the liquid pressure loss (for example, less than 0.2 MPa) is reduced. The first concentrated water is supplied to the second chamber 22 of the FO membrane module 2 at a predetermined pressure by the first liquid feeding pump 33. Here, the predetermined pressure is a pressure lower than the osmotic pressure difference between the first concentrated water and the low osmotic water, for example, 2 to 3 MPa. A decompression device can be installed upstream of the first liquid feed pump 33 to reduce the pressure and recover energy.

[Second reverse osmosis step]
In this step, as shown in FIG. 1, the first permeated water is supplied to the second reverse osmosis membrane module 12 provided with the second reverse osmosis membrane 12a, and the second permeation passes through the second reverse osmosis membrane 12a. Water and a second concentrated water, which is the concentrated first permeated water, are obtained.

Specifically, the first permeated water that has passed through the first RO membrane 11a of the first RO membrane module 11 is sent to the second booster pump 32 to be boosted to a predetermined pressure and supplied to the second RO membrane module 12. Here, the predetermined pressure is a pressure higher than the osmotic pressure of the first permeated water, for example, 0.5 to 1 MPa.

The second RO membrane module 12 separates the second permeated water from the first permeated water boosted to a predetermined pressure by the second booster pump 32 via the second RO membrane 12a. By passing through the second RO membrane 12a, it is possible to obtain the second permeated water from which the salts, impurities and the like contained in the first permeated water have been further removed. The separated second permeated water is sent to the next refining step or the like as necessary to become production water (drinking water or the like).

The remaining concentrated second concentrated water is discharged from the second RO membrane module 12 and supplied to the first chamber 21 of the FO membrane module 2 by the second liquid feed pump 34.

In order to improve the removal efficiency of impurities and the like in the second RO membrane module 12, the pH of the first permeated water may be adjusted between the first reverse osmosis step and the second reverse osmosis step. For example, the pH of the first permeated water may be adjusted to alkaline with a pH adjuster. Since boron exists in a molecular state near neutrality, it is difficult to remove it by an RO membrane, but when the pH is 9 or higher, boron mainly exists in an ionic state (anion) and is generally negatively charged. This is because the removal rate of RO membranes and NF films such as polyamides can be significantly increased. As a result, the boron content in the production water can be kept below the WHO tap water quality standard. In this case, it is desirable to use an RO membrane (for example, a polyamide membrane) that can withstand high pH.

Further, the amount of the first permeated water to be treated by the second RO membrane module 12 may be appropriately adjusted according to the water quality of the first permeated water.

In the present invention, the shapes of the RO membrane (first RO membrane 11a, second RO membrane 12a) and semipermeable membrane (FO membrane) 2a are not particularly limited, and examples thereof include a flat membrane, a spiral membrane, and a hollow fiber membrane. .. The first RO membrane 11a is preferably a hollow fiber membrane. The second RO film 12a is preferably a spiral film. The semipermeable membrane 2a is preferably a hollow fiber membrane. In FIG. 1, the flat membrane is simplified and drawn as the RO membrane and the FO membrane, but the shape is not particularly limited to such a shape. The hollow fiber membrane (hollow fiber type semipermeable membrane) can increase the membrane area per module as compared with the spiral type semipermeable membrane, and can improve the efficiency of reverse osmosis and forward osmosis. Is advantageous.

The material of the RO membrane and the FO membrane is not particularly limited, and examples thereof include cellulose acetate, polyamide, and polysulfone. The materials of the RO film (first RO film 11a, second RO film 12a) and the FO film 2a may be the same or different. Since cellulose acetate has excellent chlorine resistance, when cellulose acetate is used, a chlorine-based disinfectant can be added as a disinfectant to the water supplied to each module.

The form of the RO membrane module (1st RO membrane module 11, 2nd RO membrane module 12) and the FO membrane module 2 is not particularly limited, but when a hollow fiber membrane is used, a module in which the hollow fiber membrane is arranged straight or a module or a module in which the hollow fiber membrane is arranged straight is used. , A cross-wind type module in which a hollow fiber membrane is wound around a core tube and the like. When a flat film is used, examples thereof include a laminated module in which flat films are stacked, and a spiral module in which the flat film is wound around a core tube in an envelope shape.

[Energy recovery process]
In this step, the energy generated by the osmotic pressure difference between the first concentrated water and the low osmotic water containing the second concentrated water is recovered. In this energy recovery step, as shown in FIG. 1, the first concentrated water and the low osmotic water are brought into contact with each other via the semipermeable membrane 2a, and the amount of the first concentrated water is reduced by a forward osmosis phenomenon. It is preferable to recover the energy of the first concentrated water in which the amount of water is increased by increasing the amount.

In FIG. 1, the forward osmosis (FO) membrane module 2 used for the forward osmosis treatment includes a semipermeable membrane, a forward osmosis (FO) membrane 2a, a first chamber 21 to which a feed solution (FS) is supplied, and a draw solution. A second chamber 22 to which (DS) is supplied is provided, and the first chamber 21 and the second chamber 22 are separated by an FO film 2a.

As described above, the first concentrated water discharged from the first RO membrane module 11 is supplied to the second chamber 22 of the FO membrane module 2 by the first liquid feed pump 33. Further, low osmotic water containing the second concentrated water is supplied to the first chamber 21 of the FO membrane module 2 by the second liquid feeding pump 34. Then, due to a forward osmosis phenomenon, water flows into the first concentrated water in the second chamber 22 from the first chamber 21 side via the FO membrane 2a, and is used as diluted salt water (diluted salt water) in the second chamber 22. It is discharged from the outlet of.

The diluted salt water (in which the pressure is increased) increased in the FO membrane module 2 is supplied to the energy recovery device 5. The diluted salt water after the energy is recovered by the energy recovery device 5 is discharged to the ocean after being treated with wastewater.

Since the diluted salt water discharged from the FO membrane module 2 has a lower salt concentration than the first concentrated water, it is possible to reduce the influence on the environment due to the discharge of the high salt concentration water.

Normally, when river water or wastewater is used as an FS (particularly when a hollow fiber membrane is used as the semipermeable membrane), it is necessary to perform pretreatment to prevent clogging of the semipermeable membrane. On the other hand, in the present embodiment, the low osmotic water supplied as FS to the first chamber 21 of the FO membrane module 2 is the second concentrated water that has passed through the RO membrane. Therefore, the second concentrated water can be supplied to the FO membrane module 2 without performing a special pretreatment.

As an energy recovery method using the osmotic pressure difference, in addition to the osmotic power generation (PRO) in which power is generated by the water flow passing through the forward osmosis membrane, reverse electricity that directly converts the ion flow due to the concentration difference energy into electrical energy. You may use dialysis (RED) or the like.

(Energy recovery device)
In the above energy recovery step, the following energy recovery device (ERD) 5 is used to recover the energy of the diluted salt water whose amount has been increased (pressure has been increased) in the FO membrane module 2.

In the present embodiment, the energy recovered by the ERD 5 is supplied to the booster pump (first booster pump 31 or second booster pump 32) in the seawater desalination apparatus, thereby consuming the energy of the entire seawater desalination apparatus. The increase in quantity can be suppressed.

Further, when the energy recovered by ERD5 is mainly supplied to other facilities as electric power, it is possible to suppress an increase in energy consumption as a whole including not only the seawater desalination device but also the electric power supply facility and the like. ..

Examples of the energy recovery device (ERD) include a mechanical ERD or an electric ERD.

A mechanical ERD is a device that mechanically recovers the energy of salt water. The mechanical ERD has the advantages of less energy conversion loss and higher energy recovery efficiency than the electric ERD. Therefore, by adopting the mechanical ERD as the ERD, the power consumption of the booster pump or the like can be further reduced. Examples of the mechanical ERD include a power transmission type ERD or a pressure transmission type ERD.

The power transmission type ERD is a device that recovers the flow rate (pressure) energy of diluted salt water as power. Examples of the power transmission type ERD include a turbocharger or a water turbine coaxially coupled to the drive shaft of a booster pump.

In general, a turbocharger has an advantage that it is suitable for mass processing because it has a wider flow rate range that can be processed than a water turbine or the like coaxially coupled to the drive shaft of a booster pump.

When a water turbine coaxially connected to a drive shaft (motor shaft) of a booster pump or the like is used as the ERD, a buffer water turbine, a reaction water turbine, or the like can be used as the water turbine. Examples of the buffering turbine include a Pelton turbine, a Turgo impulse turbine, and a cross-flow turbine. Among these, it is preferable to use a Pelton turbine from the viewpoint of recovery efficiency and ease of maintenance.

A clutch may be provided between the booster pump or the like and the water turbine (ERD5). As a result, by disengaging the clutch in the initial state from the start of the water production system to the steady state, it is possible to prevent the turbine from becoming a load on the booster pump or the like even in the initial state.

A pressure transfer type ERD (Pressure Exchanger) is a device that converts the pressure of diluted salt water into the pressure of a fluid that is boosted by a pressure pump. The pressure transfer type ERD generally has a smaller conversion loss than the power transfer type ERD and is excellent in energy recovery efficiency.

An electric ERD is a device that recovers energy as electricity. Examples of the electric ERD include a water flow generator using a turbine or the like. The electric ERD has an advantage that the degree of freedom in design is high because the generated electricity may be supplied to a booster pump or the like via wiring and the electricity can be supplied to other facilities.

The FO membrane module 2 and the energy recovery device 5 described above can recover the flow rate (pressure) energy (also referred to as osmotic energy) of the first concentrated water discharged from the first RO membrane module 11. By using the recovered energy, it is possible to reduce the energy consumption of the booster pump and the like, and reduce the energy consumption of the seawater desalination apparatus.

<Embodiment 2>
The seawater desalination method of the present embodiment uses low osmotic water containing river water or wastewater in addition to the second concentrated water discharged from the second RO module 12 as the FS supplied to the FO membrane module 2. Therefore, it is different from the first embodiment. Since the other points are basically the same as those in the first embodiment, the overlapping detailed description will be omitted.

Low osmotic water is not only river water (including lake water) or wastewater (for example, industrial wastewater, domestic wastewater (treated sewage), etc.), but also low-concentration salt water (for example, brackish water) having a lower concentration than seawater. , Brackish water), fresh water containing impurities (for example, treated sewage water) and the like.

With reference to FIG. 2, in the seawater desalination method according to the second embodiment of the present invention, seawater boosted to a predetermined pressure higher than the osmotic pressure of seawater by the first booster pump 31 is supplied to the first RO membrane module 11. Then, by passing through the first RO membrane 11a, the first permeated water from which most of the salt and the like have been removed from the seawater and the first concentrated water in which the seawater is concentrated are taken out. Subsequently, the first permeated water boosted to a predetermined pressure higher than the osmotic pressure of the first permeated water by the second booster pump 32 is supplied to the second RO film module 12 and passed through the second RO film 12a. The second permeated water (produced water) from which the salt, impurities and the like contained in the permeated water have been further removed, and the second concentrated water in which the first permeated water is concentrated are taken out.

The first concentrated water discharged from the first RO membrane module 11 is supplied to the second chamber 22 of the forward osmosis (FO) membrane module 2 by the first liquid feed pump 33. The second liquid feed pump 34 supplies the second concentrated water and low osmotic water containing river water or drainage to the first chamber 21 of the FO membrane module 2. Due to the forward osmosis phenomenon, water flows into the first concentrated water in the second chamber 22 from the first chamber 21 side via the FO membrane 2a, and flows as diluted salt water (diluted salt water) in the second chamber 22. It is discharged from the outlet.

The diluted salt water (in which the pressure is increased) increased in the FO membrane module 2 is supplied to the energy recovery device 5. The diluted salt water after the energy is recovered by the energy recovery device 5 is discharged to the ocean after being treated with wastewater.

In the FO membrane treatment, by using river water or low osmotic water containing wastewater as the FS, the FS can be stably supplied to the first chamber 21 of the FO membrane module, so that the first concentrated water is sufficient. It can be diluted to increase the efficiency of energy recovery. Further, even when the second concentrated water and the low osmotic water containing river water or drainage are used as the FS, it is before the case where the low osmotic water consisting only of river water, drainage, etc. is used as in the conventional case. The amount of processing can be reduced.

The embodiments disclosed this time should be considered to be exemplary in all respects and not restrictive. The scope of the present invention is shown by the scope of claims rather than the above description, and it is intended to include all modifications within the meaning and scope equivalent to the scope of claims.

11 1st reverse osmosis membrane module (1st RO membrane module), 11a 1st reverse osmosis membrane (1st RO membrane), 12 2nd reverse osmosis membrane module (2nd RO membrane module), 12a 2nd reverse osmosis membrane (2nd RO membrane) ), 2 Reverse osmosis membrane module (FO membrane module), 2a Reverse osmosis membrane, 21 1st chamber, 22 2nd chamber, 31 1st booster pump, 32 2nd booster pump, 33 1st liquid feed pump, 34 2nd Liquid transfer pump, 5 energy recovery device (ERD).

Claims (6)

Seawater is supplied to a first reverse osmosis membrane module provided with a first reverse osmosis membrane to obtain first permeated water that has passed through the first reverse osmosis membrane and first concentrated water that is the concentrated seawater. 1 reverse osmosis process and
The first permeated water is supplied to the second reverse osmosis membrane module provided with the second reverse osmosis membrane, and the second permeated water that has passed through the second reverse osmosis membrane and the concentrated first permeated water are the second. The second reverse osmosis step to obtain concentrated water,
An energy recovery step of recovering energy generated by an osmotic pressure difference between the first concentrated water and the low osmotic pressure water containing the second concentrated water.
Only including,
The second reverse osmosis membrane Ru polyamide membrane der, desalination method. In the energy recovery step, the first concentrated water and the low osmotic water are brought into contact with each other via a semipermeable membrane, and the amount of the first concentrated water is increased by a forward osmosis phenomenon, so that the amount of water is increased. 1 The seawater desalination method according to claim 1, wherein the energy of the concentrated water is recovered. The seawater desalination method according to claim 1 or 2, wherein the pH of the first permeated water is adjusted to alkaline by a pH adjusting agent between the first reverse osmosis step and the second reverse osmosis step. The seawater desalination method according to claim 2, wherein the semipermeable membrane is a hollow fiber membrane. The method for desalinating seawater according to any one of claims 1 to 4, wherein the low osmotic water includes river water or wastewater. A seawater desalination system used in the seawater desalination method according to any one of claims 1 to 5.
The first reverse osmosis membrane module having the first reverse osmosis membrane and
With the second reverse osmosis membrane module having the second reverse osmosis membrane,
An energy recovery device that recovers energy generated by the osmotic pressure difference between the first concentrated water and the low osmotic water containing the second concentrated water.
A seawater desalination system equipped with.

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