JP2019072660A - Seawater desalination method and seawater desalination system - Google Patents

Abstract

To provide a seawater desalination method which, while, in a two-stage RO membrane treatment system, concentrated salt water (first concentrated water) generated in first-stage RO membrane treatment is discharged after diluted with low osmotic pressure water and energy is recovered utilizing osmotic pressure difference between the concentrated salt water and the low osmotic pressure water, can reduce a pretreatment amount of the low osmotic pressure water and effectively utilize concentrated water (second concentrated water) generated in second-stage RO membrane treatment.SOLUTION: A seawater desalination method comprises: a first reverse osmosis step of supplying seawater to a first reverse osmosis membrane module provided with a first reverse osmosis membrane to obtain first permeated water passing through the first reverse osmosis membrane and first concentrated water which is the concentrated seawater; a second reverse osmosis step of supplying the first permeated water to a second reverse osmosis membrane module provided with a second reverse osmosis membrane to obtain second permeated water passing through the second reverse osmosis membrane and second concentrated water which is the concentrated first permeated water; and an energy recovery step of recovering energy generated by osmotic pressure difference between the first concentrated water and low osmotic pressure water including the second concentrated water.SELECTED DRAWING: Figure 1

Description

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

  A fresh water production system (seawater desalination system) that produces fresh water from seawater supplies seawater pressurized to a predetermined pressure by a pressure pump to a reverse osmosis (RO) membrane module and passes the RO membrane. , A system to remove salt water etc. in seawater and take out fresh water. The remaining brine is drained from the RO membrane module as concentrated brine (brine).

  Water treated with RO membranes is also used for drinking water, but compliance with water quality standards is required as safety awareness rises. In order to achieve WHO water quality standard value, RO membrane treatment (high pressure RO membrane treatment) permeated water is treated with RO membrane again (low pressure RO membrane treatment) and a two-stage RO membrane treatment system is known to remove boron etc. (Patent Document 1: Japanese Patent Application Laid-Open No. 2004-97911).

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

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

  In the conventional two-stage RO membrane processing system, the concentrated salt water (first concentrated water) generated by the first stage RO membrane processing is mixed with low osmotic pressure water such as river water and drainage to dilute the salinity concentration, and so on It was released to However, it has been studied to recover the energy generated by the osmotic pressure difference between the concentrated brine and the low osmotic pressure water and 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, membrane filtration such as ultrafiltration (UF) membrane microfiltration (MF) membrane, etc. 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.

  The present invention, in the two-stage RO membrane treatment system, dilutes the concentrated brine (first concentrated water) produced by the first stage RO membrane treatment with hypotonic water and then drains it, and combines the concentrated brine and the hypotonic water with Reduce the amount of low osmotic pressure water pretreatment and effectively use the concentrated water (second concentrated water) produced by the second stage RO membrane treatment when recovering energy using the osmotic pressure difference of It aims to provide a seawater desalination method that can

[1] Seawater is supplied to a first reverse osmosis membrane module having a first reverse osmosis membrane, and a first permeated water which has passed through the first reverse osmosis membrane and a first concentrated water which is the concentrated sea water Obtaining a first reverse osmosis step;
The first permeated water is supplied to a second reverse osmosis membrane module having a second reverse osmosis membrane, and the second permeated water which has passed through the second reverse osmosis membrane and the second permeated water which is the concentrated first permeated water Obtaining a concentrated water, a second reverse osmosis step,
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 pressure water are brought into contact with each other through a semipermeable membrane to increase the amount of water of the first concentrated water by forward osmosis and the amount of water increases The seawater desalination method according to [1], wherein 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 alkali 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 seawater desalination method according to any one of [1] to [4], wherein the low osmotic pressure water includes river water or drainage.

[6] A seawater desalination system for use in the seawater desalination method according to any one of [1] to [5], wherein
The first reverse osmosis membrane module having the first reverse osmosis membrane;
The second reverse osmosis membrane module having the second reverse osmosis membrane;
An energy recovery device for recovering energy generated by an osmotic pressure difference between the first concentrated water and the low osmotic pressure water containing the second concentrated water;
Desalination system, equipped with

  According to the present invention, in the two-stage RO membrane processing system, the concentrated brine (first concentrated water) generated in the first stage RO membrane treatment is diluted with hypotonic water and then drained, and concentrated brine and hypotonicity When energy is recovered using the osmotic pressure difference with water, the pre-treatment amount of low osmotic pressure water is reduced, and the concentrated water (second concentrated water) generated by the second stage RO membrane treatment is effectively used can do.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram which shows the structure of the seawater desalination method (seawater desalination system) which concerns on Embodiment 1. FIG. It is a schematic diagram which shows the structure of the seawater desalination method (seawater desalination system) which concerns on Embodiment 2. FIG.

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

First Embodiment
The seawater desalination method of the present embodiment includes the following first reverse osmosis step, a second reverse osmosis step, and an 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, and concentration is performed. And the first concentrated water which is the seawater.

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

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

  The first RO membrane module 11 separates the first permeated water from the seawater pressurized to a predetermined pressure by the first pressurizing pump 31 via the first RO membrane 11 a. By passing through the first RO membrane 11a, it is possible to obtain a 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 salt in seawater is removed. it can. The first permeated water is sent to the second pressure rising pump 32 and supplied to the second RO membrane module 12.

  The concentrated seawater (first concentrated water) without passing through the membrane is discharged from the first RO membrane module 11 while maintaining a high pressure, and the pressure is, for example, It is a pressure at which the liquid pressure loss (for example, less than 0.2 MPa) is reduced. The first concentrated water is supplied by the first liquid feed pump 33 to the second chamber 22 of the FO membrane module 2 at a predetermined pressure. Here, the predetermined pressure is a pressure lower than the osmotic pressure difference between the first concentrated water and the low osmotic pressure water, and is, for example, 2 to 3 MPa. A pressure reducing device may be installed upstream of the first liquid delivery 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 that has passed through the second reverse osmosis membrane 12a. Water and a second concentrated water, which is the first permeated water concentrated, are obtained.

  Specifically, the first permeated water that has passed through the first RO membrane 11 a of the first RO membrane module 11 is sent to the second pressure-rising pump 32, pressurized 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, and is, for example, 0.5 to 1 MPa.

  The second RO membrane module 12 separates the second permeated water from the first permeated water pressurized to a predetermined pressure by the second pressure rising pump 32 via the second RO membrane 12a. By passing through the second RO membrane 12a, it is possible to obtain a second permeated water from which salts, impurities and the like contained in the first permeated water are further removed. The separated second permeated water is, if necessary, sent to the next purification step or the like to be produced water (such as drinking water).

  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 of the second RO membrane module 12, the pH adjustment of the first permeated water may be performed between the first reverse osmosis step and the second reverse osmosis step. For example, the pH of the first permeate may be adjusted to alkali with a pH adjuster. Boron is difficult to remove by the RO membrane because it exists in the molecular state near neutrality, but when the pH is 9 or more, boron exists mainly in the ionic state (anion) and is generally negatively charged. This is because it is possible to significantly increase the removal rate of an RO membrane such as polyamide and an NF membrane. Thereby, the content of boron in the produced water can be made equal to or lower than the WHO water quality standard. In this case, it is desirable to use an RO membrane that withstands high pH (eg, a polyamide membrane, etc.).

  In addition, 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 (the first RO membrane 11a and the second RO membrane 12a) and the semipermeable membrane (FO membrane) 2a are not particularly limited, and for example, flat membrane, spiral membrane or hollow fiber membrane can be mentioned. . 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 addition, in FIG. 1, although flat film is simplified and drawn as RO film | membrane and FO film | membrane, it is not specifically limited to such a shape. A hollow fiber membrane (hollow fiber type semipermeable membrane) can increase the membrane area per module compared to a spiral type semipermeable membrane etc., 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 (the first RO film 11a and the second RO film 12a) and the FO film 2a may be the same or different. Since cellulose acetate is excellent in chlorine resistance, when cellulose acetate is used, a chlorine-based bactericide can be added to the water supplied to each module as a bactericide.

  In addition, the form of the RO membrane module (the first RO membrane module 11 and the second RO membrane module 12) and the FO membrane module 2 is not particularly limited, but in the case of using a hollow fiber membrane, a module in which hollow fiber membranes are arranged straight or And crosswind type modules in which a hollow fiber membrane is wound around a core tube. When a flat membrane is used, a stacked module in which flat membranes are stacked, or a spiral type module in which a flat membrane is enveloped and wound around a core tube may, for example, be mentioned.

[Energy recovery process]
In this step, the energy generated by the osmotic pressure difference between the first concentrated water and the low osmotic pressure 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 pressure water are brought into contact with each other through the semipermeable membrane 2a, and the amount of water of the first concentrated water is obtained by forward osmosis. It is preferable to recover the energy of the first concentrated water which has been increased and the amount of water has been increased.

  In FIG. 1, a forward osmosis (FO) membrane module 2 used for forward osmosis treatment includes a forward osmosis (FO) membrane 2a which is a semipermeable membrane, 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 partitioned by the 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. In addition, low osmotic pressure water including the second concentrated water is supplied to the first chamber 21 of the FO membrane module 2 by the second liquid feed pump 34. Then, water flows into the first concentrated water in the second chamber 22 from the side of the first chamber 21 via the FO membrane 2a by forward osmosis, and the second chamber 22 is used as diluted saline (diluted saline). Discharged from the outlet of the

  The diluted (pressure-increased) diluted brine in the FO membrane module 2 is supplied to the energy recovery device 5. The diluted brine after the energy is recovered by the energy recovery device 5 is discharged to the ocean after being subjected to wastewater treatment.

  Since the diluted brine discharged from the FO membrane module 2 has a lower salt concentration than the first concentrated water, the environmental impact due to the discharge of high-salt water can be reduced.

  In general, when used as FS (especially when a hollow fiber membrane is used as the semipermeable membrane), river water or drainage needs to be pretreated to prevent clogging of the semipermeable membrane. On the other hand, in the present embodiment, the low osmotic pressure 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 any special pretreatment.

  As an energy recovery method using an osmotic pressure difference, reverse electricity which directly converts the flow of ions by concentration difference energy into electrical energy besides the above-mentioned osmotic power generation (PRO) which generates electric power by water flow passing through the forward osmosis membrane Dialysis (RED) or the like may be used.

(Energy recovery device)
In the above energy recovery process, the energy recovery apparatus (ERD) 5 described below is used to recover the energy of the diluted (increased pressure) diluted brine in the FO membrane module 2.

  In the present embodiment, the energy consumption of the entire seawater desalination apparatus is supplied by supplying the energy recovered by the ERD 5 to the pressure boosting pump (the first pressure boosting pump 31 or the second pressure boosting pump 32) or the like in the seawater desalination apparatus. An increase in the amount can be suppressed.

  Moreover, also when supplying the energy collect | recovered by ERD5 mainly to other facilities as electric power, the increase in energy consumption can be suppressed as the whole including not only a seawater desalination apparatus but an electric power supply facility etc. .

  Examples of the energy recovery device (ERD) include mechanical ERD and electrical ERD.

  Mechanical ERD is a device that mechanically recovers the energy of salt water. The mechanical ERD has the advantages of lower energy conversion loss and higher energy recovery efficiency than the electrical ERD. Therefore, by adopting mechanical ERD as the ERD, it is possible to further reduce the consumption power of the pressure rising pump and the like. Examples of mechanical ERDs include a power transmission ERD or a pressure transmission ERD.

  The power transmission ERD is a device that recovers, for example, flow rate (pressure) energy of diluted salt water as power. The power transmission ERD may be, for example, a turbocharger or a water wheel coaxially connected to a drive shaft of a pressure rising pump.

  In general, a turbocharger has an advantage that it is suitable for a large amount of processing because the processable flow rate range is wide compared to a water wheel or the like coaxially coupled to the drive shaft of the pressure rising pump.

  When a water wheel coaxially coupled to a drive shaft (motor shaft) such as a pressure rising pump is used as the ERD, a buffer water wheel, a reaction water wheel, or the like can be used as the water wheel. As a buffer water turbine, a Pelton water turbine, a Tago impulse water turbine, a cross flow water turbine etc. are mentioned, for example. Among these, from the viewpoint of recovery efficiency and ease of maintenance, it is preferable to use a Pelton turbine.

  A clutch may be provided between the boost pump or the like and the water wheel (ERD 5). As a result, in the initial state from the start of the water production system to the steady state, the water turbine can be prevented from becoming a load such as the pressure rising pump even in the initial state by disconnecting the clutch.

  The pressure transfer type ERD (Pressure Exchanger) is a device for converting the pressure of diluted salt water into the pressure of fluid to be boosted by a pressure pump. The pressure transfer type ERD generally has smaller conversion loss and superior energy recovery efficiency than the power transfer type ERD.

  The electrical 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 electrical ERD may supply the generated electricity to a boost pump or the like through a wire, and can also supply the electricity to other facilities, so it has an advantage of high freedom of design.

  The above-described FO membrane module 2 and energy recovery device 5 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 to reduce the energy consumption of the seawater desalination apparatus.

Second Embodiment
In the seawater desalination method of this embodiment, in addition to the second concentrated water discharged from the second RO module 12, low osmotic pressure water including river water or drainage is used as FS supplied to the FO membrane module 2. This is different from the first embodiment. The other points are basically the same as in the first embodiment, and thus the detailed description will not be repeated.

  In addition to river water (including lake water) or drainage (for example, industrial drainage, domestic drainage (sewage treated water), etc.), low osmotic pressure water is, for example, low concentration salt water (eg, , Brackish water), fresh water containing impurities (eg, treated sewage water), and the like.

  Referring to FIG. 2, in the seawater desalination method according to the second embodiment of the present invention, seawater pressurized 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. By passing the first RO membrane 11a, the first permeated water from which most of salt and the like are removed from the seawater and the first concentrated water in which the seawater is concentrated are taken out. Subsequently, the first permeated water pressurized 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 membrane module 12 and allowed to pass through the second RO membrane 12a. 1) A second permeated water (produced water) from which salts, impurities and the like contained in the permeated water are further removed, and a second concentrated water from 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. In the first chamber 21 of the FO membrane module 2, low osmotic pressure water including the second concentrated water and river water or drainage is supplied by the second liquid feed pump 34. Water flows into the first concentrated water in the second chamber 22 from the side of the first chamber 21 through the FO membrane 2a due to forward osmosis, and flows in the second chamber 22 as diluted brine It is discharged from the outlet.

  The diluted (pressure-increased) diluted brine in the FO membrane module 2 is supplied to the energy recovery device 5. The diluted brine after the energy is recovered by the energy recovery device 5 is discharged to the ocean after being subjected to wastewater treatment.

  In FO membrane treatment, by using low osmotic pressure water including river water or drainage as FS, FS can be stably supplied to the first chamber 21 of the FO membrane module, so the first concentrated water is sufficient. Can be diluted to enhance the efficiency of energy recovery. Even when low osmotic pressure water including second concentrated water and river water or drainage is used as FS, it is possible to use the low osmotic pressure water consisting only of river water, drainage, etc. as in the prior art. The amount of processing can be reduced.

  It should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is indicated not by the above description but by the claims, and is intended to include all the modifications within the meaning and scope equivalent to the claims.

  11 first reverse osmosis membrane module (first RO membrane module), 11a first reverse osmosis membrane (first RO membrane), 12 second reverse osmosis membrane module (second RO membrane module), 12a second reverse osmosis membrane (second RO membrane) 2) Forward osmosis membrane module (FO membrane module) 2a semipermeable membrane 21 first chamber 22 second chamber 31 first pressure pump 32 second pressure pump 33 first liquid delivery pump 34 second Delivery pump, 5 energy recovery unit (ERD).

Claims (6)

Supplying seawater to a first reverse osmosis membrane module having a first reverse osmosis membrane, and obtaining a first permeated water passing through the first reverse osmosis membrane and a first concentrated water which is the concentrated seawater; 1 reverse osmosis process,
The first permeated water is supplied to a second reverse osmosis membrane module having a second reverse osmosis membrane, and the second permeated water which has passed through the second reverse osmosis membrane and the second permeated water which is the concentrated first permeated water Obtaining a concentrated water, a second reverse osmosis step,
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:   In the energy recovery step, the first concentrated water and the low osmotic pressure water are brought into contact with each other through a semipermeable membrane to increase the amount of water of the first concentrated water by a forward osmosis phenomenon, thereby increasing the amount of water. The seawater desalination method according to claim 1, wherein energy of 1 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 alkali with a pH adjuster 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 seawater desalination method according to any one of claims 1 to 4, wherein the low osmotic pressure water comprises river water or drainage. It is the seawater desalination system used for the seawater desalination method of any one of Claims 1-5, Comprising:
The first reverse osmosis membrane module having the first reverse osmosis membrane;
The second reverse osmosis membrane module having the second reverse osmosis membrane;
An energy recovery device for recovering energy generated by an osmotic pressure difference between the first concentrated water and the low osmotic pressure water containing the second concentrated water;
Desalination system, equipped with

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