JP7102706B2 - Seawater desalination method and seawater desalination system - Google Patents

Description

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

Forward osmosis (FO: forward osmosis) is a solution (draw solution) in which water on the side of water to be treated (feed solution) with low concentration (low osmosis pressure) has a high concentration (high osmosis pressure) through a forward osmosis membrane. It is a phenomenon of moving toward. On the other hand, in the field of water treatment, a water treatment method using a reverse osmosis (RO) step has been conventionally known. The reverse osmosis step is a step of moving water from a high-concentration water to be treated to a low-concentration solution side by artificially applying a strong pressure, contrary to forward osmosis.

In a desalination system (seawater desalination system) that produces freshwater from seawater, seawater that has been boosted to a predetermined pressure by a booster pump is supplied to the RO membrane module and permeated through the RO membrane to remove salt in the seawater. A reverse osmosis step is performed to remove and take out fresh water (produced water). At this time, the remaining salt water that did not permeate the RO membrane is discharged from the RO membrane module as concentrated salt water (brine) (Patent Document 1: Japanese Patent Application Laid-Open No. 2004-97911).

Japanese Unexamined Patent Publication No. 2004-97911

However, the salt concentration of seawater is about 3.5 w / v%, and its osmotic pressure is as high as 2.5 to 3 MPa. Therefore, the reverse osmosis step in the seawater desalination apparatus requires a strong pressure (preferably 6 to 8 MPa) higher than the osmotic pressure of seawater, and the energy consumption is extremely large, so that the energy efficiency is low.

Therefore, an object of the present invention is to provide a seawater desalination method capable of reducing the energy consumption of the reverse osmosis step.

[1] A seawater desalination method for obtaining fresh water from seawater.
A forward osmosis step in which seawater supplied from a seawater tank is brought into contact with low osmotic water via a semipermeable membrane to obtain diluted salt water in which the seawater is diluted.
A reverse osmosis step in which a reverse osmosis membrane is used to obtain a production water that has permeated the reverse osmosis membrane and a concentrated salt water that is the concentrated diluted salt water from the diluted salt water obtained in the forward osmosis step.
Seawater desalination methods, including.

[2] The seawater desalination method according to [1], wherein the low osmotic water is wastewater.
[3] The seawater desalination method according to [1] or [2], wherein the concentrated salt water obtained in the reverse osmosis step is returned to the seawater tank and reused in the forward osmosis step.

[4] The seawater desalination method according to any one of [1] to [3], wherein the seawater is pretreated seawater.

[5] The seawater desalination method according to any one of [1] to [4], wherein the salt concentration of the concentrated salt water is 2 w / v% or more.

[6] A seawater desalination system used in the seawater desalination method according to any one of [1] to [5].
A forward osmosis treatment device having a semipermeable membrane, in which seawater supplied from a seawater tank is brought into contact with low osmotic water via the semipermeable membrane, and diluted salt water in which the seawater is diluted is discharged.
A reverse osmosis treatment in which the production water that has a reverse osmosis membrane and has permeated the reverse osmosis membrane and the concentrated salt water that is the concentrated diluted salt water is discharged from the diluted salt water discharged from the forward osmosis treatment apparatus. Equipment and
A seawater desalination system equipped with.

According to the present invention, it is possible to reduce the energy consumption of the reverse osmosis step in the seawater desalination method.

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

Hereinafter, the seawater desalination method and the seawater desalination system according to the embodiment of the present invention will be described with reference to FIG. In the drawings, the same reference numerals indicate the same parts or corresponding parts.

(Forward penetration process)
In this step, referring to FIG. 1, the seawater supplied from the seawater tank 6 is brought into contact with the low osmotic water via the semipermeable membrane 1a to obtain diluted salt water in which the seawater is diluted.

The forward osmosis treatment apparatus used in the forward osmosis step of the present invention has a semipermeable membrane 1a, and the seawater supplied from the seawater tank 6 is brought into contact with the low osmosis pressure water via the forward osmosis membrane 1a to bring the seawater into contact with the low osmosis pressure water. Drain the diluted salt water diluted with.

Specifically, the forward osmosis processing apparatus includes a forward osmosis (FO) membrane module 1. The FO membrane module 1 includes an FO membrane 1a, which is a semipermeable membrane used for the forward osmosis treatment, a first chamber 11 to which a feed solution (FS) is supplied, and a second chamber 11 to which a draw solution (DS) is supplied. It has a chamber 12, and the first chamber 21 and the second chamber 12 are separated by an FO film 1a. Further, the forward osmosis treatment apparatus includes a pump 31 that supplies FS to the first chamber 11 of the FO membrane module 1 and a pump 32 that supplies DS from the seawater tank 6 to the second chamber 12 of the FO membrane module 1. Be prepared.

In this embodiment, DS is seawater. The FS is not particularly limited as long as it is a liquid having an osmotic pressure lower than that of the DS (low osmotic water), but is more than, for example, fresh water (for example, river water, wastewater (industrial wastewater, treated sewage water, etc.)) and seawater. Salt water with a low salt concentration (for example, brackish water, brackish water) or the like is used. Low osmotic water is preferably drainage. When wastewater is used as low osmotic water, the amount of wastewater discarded as industrial waste can be reduced.

In the forward osmosis step, low osmotic water is supplied to the first chamber 11 of the FO membrane module 1 via the pump 31, and the low osmotic water is brought into contact with one surface of the semipermeable membrane 1a and via the pump 32. The seawater pumped up from the seawater tank 6 is supplied to the second chamber 12 of the FO membrane module 1 and brought into contact with the opposite surface of the semipermeable membrane 1a.

Since there is an osmotic pressure difference between the seawater that comes into contact with the semipermeable membrane and the low osmotic water, the water contained in the low osmotic water permeates the semipermeable membrane 1a due to the forward osmosis phenomenon, and the seawater. Move to. The diluted seawater (diluted salt water) having a reduced salt concentration is discharged from the FO membrane module 1 and supplied to the RO membrane module 2 via the booster pump 33.

(Reverse osmosis process)
In this step, referring to FIG. 1, from the diluted salt water obtained in the forward osmosis step, the production water permeated through the reverse osmosis membrane 2a using the reverse osmosis membrane 2a, and the concentrated salt water which is the concentrated diluted salt water. And get.

The reverse osmosis treatment apparatus used in the reverse osmosis step of the present invention has a reverse osmosis membrane 2a, and from the diluted salt water discharged from the forward osmosis treatment apparatus, the production water that has permeated the reverse osmosis membrane and the concentrated diluted salt water. Concentrated salt water is discharged.

Specifically, the reverse osmosis treatment apparatus is a reverse osmosis (RO) membrane module 2 having a reverse osmosis (RO) membrane 2a, and a booster pump that supplies diluted seawater (diluted salt water) to the RO membrane module 2. 33 is provided.

In the present invention, the shapes of the FO membrane 1a and the RO membrane 2a are not particularly limited, and examples thereof include a flat membrane, a spiral membrane, and a hollow fiber membrane. In FIG. 1, the flat membrane is simplified and drawn as the FO membrane and the RO 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 FO membrane and the RO membrane is not particularly limited, and examples thereof include cellulose acetate, polyamide, and sulfonated polysulfone. The materials of the FO film 1a and the RO 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 FO membrane module 1 and the RO 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 cloth in which the hollow fiber membrane is wound around a core tube is used. Wind type modules and the like can be mentioned. 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.

In the reverse osmosis step, the diluted salt water discharged from the FO membrane module 1 is boosted to a pressure higher than the osmotic pressure of the diluted salt water by the booster pump 33 and supplied to the RO membrane module 2. The diluted salt water supplied to the RO membrane module 2 permeates the RO membrane 2a to obtain fresh water from which salts, impurities and the like have been removed from the diluted seawater. The remaining diluted salt water that has not permeated the RO membrane 2a is concentrated and discharged from the RO membrane module 2 as concentrated salt water. The obtained fresh water is sent to the next refining process or the like as necessary to become production water.

Diluted salt water has a lower salt concentration and lower osmotic pressure than seawater. Therefore, in the reverse osmosis step, the amount of energy consumed by the booster pump 33 can be suppressed. The salt concentration of the diluted salt water is not particularly limited, but is, for example, 0.5 w / v% to 2.0 w / v%. As a method (means) for adjusting the concentration of the diluted salt water to the above range, the diluted salt water discharged from the second chamber 12 of the FO membrane module 1 is diluted in the flow path (path) until it is supplied to the RO membrane module 2. A method of installing a measuring device for measuring the concentration, osmotic pressure or flow rate of salt water and controlling the output (pressure) of the pumps 31 and 32 based on the value measured by the measuring device, and if necessary, the FO membrane. A method of adjusting the opening degree of a flow rate adjusting valve (not shown) installed in a flow path (path) until the diluted salt water discharged from the second chamber 12 of the module 1 is supplied to the RO membrane module 2 is used. Can be mentioned. In addition, for example, by lowering the output of the pump 32, the salt concentration of the diluted salt water can be lowered. Further, for example, by increasing the output of the pump 31, the salt concentration of the diluted salt water can be reduced.

The present invention is particularly useful when applied to an existing seawater desalination system using a reverse osmosis treatment (apparatus).

In the present invention, it is preferable that the concentrated salt water concentrated in the reverse osmosis step is returned to the seawater tank 6 and reused in the forward osmosis step.

Specifically, in FIG. 1, the concentrated salt water discharged from the RO membrane module 2 passes through the circulation path 4 and is supplied to the seawater tank 6. The concentrated salt water supplied to the seawater tank 6 is supplied to the second chamber 12 of the FO membrane module 1 via the pump 32, and is used again as the DS.

Further, the seawater is preferably seawater that has been pretreated by the seawater pretreatment device 5. If seawater is not pretreated, suspended turbid substances, soluble organic substances, microorganisms adhering to the membrane surface, etc. are generated, causing membrane contamination. Seawater pretreatment methods include sand filtration, coagulation filtration, pressurized levitation separation, UF membrane (Ultrafiltration), MF membrane (Microfiltration), cartridge filter, etc., for seawater taken with a pump (not shown). There is a method of treatment, which can remove the pollution source and obtain seawater having a water quality suitable for the FO membrane module 1.

Large equipment is required for these pretreatments. In addition, detailed operation management by skilled technicians is also required in order to perform sufficient treatment according to the water quality of the seawater taken in. When the concentrated salt water is circulated and reused, the amount of pretreatment of seawater can be reduced and the pretreatment equipment can be miniaturized. Conventionally, for example, when pretreatment of seawater of 30,000 tons / day is required, the treatment amount can be reduced to, for example, 500 tons / day by circulating concentrated salt water.

When starting the operation of the seawater desalination system, the seawater required to start all trains (the minimum unit that can be independently switched between operation and stop, including multiple RO membrane modules) is pretreated at one time. do not have to. The seawater pretreatment device 5 may be used to start up one train at a time, and in this case, the pretreatment device can be further miniaturized. Pretreatment can be partially omitted by using an aqueous NaCl solution that does not require pretreatment instead of seawater.

When the concentrated salt water is circulated and reused, the salt concentration of the concentrated salt water is preferably 2 w / v% or more, more preferably 2 w / v% or more and 7 w / v% or less, and further preferably 3 w. It is / v% or more and 4w / v% or less. When the salt concentration is in this range, the forward osmosis phenomenon is likely to occur because the osmotic pressure difference from the low osmotic water is sufficient in the forward osmosis treatment step after returning to the seawater tank 6. Further, when the salt concentration is higher than this range, the osmotic pressure rises and the energy consumption in the booster pump 33 increases, which is not preferable.

As a method (means) for adjusting the salt concentration of the concentrated salt water within the above range, the concentration (osmotic pressure) of the concentrated salt water is measured in the path (flow path) or the circulation path 4 in which the concentrated salt water is discharged from the RO membrane module 2. A method of adjusting the output of the booster pump 33 based on the value measured by the measuring device, and a flow path or circulation path in which concentrated salt water is discharged from the RO membrane module 2 as needed. A method of adjusting the opening degree of the flow rate adjusting valve (not shown) installed in No. 4 can be mentioned. For example, by increasing the output of the booster pump 33, the salt concentration of the concentrated salt water can be increased.

A measuring device for measuring the concentration, osmotic pressure, or flow rate of the diluted salt water is installed in the flow path (path) through which the diluted salt water discharged from the second chamber 12 of the FO membrane module 1 is supplied to the RO membrane module 2. Based on the values installed and measured by the measuring device, the outputs (pressure) of the pumps 31, 32 and 33 of the booster pump (and the opening of the flow rate adjustment valve provided in each flow path if necessary) are integrated. It is also possible to use a method of controlling to.

However, since the salt slightly passes through the FO membrane 1a and the RO membrane 2a, the salt content in the circulating seawater gradually decreases. At this time, seawater may be newly drawn from the ocean, pretreated by the seawater pretreatment device 5, and then replenished in the seawater tank 6.

Further, if necessary, additives such as a scale inhibitor, a chlorine-based bactericide, and a pH adjuster can be added to seawater.

The scale inhibitor is not particularly limited, but for example, inorganic polyphosphates such as sodium hexametaphosphate and sodium tripolyphosphate, and phosphonic acids such as aminomethylphosphonic acid and phosphonobutane tricarboxylic acid for preventing calcium-based scale. Antiscale agents containing phosphorus, copolymers of maleic acid and isobutylene, tarpolymers of maleic acid, vinyl acetate and ethyl acrylate, AA (acrylic acid) -AMPS (2-acrylamide-2-methylpropylsulfonic acid) copolymer and PMA ( Examples include phosphorus-free antiscale agents such as concomitant agents with polymaleic acid). By adding these anti-scale agents, scale damage of the film can be suppressed.

The chlorine-based disinfectant is not particularly limited, and examples thereof include chlorine gas, free chlorine such as sodium hypochlorite and calcium hypochlorite, combined chlorine such as monochloramine, and chlorine dioxide. By adding these chlorine-based fungicides, it is possible to suppress the growth of microorganisms that cause biofouling and prevent deterioration of membrane performance.

Since these additives are difficult to permeate through the FO membrane and RO membrane, when the concentrated salt water discharged from the RO membrane module 2 is circulated and reused, it is not necessary to re-add the additives or new seawater is used. The amount of the additive added can be reduced as compared with the case of treatment.

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.

1 Normal osmosis membrane module (FO membrane module), 1a Semi-permeable membrane (FO membrane), 11 1st chamber, 12 2nd chamber, 2 Reverse osmosis membrane module (RO membrane module), 2a Reverse osmosis membrane (RO membrane), 31, 32 pumps, 33 boost pumps, 4 circulation paths, 5 seawater pretreatment equipment, 6 seawater tanks.

Claims (5)

A method of desalination of seawater that obtains freshwater from seawater.
A forward osmosis step in which seawater supplied from a seawater tank is brought into contact with low osmotic water via a semipermeable membrane to obtain diluted salt water in which the seawater is diluted.
A reverse osmosis step in which a reverse osmosis membrane is used to obtain a production water that has permeated the reverse osmosis membrane and a concentrated salt water that is the concentrated diluted salt water from the diluted salt water obtained in the forward osmosis step.
Including
The shapes of the semipermeable membrane and the reverse osmosis membrane are hollow fiber membranes.
The concentrated salt water obtained in the reverse osmosis step is returned to the seawater tank and reused in the forward osmosis step.
In the forward osmosis step, the low osmotic water is supplied via the first pump, and the seawater is supplied via the second pump.
In the reverse osmosis step, the diluted salt water is supplied via a pressure pump.
The outputs of the first pump, the second pump, and the booster pump are integrally controlled based on the values measured by the measuring device for measuring the concentration, osmotic pressure, or flow rate of the diluted salt water.
A seawater desalination method in which at least one selected from the group consisting of a scale inhibitor, a chlorine-based bactericide and a pH adjuster is added to the seawater. The seawater desalination method according to claim 1, wherein the low osmotic water is wastewater. The method for desalination of seawater according to claim 1 or 2, wherein the seawater is pretreated seawater. The seawater desalination method according to any one of claims 1 to 3, wherein the salt concentration of the concentrated salt water is 2 w / v% or more. A seawater desalination system used in the seawater desalination method according to any one of claims 1 to 4.
A forward osmosis treatment device having a semipermeable membrane, in which seawater supplied from a seawater tank is brought into contact with low osmotic water via the semipermeable membrane, and diluted salt water in which the seawater is diluted is discharged.
A reverse osmosis treatment in which the production water that has a reverse osmosis membrane and has permeated the reverse osmosis membrane and the concentrated salt water that is the concentrated diluted salt water is discharged from the diluted salt water discharged from the forward osmosis treatment apparatus. Equipment and
A seawater desalination system equipped with.

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