JP5575015B2 - Fresh water production system - Google Patents

Description

  The present invention relates to a fresh water production system for obtaining fresh water from seawater or sewage effluent using a forward osmosis membrane.

  In recent years, seawater desalination apparatuses that use a reverse osmosis membrane and perform filtration treatment tend to increase. The reverse osmosis membrane is made of materials such as cellulose and polyamide. The reverse osmosis membrane is applied with a pressure more than twice the osmotic pressure of seawater (about 2.5 MPa), and water permeates the membrane, Fresh water can be obtained without permeating the membrane. The power of the high-pressure pump for applying a pressure more than twice the osmotic pressure of seawater to the reverse osmosis membrane accounts for most of the cost of seawater desalination by the reverse osmosis membrane.

  For example, [Patent Document 1] discloses a method of obtaining fresh water from seawater using a reverse osmosis membrane.

A high-pressure pump that supplies raw water to a predetermined high pressure, a high-pressure reverse osmosis device equipped with a high-pressure reverse osmosis membrane that concentrates the salinity of the high-pressure supply water set to high pressure, and a permeate line that supplies permeate to the downstream side A first drainage valve that is interposed and temporarily drains the permeated water in the initial stage of startup from the drainage line; and a permeate water line that is downstream of the first drainage valve, and passes the permeate to a predetermined low pressure. And a low pressure reverse osmosis device equipped with a low pressure reverse osmosis membrane for concentrating salt in the low pressure feed water that has been made low pressure by the low pressure pump, and a low pressure reverse osmosis device. It has a configuration including a second drain valve that temporarily discharges the low-pressure feed water at the start-up time supplied to the reverse osmosis device.
It is said that high-quality water can be produced by reverse osmosis of high pressure and low pressure.

  However, in order to obtain this, high-pressure reverse osmosis is performed, and there is a problem that a large amount of power is required for this purpose.

  As another method, water in seawater is once recovered into a high-concentration (high osmotic pressure) solution through an osmotic membrane made of a material such as cellulose, and then salted from the high osmotic pressure solution. There is a way to remove. This method is called the forward osmosis membrane method because water flows in the solution side where the osmotic pressure is high, that is, in the positive direction. In addition to utilizing the driving force in the positive direction, energy for freshwater production may be reduced by selecting a salt that can be easily separated from the solution as a salt added to the hyperosmotic pressure solution.

[Non-Patent Document 1] discloses a method of recovering water from seawater using a forward osmosis membrane. An NH ₄ HCO ₃ solution in which NH ₃ and CO ₂ are dissolved is used as a high osmotic pressure solution for recovering water. The concentration of the hyperosmotic pressure solution is tested in the range of 1.1 to 6 mol / L, and the flux using the osmotic pressure as the driving force and the salt (NaCl) concentration to be measured are measured. According to this method, 95% or more of salt in NaCl solution simulating seawater can be separated. In the final freshwater recovery process, a process for separating NH ₃ and CO ₂ from a high osmotic pressure solution is proposed.

In this method, there is a problem that corrosion of the metal structure and deterioration of the permeable membrane occur due to the high pH ammonia water derived from NH ₃ . Further, since the salt removal rate by the forward osmosis membrane is not 100%, when the high osmotic pressure solution is circulated and used for a long period of time, the salt that is not removed in the NH ₃ and CO ₂ separation step is concentrated, and the resulting fresh water is obtained. There is also a problem that the purity is lowered, and cations and carbonates in seawater are formed and deposited in the apparatus.

  In [Patent Document 2], water to be treated is brought into contact with one surface of the first semipermeable membrane, and an intermediate solution whose solubility depends on temperature is kept at a relatively high temperature in the other side of the first semipermeable membrane. A moisture absorption step of contacting the surface and absorbing the moisture of the water to be treated into the mediator solution through the first semipermeable membrane; A precipitation step for precipitating the solute), a moisture releasing step for bringing the mediator solution after the depositing step into contact with the second semipermeable membrane at a fluid pressure at which the moisture of the mediator solution can be released through the second semipermeable membrane, An apparatus for producing fresh water is described. And the separation process which isolate | separates the deposit of the mediation solute deposited by the precipitation process from the mediation solution which is provided for the moisture release process, and the separated precipitate is mixed with the mediation solution after the moisture release process and used for the dissolution process It is described that it is preferable to further include a mixing step.

JP 2010-125395 A JP 2010-162527 A

J. R. McCutcheon, R. L. McGinnis, and M. Elimelech, J. Membrane Science, 278 (2006), pp114-123.

The conventional technique described in [Patent Document 2] describes that an alum aqueous solution is used as a mediating solute, that is, a hyperosmotic solution. This alum aqueous solution, for example, AlK (SO ₄ ) ₂ has a characteristic that the concentration with respect to the temperature rises slowly, and it is necessary to raise the temperature to about 40 ° C. There is a problem that the cost is high. Further, AlK (SO ₄ ) ₂ has a problem that the pH is close to 3 and the apparatus is easily corroded.

  An object of the present invention is to provide a fresh water production system capable of obtaining fresh water quality better than a predetermined water quality and reducing running costs.

In order to achieve the above object, the present invention comprises a forward osmosis membrane treatment means for removing salt from raw water through a forward osmosis membrane to obtain permeated water into a solution having an osmotic pressure higher than that of the raw water, Separation means for separating the solute component of the high osmotic pressure solution from the high osmotic pressure solution containing the permeated water obtained by the forward osmosis membrane treatment means and the solute component is either Na ₂ HPO ₄ or ethylene carbonate, and separation means The solution after separating the solute component in the raw water is desalted with a reverse osmosis membrane and reverse osmosis membrane treatment means to obtain fresh water, a pump for supplying the raw water to the reverse osmosis membrane treatment means, and separation and recovery by the separation means This is a fresh water production system provided with a pipe for mixing the solute component with the high osmotic pressure solution supplied to the forward osmosis membrane treatment means.

  Moreover, it is a fresh water production system that uses a crystallization process as a means for separating a solute from a high osmotic pressure solution, precipitates a solute component, and separates a slurry and a supernatant.

  Further, the fresh water production system operates by lowering the osmotic pressure of the raw water obtained by the separation means and supplied to the reverse osmosis membrane treatment means to be lower than the osmotic pressure of the raw water supplied to the forward osmosis membrane treatment means.

In addition, it has means for adjusting the temperature of the raw water and the high osmotic pressure solution supplied to the forward osmosis membrane treatment and the reverse osmosis membrane treatment, and when the solute component is Na ₂ HPO ₄ , the temperature of the raw water or the high osmotic pressure solution is 25. A fresh water production system with a temperature of ℃ or less.

Further, the system is a fresh water production system which has a temperature adjusting means in the separating means or in the preceding stage, and the temperature of the solution in the separating means is 10 ° C. or lower when the solute component is Na ₂ HPO ₄ .

  Also, a heat exchange means for recovering the heat of the high osmotic pressure solution discharged from the forward osmosis membrane treatment, another heat exchange means for supplying heat to the slurry or supernatant discharged from the crystallization means, and these two kinds And a means for circulating a fluid as a heat medium between the heat exchange means.

  Moreover, it is a fresh water production system provided with the pressure converter which transmits the pressure of the high osmotic pressure solution discharged | emitted from a forward osmosis membrane processing means to the raw | natural water of a reverse osmosis membrane processing means.

  According to the present invention, water can be recovered from seawater by forward osmosis membrane treatment without using a high-pressure pump, and the adjustment range of water temperature can be reduced in a fresh water production flow, and as a result, necessary energy can be reduced. .

1 is a block diagram of a fresh water production system according to a first embodiment of the present invention. It is explanatory drawing of the water treatment in 1st Embodiment of this invention. It is a block diagram of the fresh water manufacturing system which concerns on 2nd Embodiment of this invention. It is a block diagram of the fresh water manufacturing system which concerns on 3rd Embodiment of this invention.

  Embodiments of the present invention will be described below with reference to the drawings.

[First Embodiment]
FIG. 1 is a block diagram of a fresh water production system according to a first embodiment of the present invention. The fresh water production system of the present embodiment includes pretreatment means 2 for taking in seawater, forward osmosis membrane treatment means 3 connected to the pretreatment means 2, and crystallization connected via the forward osmosis membrane treatment means 3 and the cooling means 4. Means 5, crystallization means 5 and heating means 6b, reverse osmosis membrane treatment means 7 connected via pump 15, switching means 8 connected to reverse osmosis membrane treatment means 7, reverse osmosis membrane treatment means 7 and switching means And a storage tank 14 connected to the switching means 8, and the crystallization means 5 is connected to the forward osmosis membrane treatment means 3 via the heating means 6a. 8 is connected to the forward osmosis membrane treatment means 3.

  The pretreatment means 2 filters the seawater 1 taken by a pump (not shown) with an MF membrane (Microfiltlation, microfiltration membrane). Get seawater. The pretreatment means 2 measures the differential pressure of the MF membrane and the filtration duration, and when either the preset filtration duration or the preset differential pressure is exceeded, the MF membrane is backwashed for a predetermined time. To do. The sludge 9 discharged by backwashing is disposed after concentration and dehydration processes.

  The forward osmosis membrane treatment means 3 includes rooms on both sides of the forward osmosis membrane, supplies seawater filtered by the pretreatment means 2 to one room, and supplies a high osmotic pressure solution to the other room. At this time, the osmotic pressure of each liquid can be calculated from Equation 1.

  Here, Π is the osmotic pressure, R is the gas constant, T is the absolute temperature, C is the molar concentration of the solution, and i is the Fant-Hoff coefficient. The Phanto-Hoff coefficient i is a coefficient representing the effect of ionization when the solute is an electrolyte.

  Between the forward osmosis membranes, a difference in osmotic pressure between the two solutions and a driving force according to the membrane pressure are generated. The osmotic pressure of the high osmotic pressure solution is made larger than the osmotic pressure of seawater, and water is recovered from the seawater into the high osmotic pressure solution. At this time, some of the ions in the seawater are not removed and leak to the highly osmotic solution side.

The concentration of the solute is adjusted so that the osmotic pressure of the high osmotic pressure solution is at least twice that of seawater.
The driving force increases as the osmotic pressure increases, so that the recovery rate of water from the raw water can be increased, and the permeation flow rate of the membrane can be increased.

The substance used as the solute is preferably a substance that is suitable for the subsequent crystallization treatment and can obtain a high osmotic pressure (that is, a substance that can increase the molar concentration). Specific types of solutes will be described later.
A material such as cellulose acetate or polyamide can be applied to the forward osmosis membrane. The liquid temperature inside the forward osmosis membrane treatment means 3 is kept at about 30 ° C. in order to reduce the permeation resistance of the membrane. The concentrated waste liquid 10 discharged from the forward osmosis membrane treatment means 3 is discharged to the ocean.

  From the high osmotic pressure solution discharged from the forward osmosis membrane treatment means 3, a necessary amount in consideration of the fresh water recovery amount and the fresh water recovery rate in the reverse osmosis membrane treatment means 7 is distributed to the cooling means 4. The cooling means 4 is installed after the high osmotic pressure solution outlet of the forward osmosis membrane treatment means 3. The water temperature of the solution is lowered to the set temperature for the crystallization process. The water temperature is at least 20 ° C. or less, depending on the solubility of the solute in the hypertonic solution.

  The internal structure of the crystallization means 5 is composed of a crystal growth part and a solid-liquid separation part as in a general crystallization tank. A quantity of solute exceeding the solubility at the set water temperature is deposited on the seed crystal. The grown crystals settle and are drawn out at predetermined time intervals. This slurry is sent to the heating means 6a. On the other hand, the supernatant liquid separated in the solid-liquid separation part overflows and is sent to a heating means 6b different from the heating means 6a. In the heating means 6a and 6b, each solution is heated to the water temperature (50 degreeC) in a forward osmosis membrane process and a reverse osmosis membrane process.

  The supernatant is supplied to the reverse osmosis treatment means 7 from the heating means 6a by the pump 15, and filtered by reverse osmosis membrane treatment to recover the target fresh water. On the other hand, the concentrated waste liquid is sent from the reverse osmosis treatment means 7 to the switching means 8. At this time, the chloride ion concentration in the liquid is measured by the chloride ion sensor 13. When the chloride ion concentration is less than the predetermined concentration, the switching unit 8 supplies the concentrated waste liquid to the high osmotic pressure solution side of the forward osmosis membrane treatment unit 3. On the other hand, when the chloride ion concentration is equal to or higher than a predetermined concentration, the switching means 8 switches to the drainage 11 side and discharges it outside the process, and supplies a new high osmotic pressure solution from the storage tank 14 to the forward osmosis membrane treatment means 3. .

  The waste water 11 is discarded after the concentration / dehydration treatment, or is added as a flocculant in the pretreatment means 2 when the solute of the high osmotic pressure solution contains a component that functions as a flocculant such as aluminum or iron. You can also.

  FIG. 2 is an explanatory diagram of water treatment in the first embodiment of the present invention. The ion concentration is equal to the concentration of seawater at the entrance of the pretreatment means 2 and the forward osmosis membrane treatment means 3. The high osmotic pressure solution has a higher concentration than this, and water in seawater is transferred to the high osmotic pressure solution side through the membrane. Next, in the crystallization step in the crystallization means 5, the high osmotic pressure solution is cooled to precipitate a solid, thereby reducing the ion concentration. At this time, the operation is performed under such a temperature condition that the ion concentration in the crystallization supernatant is lower than the concentration of seawater. The reverse osmosis treatment means 7 further removes ions to obtain fresh water.

  In the conventional reverse osmosis membrane treatment, a high-pressure pump intended for the osmotic pressure of seawater is required, but in the first embodiment, a pump necessary for reverse osmosis treatment is provided by interposing a solution suitable for crystallization. It is possible to reduce the pressure. This has the effect of reducing the initial cost of the pump as well as the power of the pump.

Next, the high osmotic pressure solution used in the process of the first embodiment of the present invention will be described. A solution that satisfies the conditions (a) to (e) is selected as the hyperosmotic solution.
(A) The ion concentration is higher than the concentration of seawater at the water temperature for forward osmosis membrane treatment (conditions for forward osmosis membrane treatment).
(B) The temperature dependence of solubility is large (conditions for crystallization treatment).
(C) Ion concentration after crystallization treatment is lower than seawater concentration (conditions for reverse osmosis membrane treatment).
(D) The corrosiveness of the metal is low.
(E) The degree of mixing of seawater components into the high osmotic pressure solution can be determined.

  The difference in ion concentration in the forward osmosis membrane treatment affects the osmotic pressure difference, that is, the magnitude of water driving force. By selecting a solute that can increase the ion concentration, the membrane area of the forward osmosis membrane treatment means 3 can be reduced. Moreover, the temperature difference between the forward osmosis membrane treatment and the crystallization treatment can be reduced by selecting a solute whose solubility is highly temperature dependent. As a result, the capacity of the heating means and the cooling means and the energy required during operation can be reduced. And the raw | natural water of a reverse osmosis membrane process can reduce required pump power, so that it is a low ion concentration.

  In addition, by selecting a system with low corrosivity, the life of the device can be extended. In addition, for example, by selecting a system that does not contain chloride ions and sodium ions, which are main components in seawater, the impurity concentration can be monitored, and a reduction in the processing efficiency of each process due to the mixing thereof can be suppressed.

The solute of the high osmotic pressure solution used in the first embodiment of the present invention may be any substance that satisfies the above, but for example, Na ₂ HPO ₄ or ethylene carbonate can be applied.
Table 1 shows the molar concentration at each water temperature estimated from the solubility of Na ₂ HPO ₄ . The concentration of NaCl in seawater is about 0.513 mol / L (3 wt%), and satisfies the above conditions (a), (b) and (c) in the range of at least 25 ° C. to 0 ° C.

Specifically, in Equation 1, when i = 2 of NaCl and C = 0.5 mol / L, Π = 22.4 atm (0 ° C.) and 24.5 atm (25 ° C.). On the other hand, when i = 4 (maximum value) of Na ₂ HPO ₄ , Π = 9.9 atm (0 ° C., C = 0.11 mol / L), 146.7 atm (25 ° C., C = 1.5 mol / L) And the conditions (a), (b), and (c) are satisfied. Na ₂ HPO ₄ is a substance having a pH close to 7 and applicable from the viewpoint of the corrosiveness and impurities of the apparatus.

As shown in Table 1, the solubility of Na ₂ HPO ₄ is low at 0 to 20 ° C., increases rapidly at 25 ° C. or higher, and becomes 10 times or more at 25 ° C. compared to 0 ° C. That is, since the osmotic pressure 6 times higher than seawater can be obtained at a relatively low temperature (˜25 ° C.), the forward osmosis membrane treatment can be carried out efficiently. In the crystallization operation, most of the solid (hydrate) of Na ₂ HPO ₄ can be recovered simply by cooling the solution to a relatively high temperature (−10 ° C.). Therefore, if Na ₂ HPO ₄ is applied, the adjustment range of the water temperature can be reduced in a series of fresh water production flows, and as a result, the required energy can be reduced. In addition, since it shortens the time required for overheating and cooling and reduces the size of the equipment, it has the advantage that the throughput of fresh water production can be improved and the initial equipment cost can be reduced.

Said substance is a substance which exists in solid as a simple substance in the temperature range (0 degreeC-25 degreeC) which a freshwater manufacturing system assumes. These are dissolved in water to obtain a high osmotic pressure solution. In addition to such substances, substances that change phase in the above temperature range can also be used as the solute. For example, a method using a substance having a freezing point in this temperature range and mutually soluble in water. In the forward osmosis membrane treatment, a temperature higher than at least the freezing point of the solute is maintained. In the crystallization treatment, the temperature is lower than the freezing point, and precipitation of solute components and solid recovery are performed by phase change from liquid to solid. By using such a substance, it is possible to reduce the adjustment range of the water temperature. An example of such a material is ethylene carbonate (1,3-dioxolan-2-one, C ₃ H ₄ O ₃ ). Ethylene carbonate has a freezing point of 34 to 37 ° C. and is easily dissolved in water.

  The main energy consumed in the fresh water production system is the energy for feeding water by the pump, the energy for adjusting the water temperature in the crystallization process, and the energy for filtering by the high pressure pump in the reverse osmosis membrane treatment. Among these, the thermal energy used for water temperature adjustment in the crystallization step can be made smaller than that of the evaporation method because the temperature range to be adjusted is narrower than that of the evaporation method and does not require latent heat of evaporation. Efficiency is further improved by using heat exchange, seawater, or the like described in the examples described later for the cooling water.

On the other hand, the energy of the high-pressure pump in reverse osmosis is roughly proportional to the osmotic pressure of raw water. Therefore, energy can be reduced by making the osmotic pressure of the raw water of the reverse osmosis membrane treatment in the fresh water production system of the present embodiment at least lower than the osmotic pressure of seawater. That is, the concentration of the high osmotic pressure solution obtained in the crystallization step may be set to a lower concentration. Na ₂ HPO ₄ and ethylene carbonate can adjust the concentration to 1/5 or less of seawater, which is advantageous for energy reduction. In addition, since it is possible to use a pump with a low pressure (lifting) at the time of supply, there is an effect of reducing the initial equipment cost.

  In addition, the pretreatment means 2 has been described for the case of the MF membrane treatment, but any method that can achieve a level of turbidity removal performance suitable for forward osmosis membrane treatment (for example, SDI (silt concentration index) ≦ 2). Coagulation sedimentation and sand filtration may be used.

[Second Embodiment]
FIG. 3 is a block diagram of a fresh water production system according to the second embodiment of the present invention. The system of the present embodiment has a configuration in which heat exchangers 31a and 31b are provided between the crystallization unit 5 and the heating units 6a and 6b in the configuration of the first embodiment.

  In the heat exchanger 31a, heat is recovered from the high osmotic pressure solution discharged from the forward osmosis membrane treatment means 3. The recovered heat is transferred to the heat exchanger 31b through a fluid as a medium, and is again heat-exchanged with the slurry and supernatant discharged from the crystallization means 5. The fluid serving as the heat medium circulates between the heat exchangers 31a and 31b, and continuously cools the high osmotic pressure solution before the crystallization treatment and heats the slurry and the supernatant after the crystallization treatment. Used to. When the predetermined water temperature is not reached only by heat exchange, the cooling means 4 or the heating means 6a, 6b is operated to adjust the water temperature.

  The structure of the heat exchanger is a partition type multi-tubular heat exchanger, and water can be used as a fluid as a heat medium. Heat exchange methods other than this combination include direct type, heat storage type, heat pipe type, and fluids include MEA (monoethanolamine) aqueous solution, oil, heavy oil, brine (sodium chloride solution), There are organic solvents and caustic soda solutions, which can be used.

  With such a configuration, in addition to the effects described in the first embodiment, a part of the energy required for adjusting the water temperature in the membrane treatment and crystallization treatment can be circulated and used in the system, which is necessary for fresh water production. Energy can be reduced.

The present embodiment is an example in which direct heat transfer through a fluid is applied in heat recovery / reuse. There is a heat pump as another means for efficiently using heat. The heat pump is a device that assembles heat energy from a low temperature to a high temperature, and can utilize unused energy such as in the air or other exhaust heat. The heat transfer by a heat pump using a heat medium utilizes both reversible exothermic phenomenon and endothermic phenomenon, and can be applied to both heating and cooling.
Therefore, the same effect as the heat exchange can be obtained by applying the heat recovery / heating / cooling means of the present embodiment.

[Third Embodiment]
FIG. 4 is a block diagram of a fresh water production system according to the third embodiment of the present invention. In the system of the present embodiment, the pressure between the flow path connecting the forward osmosis membrane treatment means 3 and the cooling means 4 and the flow path connecting the heating means 6b and the pump 15 in the configuration of the first embodiment. The converter 41 is provided.

A high osmotic pressure solution discharged from the forward osmosis membrane treatment means 3 is introduced into the pressure converter 41. The pressure of the high osmotic pressure solution is transmitted to the supernatant water of the crystallization means 5 connected to the pressure transducer 41.
The pressurized supernatant water is supplied to the reverse osmosis membrane treatment means 7 after further obtaining a pressure necessary for the reverse osmosis membrane treatment from the pump 15. The high osmotic pressure solution whose pressure has been reduced is introduced into the crystallization means 5 through the cooling means 4.

  The method of pressure conversion is not particularly limited. For example, a motor shaft direct connection type Pelton turbine or a turbocharger can be used.

  The performance required for the pump 15 is the osmotic pressure (p3 atm) required for seawater (osmotic pressure = p1 atm), the osmotic pressure of the forward osmosis membrane treatment (osmotic pressure = p2 atm) and the raw water for the reverse osmosis membrane treatment. ) And the efficiency (α) of the pressure recovery device, and ideally, a pressure of (p2−p1) × α is applied to the raw water of the reverse osmosis membrane treatment, and this pressure value and p3 What is necessary is just to use the pump 15 of the performance which can provide a difference.

  With this configuration, in addition to the effects described in the first embodiment, a part of the energy required for the reverse osmosis membrane treatment can be recovered and used in the system, so that the energy required for freshwater production can be reduced. It becomes.

  Thus, according to each embodiment, water can be recovered from seawater by forward osmosis membrane treatment without using a high-pressure pump, and by reducing the concentration of the stock solution during reverse osmosis treatment for freshwater recovery. Since the power consumption of the high-pressure pump can be reduced, the total energy amount related to fresh water production can be reduced.

  Moreover, since it is set as the structure which utilizes the waste heat in a system for temperature control of raw | natural water or a high osmotic pressure solution, the total energy amount which concerns on freshwater manufacture can be reduced.

  Moreover, since it is set as the structure which can collect | recover and use a part of energy required for reverse osmosis membrane processing in a system, the energy amount which concerns on freshwater manufacture can be reduced. Moreover, in these processes, since the high osmotic pressure solution system with low corrosivity is applied, it is easy to maintain the soundness of the equipment.

2 Pretreatment means 3 Forward osmosis membrane treatment means 4 Cooling means 5 Crystallization means 6a, 6b Heating means 7 Reverse osmosis membrane treatment means 8 Switching means 13 Chloride ion sensor 14 Storage tank 15 Pumps 31a, 31b Heat exchanger 41 Pressure converter

Claims (5)

Forward osmosis membrane treatment means for removing salt from the raw water through the forward osmosis membrane and obtaining permeated water into a solution having an osmotic pressure higher than the osmotic pressure of the raw water;
Separation means for separating the solute component of the high osmotic pressure solution from the high osmotic pressure solution containing the permeated water obtained by the forward osmosis membrane treatment means and the solute component being Na ₂ HPO ₄ ;
A reverse osmosis membrane treatment means for obtaining fresh water by using a solution after separating the solute component by the separation means as raw water, desalting with a reverse osmosis membrane,
Means for adjusting the temperature of the raw water or high osmotic pressure solution supplied to the forward osmosis membrane treatment or reverse osmosis membrane treatment, and when the solute component is Na ₂ HPO ₄ , the temperature of the raw water or high osmotic pressure solution is 25 ° C. or less. ,
In the case where the separation means or the temperature adjusting means in the preceding stage and the solute component is Na ₂ HPO ₄ , the temperature of the solution in the separation means is 10 ° C. or less,
A pump for supplying raw water to the reverse osmosis membrane treatment means;
A fresh water production system comprising: a pipe for mixing the solute component separated and recovered by the separation means with a high osmotic pressure solution supplied to the forward osmosis membrane treatment means. The fresh water production system according to claim 1,
A crystallization process is used as the separation means, a solute component of a high osmotic pressure solution is deposited, and the slurry and the supernatant liquid are separated. The fresh water production system according to claim 1,
A fresh water production system characterized in that the osmotic pressure of raw water obtained by the separation means and supplied to the reverse osmosis membrane treatment means is lower than the osmotic pressure of raw water supplied to the forward osmosis membrane treatment means. In the fresh water production system according to claim 1 or 2,
Heat exchange means for recovering the heat of the high osmotic pressure solution discharged from the forward osmosis membrane treatment
And supplying heat to the slurry or supernatant discharged from the crystallization treatment apparatus.
The heat exchange means and the fluid that is the heat medium are circulated between these two heat exchange means.
Freshwater producing system characterized in that it comprises a means for. The fresh water production system according to claim 1,
A fresh water production system comprising a pressure converter for transmitting the pressure of a high osmotic pressure solution discharged from the forward osmosis membrane treatment means to raw water for the reverse osmosis membrane treatment.

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