JP2001252662A - Fresh water generating method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a generating method of fresh water using reverse osmotic separation which enables reduction in fresh water generating cost and besides, decrease in a washing work of the membrane. SOLUTION: When sea water is fed to a reverse osmosis membrane module unit containing plural pieces of reverse osmosis membrane elements 2 to obtain permeated water, concentration of a calcium element contained in feeding sea water is controlled to <=380 mg/L, and also a reverse osmosis membrane module unit being >=60% at a recovery ratio of permeated water, when sea water with 3.5 wt.% salt concentration is used as the feeding sea water, is used for generating fresh water.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing fresh water to which reverse osmosis separation is applied.

[0002]

2. Description of the Related Art Conventionally, a membrane separation method has been widely used as a method for selectively separating a specific substance from a mixture (mainly a solution). Among these membrane separation methods, the reverse osmosis method is widely used as a technique for desalinating seawater or brackish water (low-concentration brine) to provide industrial, agricultural, or domestic freshwater. .

[0003] This reverse osmosis method uses a reverse osmosis membrane having the property of permeating only water molecules, and applies a pressure higher than the osmotic pressure of the solution to the solution and water in osmotic equilibrium across the reverse osmosis membrane. This is a technique for transferring water molecules in a solution to the water side by adding from the side. In other words, the reverse osmosis method can take out water from the solution without causing a phase change unlike the conventional evaporation method, and therefore has the advantage of being energy-efficient and easy of operation management.

[0004] When this reverse osmosis separation is performed on a practical scale, the following reverse osmosis separation apparatus is usually used.
First, the reverse osmosis membrane is a spiral, tubular, flat membrane laminate,
Alternatively, the membrane element is processed into a hollow fiber membrane and housed in a case with a flow path material interposed therebetween to form a membrane element called an element. The elements are appropriately connected in series, housed in a pressure-resistant container to form a module, and the modules are connected in parallel to form a reverse osmosis separation module unit. Then, by applying a predetermined pressure to the entire module unit, reverse osmosis separation is performed.

[0005] When reverse osmosis separation is actually performed, salts that cannot permeate the reverse osmosis membrane stay on the solution side near the membrane surface to increase the salt concentration on the membrane surface, so that a so-called concentration polarization phenomenon occurs and the membrane is concentrated. The osmotic pressure of the surface increases. Therefore, it is practically necessary to perform reverse osmosis separation at a pressure higher than the osmotic pressure of the solution (hereinafter, referred to as “operation pressure”). Here, the difference between the operating pressure and the osmotic pressure of the solution is called “effective pressure”, and the operating pressure is usually determined so that the effective pressure at the outlet side of the module unit is about 2 MPa.

[0006] In the reverse osmosis separation, the rate of recovering fresh water from seawater (recovery rate) is determined not only in designing the entire reverse osmosis separation process but also in determining the production cost of fresh water obtained by reverse osmosis separation. It is an important factor. The higher the recovery rate, the lower the cost of desalination. Therefore, it is important to improve the recovery rate. For example, when using seawater having a salt concentration of 3.5%, the recovery in the related art is about 40%. In this case, the seawater supplied at a salt concentration of 3.5% is concentrated in the module unit and taken out from the outlet side as a concentrated water having a salt concentration of 6%, and the osmotic pressure of the seawater in the module unit is reduced. It becomes about 2.5 to 4.5 MPa. Therefore, a value (6.5 MPa) obtained by adding the above-mentioned effective pressure to the osmotic pressure is applied as an operating pressure to the entire module unit to perform the operation.

[0007]

However, in the case of the conventional technique, it is extremely difficult to realize a recovery rate exceeding 40% due to the following limiting factors. First, the first limiting factor is a fouling problem. Fouling is a phenomenon in which turbid components and the like contained in seawater adhere to the membrane surface of a reverse osmosis membrane and cause clogging, which causes a decrease in membrane life and an increase in operating costs during reverse osmosis separation. Become. Then, as the effective pressure increases, the amount of permeated water on the membrane surface increases, and the turbid component is concentrated on the membrane surface, so that fouling is likely to occur.

[0008] Then, the concentration of the seawater introduced into the inlet side of the module unit increases toward the outlet side,
Therefore, the osmotic pressure of the seawater on the outlet side is higher than that on the inlet side. On the other hand, since the operating pressure is substantially the same on the inlet side and the outlet side of the module unit, the effective pressure represented by the difference between the operating pressure and the osmotic pressure increases on the inlet side. As described above, fouling is particularly likely to occur on the inlet side of the module unit, so it is necessary to prevent the fouling by limiting the entire operation pressure (= recovery rate).

The second limiting factor is a problem that scale components (such as calcium carbonate, calcium sulfate, and strontium sulfate) in seawater and salts (sodium chloride) as a solute are deposited on the film surface. This precipitation phenomenon occurs when seawater is concentrated by reverse osmosis and the concentration of the scale component or salt exceeds its solubility, and when these are deposited on the membrane surface, fouling tends to occur. The higher the recovery rate, the higher the concentration of the scale component and the like in the concentrated water. Therefore, it is necessary to regulate the recovery rate in a range where the scale component and the salt do not precipitate. For example, the upper limit of the recovery defined by the solubility of the scale component is about 70%. Further, even when the scale component is prevented from being precipitated by using an appropriate scale inhibitor, the upper limit of the recovery rate is restricted to about 85% because the solubility of the salt is limited.

The third limiting factor is the pressure resistance of the reverse osmosis membrane and the reverse osmosis separation module unit. For example, recovery rate 4
The operating pressure when performing reverse osmosis separation at 0% is 6.5 MPa as described above. For example, when performing reverse osmosis separation at a recovery rate of 60%, the osmosis of the concentrated water (salt concentration of 8.8%) is performed. The pressure becomes 7 MPa, so the operating pressure rises to 9 MPa. Therefore, the reverse osmosis membrane and the module unit are required to have higher pressure resistance.

[0011] Among the above-mentioned limiting factors, precipitation of scale components on the membrane surface is an obstacle to performing reverse osmosis separation at a high recovery rate of about 60% or more. further,
Since concentration polarization occurs near the membrane surface, precipitation may occur even when reverse osmosis separation is performed at a lower recovery rate. Further, when the scale component is deposited on the membrane surface, it is necessary to stop the reverse osmosis separation operation and wash the membrane to remove the precipitate, thereby lowering the operation efficiency. In particular, it is difficult to completely remove the precipitate once deposited, and the precipitate remaining without being removed serves as a starting point to promote new deposition. come.

The present invention solves the above-described problems in reverse osmosis separation, improves the recovery rate of fresh water from seawater, reduces the cost of producing fresh water, and further reduces the membrane cleaning operation. The purpose of the present invention is to provide a method for producing fresh water using the method.

[0013]

In order to achieve the above object, a fresh water producing method according to the first aspect of the present invention supplies seawater to a reverse osmosis membrane module unit including a plurality of reverse osmosis membrane elements and supplies seawater to the reverse osmosis membrane unit. In obtaining water, the concentration of calcium element contained in the supplied seawater is controlled to 380 mg / L or less, and the salt concentration of the supplied seawater is 3.5% by weight.
Characterized by using a reverse osmosis membrane module unit having a recovery rate of permeated water of 60% or more when seawater is used.

[0014] The fresh water producing method according to claim 2 is characterized in that the SDI value of the supplied seawater is controlled to 1 or less by performing a membrane treatment. The fresh water producing method according to claim 3 is
Using at least two reverse osmosis membrane module units including a reverse osmosis membrane element connected in series, supplying seawater to the first-stage reverse osmosis membrane module unit, and enriching obtained from the first-stage reverse osmosis membrane module unit Seawater is supplied to the next reverse osmosis membrane module unit,
It is characterized by obtaining permeated water from each stage.

According to a fourth aspect of the present invention, in any of the reverse osmosis membrane elements, the amount of permeated water per 1 m ^{2 of} the reverse osmosis membrane is in the range of 0.07 to 1.2 m ³ / d. It is characterized by treating seawater. The fresh water producing method according to claim 5, in any reverse osmosis membrane element,
It is characterized in that seawater is treated so that the film surface velocity of supplied seawater becomes 0.03 m / s or more.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a method for producing fresh water using reverse osmosis separation according to the present invention will be described with reference to FIG. FIG.
, The reverse osmosis separation device 1 includes a reverse osmosis membrane module unit 2, a high-pressure pump 4, and the like. Then, the raw seawater 10 is subjected to membrane treatment in the membrane separation device 6 to become supply seawater 20, and the supply seawater 20 is pressurized by the high-pressure pump 4 until a predetermined operation pressure is reached and introduced into the reverse osmosis membrane module unit 2, Here, the permeated water 20a from which the salt has been removed by the reverse osmosis treatment and the concentrated water 20b in which the salt is concentrated are separated. In addition, in the case of the reverse osmosis separation device 1, reverse osmosis separation is performed in one stage with a single operating pressure. For example, as in the technique described in JP-A-8-108048, multi-stage reverse osmosis separation is performed. May go.

The permeated water 20a obtained in this manner is appropriately stored in the tank 9 and used as fresh water 50. The fresh water 50 has, for example, a predetermined drinking water standard (for example, evaporation residue of 500 mg / L or less, chloride ion concentration of 200 mg / L).
L or less) and those satisfying a predetermined purpose may be used. On the other hand, the concentrated water 20b is sent to the outside of the reverse osmosis separation device 1. In this case, the pressure energy of the concentrated water 20b taken out of the system may be recovered using, for example, a turbocharger or a Pelton turbine.

Each reverse osmosis membrane module unit 2, 4
The elements as shown in FIGS. 2 and 3 are connected in series, and as shown in FIG.
This module is configured singly or connected in parallel. Then, a predetermined operation pressure is applied to each of the module units 2 and 4 as a whole.

In FIG. 2, a membrane unit including a supply liquid flow path member 62, a reverse osmosis membrane 63 and a permeate liquid flow path material 64 is disposed around a water collecting pipe 65, and a brine seal 66 is provided at an end. It has a structure with. As shown in FIG. 3, the arrangement of the membrane unit is such that the bag-like body of the reverse osmosis membrane is provided around the water collecting pipe 65 with the supply liquid flow path member 62 and the permeate liquid flow path material 64 interposed therebetween. It is wound in a spiral shape and the whole is housed in a cylindrical case. Then, one end of the bag-like body is adhered so as to open and communicate with the through hole 65a of the water collecting pipe, and the supplied seawater 10 flows outside the bag-like body, passes through the bag-like body, and Permeated water flows into the inside and is collected in the water collection pipe through the opening. The elements having such a structure are connected in order through a joint 67 as shown in FIG.
The module 80 is housed in the pressure-resistant container 68 while being partitioned at 6. One end of the water collecting pipe is sealed by a product end cap 72.

The seawater introduced from the supply liquid port 69 provided at one end of the pressure-resistant container 68 is guided into the element 61, and the supply liquid flow path material 62, the reverse osmosis membrane 63, the permeate liquid flow path material 6
After passing in the order of 4, the water is collected in the water collecting pipe 65 and taken out from the permeated liquid port 70. The concentrated water that has not passed through the reverse osmosis membrane 63 is sequentially led to the downstream element, separated into permeated water and concentrated water in the same manner as described above, and finally discharged from the outlet 71.

In the above description, a spiral type element using a flat membrane-like reverse osmosis membrane has been described as an element, but a plate-and-frame type element, a tubular type element using a tubular membrane,
Further, a hollow fiber membrane element in which hollow fiber membranes are bundled and housed in a case can also be used. Any reverse osmosis membrane may be used as long as it can selectively permeate the solvent (water molecules) in the solution and prevent permeation of the solute (salt). As the film structure, for example, an asymmetric film having a dense layer on at least one surface of the film, and having fine pores whose diameter gradually increases from the dense layer toward the opposite surface, or a dense layer of the asymmetric film A composite membrane with a thin active layer of another material on top can be used. Further, as the form of the membrane, a flat membrane or a hollow fiber membrane can be used. As a material of the film, a polymer material such as cellulose acetate polymer, polyamide, polyester, polyimide, and vinyl polymer can be used. Typical reverse osmosis membranes include cellulose acetate-based or polyamide-based asymmetric membranes, and
A composite film having a polyamide-based or polyurea-based active layer can be used. In particular, it is preferable to use a cellulose acetate-based asymmetric membrane, a composite membrane having a polyamide-based active layer, and a composite membrane having an aromatic polyamide-based active layer.

Incidentally, calcium carbonate, calcium sulfate, strontium sulfate, and the like contained in the supplied seawater are called scale components, which are concentrated during reverse osmosis separation and deposited on the membrane surface. In this case, the scale component precipitates when reverse osmosis separation is performed at a recovery rate of more than about 60%. However, when concentration polarization is remarkable, the scale component may precipitate even at a lower recovery rate.

Accordingly, the present invention performs reverse osmosis separation by removing scale components from the supplied seawater to reduce the concentration thereof, and suppresses precipitation of the scale components when reverse osmosis separation is performed at a high recovery rate. Since the value of the recovery rate varies depending on the salt concentration of the supplied seawater, in the present invention, the recovery rate is defined as a value obtained when seawater having a salt concentration of 3.5% by weight is used as the supplied seawater.

The above seawater having a salt concentration of 3.5% by weight refers to seawater containing 3.5% by weight of a soluble solid substance such as sodium chloride or magnesium chloride. The actual salt concentration in seawater ranges from 0.7% by weight (Baltic Sea) to 4.5%.
X and Y are obtained by converting the recovery rate according to the following formula, since the range is wide up to the weight% (Gulf of Persia). When seawater having a salt concentration of 3.5% by weight is supplied, the recovery rate (%) of each stage = (1− (3.5 / the salt concentration of the concentrated water of each stage)) × 100 For example, the salt of the concentrated water When performing reverse osmosis separation until the concentration becomes 8.8% by weight, the recovery rate is used as about 60% regardless of the salt concentration of the supplied seawater.

The scale component in the present invention includes, for example, the above-mentioned calcium carbonate, calcium sulfate,
In addition to strontium sulfate, barium sulfate, barium carbonate, and the like can be given. However, most of the scale components are composed of calcium compounds. Next, the relationship between the concentration of the scale component in the supplied seawater and the recovery rate will be described. First, consider the case where the concentration of scale components in the supplied seawater before reverse osmosis separation is C ₀ , reverse osmosis separation is performed at a recovery rate X (%), and the concentration of scale components is concentrated to its solubility C _S. In this case, the calculation of the scale component concentration yields the following relationship: X = 100 × (1−C ₀ / C _S ) (1) That is, as shown in FIG. 5, the recovery rate X can be increased as the concentration C ₀ of the precipitated component is reduced.

In FIG. 5, the concentration of the element that is the main component of the conventional scale component of the supplied seawater is usually about 380 mg / L, and the recovery rate must be limited to 60% or less in order to prevent precipitation. . On the other hand, when the concentration of the element that is the main component of the scale component in the supplied seawater is 380 mg / L or less, C ₀ in the formula (1) becomes small, so that the recovery rate at which the scale component does not precipitate is improved. (60% or more).

Further, when the concentration of the element that is the main component of the scale component in the supplied seawater is 300 mg / L, the recovery rate can be further improved (for example, 70% or more). In this case, it is possible to obtain an upper limit recovery rate (about 85%) defined by the solubility of the salt (sodium chloride) as a solute. Here, in order to make the concentration of the element that is the main component of the scale component in the supplied seawater 20 380 mg / L or less, it is preferable that the raw seawater 10 be subjected to membrane treatment by the membrane separation device 6. As the membrane treatment, it is particularly preferable to apply NF (nanofiltration). In this case, since the concentrated solution 6b in which the scale component is concentrated remains in the inside of the membrane separation device 6, the concentrated solution 6b may be taken out of the system by performing appropriate back washing or the like. As the NF membrane, for example, a membrane element SU-620 (salt rejection rate 55%) manufactured by Toray Industries, Inc. can be used.

Then, by performing nanofiltration, the concentration of the element which is the main component of the scale component is about 3 to 150 mg / L.
To be reduced. Note that, here, the element that is the main component of the scale component refers to an element such as calcium, strontium, and barium among scale components such as calcium carbonate and calcium sulfate, strontium sulfate, barium sulfate, and barium carbonate. The concentration can be measured by an atomic absorption method, a plasma emission analysis method, or the like.

Further, it is preferable that the raw seawater 10 is subjected to a pre-treatment beforehand and then subjected to the above membrane treatment. As the pre-processing, for example, the following operation can be performed. First, raw seawater is drawn into the sedimentation basin using the so-called deep seawater that pumps out the seawater of the deep sea layer, the osmotic water intake method using the seabed sand layer as a filter, or the open intake method that simply puts a pump into the sea. Then, a bactericide such as chlorine is added to precipitate and remove particles in seawater and sterilize microorganisms. Next, a coagulant such as iron chloride is added to the seawater to coagulate the turbid component, which is removed by sand filtration. And
A pH adjuster such as sulfuric acid is added to the treated seawater to add p
After lowering H to prevent precipitation of calcium and the like, a reducing agent such as sodium sulfite is added to remove the bactericide, and further passed through a security filter.

In the case of the present invention in which reverse osmosis separation is performed using the supplied seawater from which the scale components have been removed, even if the operation is performed at a high recovery rate of 60% or more, the scale components do not precipitate and the high recovery is achieved. The rate can be realized, and the production cost of fresh water (desalination cost) can be reduced. Further, in the present invention, since precipitation of scale components is reliably suppressed,
The work of cleaning the film to remove the precipitates is reduced, and a decrease in operation efficiency can be prevented.

The operating pressure in the actual reverse osmosis separation is determined as follows. First, the salt concentration of the concentrated water when reverse osmosis separation is performed at a specified recovery rate is calculated, and the osmotic pressure is obtained by an osmotic pressure equation. Then, a predetermined (minimum) effective pressure is added to the osmotic pressure to obtain an operating pressure. Examples of the osmotic pressure method include a van't Hoff method, a Miyake method, and a Stoughton method.

As a method of operating the recovery rate to a predetermined value, for example, the obtained concentrated water amount and the permeated water amount are sequentially monitored, the recovery rate is calculated from the ratio, and the value is set as a target recovery rate. In the case where the ratio deviates from the rate, control for changing the operation pressure to increase or decrease the amount of permeated water can be performed. Incidentally, it has been found that when the above-described treatment with the NF membrane is performed, not only the scale components in the seawater but also the turbid components are removed at the same time. In this case, fouling is less likely to occur as the amount of turbid components in the seawater decreases, and accordingly, the recovery rate can be improved. An SDI value can be used as an index indicating the concentration of such a turbid component. Where SDI
The value (FI value) is expressed as SDI value = (1−T ₀ / T ₁₅ ) × 100/15 (2) and indicates the fine turbid concentration in the target water (where T ₀ = 0). time required for filtration of the sample water for the first 500ml of sample water by using a microfiltration membrane when subjected to pressure filtration at 0.2 MPa / cm ² of .45μm, T _{15: 15} at the same conditions after T ₀ Time required for filtration of 500 ml of sample water after filtration for 5 minutes). The SDI value when there is no turbidity is 0
And the maximum value in the most dirty water is 6.67.

Hereinafter, the relationship between the SDI value and the recovery rate will be described with reference to FIGS. 6, a salt concentration of 3.5% of the feed seawater recovery of 40%, when the reverse osmosis separation at operating pressure P _1, the effective pressure U ₀ of the module unit outlet side, necessary to perform the reverse osmosis separation It will be the minimum value (about 2 MPa). On the other hand, the effective pressure U ₁ module unit inlet side is higher than that U ₀ from the relationship of the osmotic pressure. Since the amount of permeated water increases as the effective pressure increases, the amount of permeated water is larger on the inlet side of the module unit than on the outlet side. For example, under the above operating conditions, U ₁ is 3.9 MPa, and the amount of permeated water on the module unit inlet side is about 0.6 per unit membrane area (unit: m ² ).
m ³ / d.

Next, the recovery rate was set to 60%, and the operating pressure P ₂
When performing reverse osmosis separation at (P ₂ > P ₁ ), the effective pressure at the outlet of the module unit is U ₀ as described above. On the other hand, since the operating pressure P ₂ is higher than P ₁ , the effective pressure U ₂ on the module unit inlet side is higher than U _1, and the amount of permeate increases accordingly. For example, under the above-mentioned operating conditions, U ₂ is 6.5 MPa, and the amount of permeated water on the module unit inlet side is a unit membrane area (unit: m ² ).
About 1.2 m ³ / d.

Here, in order to reduce the equipment cost and operation cost of reverse osmosis separation, it is preferable to increase the amount of permeated water as much as possible to increase the amount of fresh water produced per membrane area, but the amount of permeated water becomes too large. Fouling occurs. Therefore, the maximum amount of permeated water in which fouling does not occur (hereinafter, referred to as “critical permeated water amount”) is defined, and reverse osmosis separation is performed while maintaining this critical permeated water amount.

As shown in FIG. 7, the limit permeated water amount and the SDI value of the supplied seawater are in inverse proportion. In this case, the SDI value of the raw seawater without any treatment is about 5 to 6, and when the ordinary pretreatment is performed by adding a flocculant or sand filtration, the SDI value is 3 to 4. Become. That is, the SDI value of the conventional supply seawater is about 3, and FIG.
According to this, the limit permeate water amount of the supplied seawater is about 0.6 m ³ / d per unit membrane area (unit: m ² ). On the other hand, the amount of permeated water on the inlet side of the module unit when reverse osmosis is separated at a recovery rate of 40% is about 0.6 m ³ / d. That is, the recovery rate of the conventional reverse osmosis separation is limited to about 40%.

On the other hand, when the SDI value of the supplied seawater is 1 or less, the critical permeated water amount increases to about 1.2 m ³ / d. On the other hand, when reverse osmosis separation is performed at a recovery rate of 60%, the amount of permeated water on the inlet side of the module unit becomes about 1.2 m ³ / d, which is the same as the above-mentioned limit permeated water amount. That is, when the SDI value of the supplied seawater is 1 or less, reverse osmosis separation can be performed with a recovery rate exceeding 40% (for example, 60%).

When the amount of permeated water exceeds 1.2 m ³ / d, concentration polarization on the membrane surface becomes remarkable, and the osmotic pressure there rises, so that it is not possible to obtain any more permeated water. Therefore, in FIG. 7, in the region where the SDI value is 1 or less, the limit permeated water amount becomes a constant value (about 1.2 m ³ / d). As described above, when both the scale component and the turbid component in seawater are removed, the following effects are obtained. First, by removing the scale component, the deposition of the scale component in the portion where the concentration polarization occurs as described above is effectively prevented. Further, fouling is effectively prevented by removing the turbid component. Since these limiting factors are removed, the recovery rate can be improved accordingly. In particular, fouling tends to occur on the inlet side of the module unit, while concentration polarization tends to occur on the outlet side of the module unit.To prevent these at the same time, scale components and turbid components in seawater must be removed. It is effective to remove them together.

In the present invention, since the recovery rate is 60% or more (60% to 85%), the operating pressure is 7 to 2%.
It is better to specify 1 MPa. The salt concentration of the concentrated water obtained by reverse osmosis separation at the above-mentioned recovery rate is 8 to 25% by weight. Meanwhile, in reverse osmosis separation, it is extremely important to prevent fouling as described above. In particular, the items described below have a great effect on fouling, so it is preferable to determine the management range as follows.

First, it is preferable that the operating pressure be higher than the osmotic pressure of the concentrated water by 0.5 to 5 MPa. If the operating pressure is less than 0.5 MPa, the utilization efficiency of the membrane may deteriorate, and if it exceeds 5 MPa, the amount of permeated water may increase and fouling may occur. Also,
The amount of permeated water at the time of reverse osmosis separation at each stage is determined by the unit membrane area (unit: m ² ) in any reverse osmosis membrane element.
It is preferably 0.07 to 1.2 m ³ / d. Here, when the amount of permeated water is less than 0.07 m ³ / d,
Since a sufficient amount of fresh water cannot be recovered, there is a possibility that the operation cost and the equipment cost of reverse osmosis separation increase. When the flow rate exceeds 1.2 m ³ / d, the speed at which the permeated water passes through the membrane surface increases, so that the turbid matter is attracted to the membrane surface from the supply water side and adheres to the membrane surface, and fouling occurs. Rings are more likely to occur.

Further, in addition to the above, back pressure may be applied to the permeated water side of the reverse osmosis membrane. Usually, on the inlet side of the reverse osmosis membrane module unit, the effective pressure is high, so that the amount of permeated water is increased, so that fouling is likely to occur. Therefore, by applying a back pressure to the permeated water side of the reverse osmosis membrane on the inlet side, the effective pressure in this portion can be reduced, and fouling can be prevented. And, usually, the module unit consists of elements connected in series,
A back pressure may be selectively applied to the upstream element that may cause fouling. The value of the back pressure may be, for example, 0.1 to 1.5 MPa.

When performing two or more reverse osmosis separations, a back pressure may be applied to the permeated water side of a predetermined stage. For example, if the seawater temperature rises and the salt rejection ability of the reverse osmosis membrane in any of the stages decreases, the water quality deteriorates. Therefore, the effective pressure (operating pressure) of the stage is usually raised to prevent this. I have. However, when a turbocharger is used as the pressure boosting pump, the pressure of the concentrated water in the preceding stage plus a certain amount of boosted pressure becomes the operating pressure in the subsequent stage.Therefore, in order to increase the operating pressure in the latter stage, the operating pressure in the preceding stage must be increased. Also need to be higher. In such a case, fouling is likely to occur in the preceding stage in the same manner as described above, and therefore it is preferable to apply a back pressure to the preceding stage, more specifically, the preceding stage of the stage where the membrane performance is reduced.

When the concentration polarization becomes remarkable, the separation performance of the membrane is lowered, and the possibility that the scale components and salts are deposited on the membrane surface to cause fouling is increased. Therefore, it is preferable to operate under the operation condition in which the concentration polarization is suppressed. This concentration polarization is more likely to occur as the membrane flow velocity of the concentrated water in the reverse osmosis membrane is smaller. In particular, since the flow velocity is smaller at the outlet side than at the inlet side of the module unit, the concentration polarization is remarkable at the outlet side. Tends to occur. For this reason, it is preferable that the membrane surface flow velocity of the supply water in any of the reverse osmosis membrane elements be 0.03 m / s or more from the viewpoint of suppressing concentration polarization.

When the film surface flow rate is less than 0.03 m / s, concentration polarization occurs remarkably on the film surface as described above, and fouling occurs. Here, the membrane surface flow velocity of the feed water in the reverse osmosis membrane element is an average feed water flow rate per unit time passing through the inside of the element. It means the value divided by the cross-sectional area that can pass (hereinafter referred to as the cross-sectional area of the supply water flow path). This feedwater flow path cross-sectional area is, for example,
In the case of an element using a hollow fiber membrane or a tubular membrane, it can be calculated from the inner diameter and the outer diameter of each membrane. In the case of a spiral type element, feed water ( (Raw water) Since the flow path material is used, the flow rate may be calculated based on the porosity of the raw water flow path material. Here, the porosity refers to the ratio of the range in which the raw water can pass, excluding the volume of the members constituting the raw water flow path material, from the total volume occupied by the raw water flow path material, and the above-mentioned membrane surface flow rate is It is calculated by multiplying the length, thickness and porosity of the raw water flow path material and the number of used raw water flow path materials when viewed from the cross section of the element. In the present invention, the supply water refers to seawater to be treated in the first stage, and refers to concentrated seawater obtained from the previous stage in each stage other than the first stage.

In order to control the film surface velocity to 0.03 m / s or more, for example, the supply pressure per unit time may be increased by increasing the supply pressure of the supply water supplied to each stage,
In addition, the later the element where the membrane surface flow rate tends to decrease,
This can be realized by reducing the cross-sectional area of the supply water flow path. In the case of an element using a hollow fiber membrane or a tubular membrane, the feed water flow passage cross-sectional area was adjusted by adjusting the diameter of each membrane or the number of membranes used per element, and using a raw water flow path material. In the case of a spiral type element, it can be changed by adjusting the number, length, porosity, etc. of the raw water flow path material.

Further, in addition to controlling the membrane surface flow rate to 0.03 m / s or more, if the flow of the supply water is disturbed by devising the shape of the flow path material, the thickness of the concentration polarization layer becomes small. Concentration polarization can be further suppressed. Specifically, for example, the flow of the supply water may be disturbed by using a rhombic net as a raw water flow path material. In the above description, the case where one stage of the reverse osmosis membrane module unit is mainly provided has been described. However, it is also preferable to connect multiple stages in series to obtain permeated water from each stage. In this case, while supplying seawater to the first-stage reverse osmosis membrane module unit, and supplying concentrated seawater obtained from the first-stage reverse osmosis membrane module unit to the next-stage reverse osmosis membrane module unit, a so-called concentrated water multistage method may be used. . In the concentrated water multi-stage method, the concentration of the supplied seawater is higher in the later stage, and therefore, the supply pressure needs to be increased in the latter stage. At this time, since the concentrated water finally discharged contains high pressure energy, it is preferable that this energy be recovered by a turbocharger or the like and used for pressurizing the supply water in the preceding stage. Also, since the pressure energy of the concentrated water increases in the later stages, it is most efficient and preferable to recover the pressure of the concentrated water in the final stage.

[0047]

EXAMPLES Examples 1 and 2, Comparative Example Reverse osmosis separation was performed using the reverse osmosis separation apparatus shown in FIG.
Fresh water was produced. Here, the raw seawater has a salt concentration of about 3.5% by weight, a concentration of calcium, which is an element constituting a scale component, is about 400 mg / L, and a concentration of sulfate ion is about 2,550 mg / L. , A bactericide (NaOCl) and a coagulant (FeCl ₃ ) were sequentially added thereto, and the mixture was subjected to pretreatment by filtration such as sand filtration. The calcium concentration was measured by an atomic absorption method, and the sulfate ion concentration was measured by an ion chromatograph.

Next, this seawater is passed through an NF membrane module housed in a pressure-resistant container with six NF membrane elements (SU-620, salt removal rate: 55%) connected in series at a predetermined pressure. The filtration treatment was performed under the conditions shown in Table 1. Next, the seawater that has been subjected to this filtration treatment is desalted at a rate of 99.75.
%, A freshwater production capacity per element incorporating a polyamide reverse osmosis membrane having a membrane area of 28 m ² is 12 m ³ /
The reverse osmosis treatment was performed on the one-stage reverse osmosis membrane module unit in which six elements d were connected in series and housed in a pressure-resistant container under the conditions shown in Table 1. After operating continuously for about three months, the element was extracted and the presence or absence of scale component deposition on the reverse osmosis membrane surface was observed. Table 2 shows the evaluation results including an example in which the treatment with the NF film was not performed.

[0049]

[Table 1]

[0050]

[Table 2]

[0051]

As is clear from the above description, according to the method for producing fresh water using reverse osmosis separation according to the present invention, the total concentration of the scale component containing at least the calcium compound is 380.
Since reverse osmosis separation is performed using seawater supplied at a concentration of not more than mg / L, scale components do not precipitate even if reverse osmosis separation is performed at a high recovery rate of 60% or more. As a result, reverse osmosis separation can be performed at a high recovery rate, so that fresh water production costs can be reduced.

Further, according to the present invention, since the precipitation of scale components is reliably suppressed, the work of cleaning the membrane to remove the precipitate can be reduced, and a decrease in operation efficiency can be prevented.

[Brief description of the drawings]

FIG. 1 is a diagram showing a flow of a fresh water producing method using reverse osmosis separation according to the present invention.

FIG. 2 is a partially cutaway perspective view showing a spiral reverse osmosis membrane element.

FIG. 3 is a sectional view taken along line III-III in FIG. 2;

FIG. 4 is a schematic diagram showing a reverse osmosis membrane module.

FIG. 5 is a graph showing a relationship between the concentration of an element that is a main component of scale components in supplied seawater and a recovery rate.

FIG. 6 is a graph showing a relationship between a recovery rate and an operation pressure, and an effective pressure.

FIG. 7 is a graph showing a relationship between an SDI value of supplied seawater and a limit permeate water amount.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 reverse osmosis separator 2 reverse osmosis membrane module unit 4 high pressure pump 6 membrane separator 10 raw seawater 20 supply seawater 20a permeate 20b concentrated water 50 freshwater 61 spiral reverse osmosis membrane element 62 supply liquid channel material 63 reverse osmosis membrane 64 Permeate flow path material 65 Water collecting pipe 65a Through hole 66 Brine seal 67 Joint 68 Pressure vessel 69 Supply liquid port 70 Permeate liquid port 71 Drain port 72 Product end cap 80 Module

────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] July 28, 2000 (2007.2
8)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Claims

[Correction method] Change

[Correction contents]

[Claims]

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0013

[Correction method] Change

[Correction contents]

[0013]

In order to achieve the above object, a fresh water producing method according to the first aspect of the present invention supplies seawater to a reverse osmosis membrane module unit including a plurality of reverse osmosis membrane elements and supplies seawater to the reverse osmosis membrane unit. In obtaining water, the concentration of calcium element contained in the supplied seawater is controlled to 380 mg / L or less, and the salt concentration of the supplied seawater is 3.5% by weight.
Characterized by using a reverse osmosis membrane module unit having a recovery rate of permeated water of 60% or more when seawater is used. The desalination method according to claim 2 performs the membrane treatment.
The concentration of calcium element in the supplied seawater
It is controlled to 380 mg / L or less.

[Procedure amendment 3]

[Document name to be amended] Statement

[Correction target item name] 0014

[Correction method] Change

[Correction contents]

[0014] The desalination method according to claim 3 is characterized in that the SDI value of the supplied seawater is controlled to 1 or less by performing a membrane treatment. The fresh water producing method according to claim 4 ,
Provided connected in series, with at least two reverse osmosis membrane module unit comprising a reverse osmosis membrane element, it supplies the seawater reverse osmosis membrane module unit of the first stage, obtained from the reverse osmosis membrane module unit of the preceding stage Supply concentrated seawater to the next reverse osmosis membrane module unit,
It is characterized by obtaining permeated water from each stage.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0015

[Correction method] Change

[Correction contents]

According to a fifth aspect of the present invention, in any of the reverse osmosis membrane elements, the amount of permeated water per 1 m ^{2 of} the reverse osmosis membrane is in the range of 0.07 to 1.2 m ³ / d. It is characterized by treating seawater. The fresh water producing method according to claim 6 , in any reverse osmosis membrane element,
It is characterized in that seawater is treated so that the film surface velocity of supplied seawater becomes 0.03 m / s or more. ────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] March 2, 2001 (2001.3.2)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0032

[Correction method] Change

[Correction contents]

As a method of operating the recovery rate to a predetermined value, for example, the obtained concentrated water amount and the permeated water amount are sequentially monitored, the recovery rate is calculated from the ratio, and the value is set as a target recovery rate. In the case where the ratio deviates from the rate, control for changing the operation pressure to increase or decrease the amount of permeated water can be performed. Incidentally, it has been found that when the above-described treatment with the NF membrane is performed, not only the scale components in the seawater but also the turbid components are removed at the same time. In this case, fouling is less likely to occur as the amount of turbid components in the seawater decreases, and accordingly, the recovery rate can be improved. An SDI value can be used as an index indicating the concentration of such a turbid component. Where SDI
The value (FI value) is expressed as SDI value = (1−T ₀ / T ₁₅ ) × 100/15 (2) and indicates the fine turbid concentration in the target water (where T ₀ = 0). The time required for the first 500 ml of sample water to be filtered when the sample water was pressure-filtered at 0.2 MPa using a 45 μm microfiltration membrane. After T ₁₅ : T ₀ , filtration was further performed under the same conditions for 15 minutes. (Time required for filtration of 500 ml of sample water). Then, the SDI value when there is no turbidity is 0, and the maximum value in the most dirty water is 6.67.

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] Explanation of sign

[Correction method] Change

[Correction contents]

[Description of Signs] 1 Reverse osmosis separator 2 Reverse osmosis membrane module unit 4 High pressure pump 6 Membrane separator 9 Tank 10 Raw seawater 20 Supply seawater 20a Permeate 20b Concentrated water 50 Fresh water 61 Spiral reverse osmosis membrane element 62 Supply liquid flow Road material 63 Reverse osmosis membrane 64 Permeate liquid flow path material 65 Water collecting pipe 65a Through hole 66 Brine seal 67 Joint 68 Pressure-resistant container 69 Supply liquid port 70 Permeate liquid port 71 Discharge port 72 Product end cap 80 Module

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Toshiro Miyoshi 52-4, Fujimidai, Otsu-shi, Shiga F-term (reference) 4D006 GA03 HA62 HA63 HA65 JA25A JA58A KA01 KA02 KA03 KA12 KA57 KA62 KA63 KB13 KB14 KB15 KD02 KD06 KD08 KD08 KD11 KD23 KD24 KE03Q KE03R KE05Q KE05R KE08Q KE30P KE30R MA03 MC18 MC22 MC48 MC52 MC54 MC58 PA01 PB03

Claims (5)

[Claims] 1. When supplying seawater to a reverse osmosis membrane module unit including a plurality of reverse osmosis membrane elements to obtain permeate, the concentration of calcium element contained in the supplied seawater is controlled to 380 mg / L or less. A method for producing fresh water, comprising using a reverse osmosis membrane module unit having a recovery rate of permeated water of 60% or more when seawater having a salt concentration of 3.5% by weight is used as seawater to be supplied. 2. The fresh water producing method according to claim 1, wherein the SDI value of the supplied seawater is controlled to 1 or less by performing a membrane treatment. 3. Using a reverse osmosis membrane module unit comprising at least two reverse osmosis membrane module units including a reverse osmosis membrane element provided in series, supplying seawater to the first-stage reverse osmosis membrane module unit, and using the reverse osmosis membrane module in the first stage. The concentrated seawater obtained from the unit is supplied to a reverse osmosis membrane module unit in the next stage, and permeated water is obtained from each stage.
The desalination method according to 1. 4. In any reverse osmosis membrane element, the amount of permeated water per 1 m ^{2 of} reverse osmosis membrane is 0.07 to 1.2.
m ³ / d for processing the seawater to be in the range of, desalination method as claimed in any of claims 1 to 3. 5. The desalination method according to claim 1, wherein the seawater is treated so that the membrane surface flow rate of the supplied seawater is 0.03 m / s or more in any of the reverse osmosis membrane elements. .