JP2001321641A - Fresh water generating method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fresh water generating method using a reverse osmosis separation by which the cost of generating fresh water is reduced and a stabilized operation for a long period is attained. SOLUTION: In the fresh water generating method, two or more stages of reverse osmosis separations having different operational pressures are performed, and supplied sea water 10 is separated at the first stage by reverse osmosis to obtain concentrated water 20b, and thereafter, the operational processes in which the pressure of the concentrated water obtained at the front stage is further raised and then the resultant concentrated water is separated at the subsequent stage by reverse osmosis are sequentially performed, and the permeated water batches 20a, 40a obtained at respective stages are recovered to obtain fresh water. In the method, operational processes satisfying the following equation are conducted: S×F<=5 (here, S is an SDI value of supplied sea water, F: the maximum value (m3/(m2.d)) of a permeated water quantity per one m2 of a reverse osmosis membrane).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing fresh water.
In particular, the present invention relates to a method for producing freshwater from seawater using two or more stages of reverse osmosis separation having different operation pressures.

[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 desalination cost. 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 effective pressure to the osmotic pressure is applied to the entire module unit as an operating pressure, and the operation is performed.

[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. And, as the effective pressure increases, the amount of permeated water on the membrane surface increases, and the turbid component is attracted to the membrane surface.
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, but when performing reverse osmosis separation at a recovery rate of 60%, the osmotic pressure of the concentrated water (salt concentration 8.8%). Is 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, the problem of fouling is a major obstacle in improving the recovery rate in reverse osmosis separation. For this reason, Japanese Unexamined Patent Publication No. Hei 8-108048 discloses that the concentrated water obtained by the reverse osmosis separation in the previous stage is further pressurized by using reverse osmosis membrane module units arranged in multiple stages, and the reverse osmosis separation is performed in the next stage. A technique for performing this is disclosed. In this technique, the overall recovery rate (hereinafter, referred to as “total recovery rate”) is represented by the sum of the recovery rates at each stage, and a high total recovery rate of, for example, 60% can be achieved.

By the way, when performing a multistage reverse osmosis separation,
Even if a trouble such as fouling occurs in any one of the stages, the entire stable operation is affected. Therefore, it is necessary to establish operating conditions for reliably preventing fouling in each stage. In particular, in the operation of multi-stage reverse osmosis separation, since the recovery rate in each stage and the salt concentration of seawater sent to each stage are different, operating conditions that can prevent fouling in all stages have been conventionally set. It was difficult to find. And once fouling occurs,
Since it is necessary to stop the operation and clean or replace the membrane at that stage, there has been a problem that the operation efficiency is lowered and the operation cost is increased.

The present invention solves the above-mentioned problems in reverse osmosis separation, improves the recovery rate of fresh water from seawater, reduces operating costs and lowers fresh water production costs, and achieves long-term stable operation. An object of the present invention is to provide a method for producing fresh water using reverse osmosis separation.

[0014]

In order to achieve the above object, a fresh water producing method according to claim 1 comprises at least two reverse osmosis membranes including a reverse osmosis membrane element provided in series. Using the module unit, seawater is supplied to the first-stage reverse osmosis membrane module unit, and concentrated seawater obtained from the first-stage reverse osmosis membrane module unit is supplied to the next-stage reverse osmosis membrane module unit. in obtaining, any of the in the reverse osmosis membrane element, to control the film surface velocity of the feed seawater or 0.03 m / s, transmission of the reverse osmosis membrane 1 m ² per in SDI value S and the reverse osmosis membrane element of the feed seawater Maximum amount of water F
(M ³ / (m ² · d)) is characterized by treating seawater under conditions that satisfy the following equation: S × F ≦ 5.

It is preferable to treat seawater having an SDI value of 4 or less (claim 2). Further, the seawater may be treated while recovering the pressure energy of the concentrated seawater at the final stage by using a turbocharger.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a method for producing fresh water for performing reverse osmosis separation in two stages according to the present invention will be described with reference to FIG. In FIG. 1, a reverse osmosis separation device 1 includes a first-stage reverse osmosis membrane module unit 2 and a second-stage reverse osmosis membrane module unit 4 having different operation pressures.

Then, the supplied seawater 10 is introduced into the reverse osmosis separation device 1 to perform reverse osmosis separation, whereby fresh water 50 is produced. In this case, the supplied seawater 10 is supplied to the high-pressure pump 6
The first stage permeated water 20a in which the pressure is increased to the first stage operating pressure and is introduced into the first stage reverse osmosis membrane module unit 2 where the salt is removed by the reverse osmosis treatment, and the first stage concentrated water 20b in which the salt is concentrated Is separated into Next, the first-stage concentrated water 20b is introduced into the second-stage reverse osmosis membrane module unit 4. Specifically, the first-stage concentrated water 20b is boosted to a second-stage operating pressure by a turbocharger 8 described below and subjected to reverse osmosis treatment. It is separated into concentrated water 40b.

The first stage permeated water 20a thus obtained
And the latter permeated water 40 a are mixed and appropriately stored in the tank 9, and used as fresh water 50. On the other hand, the second-stage concentrated water 40b is used as a power source of the turbocharger 8, and then is sent to the outside of the reverse osmosis separation device 1. The fresh water 50 may be any as long as it satisfies, for example, a predetermined drinking water standard (for example, evaporation residue of 500 mg / L or less, chloride ion concentration of 200 mg / L or less).

As the supplied seawater 10 to be subjected to reverse osmosis separation, raw seawater may be used as it is, but it is preferable to perform a pretreatment to remove turbid components and the like contained in the raw seawater. As the pre-processing, for example, the following operation can be performed.
First, so-called deep seawater, which pumps out the seawater from the deep sea,
Introduce raw seawater that has been withdrawn by the osmotic water intake method using a seabed sand layer or the like as a filter, or the open intake method in which a pump is simply put into the sea, add a germicide such as chlorine, and add particles in the seawater. And disinfect microorganisms. Next, a coagulant such as iron chloride is added to the seawater to coagulate the turbid component, which is removed by sand filtration. Then, a pH adjuster such as sulfuric acid is added to the treated seawater to lower the pH of the seawater to prevent precipitation of calcium and the like, and then a reducing agent such as sodium sulfite is added to remove the bactericide, and further secure. Pass through the filter.

[0020] Calcium carbonate, calcium sulfate, strontium sulfate, and the like in the supplied seawater are concentrated during reverse osmosis separation and easily precipitate on the membrane surface, and these are called scale components. Therefore, in addition to the above pretreatment, a scale inhibitor may be added to the supplied seawater to suppress the precipitation of the scale component. The scale inhibitor forms a complex with the metal (ion) in the scale component to solubilize the complex, and for example, a polymer or a monomer composed of an organic ion or an inorganic ion can be used. Further, the above-mentioned scale component may be removed by using an NF film or the like. When the precipitation of scale components is suppressed in this way, the recovery rate can be further improved, and a total recovery rate of about 85%, which is the precipitation limit of salt (sodium chloride), which is a main component of the solute, can be realized. Is also possible.

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 thereof. 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 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. However, 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.

The turbocharger 8 utilizes the fact that the permeated water and the concentrated water separated by reverse osmosis have high pressure energy, and applies the high-pressure fluid to a turbine impeller to serve as a drive source of a pump. For example, JP-A 1-2949
No. 03 can be used. If the turbocharger 8 is used, the pressure of the low-pressure fluid sent to the turbocharger can be increased without using electric energy or the like. That is, the pressure energy of the concentrated water can be recovered. Here, 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.

Instead of the turbocharger 8, the reverse osmosis separation of the second and subsequent stages may be performed using a normal high-pressure pump, or the turbocharger may be used for the reverse osmosis separation of the first stage. Instead of using the turbocharger 8, the final stage concentrated water may be applied to, for example, a Pelton turbine to recover the pressure energy. Then, using the reverse osmosis separator 1, for example, the recovery rate of the first stage with respect to the supplied seawater (salt concentration of 3.5%) is 40%, and the recovery ratio of the second stage is 33% (the second stage permeate with respect to the first stage concentrated water sent to the second stage) When the total recovery rate indicating the ratio of the total amount of permeated water obtained in each stage to the supplied seawater is set to 60%, the actual reverse osmosis separation is performed as follows.

First, the first-stage reverse osmosis membrane module unit 2
, And reverse osmosis separation is performed by applying an operating pressure (6.5 MPa) corresponding to a recovery rate of 40%. Next, the obtained first-stage concentrated water 20b was introduced into the second-stage reverse osmosis membrane module unit 4 and operated at an operating pressure (9%) corresponding to a total recovery of 60%.
MPa) is applied to perform reverse osmosis separation in the subsequent stage, and the permeated water 20a, 40a obtained in each stage is recovered at the above recovery rate to obtain fresh water.

By the way, the value of the recovery rate in the present invention means a value in the case where seawater having a salt concentration of 3.5% by weight is used in consideration of the variation depending on the salt concentration of the supplied seawater. The recovery rate of each stage is calculated by the ratio of permeated water of each stage to seawater introduced into the first stage. 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 of seawater ranges from 0.7% by weight (Baltic Sea) to 4.5% by weight (Persian Gulf)
The recovery rate is determined by converting the recovery rate according to the following equation.

When seawater having a salt concentration of 3.5% by weight is supplied, the recovery ratio (%) of each stage = (1− (3.5 / salt concentration of concentrated water in each stage)) × 100 When performing reverse osmosis separation until the salt concentration of water reaches 8.8% by weight, the recovery rate is used as about 60% regardless of the salt concentration of the supplied seawater. The operating pressure of each stage is
Set as follows. First, the salt concentration of the concentrated water at the time of reverse osmosis separation at a predetermined recovery rate is calculated within a range not exceeding the upper limit salt concentration described above, and the osmotic pressure is obtained by an osmotic pressure equation. Then, an operating pressure is obtained by adding a predetermined effective pressure to the osmotic pressure. Examples of the osmotic pressure type include a van't Hoff type, a Miyake type, and a Stoughton type.

As a method of operating so that the recovery rate of each stage becomes a set value, for example, the amount of concentrated water and the amount of permeated water obtained in each stage are sequentially monitored, and the recovery ratio is calculated from the ratio. When the value deviates from the target recovery rate, control for changing the operation pressure to increase or decrease the amount of permeated water can be performed. When the reverse osmosis separation is performed in multiple stages in this manner, the operating conditions in each stage are greatly different as described above. For example, in the first stage, reverse osmosis separation is performed at a recovery rate of 40% using supplied seawater having a salt concentration of 3.5%, but in the subsequent stage, a recovery rate of 3% is obtained using a first-stage concentrated water having a salt concentration of 6%.
Reverse osmosis separation is performed at 3%.

From the above, the present invention focuses on the turbidity of the supplied seawater and the amount of permeated water in each stage as factors affecting fouling, and optimizes these values to make each stage possible. It is possible to effectively prevent fouling in all stages even if the operating conditions are different. That is, in the present invention, the following formula: S × F ≦ 5 (1) (where, S: SDI value of supplied seawater, F: permeated water amount per m ² of reverse osmosis membrane in the reverse osmosis membrane element (m ³ / ( m
Operation is performed while satisfying the relationship of ² ) d)). Here, the SDI value (FI value) indicates a fine turbid concentration in the target water, and is (1−T ₀ / T ₁₅ ) × 100/15.
In represented by (wherein, T _0: initial 50 when subjected to pressure filtration at 0.2MPa the sample water using a 0.45μm microfiltration membranes
Time required for filtration of ₀ ml of sample water, time required for filtration of 500 ml of sample water after further filtration for 15 minutes under the same conditions after T ₁₅ : T ₀ ). When there is no turbidity, the value is 0, and the maximum value in the most dirty water is 6.67.

The product of the SDI value (S) expressed by the formula (1) and the maximum value of the permeated water amount (F) indicates the amount of fine particles accumulated (attached) to the reverse osmosis membrane per unit time. It has been found that by performing an operation in which this value is maintained at a predetermined value of 5 or less, fouling in each stage can be reliably prevented. On the other hand, if the above product is 5
If it exceeds, the amount of fine particles accumulated (adhered) to the reverse osmosis membrane per unit time increases, and fouling occurs.

In this case, as shown in FIG. 5, the amount of permeated water and the SDI value of the supplied seawater are in inverse proportion. And the SDI value of the raw seawater without any pretreatment is about 5,
The SDI value when this is subjected to a normal pretreatment by adding a flocculant, filtering with sand, or the like is 3 to 4. Therefore, according to FIG. 5, when the supply seawater having the SDI value of 3 is used, the upper limit of the permeated water amount is about 1.6 m ³ per unit membrane area (unit: m ² ).
/ (M ² · d).

On the other hand, if the SDI value of the supplied seawater is 1, for example, the upper limit of the amount of permeated water is about 5 m ³ / (m ² · d).
In other words, if the supplied seawater having a small SDI value is used, the amount of permeated water can be increased and the operating pressure can be increased, and as a result, the recovery rate can be improved. In addition, since the restriction on the amount of permeated water is relaxed, the degree of freedom in designing when determining the operating conditions of each stage can be increased. Further, the amount of permeated water can be increased to increase the amount of water produced per membrane area, and the equipment cost of reverse osmosis separation can be reduced. From the above, it is better to set the SDI value of the supplied seawater to 4 or less.
Preferably it is 3 or less, more preferably about 1.

The maximum value of the amount of permeated water is 5 m ³ / (m ² ···
If d) is exceeded, the concentration polarization on the membrane surface becomes remarkable, and the osmotic pressure there rises, making it difficult to obtain more permeated water. Therefore, in FIG. 5, in the region where the SDI value is 1 or less, the amount of permeated water becomes smaller than the value calculated by the equation (1), and the value eventually saturates. In the case of the present invention in which reverse osmosis separation is performed in multiple stages as described above, since the operating pressure increases in the later stages, for example, the above-described total recovery rate of 60
%, Even when operating at high operating pressures (9MP
a) is not applied to the first stage, and the operating pressure (6.5 MPa) is applied to the first stage so as not to exceed the permeated water amount defined by the above equation (1). That is, in the present invention, even when the operation is performed at a high recovery rate (for example, 60%), the amount of permeated water does not increase and fouling does not occur as in the case of performing the reverse osmosis separation in the conventional single stage. Reverse osmosis separation can be performed with high recovery. As a result, the production cost of fresh water (desalination cost) can be reduced.

Further, if the supplied seawater having a small SDI value is used, the operating pressure can be increased by increasing the permeated water amount according to the equation (1), and a higher recovery rate can be realized. In order to reduce the SDI value of the supplied seawater to 1 or less, the raw seawater (or the raw seawater subjected to the above-described pretreatment) is used.
Is preferably subjected to a membrane treatment by a predetermined membrane separation device. As the membrane treatment, for example, UF (ultrafiltration) and MF (microfiltration) can be applied. In this case, since the concentrated liquid in which the turbid components and the like are trapped and concentrated remains in the inside of the membrane separation apparatus, it is preferable to take out the concentrated liquid to the outside of the membrane separation apparatus by performing appropriate back washing or the like.

In the present invention, the value of the above-mentioned total recovery rate is not particularly limited, but is preferably 30 to 85%. If it is less than 30%, the effect of reducing the desalination cost is not sufficient as compared with the conventional reverse osmosis separation, and the necessity of applying the present invention is reduced. On the other hand, if it exceeds 85%, it exceeds the precipitation limit of salt as a solute, so that it is difficult to operate at a higher recovery rate.

In multistage reverse osmosis separation, it is extremely important to prevent fouling in each stage 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, assuming that the SDI value of the supplied seawater is 3 as a standard, the amount of permeated water at the time of reverse osmosis separation in each stage is calculated per unit membrane area (unit: m ² ) in any reverse osmosis membrane element in each stage. 0.07-
It is preferably 1.6 m ³ / (m ² · d). Here, when the permeated water amount is less than 0.07 m ³ / (m ² · d), a sufficient amount of fresh water cannot be recovered, and the operating cost and equipment cost of reverse osmosis separation increase. There is a fear. When the flow rate exceeds 1.6 m ³ / (m ² · d), the speed at which the permeated water passes through the membrane surface is increased. Fouling is likely to occur.

Next, it is preferable that the operating pressure in each stage be higher than the osmotic pressure of the concentrated water in the stage by 0.5 to 5 MPa. If the difference between the operating pressure and the osmotic pressure is less than 0.5 MPa, the reverse osmosis separation may not proceed sufficiently due to the concentration polarization of the membrane surface, and if it exceeds 5 MPa, the amount of permeated water may increase. This is because fouling may occur. When the concentration polarization becomes remarkable, the separation performance of the membrane decreases,
Further, there is a high possibility that the scale components and salts are deposited on the film surface to cause fouling. 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, in the present invention, the membrane surface flow rate of the concentrated water is 0.03 m / s or more in any reverse osmosis membrane of each stage.

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 supply seawater in the reverse osmosis membrane element is an average supply seawater flow rate per unit time passing through the inside of the element, and the cross-sectional area of the supply seawater perpendicular to the supply seawater flow direction 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 seawater flow path). For example, in the case of an element using a hollow fiber membrane or a tubular membrane, the cross-sectional area of the supply seawater flow path can be calculated from the inner and outer diameters of each membrane. In order to use the supplied seawater (raw water) flow path material for the flow path,
What is necessary is just to calculate based on the porosity of this 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 supplied seawater refers to seawater to be treated in the first stage, and refers to concentrated seawater obtained from the preceding stage in each stage other than the first stage.

In order to control the above-mentioned membrane surface flow rate to 0.03 m / s or more, for example, the supply pressure per unit time is increased by increasing the supply pressure of the supply seawater supplied to each stage, The latter element in which the membrane surface flow rate tends to decrease can be realized by reducing the cross-sectional area of the supplied seawater flow path. In the case of an element using a hollow fiber membrane or a tubular membrane, the cross-sectional area of the supplied seawater flow path can be adjusted by adjusting the diameter of each membrane and the number of membranes used per element, and using the raw water flow path material. In the case of the spiral type element, the number and length of the raw water flow path material, the porosity and the like can be changed by adjusting.

Further, in addition to controlling the membrane surface flow velocity to 0.03 m / s or more, if the flow 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 supplied seawater may be disturbed by using a rhombic net as a raw water flow path material. Further, in addition to the above, a 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
Normally, since the module unit is formed by connecting the elements in series, it is only necessary to selectively apply back pressure to a predetermined upstream element that may cause fouling. The value of the back pressure may be, for example, 0.1 to 1.5 MPa.

Also, 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 blocking ability of any of the reverse osmosis membranes decreases, the water quality deteriorates.
Usually, the effective pressure (operating pressure) of the stage is raised to prevent this. 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 previous stage in the same manner as described above, so that a flow control valve is provided in the previous stage, more specifically, in the permeated water line in the previous stage of the stage in which the membrane performance is reduced, It is preferred to apply back pressure by squeezing the valve.

By the way, when performing reverse osmosis separation in multiple stages,
In order to further reduce the desalination cost, it is important to reduce the operation cost. Here, the operating cost is the replacement cost of the membrane, the power cost of the equipment (electric power consumption).
And the like, but the majority of this is occupied by the power costs of the high-pressure pumps arranged in each stage. Since the power cost of the pump is affected by the operating pressure of each stage and, consequently, the recovery rate of each stage, it is possible to reduce the power cost of the pump by appropriately setting the recovery rate of each stage. Become.

In this case, in the first stage, the supplied seawater having a relatively low salt concentration may be subjected to reverse osmosis separation. Therefore, the quality of the permeated water in the first stage is usually superior to those in subsequent stages. Therefore, it is preferable to increase the recovery rate of the first stage as much as possible from the viewpoint of improving the quality of freshwater. Therefore, it is particularly important to set the recovery ratio of the first stage. For this reason, by focusing on the total recovery X and the recovery Y in the first stage reverse osmosis separation and defining a predetermined relationship between the two, the power cost of the pump can be reduced. This will be described with reference to FIG. In the following description, a two-stage reverse osmosis separation is performed. However, in a three or more-stage reverse osmosis separation, the reverse osmosis separation is divided into two stages, a first stage and a subsequent stage. The present invention can be applied.

In FIG. 6, the reverse osmosis separation at the first stage is set at the recovery rate Y (%) and the operating pressure P _1, and the reverse osmosis separation at the second stage is set at the operating pressure P ₂ as described above. X (%) is set. In this case, the power cost of the pumps is determined by the amount of seawater introduced into each pump and the product of the pressure difference between the inlet and the outlet of the pump. Then, in the first stage of the pump, the pressure difference is represented by the operating pressure P ₁ on the vertical axis of FIG. The amount of seawater is equal to the total amount of supplied seawater (100%).
Is a value proportional to the length of 0 to 100 (side L) on the horizontal axis. That is, the power cost A of the first stage pump is P ₁ × L
Is proportional to

On the other hand, the pressure difference in the latter stage pump is represented by (P ₂ −P ₁ ) since the concentrated water that has been increased to the pressure P ₁ in the first stage may be increased to the pressure P ₂ . The amount of seawater is equal to the amount of concentrated water in the first stage, which is a value obtained by subtracting the permeated water (0 to Y on the horizontal axis) obtained at the recovery rate Y from the total amount of supplied seawater (0 to 100 on the horizontal axis). That is, the value is proportional to the length of 100 (side M) from Y on the horizontal axis. That is,
The power cost B of the subsequent pump is proportional to (P ₂ −P ₁ ) × M.

Therefore, the total power cost E of the pump, which is the sum of A and B, is expressed by E 表 P ₁ × L + (P ₂ −P ₁ ) × M (2) On the other hand, the power cost E ′ of the pump when reverse osmosis separation is performed using the conventional one-stage method at the recovery rate X and the operating pressure P ₂ is expressed by E′∝P ₂ × L (3) You.

That is, when reverse osmosis separation is carried out in multiple stages, the amount of EE ′ = (P ₂ −P ₁ ) × (LM) (4) is proportional to the conventional method. Therefore, the power cost of the pump can be reduced. Here, the magnitude of EE ′ is represented by the area S in FIG. 6, and as the area S increases, the power cost (operating cost) of the pump decreases. In this case, the size of the area S varies depending on the operation pressures P ₁ and P ₂ and the values of the side M, and these values are determined by the recovery rates X and Y.
That is, the value of the area S is determined by setting the ratio of X and Y, more specifically, the ratio of Y to X.

For this reason, it is necessary to set X and Y that maximize the area S, but the following fouling problem is a limiting condition in setting them. This will be described with reference to FIGS. FIG.
In this operating condition, the ratio of Y to X is set to be small. Then, the effective pressure of the first-stage module unit inlet side U _1, the effective pressure on the outlet side they become U _0, and, U ₁ is higher than U _0. Similarly, the effective pressure of the subsequent stage module unit inlet side U _2, the effective pressure on the outlet side they become U _0, U ₂ is higher than U _0. Here, U ₂ is represented by U ₂ = (P ₂ −P ₁ ) + U ₀ (5), and U ₀ indicates the minimum effective pressure (usually about 2 MPa).

In this case, since the ratio of Y to X is small, P ₁ is much smaller than P ₂ . As a result, (P ₂ −P ₁ ) greatly increases, and accordingly, the value of U ₂ represented by the equation (5) increases, so that fouling occurs on the inlet side of the subsequent module unit. Become.
On the other hand, in FIG. 8, under these operating conditions, the ratio of Y to X is set to be large. And
Therefore, P ₁ increases to a value close to P ₂ , and accordingly, the value of U ₁ increases, so that fouling occurs at the entrance side of the first-stage module unit.

In particular, the problem of fouling described above becomes remarkable when the value of X, that is, P ₂ is increased in order to realize a high recovery rate. Y must be specified. In consideration of the above, in the present invention, it is preferable to operate with X and Y satisfying the relational expression of 0.2X ≦ Y ≦ 0.8X (6). And
When X and Y have the relationship of Expression (6), the area S increases to about 15 to 25% of the operating cost, and the operating cost decreases accordingly. Further, in this case, the permeated water amount is in the above-mentioned range in any stage, and fouling is prevented, so that the multistage reverse osmosis separation can be stably operated for a long time.

Here, when the recovery rate Y is less than 0.2X, the ratio of Y to X decreases, and as described above, the effective pressure increases on the inlet side of the subsequent module unit, and fouling occurs. It is not preferable because it occurs. Further, when the value of Y decreases, (L−
Since the value of M) decreases and the area S decreases, the operating cost increases. Furthermore, since the ratio of the first stage permeated water to the total permeated water is reduced, the quality of fresh water is reduced.

If the recovery rate Y exceeds 0.8X, the ratio of Y to X increases, and the effective pressure on the inlet side of the first-stage module unit increases, which is not preferable because fouling occurs. Also, when the value of Y increases, the value of (P ₂ −P ₁ ) in equation (4) decreases and the area S decreases, so that the operating cost increases. In addition,
Regarding X and Y, it is more preferable to operate while satisfying the relational expression of 0.4X ≦ Y ≦ 0.7X (7).

In particular, X is 45-65%, and Y is 25-65%.
Preferably, it is 45%. In this case, the area S is maximized, and the operation cost is minimized, so that the fresh water production cost can be further reduced. Further, fouling in each stage can be prevented, and the quality of fresh water can be improved. When the above-described turbocharger is used as the high-pressure pump, the power cost of the pump at that stage is zero. However, since the energy for operating the turbocharger is obtained by recovering the pressure energy of the high-pressure pump in another stage, the power cost of the turbocharger is actually transferred to the power cost of the pump in another stage. You. The power cost of the entire pump in this case is a value obtained by subtracting the pressure energy recovered for operating the turbocharger from the power cost of using a high-pressure pump in each stage. For this reason, even when a turbocharger is used, in order to reduce the power cost of the entire pump, it is necessary to perform the operation while satisfying the relationship of Expression (6).

Further, it is preferable that the ratio between the operating pressure in the first stage and the operating pressure in the last stage is in the range of 1.25 to 1.8. In this case, if the ratio is less than 1.25, the ratio of the recovery Y in the first stage to the total recovery X becomes extremely small, and fouling occurs in the latter stage as described above, which is not preferable. On the other hand, when the ratio exceeds 1.8, the ratio of the recovery Y at the first stage to the total recovery X becomes extremely large, and fouling occurs at the first stage as described above, which is not preferable.

The salt concentration of the concentrated water in the first stage is 3.7 to 1
It is preferably in the range of 1% by weight, more preferably in the range of 5 to 6.5% by weight. further,
The salt concentration of the second stage (final stage) concentrated water is preferably in the range of 5 to 24% by weight, and more preferably in the range of 7 to 10% by weight.

[0058]

EXAMPLES Examples 1 to 3 and Comparative Examples Two-stage reverse osmosis separation was performed using the reverse osmosis separation apparatus shown in FIG. 1 to produce fresh water. Here, each element is constructed by incorporating a polyamide-based reverse osmosis membrane having a desalination rate of 99.75% and a membrane area of 28 m ² , and can produce 12 m ³ / d of fresh water per element. Six modules are connected in series and housed in a pressure-resistant container to form a module. Two modules are arranged in parallel in the first module unit, and one module is used in the second module unit. That is, the film area ratio between the first stage and the second stage is 2: 1.

The supplied seawater was prepared by adding a disinfectant (NaOCl), a coagulant (FeCl ₃ ), p-water to raw water having a salt concentration of about 3.5%.
After an H adjuster (H ₂ SO ₄ ) was sequentially added, the mixture was filtered to prepare. Then, the first stage reverse osmosis separation was performed at the operation pressures shown in Table 1, and the second stage reverse osmosis separation was performed on the obtained concentrated water. Table 1 shows the SDI value of the supplied seawater, the amount of permeated water at each stage, the recovery rate, and the total recovery rate. Also,
Permeated water and concentrated water obtained in each stage in each Example,
In addition, the quality of fresh water is shown in Table 2 (for the first stage) and Table 3 (for the latter stage).

It should be noted that the value of the membrane flow velocity is the value of the element located at the most downstream position in the membrane module constituting each stage (therefore, the value of the membrane flow velocity is the smallest in the stage). The value was shown.

[0061]

[Table 1]

[0062]

[Table 2]

[0063]

[Table 3]

As is apparent from Tables 1 to 3, in Examples 1 to 3 in which the product of the SDI value of the supplied seawater and the maximum value of the permeated water amount is 4 or less, fouling occurs in each of the stages. And a high total recovery of more than 60% is obtained. Further, changes in the operating pressure and the permeated water amount after the operation was continued for 6 months without maintenance were smaller than those in the comparative example, and stable operation could be performed.

[0065]

As is clear from the above description, according to the fresh water producing method of the present invention, since the reverse osmosis separation is carried out by gradually increasing the operating pressure in each stage, a high recovery rate is set as a whole. Even in such a case, the high operating pressure of the next stage is not directly applied to the previous module unit. Then, the effective pressure in the preceding stage does not increase and the amount of permeated water does not become excessive, and fouling can be reliably prevented.

In this case, even if the operating conditions (recovery rate and salt concentration of the seawater to be treated) at each stage are different, the membrane surface flow rate of the supply water of the reverse osmosis membrane element is set to 0.03 m / s or more, and By keeping the product of the SDI value of the supplied seawater and the maximum value of the permeated water amount at or below a predetermined value, fouling in all stages can be prevented. In other words, multi-stage reverse osmosis separation,
It can be performed stably with a high recovery rate, and as a result, the cost of fresh water and the cost of operation can be reduced.

Further, when the SDI value of the supplied seawater is reduced to about 1, the amount of permeated water in each stage can be increased without fouling, and the recovery rate can be further improved.

[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 the relationship between the SDI value of supplied seawater and the amount of permeated water.

FIG. 6 is a graph showing a relationship between a recovery rate and an operating pressure, and a region showing a power cost of a pump in each stage.

FIG. 7 is a diagram showing the effective pressure in each module unit when the recovery rate Y of the first stage is defined at a predetermined rate with respect to the total recovery rate X.

FIG. 8: Recovery rate Y in the first stage at a different rate from total recovery rate X
It is a figure which shows the effective pressure in each module unit when it defines.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Reverse osmosis separation apparatus 2 First stage reverse osmosis membrane module unit 4 Second stage reverse osmosis membrane module unit 6 High pressure pump 8 Turbocharger 9 Tank 10 Supply seawater 20a (First stage) Permeated water 20b (First stage) Concentrated water 40a (Second stage) Permeated water 40b ( 2) Concentrated water 50 Fresh water 61 Spiral reverse osmosis membrane element 62 Feed liquid flow path 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 vessel 69 Supply liquid port 70 Permeate Mouth 71 Outlet 72 Product end cap 80 Module

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yoshisuke Nakamura 1-1-1, Sonoyama, Otsu-shi, Shiga Toray Industries, Inc. Shiga Plant (72) Inventor Toshiro Miyoshi 52-4 Fujimidai, Otsu-shi, Shiga

Claims (3)

[Claims] A seawater is supplied to a first-stage reverse osmosis membrane module unit using at least two reverse osmosis membrane module units including a reverse osmosis membrane element provided in series, and a first-stage reverse osmosis membrane unit is provided. When the concentrated seawater obtained from the module unit is supplied to the reverse osmosis membrane module unit at the next stage, and the permeated water is obtained from each stage, the membrane surface flow rate of the supplied seawater is 0.03 m / s in any reverse osmosis membrane element. In addition to the above control, the SDI value S of the supplied seawater and the reverse osmosis membrane 1 m in the reverse osmosis membrane element
A fresh water producing method characterized by treating seawater under a condition that a maximum permeated water amount per ² F (m ³ / (m ² · d)) satisfies the following expression. S × F ≦ 5 2. The fresh water producing method according to claim 1, wherein seawater having an SDI value of 4 or less is treated. 3. Treating seawater while recovering the pressure energy of the final stage concentrated seawater using a turbocharger.
The fresh water producing method according to claim 1.