JP2001269544A - Membrane separation device and method for separating highly concentrated solution - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a membrane separation device which is capable of obtaining fresh water efficiently and stably with a small amount of energy and at a high recovery rate such as 40% or more, especially from seawater and removing boron, without causing an issue of scale formation as well as provide a membrane separation method. SOLUTION: This membrane separation device comprises a reverse osmosis membrane module unit A using a membrane (a) which shows a performance at a desalting rate of 90% or more when brine of 3.5% concentration with a pH value of 6.5 at a temperature of 25 deg.C is supplied at a pressure of 56 kgf/cm2 and is measured and a loose R0 membrane module unit B using a membrane (b) which shows a transmission flux of 0.8 m3/m2.day or more when brine of 1,500 ppm concentration with a pH value of 6.5 at a temperature of 25 deg.C is supplied at a pressure of 15 kgf/cm2 and is measured, arranged in a multi-stage fashion.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel membrane separation device for reverse osmosis separation of a highly concentrated solution and a method for reverse osmosis separation of a highly concentrated solution. The apparatus and method of the present invention can be used for desalination of brackish water, desalination of seawater, treatment of wastewater, recovery of useful substances, and the like. In particular, the present invention is a high-concentration solution containing a lot of scale components such as calcium carbonate, calcium sulfate, and silica, and a case where a low-concentration solution is obtained from a high-concentration solution containing a large amount of boron or a high-concentration solution is further increased in concentration. It is effective when concentrating.

[0002]

2. Description of the Related Art With respect to separation of a mixture, there are various techniques for removing substances (eg, salts) dissolved in a solvent (eg, water). In recent years, however, membrane separation has been used as a process for saving energy and resources. The law is being used. Microfiltration (MF; Micro)
Filtration method, ultrafiltration (UF; Ultr)
afiltration method, reverse osmosis (RO; Rev)
rse Osmosis) method. More recently, a membrane separation (Loose R) located between reverse osmosis and ultrafiltration
A membrane separation method of the concept of O or NF (Nanofiltration) has appeared and has been used. For example, reverse osmosis is used to desalinate seawater or low-concentration saline water (canned water) to provide industrial, agricultural, or domestic water. According to the reverse osmosis method, desalinated water can be produced by permeating water containing salt through a reverse osmosis membrane at a pressure higher than the osmotic pressure. This technology can be used, for example, to obtain drinking water from seawater, can water, and water containing harmful substances, and has also been used in the production of industrial ultrapure water, wastewater treatment, and recovery of valuable resources. Was. In particular, seawater desalination using a reverse osmosis membrane has a feature that there is no phase change such as evaporation, is advantageous in energy, is easy in operation management, and has begun to spread widely.

When a solution is separated by a reverse osmosis membrane, the solution is separated by a pressure higher than the difference between the chemical potentials of the solution itself (which can be expressed by osmotic pressure) determined by the solute concentration of each solution in contact with both sides of the membrane. Must be supplied to the reverse osmosis membrane surface. For example, when seawater is separated by a reverse osmosis membrane module to obtain fresh water, a pressure of at least about 30 atm or more and at least about 50 to 60 atm is necessary in consideration of practicality. If the supply liquid is not pressurized to a pressure higher than that by a pressure pump, sufficient reverse osmosis separation performance will not be exhibited.

[0004] Taking the case of seawater desalination using a reverse osmosis membrane as an example, in a normal seawater desalination technique, the rate of recovering fresh water from seawater (recovery rate) is at most 40%, which is 40% of the supplied amount of seawater. % Equivalent of fresh water is obtained through the membrane,
Seawater concentration of 3.5% to 6% in reverse osmosis membrane module
It will be concentrated to a degree. In order to perform the reverse osmosis separation operation of obtaining fresh water with a recovery rate of 40% from seawater in this manner, a pressure higher than the osmotic pressure corresponding to the concentration of concentrated water (about 45 atm for a 6% concentration of concentrated seawater). is necessary. The quality of fresh water can correspond to the so-called drinking water level,
In order to obtain a sufficient amount of water, it is actually necessary to apply a pressure higher than the osmotic pressure corresponding to the concentration of the concentrated water by about 20 atm (this pressure is called an effective pressure) to the reverse osmosis membrane. In general, a reverse osmosis membrane module for seawater desalination is operated under a condition of a recovery rate of 40% under a pressure of about 60 to 65 atm.

[0005] The recovery rate of fresh water with respect to the supplied amount of seawater directly contributes to the cost. The higher the recovery rate, the better, but there is a limit in terms of operation and operation to actually increase the recovery rate. In other words, when the recovery rate is increased, the concentration of the seawater component in the concentrated water increases, and at a certain recovery rate or higher, the concentration of salts such as calcium carbonate and calcium sulfate, the so-called scale component, becomes higher than the solubility and the concentration on the surface of the reverse osmosis membrane increases. This is because there is a problem of deposition and clogging of the film.

[0006] At a current recovery rate of about 40% (which is widely recognized as the highest recovery rate), the pH of feed water is reduced to 7%.
If kept below, there is little concern about precipitation of these scale components and no special measures are required, but a higher recovery rate,
Alternatively, when the operation of the reverse osmosis membrane is to be performed at an alkaline pH, it is necessary to add a scale inhibitor that enhances the solubility of the salt in order to prevent precipitation of these scale components. Representative scale inhibitors include ethylenediaminetetraacetic acid and sodium hexametaphosphate. In ethylenediaminetetraacetic acid, two nitrogen atoms and four oxygen atoms form a stable chelate complex with a divalent cation to prevent the generation of scale. on the other hand,
The effect of sodium hexametaphosphate is called the marginal treatment effect, because the oxygen-phosphorus-oxygen bonds in sodium hexametaphosphate are geometrically consistent with the scale crystal lattice.
It is said that the growth of the scale is suppressed by inactivating the nucleation surface by being adsorbed on the scale surface.

However, even if a scale inhibitor is added, the precipitation of the above scale components can be suppressed by p
In the case of H7 or less, the concentration of the concentrated water is about 10 to 11%, and this decreases as the pH of the supply water becomes larger than 7. Therefore, when desalinating seawater with a seawater concentration of 3.5% and a pH of 7 or less, the recovery rate is limited to about 65 to 68% in terms of material balance. In consideration of this, it is recognized that the actual recovery limit at which the reverse osmosis membrane seawater desalination plant can be operated stably is about 60%. When seawater desalination is practically performed using a normal reverse osmosis membrane, as described above, it is necessary to apply a pressure about 20 atm higher than the concentrated water osmotic pressure determined by the concentrated water concentration to the reverse osmosis membrane module. . Recovery rate 60 when the seawater concentration is 3.5%
% Is 8.8% and the osmotic pressure is about 70 atm. As a result, 90a
It is necessary to apply a pressure of about tm.

On the other hand, among the reverse osmosis methods, in the fields of canned water desalination and ultrapure water production, the pressure has been reduced in recent years, and low-pressure reverse osmosis membranes operated at a pressure of 20 atm or less have been marketed and used. As these low pressure reverse osmosis membranes, composite reverse osmosis membranes using a crosslinked wholly aromatic polyamide as a separation functional layer are mainstream, and achieve high water production and high salt rejection at an effective pressure of several atm to several tens atm. . More recently, loose RO membranes located between the reverse osmosis membrane and the ultrafiltration membrane have emerged and are being used. Loose RO membrane has a molecular weight of several hundred to several thousand or more medium to high molecular weight, calcium,
High rejection of divalent ions such as magnesium and polyvalent ions such as heavy metal ions, but it is a membrane that has the property of permeating monovalent ions and low molecular weight substances, and softens hard water containing a large amount of divalent ions. It is used for Another characteristic of this loose RO membrane is that the membrane has a high permeation rate and can be separated at an ultra-low pressure of 10 atm or less with an aqueous solution having a low concentration of about 0.1%. Has been devised. Japanese Patent Application Laid-Open No. 4-150923 discloses a method of separating a high-concentration stock solution into a higher-concentration solution and a middle-concentration solution using a loose RO membrane. However, it is difficult for a loose RO membrane to obtain fresh water from a highly concentrated solution in one step because of its separation characteristics.
Therefore, as a method of using the loose RO membrane, a method of combining with another separation method or performing multi-stage membrane separation has been proposed. For example, JP-A-61-200810, 6
Japanese Patent Application Laid-Open No. 1-200813 discloses a separation apparatus having a loose RO membrane in two stages. Japanese Patent Application Laid-Open No. 3-278818 discloses a method for concentrating a dilute aqueous solution of an organic substance of 1% or less by using a multi-stage film having a low rejection rate of 20 to 70% for rejection of an organic substance. Japanese Patent Application Laid-Open No. 53-58974 discloses a concentration method in which a reverse osmosis membrane module having a lower rejection performance than the preceding stage is arranged in multiple stages in the latter stage. Japanese Patent Application Laid-Open No. 54-124875 also discloses that concentration is performed using a high exclusion rate reverse osmosis membrane in the first stage.
In the method of further concentrating a concentrated solution using a loose RO membrane at the stage, Japanese Patent Application Laid-Open No. 3-21326 discloses a reverse osmosis membrane module unit arranged in series, and a reverse osmosis membrane having high rejection performance is provided on the upstream side. An apparatus for disposing a loose RO film on the side is disclosed. The separation method in which these loose RO membrane modules are multistage has the advantage that operation at a low pressure is possible and a high concentration concentrate can be obtained at a relatively low pressure. However, there is a problem that a very large number of stages are required to improve the quality of the permeated water, and it is difficult to increase the efficiency.

As a method for obtaining fresh water by combining a loose RO membrane, Japanese Patent Application Laid-Open No. 62-91287 discloses a method in which a feed solution is first treated with a membrane having a higher rejection rate of divalent ions than monovalent ions. An apparatus for producing pure water in which the pH of a permeate is adjusted and then treated with a normal reverse osmosis membrane is disclosed.

Japanese Patent Application Laid-Open No. 62-102887 discloses that separation of seawater using a loose RO membrane results in a solution having a low concentration of scale components on the permeate side.

On the other hand, in a recent reverse osmosis membrane seawater desalination plant, removal of boron has been attracting attention as a technical problem in addition to aiming at high recovery operation. Boron exists as boric acid in seawater, and is
-5 ppm. Boric acid has a dissociation constant of 9 at pKa, and is almost non-dissociated in seawater. None of the reverse osmosis membranes for seawater desalination currently on the market can satisfy the rejection rate of boric acid sufficiently under conventional seawater desalination conditions, and therefore the guideline value of boron concentration specified in the tap water quality monitoring item (0.2 mg / L) or less.

As a method for removing boron, other than the reverse osmosis method, a method of removing by adsorption with a strongly basic anion exchange resin or a method of absorbing and removing with a resin obtained by bonding N-methylglucamine to a styrene-divinylbenzene copolymer is used. It has been known. In the former case, if a large amount of salt other than boric acid is present, the amount of boron adsorbed on the ion exchange resin is significantly reduced, and it is economically impossible to treat a large amount of seawater with the ion exchange resin. On the other hand, the latter method has the characteristic that two highly hydroxyl groups in glucamine bound to the resin and boron form a chelate and are adsorbed, so that separation with very high selectivity can be performed. It is used for the recovery of boron from contained wastewater. However, when boron is removed from seawater using a glucamine-bound resin, the treatment cost including the cost of resin regeneration is high, so that applying this method to seawater desalination is economical. There is a problem from the point.

On the other hand, a composite reverse osmosis membrane having a cross-linked wholly aromatic polyamide as a separation functional layer, which is a typical reverse osmosis membrane currently on the market, has an unreacted carboxyl group and amino group in the separation functional layer. As such, it has the property of better removing ionic substances than neutral substances. Therefore, if the reverse osmosis separation is performed by adjusting the supply liquid to the reverse osmosis membrane to a pH of 9 or more at which boric acid is dissociated and ionized, the pH is higher than that in a neutral region where boric acid is not yet dissociated. Can also be expected to greatly improve the rejection of boron.

However, when performing a reverse osmosis separation of a highly concentrated solution containing a large amount of scale components such as seawater in an alkaline region of pH 9 or more, formation of scale as described above and divalent cations such as magnesium hydroxide are performed. Clogging of the film due to precipitation of the hydroxide of ions occurs, causing problems such as lowering the amount of fresh water. Therefore, also in the case of removing boron by the present method, as in the case of operating at a high recovery rate as described above, prevention of scale generation is an important issue.

[0015]

When a reverse osmosis membrane desalination plant is operated at about 40% of the conventional maximum recovery level, a plurality of modules are simply arranged in parallel and a pressure of 65 atm (supply water temperature of 20%). ° C), supply water pH7
By operating under the following conditions and setting the supply seawater amount to 2.5 times the total amount of permeated water, the above conditions for preventing fouling and concentration polarization are sufficiently satisfied, and stable operation is performed. Have been. Further, it was not necessary to consider the balance of the permeated water of each element inside the module and the scale component precipitation of the concentrated water.

In order to further reduce the cost of a reverse osmosis membrane seawater desalination plant, high recovery operation with a higher recovery rate is an issue. As described above, desalination of seawater is carried out by a normal method. When this is performed, it is desirable to increase the recovery rate to about 60% as a seawater desalination recovery rate with a seawater concentration of 3.5%, and on the assumption that an appropriate amount of a scale inhibitor is added, the operating pressure of the RO membrane is usually concentrated. It is necessary to operate at a pressure of 90 atm about 20 atm higher than the osmotic pressure of water.

However, in the conventional separation using one kind of membrane, it is necessary to add 9% to the feed liquid in order to operate at a recovery rate of 60%.
It is necessary to apply a pressure of 0 atm at a time, which causes fouling on the membrane surface to be too large, and also causes problems such as deteriorating the membrane depending on fouling substances such as heavy metals, and generation of scale on the concentrated solution side. Is also a problem.

Similarly, when the supply water is made alkaline at pH 9 or more and the reverse osmosis separation is carried out for the purpose of improving the boron exclusion rate of the reverse osmosis membrane, scale is generated and hydroxide is precipitated, which is a serious problem. .

Several methods for concentrating and removing scale components from seawater have been devised by combining a loose RO membrane. However, a specific method for obtaining fresh water from a highly concentrated solution such as seawater with a high recovery rate has been proposed. The fact is that there is still no way to solve the problem.

According to the present invention, the formation of scale components in a high-concentration solution on a membrane surface is prevented by a reverse osmosis method, so that a low-concentration solution can be obtained at a high recovery rate, more stably, with less energy and at a lower cost. An object of the present invention is to provide an apparatus and a separation method that can obtain the water efficiently and, particularly, to obtain fresh water efficiently and stably with low energy at a high recovery rate of 40% or more from seawater and a conventional reverse osmosis. An object of the present invention is to provide an apparatus and a separation method for improving the removal of boron, which has been insufficiently removed by the method, without causing the problem of scale formation.

[0021]

To solve the above problems, the present invention has the following arrangement. "Temperature 25 ° C, pH6.
5. A reverse osmosis membrane module unit A using a membrane a having a salt rejection rate of 90% or more when a salt solution having a concentration of 3.5% is supplied at a pressure of 56 kgf / cm ² , at a temperature of 25 ° C. and a pH of
6.5, salt solution with a concentration of 1,500ppm, pressure 15kg
The permeation flux when supplied at f / cm ² is 0.8 m ³ / m ²
-A membrane separation apparatus characterized by arranging loose RO membrane module units B using membranes b that are days or more in multiple stages. In the present invention, the permeation flux refers to a salt solution obtained by dissolving 1500 ppm of salt in distilled water or pure water at 15 kgf /
500 ppm of sodium chloride was dissolved in per unit membrane area, per unit time of membrane permeation, or distilled water or pure water when reverse osmosis separation was performed under the conditions of cm ² , 25 ° C., pH 6.5, and a recovery rate of 15% or less. 5 kgf / cm using solution
^{2. The} amount of permeated water per unit time per unit membrane area per unit time when membrane separation is performed at 25 ° C., pH 6.5, and a recovery rate of 15% or less.

Here, the rejection rate is a value calculated by the following equation.

Exclusion rate (%) = (concentration of feed solution−concentration of permeate solution) / concentration of feed solution × 100 The concentration of the feed solution and the concentration of the permeate solution can be determined by measuring the electric conductivity of the solution. The recovery rate is a ratio of the amount of the permeated liquid to the amount of the liquid supplied to the membrane, and is defined by the following equation.

Recovery rate (%) = amount of permeate / amount of feed solution ×
100

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, a membrane a is a semipermeable membrane capable of substantially reverse osmosis separation, which allows some components in a liquid mixture to be separated, for example, a solvent to pass and other components not to pass. Polymeric materials such as cellulose acetate polymers, polyamides, polyesters, polyimides, and vinyl polymers are often used as the material. The membrane structure has a dense layer on at least one surface of the film, and an asymmetric film having fine pores having a large pore diameter gradually from the dense layer toward the inside of the film or the other surface. There is a composite membrane having a very thin separation functional layer formed of a material. The membrane form includes a hollow fiber and a flat membrane. However, the method of the present invention can be used irrespective of the material, membrane structure and membrane form of the reverse osmosis membrane, and all are effective. Representative reverse osmosis membranes include, for example, cellulose acetate-based and polyamide-based asymmetric membranes and composite membranes having polyamide- and polyurea-based separation functional layers. Among these, the apparatus and method of the present invention are effective for cellulose acetate-based asymmetric membranes and polyamide-based composite membranes.

The working pressure of the membrane a is not particularly limited, but it is preferably operated at 50 kgf / cm ² or more, more preferably at 80 kgf / cm ² or more, in order to obtain a high recovery rate. . Therefore, the reverse osmosis membrane used here is preferably a membrane used under high pressure conditions such as seawater desalination and valuable resource recovery, and has a denser separation function layer and is a membrane having high pressure resistance. Is preferred.

In the present invention, the properties that the film a should have are:
3.5% saline, 56 kgf / cm ² , 25 ° C., pH
It is a membrane having a separation performance of 90% or more, preferably 95% or more, more preferably 99% or more as measured by 6.5. The higher the rejection rate, the lower the concentration of chlorine ions in the permeated water. 90 salt rejection
%, The amount of chloride ions in the permeate increases, making it difficult to use the permeate as it is as drinking water or industrial water.

Further, the membrane a is made of 3.5% saline, 56 k
gf / cm^TwoAt 25 ° C. and pH 6.5
1.5m overflux^Three/ M^TwoDays or less, more preferably 0.
5m ^Three/ M^Two・ 1.0m or more for days^Three/ M^Two・ It must be less than days
Is preferred. 1.5m^Three/ M^Two・ When it exceeds days,
Including salt rejection performance and reduced pressure resistance, and 0.5m^Three/ M^Two
・ Less than a day requires a large membrane area, resulting in high membrane cost
And it is difficult to obtain a high recovery rate.
You.

In the present invention, the film b is a loose RO film.

Loose RO membranes have high rejection performance for polyvalent ions such as medium to high molecular weight molecules, divalent ions and heavy metal ions, but have a high molecular weight of several hundreds to several thousands or more, but have high monovalent ions and low molecular weight substances. Is a membrane having the property of permeability, and polyamide, polypiperazinamide, polyesteramide, or a crosslinked water-soluble vinyl polymer is often used as the material. The membrane structure has a dense layer on at least one surface of the film, and an asymmetric film having fine pores having a large pore diameter gradually from the dense layer toward the inside of the film or the other surface. There is a composite membrane having a very thin separation functional layer formed of a material. The membrane form includes a hollow fiber and a flat membrane. However, the method of the present invention can be used irrespective of the material of the reverse osmosis membrane, the membrane structure and the membrane form, and both are effective. Membranes are preferred. More preferably, a polyamide-based composite membrane is used, and a piperazine polyamide-based composite membrane or the like is more suitable in terms of the amount of permeated water, chemical resistance, and the like.

In the present invention, the loose RO film should have
Characteristics are 500 ppm saline, 5 kgf / cm^Two, 2
A permeation flux of 0.5 m when measured at 5 ° C. and pH 6.5
^Three/ M ^TwoA membrane having more than 500 days is preferred;
ppm saline, 5kgf / cm^Two, 25 ° C, pH6.
The salt rejection as measured at 5 is 80% or less, preferably
60% or less and 1000 ppm magnesium sulfate
Aqueous solution, 5kgf / cm^TwoMeasured at 25 ° C and pH 6.5
90% or more, preferably 98% or less
It is preferable that the membrane has the above separation performance.

The membrane element is formed by actually using the above membrane, and the flat membrane is incorporated into a spiral, tubular, plate and frame element, and the hollow fibers are bundled to form an element. However, the present invention does not depend on the form of these membrane elements.

The reverse osmosis membrane or loose RO membrane module unit is a module in which one or several of the above-mentioned membrane elements are placed in a pressure vessel in parallel, and the combination, number and arrangement of the modules are determined according to the purpose. Arbitrarily.

Next, the configuration of the apparatus of the present invention will be described with reference to the drawings. In the present invention, the membrane separation device includes at least a water intake portion for a supply liquid, a pretreatment portion, and a membrane portion. The membrane part is a part that supplies the liquid to be treated to the membrane module under pressure for the purpose of water production, concentration, separation, etc., and separates it into a permeate and a concentrate, and usually consists of a membrane element and a pressure vessel. It is composed of a unit in which modules are arranged, a pressure pump and the like. The liquid to be separated supplied to the membrane portion is a pretreatment portion, which is usually a chemical solution such as a bactericide, a flocculant, a reducing agent, a pH adjuster and the like, and a pretreatment by sand filtration, activated carbon filtration, security filter, etc. Component removal). For example, in the case of desalination of seawater, after taking in seawater at a water intake portion, particles and the like are separated in a sedimentation basin, and a bactericide is added here to perform sterilization. Further, sand filtration is performed by adding a flocculant such as iron chloride. The filtrate is stored in a storage tank, and after adjusting the pH with sulfuric acid or the like, is sent to a high-pressure pump. Add a reducing agent such as sodium bisulfite into this solution to eliminate germicides that cause deterioration of the membrane material, pass through the security filter, and then increase the pressure with a high-pressure pump and supply it to the membrane module. Is often done. However, these pretreatments are appropriately adopted depending on the type of the supply liquid used and the application.

FIG. 1 is a diagram of an apparatus for supplying permeated water of a loose RO membrane module unit B to a reverse osmosis membrane module unit A. High-concentration feedwater such as seawater is first pretreated in the pretreatment section, and then the first stage RO
It is supplied to the membrane module unit B. The loose RO membrane module unit B has a low rejection rate of monovalent ions, so that the osmotic pressure difference between the concentrated water and the permeated water is reduced, and as a result, it can be operated at a relatively low pressure even in a highly concentrated solution such as seawater. . At the first stage, polyvalent ions such as scale components and medium to high molecular weight substances are separated into monovalent ions and low molecular weight substances. The concentrated water in which the polyvalent ions are concentrated is released as it is, and the permeated water containing no scale component is pressurized and supplied to the second reverse osmosis membrane module unit A. In the reverse osmosis membrane module unit A, there is no fear of scale generation, so that separation can be performed at a high recovery rate.

Here, a method of performing a high recovery operation in the unit A following the unit B is shown in FIG. In order to obtain a high recovery rate, a method in which the reverse osmosis membrane module units A are arranged in multiple stages and the concentrated water of the previous reverse osmosis membrane module units A is supplied to the next reverse osmosis membrane module units A It is preferable to carry out the method by a concentrated water pressurization method) from the viewpoint of preventing fouling of the membrane. The pre-processing portion and the first-stage loose RO membrane module unit B are the same as above. The permeated water of the loose RO membrane module unit B is first supplied to the reverse osmosis membrane module unit A in the preceding stage at the same operating pressure as that of ordinary seawater desalination (60 atm).
) And recovery (about 40%) to obtain fresh water. Next, the concentrated water is pressurized to 80 atm or more and supplied to the reverse osmosis membrane module unit A at the next stage. In addition, the operating pressure and the recovery rate shown here are shown as an example, and are not limited thereto. The same membrane may be used as the membrane a used in the previous stage and the next stage of the reverse osmosis membrane module unit A, but it is more preferable to use membranes having different characteristics. Further, the number of stages of the reverse osmosis membrane module unit A is not limited. However, as the number of stages increases, a pump for increasing the pressure is required. Therefore, it is preferable to use two stages in consideration of equipment costs and operation costs. In addition, in order to improve the boron removal performance, a device for injecting alkali into the feed water of the reverse osmosis membrane module unit A is provided, and the pH at which boric acid in the feed water is dissociated to become anions.
Can also be prepared. At this time, the pH is preferably 9 or more, more preferably 9.5 or more, and 11 or less. Even when operating under such high alkali conditions, there is little danger of scale formation because scale components have been removed in advance. As the alkali, a concentrated aqueous solution of an alkali salt such as sodium hydroxide or sodium carbonate is used, and the permeated water of the loose RO membrane module unit B is supplied by a chemical injection pump.
That is, it is injected into the supply water of the reverse osmosis membrane module unit A.

FIG. 3 shows a case where the concentrated water of the reverse osmosis membrane module unit A is supplied to the loose RO membrane module unit B, and the permeated water of the loose RO membrane module unit B is mixed with the supply water of the reverse osmosis membrane module unit A. It is a figure of an apparatus. First, the pretreated seawater is supplied to the first-stage reverse osmosis membrane module unit A, where fresh water is separated from a highly concentrated solution such as seawater. The concentrated water may be directly supplied to the loose RO membrane module unit B, but it is preferable to use the concentrated water pressurization method in order to obtain a high recovery rate. After that, the concentrated water at the final stage of the reverse osmosis membrane module unit A is supplied to the loose RO membrane module unit B. At this time, there is no need to pressurize the concentrated water itself because it has a pressure. The loose RO membrane module unit B is further separated into concentrated water containing scale components and permeated water having a small salt concentration and containing no scale components. Loose R
The concentrated water of the O membrane module unit B is discharged as it is, and the permeated water is returned to the supply water of the first-stage reverse osmosis membrane module unit A and mixed. At this time, since the scale component concentration of the first-stage feed water is reduced by the permeated water of the loose RO membrane module unit B, the reverse osmosis membrane module unit A can be operated at a recovery rate higher than the normal 40%. . Further, in the case of adding the scale inhibitor in the case of FIG. 3, the scale inhibitor only needs to be added to the feed water of the membrane b, and the feed water of the loose RO membrane module unit B is smaller than the total feed water. Therefore, there is an advantage that the amount of the scale inhibitor can be reduced as a whole.

In the present invention, the scale inhibitor to be added to the feed liquid of the reverse osmosis membrane device forms a complex with a scale component such as a polyvalent metal ion in the solution and suppresses the generation of scale. And inorganic ionic polymers or monomers can be used. As the ionic polymer, synthetic polymers such as polyacrylic acid, sulfonated polystyrene, polyacrylamide, and polyallylamine, and natural polymers such as carboxymethylcellulose, chitosan, and alginic acid can be used. Ethylenediaminetetraacetic acid or the like can be used as the organic monomer. Polyphosphates and the like can be used as the inorganic scale inhibitor. Among these scale inhibitors, polyacrylic acid-based polymers, polyphosphates, ethylenediaminetetraacetic acid (E) are particularly preferred in terms of availability, operability such as solubility, and price.
DTA) and the like are suitably used in the present invention. The polyphosphate refers to a polymerized inorganic phosphoric acid-based substance having two or more phosphorus atoms in a molecule represented by sodium hexametaphosphate and bonded to an alkali metal or an alkaline earth metal by a phosphoric acid atom or the like. Typical polyphosphates include pyrophosphate 4
Examples include sodium, disodium pyrophosphate, sodium tripolyphosphate, sodium tetrapolyphosphate, sodium heptapolyphosphate, sodium decapolyphosphate, sodium metaphosphate, sodium hexametaphosphate, and potassium salts thereof.

It is sufficient that the concentration of the scale inhibitor added is at least an amount capable of taking in the scale components in the feed solution. However, in consideration of the operability such as the cost and the time required for dissolution, it is generally 0. Although it depends on the quality of feed water, it is usually 0.1 to 100 ppm, more preferably 1 to 50 ppm in seawater. When the addition amount is less than 0.01 ppm, the generation of scale cannot be sufficiently suppressed, so that the film performance deteriorates. 1000 ppm
Above-mentioned is not preferable because the scale inhibitor itself is adsorbed on the membrane surface to reduce the amount of fresh water or deteriorate the water quality.
Dozens to hundreds of ppm for feed solutions containing large amounts of scale components
May need to be added.

If an ultrafiltration membrane is used in the pretreatment part of the apparatus and the separation method of the present invention, the apparatus of the present invention can be operated more stably, and thus is preferably used. The ultrafiltration membrane is used, for example, as a hollow fiber membrane module formed by bundling a plurality of hollow fiber membranes, and is used in combination with sand filtration or alone. Further, in operation of the apparatus, it is necessary to use a hollow fiber membrane which can be used for a long period of time while removing dirt on the surface of the hollow fiber membrane by physical washing means in operation of the apparatus. As the physical cleaning means, reverse flowing water cleaning of filtered water, air flushing with air or scrubbing cleaning can be used.

The hollow fiber membrane module used in the present invention is a hollow fiber membrane module in which the inside of the hollow fiber membrane is opened by hardening the end of the hollow fiber membrane bundle with an adhesive and then cutting it. However, the optimum shape can be adopted in combination with the physical cleaning means. Particularly preferably, a module having a shape in which a plurality of hollow fiber membrane elements are loaded in a tank-shaped container is suitable for increasing the capacity, and is most preferable. The hollow fiber membrane constituting the hollow fiber membrane module is not particularly limited as long as it is a porous hollow fiber membrane,
Polyethylene, polypropylene, polysulfone, polyvinyl alcohol, cellulose acetate, polyacrylonitrile, and other materials can be selected. Among these, a particularly preferable hollow fiber membrane material is a hollow fiber membrane made of a polymer containing acrylonitrile as at least one component. Among the most preferred acrylonitrile-based polymers, acrylonitrile has at least 5
Acrylonitrile copolymer comprising 0 mol% or more, preferably 60 mol% or more, and 50% or less, preferably 0 to 40 mol% of one or more vinyl compounds copolymerizable with the acrylonitrile. It is. Further, a mixture of two or more of these acrylonitrile-based polymers, and a mixture with another polymer may be used. The vinyl compound is not particularly limited as long as it is a known compound having copolymerizability to acrylonitrile, and preferred copolymerization components include acrylic acid, itaconic acid, methyl acrylate, methyl methacrylate, and acetic acid. Examples thereof include vinyl, sodium allyl sulfonate, and sodium p-styrene sulfonate.

According to the apparatus and the separation method of the present invention, the reverse osmosis membrane module unit A can be operated at a higher recovery rate than a normal recovery rate, and considering the cost of separation, the higher the recovery rate is, the better. preferable. In the separation method of the present invention, the recovery rate can be set to a value exceeding normal 40%. In order to further reduce the cost, it is preferable to perform the separation at a recovery rate of 50% or more, more preferably 60%.

The apparatus and the separation method of the present invention are suitable for separating a supply liquid having a high concentration. In particular, it has an effect on separating a solution having a solute concentration of 0.5% or more, and has a great effect on desalination of seawater.

[0044]

EXAMPLES The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

Table 1 shows the properties of the film used in the present invention. In the present invention, for these two types of film a and one type of film b,
A membrane element having a membrane area of 7 m ³ was prepared, and one or several pressure vessels loaded with one or several of these elements were arranged in parallel to form a reverse osmosis membrane module unit or a loose RO membrane module unit. Desalination was performed. Seawater of Seto Inland Sea adjusted to a salt concentration of 3.5% was used. The amount of boron was determined by curcumin absorption spectrophotometry.

[0046]

[Table 1]

Example 1 An apparatus shown in FIG. 1 was manufactured using a reverse osmosis membrane module unit using a membrane b-1 and a membrane a-1 and a loose RO membrane module unit. Using this apparatus, first, seawater having a salt concentration of 3.5% was subjected to 25 ° C., pH 6.
After the preparation to 7, the mixture was treated with a hollow fiber ultrafiltration membrane module. Thereafter, the pressure is increased to 25 kgf / cm ² and loose RO
The feed water was supplied to the membrane module unit B to obtain permeated water having a salt concentration of 1.9% and concentrated water having a salt concentration of 5.9%. The permeated water of the loose RO membrane module unit B contained a trace amount of polyvalent ions. This permeated water was pressurized to 90 atm and supplied to the reverse osmosis membrane module unit A. The ratio of the permeated water amount of the reverse osmosis membrane module unit A to the supplied water amount is 47%.
And the salt concentration of the permeated water was 266 ppm. The amount of fresh water produced by the reverse osmosis membrane module unit A is 6.7 m.
³ / day, and no decrease in the amount of permeated water was observed even after lapse of 2000 hours. Example 2 In Example 1, the unit A was changed from a single stage to two stages. That is, the permeated water of the loose RO membrane module unit B is supplied to the reverse osmosis membrane module unit A using the membrane a-1, the concentrated water is pressurized, and the reverse osmosis membrane module unit using the membrane a-2 is supplied. A method using the pressurization of concentrated water supplied to A was performed by the apparatus shown in FIG. The operating pressure of membrane a-1 was operated at 52 atm, and membrane a-2 was operated at 80 atm. The recovery rate of the reverse osmosis membrane module unit A was 63% for the first and second stages, and the salt concentration of the permeated water was 185 ppm. Further, in the reverse osmosis membrane module unit A, no decrease in the permeated water amount was observed even after 1600 hours had elapsed in both the former stage and the latter stage. Example 3 An apparatus shown in FIG. 3 was produced using a reverse osmosis membrane module unit using a membrane a-1 and a membrane b-1 and a loose RO membrane module unit. Using this apparatus, first, seawater having a salt concentration of 3.5% was subjected to 25 ° C., pH 6.
After adjusting to 7, water was supplied through treatment with a hollow fiber ultrafiltration membrane module. Thereafter, the supply water and the permeated water of the loose RO membrane module unit B were mixed, the pressure was increased to 90 atm, and the mixture was supplied to the reverse osmosis membrane module unit A. The concentrated water of the reverse osmosis membrane module unit A had a salt concentration of 5.6%. Add 10p of sodium hexametaphosphate to this concentrated water
pm, and supplied to the loose RO membrane module unit B. In the loose RO membrane module unit B, concentrated water having a salt concentration of 8.9% and permeated water having a low salt concentration of 2.9% were obtained. The concentrated water of the loose RO membrane module unit B was taken out of the apparatus, and the permeated water was circulated and mixed with the feed water of the reverse osmosis membrane module unit A. The ratio of the permeated water amount of the reverse osmosis membrane module unit A to the supplied seawater amount was 60%, and the salt concentration of the permeated water was 227 ppm. Further, the amount of permeated water of the reverse osmosis membrane module unit A is 29.9 m ³ / day,
No decrease in the amount of permeated water was observed even after lapse of 2,000 hours. Example 4 In Example 3, instead of the unit A which was a single stage, 2
The unit A of the step was used. That is, the reverse osmosis membrane module unit A using the membrane a-1 was used in the first stage, and the reverse osmosis membrane module unit A using the membrane a-2 was used in the second stage, using a concentrated water pressurization method. Therefore, the unit configuration was {A (concentrated water) → A (concentrated water) → B}. Return the permeated water of the loose RO membrane module unit B to the feed water and mix,
The pressure was increased to 0 atm and supplied to the reverse osmosis membrane module unit A in the preceding stage. The pressure of the concentrated water of the reverse osmosis membrane module unit A in the first stage was increased to 90 atm, and supplied to the second reverse osmosis membrane module unit A. The salt concentration of the concentrated water in the reverse osmosis membrane module unit A in the latter stage was 6.3%. Sodium hexametaphosphate was added to the concentrated water to a concentration of 10 ppm, and supplied to the loose RO membrane module unit B. In the loose RO membrane module unit B, concentrated water having a salt concentration of 9.2% and permeated water having a low polyvalent ion concentration at a salt concentration of 3.6% were obtained. The concentrated water of the loose RO membrane module unit B was taken out of the apparatus, and the permeated water was circulated and mixed with the feed water of the preceding reverse osmosis membrane module unit A. The ratio of the amount of permeated water of the reverse osmosis membrane module unit A to the amount of seawater to be supplied is 64% in the former stage and the latter stage.
And the salt concentration of the permeated water was 197 ppm. Further, the permeated water amount of the reverse osmosis membrane module units A in the first and second stages did not decrease even after 1600 hours. Comparative Example 1 Using a reverse osmosis membrane module unit A using the membrane a-1 of Example 1, supplied with seawater (salt concentration: 3.5%) subjected to coagulated sand filtration, and separated at 90 atm. Was performed. 10 pp sodium hexametaphosphate in feed water
m, and the operation was performed with the ratio of the amount of permeated water to the amount of supplied seawater being 60%. As a result, the chlorine ion concentration of the permeated water was 306 ppm.
Furthermore, permeate flow rate is at 21.7 m ³ / day, permeate flow after lapse of 2000 hours dropped 19.3 m ³ / day and 11%. Comparative Example 2 Using reverse osmosis membrane module unit A using membrane a-1 of Example 1, supplied with seawater (salt concentration: 3.5%) subjected to coagulated sand filtration, and separated at 63 atm. Was performed. 10 pp sodium hexametaphosphate in feed water
m, and the operation was performed with the ratio of the amount of permeated water to the amount of supplied seawater being 42%. The chlorine ion concentration of the permeated water was 306 ppm, and the concentration of boron was 1.3 ppm. It exceeded the guideline value of the tap water quality monitoring item. Comparative Example 3 In Comparative Example 2, the pH was adjusted to 9 by injecting an alkali into the feed water. As a result, a large amount of divalent cation hydroxide was precipitated, and the amount of permeated water of the reverse osmosis membrane module unit A was sharply reduced, so that the operation became impossible.

[0048]

According to the present invention, there is provided an apparatus and a separation method capable of stably obtaining a low-concentration solution from a high-concentration solution, particularly from seawater, at a high recovery rate, with a small amount of energy, at a lower cost and with a sufficient boron concentration removed. Can be provided.

[Brief description of the drawings]

FIG. 1 is a flowchart of a membrane separation apparatus in a case where a unit configuration is {B → A}. (Example 1)

FIG. 2 is a flow chart of the membrane separation apparatus in the case of unit configuration {B → A (concentrated water) → A}. (Example 2)

FIG. 3 is a unit configuration having scale prevention means.
It is a flow figure of the membrane separation device when (concentrated water) → B｝. (Example 3)

[Explanation of symbols]

1: High-concentration solution (eg, seawater) 2: Pretreatment part 3: Pressurizing pump 4: Reverse osmosis membrane module unit A using membrane a 5: Concentrated water of reverse osmosis membrane module unit A using membrane a 6 : Permeate water of reverse osmosis membrane module unit A using membrane a 7: Loose RO membrane module unit B using membrane b 8: Concentrated water of loose RO membrane module unit B using membrane b 9: Use membrane b Permeated water of loose RO membrane module unit B 10: Means for adding scale inhibitor

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C02F 1/44 C02F 1/44 G K 5/00 610 5/00 610F 620 620B 620C 5/08 5/08 F5 / 10 620 5/10 620A 620Z 630 630 5/12 5/12 (72) Inventor Toshihiro Ikeda 1-1-1, Sonoyama, Otsu, Shiga Prefecture Toray Industries, Inc. Shiga Plant (72) Inventor Yu Kurihara 1-1-1, Sonoyama, Otsu City, Shiga Prefecture Toray Industries, Inc. Shiga Plant

Claims (17)

[Claims] 1. A reverse osmosis membrane module using a membrane a having a salt rejection of 90% or more when a saline solution having a temperature of 25 ° C., a pH of 6.5 and a concentration of 3.5% is supplied at a pressure of 56 kgf / cm ^2. Unit A, temperature 25 ° C, pH 6.5, concentration 1,5
A membrane b having a permeation flux of 0.8 m ³ / m ² · day or more when 00 ppm saline is supplied at a pressure of 15 kgf / cm ^2.
A loose RO membrane module unit B using a plurality of units. 2. The operation pressure of the membrane a is 50 atm or more.
The membrane separation device according to claim 1. 3. The film b has a temperature of 25 ° C., a pH of 6.5, and a concentration of 5.
3. The membrane separation device according to claim 1, wherein the permeation flux when supplying 00 ppm saline at a pressure of 5 kgf / cm ² is 0.5 m ³ / m ² · day or more. 4. The film b has a temperature of 25 ° C., a pH of 6.5, and a concentration of 5.
When a salt solution of 00 ppm is supplied at a pressure of 5 kgf / cm ² , the salt rejection is 80% or less, and the temperature is 25%.
° C., pH 6.5, salt rejection rate when supplying the aqueous solution of magnesium sulfate concentration 1,000ppm pressure 5 kgf / cm ² is 90% or more, the membrane separation apparatus according to claim 1. 5. The membrane separation device according to claim 1, wherein the permeated water of the reverse osmosis membrane module unit A is supplied to the loose RO membrane module unit B. 6. The membrane separation apparatus according to claim 1, wherein the permeated water of the loose RO membrane module unit B is supplied to the reverse osmosis membrane module unit A. 7. An apparatus for adjusting the pH of the supply water of the reverse osmosis membrane module unit A to 9 or more.
5. The membrane separation device according to item 1. 8. The membrane separation device according to claim 1, wherein the concentrated water of the reverse osmosis membrane module unit A is supplied to the loose RO membrane module unit B. 9. The concentrated water of the reverse osmosis membrane module unit A is supplied to the loose RO membrane module unit B,
The membrane separation device according to any one of claims 1 to 4, wherein the permeated water of the O membrane module unit B is mixed with the supply water of the reverse osmosis membrane module unit A. 10. The reverse osmosis membrane module unit A is arranged in multiple stages, and the concentrated water of the previous stage reverse osmosis membrane module unit is supplied to the next stage reverse osmosis membrane module unit. The membrane separation device as described in the above. 11. The loose RO membrane module unit B according to claim 5, wherein the loose RO membrane module units B are arranged in multiple stages, and the permeated water of the preceding loose RO membrane module unit is supplied to the next loose RO membrane module unit. The membrane separation device as described in the above. 12. The membrane separation device according to claim 1, further comprising a device for adding a scale inhibitor. 13. The membrane separation apparatus according to claim 1, further comprising an apparatus for treating the first-stage supply water with a backwashable ultrafiltration membrane. 14. A method for separating a high-concentration solution, comprising using the apparatus according to claim 1. 15. The ratio of the amount of permeated water to the amount of water supplied to the reverse osmosis membrane module unit A exceeds 40%.
5. The method for separating a high-concentration solution according to 4. 16. The method for separating a high-concentration solution according to claim 14, wherein the high-concentration solution is a solution having a solute concentration of 0.5% or more. 17. The high concentration solution is seawater.
16. The method for separating a high-concentration solution according to any one of 16.