JP2005246158A - Method and system for desalinating seawater - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To maintain a high separation efficiency over a long period of time while suppressing the deterioration of a separation membrane to the minimum by preventing scale (containing carbonic acid salt) from being deposited/sedimented on a separation membrane surface of especially reverse osmosis membrane equipment of a second stage when desalinating the seawater at a high recovery rate using the reverse osmosis membrane equipment connected in two stages. <P>SOLUTION: In a method for desalinating seawater using the reverse osmosis membrane equipment connected in two stages serially, on the upstream of the reverse osmosis membrane equipment in the second stage, water penetrating the first reverse osmosis membrane equipment is subjected to scale dispersion injection and/or decarboxylation, and is subjected to alkali injection so that pH of the penetrating water becomes 9.2-9.7. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an improvement in a method and apparatus for desalinating seawater using a reverse osmosis membrane device, and more specifically, processing seawater using a reverse osmosis membrane device connected in two stages in a series direction to produce fresh water. In particular, the present invention relates to an improved method and apparatus so that the boron concentration in treated water (permeated water) can be efficiently reduced below the water quality standard concentration of the waterworks law.

  Seawater desalination using a reverse osmosis membrane device is widely used as a method for removing salt from seawater and obtaining fresh water suitable for beverages in remote islands and coastal areas where it is difficult to obtain fresh water.

  By the way, seawater usually contains about 4 to 5 mg / liter of boron, and membrane permeate treated with a normal reverse osmosis membrane device contains about 1 to 2.5 mg / liter of boron. . Although this boron is an essential element for the human body, it has been confirmed that adverse effects on the human body if too much is consumed, and the revision of the Water Supply Law regulates the boron concentration in drinking water to 1 mg / liter or less. It was to be done.

  That is, some of the permeated water obtained by desalination of seawater using reverse osmosis membrane devices that are currently in practical use do not meet the boron concentration defined by the water quality standards of the current Waterworks Law. In order to pass water quality standards, the membrane permeate must be further post-treated to remove boron and reduce the boron concentration to 1 mg / liter or less. However, when land water (for example, well water, river / lake water) containing almost no boron can be used as mixed water, the boron removal rate in the post-treatment may be determined in accordance with the use state of the land water. Therefore, some studies have been made on methods for removing boron from membrane permeated water. For example, boron is captured and removed by a chelate resin, or the permeated water is further applied to a low-pressure reverse osmosis apparatus to measure the boron concentration. A method of reducing the value below the value has been proposed.

  However, the above-described method has a problem of reclaimed wastewater treatment when regenerating a chelate resin in which boron is chelated and captured. Therefore, considering the practicality on an industrial scale, the above-mentioned method, that is, by treating the membrane permeate treated with the first-stage reverse osmosis membrane device on the downstream side through the low-pressure reverse osmosis membrane device, boron is obtained. A method of reducing the concentration below the reference value is considered effective.

  This type of method will be described more specifically. For example, the permeated water that has passed through the first-stage reverse osmosis membrane device is adjusted to pH 5.7 or higher, and this is operated at a low pressure. A method of reducing the boron content to a level of 0.2 ppm or less by treating with a reverse osmosis membrane device is disclosed (Patent Document 1). Further, when seawater is treated in two stages using a reverse osmosis membrane device, a part of the permeated water of the first-stage reverse osmosis membrane device is used as the downstream permeated water of the reverse osmosis membrane device and the remaining permeated water is used as the permeated water. By returning to the upstream side of the reverse osmosis membrane device, the supply side pH value of the first stage reverse osmosis membrane device is set to 8 or less, and the supply side pH value of the second stage reverse osmosis membrane device is set to 8 or more. A method for reducing the boron content to 0.2 ppm or less is disclosed (Patent Document 2).

  In order to obtain a high level of boron removal efficiency by this method, the operation pressure of the second-stage reverse osmosis membrane device is made lower than the operation pressure of the first-stage reverse osmosis membrane device, and the second-stage reverse osmosis membrane device is reversed. It has been clarified that it is preferable to adjust the pH of the water supplied to the osmotic membrane device to an alkalinity of 8 to 10.

  Although it is desirable to operate at a high recovery rate from an economical point of view, the first-stage reverse osmosis membrane device has a limit of a recovery rate of about 60% to prevent precipitation of calcium sulfate. On the other hand, the second-stage reverse osmosis membrane device is operated at a recovery rate as high as possible, generally 60% or more, preferably 80% or more.

  However, when the water supplied to the second-stage reverse osmosis membrane device as described above (that is, the permeated water of the first-stage reverse osmosis membrane device) is adjusted to a high pH range and operated at a high recovery rate, Calcium (approximately 2.5 mg / liter), magnesium, barium, and the like contained in the permeated water of the first-stage reverse osmosis membrane device react with carbonic acid dissolved in the permeated water to form an insoluble scale (calcium carbonate, magnesium carbonate). , Including carbonates such as barium carbonate) and depositing on the separation membrane surface of the second-stage reverse osmosis membrane device.

  In addition, the inventors have clarified that if the scale is deposited on the film surface even when a small amount of scale is present in a state where the pH of the feed water is high, it causes a chemical deterioration of the film.

The present inventors have already proposed a seawater desalination treatment method and apparatus (Patent Document 3) and another-stage reverse osmosis treatment method (Patent Document 4).
Japanese Patent Laid-Open No. 9-10766 Japanese Patent Laid-Open No. 11-10146 JP 2003-236541 A JP 2003-71252 A

  The present invention has been made paying attention to the above problems, and its purpose is to desalinate seawater with a high recovery rate using a reverse osmosis membrane device connected in two stages in a series direction. In particular, it prevents the deposition and deposition of scale components such as calcium carbonate, magnesium carbonate, and barium carbonate on the separation membrane surface of the second stage reverse osmosis membrane device, and maintains a high level of boron removal efficiency over a long period of time. It is to provide such a method and apparatus. It is another object of the present invention to provide a method capable of maintaining a high level of boron removal efficiency while minimizing the deterioration of the separation membrane.

  The present invention relates to a method for desalinating seawater with a reverse osmosis membrane device connected in two stages in a series direction, and the permeation of the first stage reverse osmosis membrane device on the upstream side of the second stage reverse osmosis membrane device. A seawater desalination treatment method having the gist of subjecting water to scale dispersant injection treatment and / or decarboxylation treatment and alkali injection treatment so that the pH of the permeate becomes 9.2 to 9.7. is there.

  In carrying out the present invention, in the alkali injection process, it is preferable to precisely control the alkali injection amount by an alkali proportional injection control method or a PID control method.

  The present invention also relates to a device for desalinating seawater using a reverse osmosis membrane device connected in two stages in a series direction, and a second-stage reverse osmosis membrane device downstream of the first-stage reverse osmosis membrane device. Is provided with a scale dispersant injection treatment device and / or a decarboxylation treatment device, and an alkali injection device is provided following any of the devices, or a scale generation prevention device is provided following the alkali injection device. Is a seawater desalination apparatus having the gist of being provided.

    According to the method and apparatus of the present invention, when seawater is desalinated with a two-stage reverse osmosis membrane apparatus, the scale (including carbonate) on the separation membrane surface generated particularly in the second-stage reverse osmosis membrane apparatus. As a result, it is possible to prevent deterioration of the separation membrane due to the precipitation of water as much as possible, and to obtain fresh water with a higher cleanliness efficiently over a long period of time under stable operability. In particular, according to the present invention, it is possible to efficiently obtain drinking water that sufficiently satisfies the water quality standard related to boron concentration, which has become stricter in recent water supply laws, over a long period of time, and a desalination technology using a reverse osmosis membrane device. Generalization can be further improved.

  The inventors of the present invention have proposed a separation membrane surface of a reverse osmosis membrane device based on the already proposed seawater desalination treatment method and apparatus (Patent Document 3) and the multistage reverse osmosis treatment method (Patent Document 4). The inventors of the present invention have intensively studied a technique capable of preventing the precipitation of scale components such as calcium carbonate and magnesium carbonate on the surface and maintaining a high level of boron removal rate over a long period of time, and have arrived at the present invention.

  As described above, in the present invention, the separation membrane surface generated in the second-stage reverse osmosis membrane device when the seawater to be treated is desalinated at a high recovery rate by the two-stage reverse osmosis membrane device connected in series. By preventing the deposition of scale (including carbonate such as calcium carbonate), the deterioration of the second stage reverse osmosis membrane is prevented, and a high level of boron removal efficiency is maintained over a long period of time. That is, by adopting the method and apparatus of the present invention, it is possible to maintain a high level of desalination treatment efficiency stably for a long period of time, and extend the life of the reverse osmosis membrane apparatus to improve the maintenance and maintainability of the equipment. Can be increased.

  Hereinafter, the present invention will be described more specifically with reference to the drawings showing typical embodiments. However, the present invention is not limited to the following illustrated examples, and may be adapted to the spirit described above and below. Of course, it is also possible to carry out by appropriately changing the range.

FIG. 1 is a flow diagram illustrating a desalination treatment method and apparatus according to the present invention, in which 1 is a seawater tank and 2 is a pretreatment apparatus for removing floating contaminants and the like contained in seawater. 3 is a first-stage reverse osmosis membrane device, 4 is a decarbonation treatment device, 5 is an intermediate tank, 6 is an alkaline liquid tank (alkali injection device), 7 is a second-stage reverse osmosis membrane device, P _{1 to} P ₃ indicates a pump, respectively.

In the desalination of seawater using this apparatus, the seawater stored in the seawater tank 1 is sent to the pretreatment apparatus 2 and the floating impurities contained in the seawater are removed in advance, and then the pump P ₁ Is supplied to the first-stage reverse osmosis membrane device 3 at a predetermined pressure. In the first-stage reverse osmosis membrane device 3, the salt content is removed by the reverse osmosis membrane mounted therein, and the permeated water is separated into non-permeated water having a high salt concentration and desalted permeated water. Discharged from ₁ .

On the other hand, the permeated water is sent to the scale dispersant injection device and / or the decarbonation device (4 in FIG. 1 indicates a decarbonation treatment device) through the line L _2, and the decarbonation treatment device 4 performs the decarbonation treatment. After reducing the total carbonic acid concentration in the permeated water, it is sent from the line L ₃ to the intermediate tank 5. At this time, the pH of the permeated water is preferably adjusted by supplying an appropriate amount of the aqueous alkaline solution from the alkaline liquid tank 6 from the pump P ₂ (in addition, the alkaline aqueous solution may be directly injected into the permeated water). Then, the pressure is increased to a predetermined pressure by the pump P ₃ and then sent to the second reverse osmosis membrane device 7 to further remove boron, particularly boron contained in the permeated water, thereby reducing the boron concentration. Preferably, it is reduced to 1 mg / liter or less which is the water quality standard of the Waterworks Law.

  In order to increase the boron removal efficiency in the second-stage reverse osmosis membrane device 7, it is recommended that the pH of the permeate supplied to the reverse osmosis membrane device 7 be adjusted to 9.2 or higher. This is because when the pH of the permeate is increased to 9.2 or higher, boric acid is dissociated and ionized, which is easily blocked by the reverse osmosis membrane. More preferably, the pH is 9.3 or more. As a result of the study by the present inventors, the higher the pH of the permeate, the higher the degree of dissociation of boric acid, but the reverse osmosis membrane tends to deteriorate and a sufficient removal rate can be obtained over a long period of time. I knew that it would be difficult to be. Therefore, in order to reduce the boron concentration to a predetermined value (for example, 1 mg / liter or less) while suppressing deterioration of the reverse osmosis membrane over a long period of time, the permeate (supply) supplied to the second-stage reverse osmosis membrane device 7 The pH of the liquid) is preferably adjusted to 9.7 or lower, more preferably 9.6 or lower.

  By the way, when the pH of the water supplied to the second-stage reverse osmosis membrane device (treated water) is adjusted to be alkaline in order to increase the boron removal rate and operated at a high recovery rate, the second-stage reverse osmosis membrane device is The production of carbonate by the reaction of carbon dioxide gas dissolved in the water to be treated (permeated water from the first-stage reverse osmosis membrane device) with Ca ions, Mg ions, Ba ions, etc. is facilitated. Therefore, when the desalination operation is advanced, the carbonate concentration in the water to be treated increases, and when the carbonate concentration exceeds the saturation solubility, the carbonate scale is deposited on the surface of the separation membrane. The amount of scale generated is not so large that it usually clogs the membrane and causes a reduction in the amount of permeate. However, when the pH of the water to be treated is high, a small amount of scale attached to the membrane surface catalyzes and promotes chemical deterioration of the membrane material.

  Accordingly, in the present invention, a scale dispersant injection treatment device and / or a decarboxylation treatment device is provided in a line connecting the first-stage reverse osmosis membrane device 3 and the second-stage reverse osmosis membrane device 7, and the scale dispersant injection is performed here. Treatment and / or decarboxylation treatment is performed. Examples of the scale dispersant injection treatment apparatus include an apparatus (scale dispersant supply apparatus) that supplies the scale dispersant to the water to be treated, but is not limited thereto. Examples of the decarboxylation device include a decarboxylation device such as a decarboxylation tower, but are not limited thereto, and the calcium concentration in the liquid to be treated and the membrane permeate recovery rate in the second-stage reverse osmosis membrane device. As long as the carbon dioxide concentration can be reduced to such an extent that no scale is deposited, any device may be used.

  Depending on the concentration of carbonic acid in the seawater or the equipment specifications of the entire desalination apparatus, it is effective to use the decarboxylation treatment and the scale dispersant injection treatment in combination (whichever comes first). Of course, the scale dispersant injection treatment and the decarboxylation treatment may be either one as long as scale precipitation can be suppressed.

As the decarboxylation treatment, a decarboxylation device 4 is provided, and the carbon dioxide gas concentration contained in the liquid to be treated may be reduced after the water to be treated is decarboxylated by the decarbonation device 4. That is, in the example of FIG. 1, the concentration of carbon dioxide in the water to be treated is reduced by blowing air into the membrane permeated water sent from the line L ₂ and bringing it into gas-liquid contact. If it does so, the carbonic acid as a production source of the carbonate which is one of the scale generation sources will decrease. As a result, precipitation of carbonate scale on the separation membrane surface is prevented or suppressed, and the problems pointed out in the prior art can be solved.

  In addition, when decarboxylation is performed by blowing air as in the example of FIG. 1, if the air is pretreated with an alkali scrubber or the like prior to blowing into the decarbonator 4, the carbon dioxide concentration in the air is reduced, This is preferable because the decarboxylation efficiency in the decarboxylation device 4 can be further increased. Further, as another means for increasing the decarboxylation efficiency, it is also effective to spray the water to be treated while reducing the pressure in the decarboxylation device 4. In addition, the saturation solubility of carbon dioxide in the water to be treated is reduced by blowing air or oxygen or nitrogen gas, so that these gases are used as gas-liquid contact gas for decarboxylation instead of air. It is also effective.

  The specific method of bringing the water to be treated and the decarbonation gas into gas-liquid contact is not particularly limited, but as a general method, a method of bubbling the decarbonation gas into the water tank to be treated, or a packed tower or shelf Examples thereof include a method in which a staged gas-liquid contact tower or the like is used and gas-liquid contact with the water to be treated is performed in a counterflow manner.

In order to effectively exhibit the carbonate precipitation prevention effect and the reverse osmosis membrane deterioration inhibiting effect by the decarboxylation treatment on a practical scale, the total carbonate concentration in the water to be treated is 4 mg / liter or less in terms of CO ₂ , It is preferably reduced to about 2 mg / liter, and it has been confirmed that precipitation of carbonate on the separation membrane surface of the second-stage reverse osmosis membrane device can be substantially eliminated. When operating the second stage reverse osmosis membrane device at a higher recovery rate, it is necessary to further reduce the total carbonate concentration in the water to be treated.

In addition, as a scale dispersant injection treatment, a scale dispersant is added to the water to be treated through a line connecting the first-stage reverse osmosis membrane device 3 and the second-stage reverse osmosis membrane device 7, and the second-stage reverse osmosis membrane It is also effective to suppress the generation of scale on the separation membrane surface of the apparatus. In the case of adopting this method, for example, as shown in FIG. 2, a scale dispersant supply unit is provided in the permeate feed pipe, and the scale dispersant may be mixed into the permeate supply pipe through the pump P ₄ from the scale dispersant tank 8. These scale dispersant tank 8 and pump P ₄ constitute scale dispersant addition means.

  The type of the scale dispersant is not particularly limited, but preferred examples include polycarboxylic acid and polyphosphoric acid. Incidentally, a polycarboxylic acid-based scale dispersant is commercially available, for example, from Biolab under the trade name “Flocon 100”. Examples of the polyphosphate scale dispersant include sodium hexametaphosphate. It is preferable to add about 1 to 5 mg / liter of these scale dispersants with respect to the water to be treated because the generation of scale on the separation membrane surface of the second-stage reverse osmosis membrane device 7 can be effectively suppressed.

  As described above, in the present invention, when seawater is desalinated with a high yield using a two-stage reverse osmosis membrane device, the precipitation of carbonate on the separation membrane surface of the second-stage reverse osmosis membrane device, It is prevented by decarboxylation on the upstream side and / or the generation of scale is suppressed by adding a scale dispersant, and the pH of the water to be treated supplied to the second-stage reverse osmosis membrane device is 9.2. It has the greatest feature in the control within the range of ~ 9.7, and other specific equipment configurations and operating conditions can be arbitrarily changed as necessary. The preferable conditions for the desalination treatment by connecting the membrane device are as follows.

  That is, preferable operating conditions of the first stage reverse osmosis membrane device are: recovery rate: 40 to 63%, treated water pH: preferably 5.0 or more, more preferably 6.0 or more, preferably 7 0.0 or less, more preferably 6.5 or less, pressure; preferably 4.0 MPa or more, more preferably 5.0 MPa or more, preferably 10.0 MPa or less, more preferably 8.3 MPa or less, second stage Preferred conditions for the reverse osmosis membrane apparatus are: recovery rate: 60 to 90%, treated water pH: preferably 9.2 or more, more preferably 9.3 or more, preferably 9.7 or less, more preferably Is 9.6 or less, and the pressure is 0.5 to 4.0 MPa.

  In the present invention, the alkali injection method is not particularly limited, and the alkali injection amount may be appropriately controlled so as to achieve the above-mentioned suitable pH. From the viewpoint of chemical degradation spinning of the membrane, it is important not to raise the pH of the feed water more than necessary. Therefore, it is necessary to precisely control the desired pH range. In particular, since the water supplied to the second-stage reverse osmosis membrane device is the membrane permeate of the first-stage reverse osmosis membrane device, the amount of dissolved impurities is extremely small and there is almost no pH buffering capacity. Therefore, as a method for adjusting pH by injecting strong alkali such as caustic soda into the water supplied to the second-stage reverse osmosis membrane device, if a simple ON-OFF control is performed, the fluctuation range of the pH value due to alkali injection is reduced. It becomes extremely large, and it is difficult to fit within a predetermined pH control range. When the pH of the second stage supply water is lower than a predetermined value, the boron concentration of the membrane permeated water may be higher than a set value (for example, 1 mg / L or less). In addition, when the pH of the second stage supply water is higher than a predetermined value, chemical deterioration of the membrane material is promoted. Therefore, as a pH control method of the second stage feed water having a low pH buffering capacity, a method of proportionally controlling alkali injection or a method of PID control is suitable. In the case of these control methods, the pH value can be controlled more precisely than the ON-OFF control, and can be suppressed within a predetermined pH control range. The pH measurement point is not particularly limited, but water supplied to the second-stage reverse osmosis membrane device is desirable. Further, the pH measurement point may be, for example, either a concentrated liquid or a permeated liquid discharged from the second-stage reverse osmosis membrane device. Based on these measured pH values, the amount of alkali added to the permeate of the first-stage reverse osmosis membrane device may be controlled.

  The alkali injection position is not particularly limited as long as it is downstream of the first-stage reverse osmosis membrane device and upstream of the second-stage reverse osmosis membrane device. You may process in reverse order. However, when the permeate of the first-stage reverse osmosis membrane device is subjected to an alkali injection treatment, carbon dioxide gas (for example, bicarbonate or carbonate) becomes an ionic state (for example, bicarbonate ion or carbonate ion). Decarboxylation may not be possible. Therefore, when performing the decarboxylation treatment and the alkali injection treatment, it is desirable to first perform the alkali injection treatment after the permeate is decarboxylated first.

  In addition, when the salt content is separated and removed by the first-stage reverse osmosis membrane apparatus after removing (pretreatment) floating contaminants in seawater as in the present invention, marine organisms propagate on the reverse osmosis membrane. Biofouling may occur and marine organisms may breed in the reverse osmosis membrane device. Therefore, depending on the operating conditions, it is desirable to add a chlorinated sterilant (for example, hypochlorous acid or the like) to the seawater supplied to the first-stage reverse osmosis membrane device in order to prevent device contamination due to biofouling or the like. In the present invention, as the first-stage reverse osmosis membrane, a reverse osmosis membrane having a desired desalination rate such as a cellulose reverse osmosis membrane such as cellulose triacetate or a polyamide reverse osmosis membrane can be used. When an agent is added, the first-stage reverse osmosis membrane may be deteriorated by the chlorine. Even if the first-stage reverse osmosis membrane is not deteriorated by chlorine, if the chlorine remains in the permeate, the second-stage reverse osmosis membrane may be deteriorated by the residual chlorine. Therefore, when a chlorine-based sterilant is added, a reducing agent such as sodium bisulfite is added to the seawater before the first-stage reverse osmosis membrane supply or the permeate after the reverse osmosis membrane treatment to remove chlorine. It is desirable.

  In addition, if an excessive reducing agent (for example, sodium bisulfite or sodium thiosulfite) is present in the liquid to be treated for the second stage reverse osmosis membrane treatment under high alkaline conditions together with the scale, the chemical deterioration of the membrane proceeds. I have confirmed that. In such a case, it is necessary to determine the pH of the liquid to be treated comprehensively in consideration of both the prevention of the film deterioration due to the high pH and the improvement of the boron removal rate due to the high pH. Specifically, the pH of the liquid to be treated supplied to the second-stage reverse osmosis membrane treatment is 9.2 or more, preferably 9.3 or more, and is 9.7 or less, more preferably 9.6 or less. It is desirable to control so that

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

Example 1
In accordance with the flow chart shown in FIG. 1, seawater is treated with a first-stage reverse osmosis membrane device, then decarboxylated by passing the permeated water through a decarboxylation device and bringing it into gas-liquid contact with air, and then pH After adjustment, the water was passed through a second-stage reverse osmosis membrane device to be desalinated. As the decarboxylation device, a packed tower with a height of 2700 mm filled with a filler for promoting gas-liquid contact is used, and air is blown from the tower bottom side while flowing water to be treated from above, and the gas-liquid is flown in a counterflow. The contact method was adopted. The air blowing amount in the decarbonation device is changed variously, and the total carbonic acid concentration in the treated water on the inlet side and the outlet side in the decarbonation device is measured with a TOC meter, and the alkaline water is added to the treated water after the decarbonation treatment. (25% caustic soda) was added to adjust the pH to 9.5 (adjusted by a proportional control method), and a desalination treatment was continuously performed. Six months later, the state of attachment of the carbonate scale to the separation membrane surface of the second-stage reverse osmosis membrane device was examined by elemental analysis using an EDX (energy dispersive X-ray fluorescence analyzer). The components of the raw water (seawater), the operating conditions of the first and second stage reverse osmosis membrane devices, etc. were as follows.
[Boron concentration in raw water (seawater)]
Boron concentration: 4.55 mg / liter [Operating conditions of the first stage reverse osmosis membrane device]
Water to be treated pH: 6.0, temperature: 30 ° C., pressure: 6.0 MPa, recovery rate: 40%
[Operating conditions of the second stage reverse osmosis membrane device]
Water to be treated pH: 9.5, temperature: 30 ° C., pressure: 1.0 MPa, recovery rate: 60%

Example 2
In Example 1 described above, a method of adding a scale dispersant to the permeated water obtained from the first-stage reverse osmosis membrane device instead of decarboxylation by gas-liquid contact with air was performed, and a similar experiment was performed. . That is, the same seawater as used in Example 1 was used, and the scale dispersant (Flocon 100) was equivalent to 2.5 ppm in the permeate treated by the first-stage reverse osmosis membrane device under the same conditions as in Example 1. Along with the injection of an aqueous solution, an aqueous alkaline solution (25% sodium hydroxide) was added to adjust the pH (adopting PID control). This was supplied to the 2nd stage reverse osmosis membrane apparatus, and the desalination process was performed continuously.

Comparative Example 1
In Example 1 above, the same experiment was performed by omitting the decarboxylation treatment by gas-liquid contact with air. That is, the same seawater as used in Example 1 was used, and an aqueous alkaline solution (25% caustic soda) was added to the permeate treated under the same conditions as in Example 1 using the first-stage reverse osmosis membrane device to adjust the pH. Was adjusted to 9.5. The alkaline aqueous solution addition method controlled ON / OFF of the alkaline aqueous solution addition pump by the pH measured by the sensor installed in the 2nd stage supply water piping downstream from the alkaline aqueous solution addition position. This was supplied to the 2nd stage reverse osmosis membrane apparatus, and the desalination process was performed continuously.

[Presence or absence of scale adhesion]
As a result of EDX analysis, in Example 1 and Example 2, no precipitation of scale was observed on the separation surface of the second-stage reverse osmosis membrane device, but in Comparative Example 1, a trace amount of calcium-based precipitate was confirmed. It was done.

[Results of pH control of feed water]
Set value pH value during operation Control method example 1 9.5 9.45 to 9.55 Proportional control method example 2 9.5 9.45 to 9.55 PID control method comparison example 1 9.5 9.2 ~ 10.0 ON-OFF control method

[result]
In the case of Example 1 and Example 2, the desalination treatment could be performed stably over a long period of 6 months or more. On the other hand, in the case of Comparative Example 1, since the pH range could not be controlled to the desired 9.2 to 9.7, a decrease in the salinity inhibition rate of the low-pressure membrane was observed after 6 months.

  When 6 months passed from the start of operation, performance tests of each membrane module were performed under the conditions shown in Table 1 below. The results are shown in Table 2.

  As shown in Table 2, in Examples 1 and 2, a high level of boron rejection was maintained over a long period of time, but in Comparative Example 1, the boron rejection was reduced. . From these experimental results, in Example 1 and Example 2, which are examples of the present invention, the deterioration of the separation membrane due to the deposition of scale (including carbonate) on the separation membrane surface can be prevented as much as possible. It can be seen that fresh water with higher cleanliness can be efficiently obtained over a long period of time under stable operability.

  In the present invention, as described above, the pH is in the range of 9.2 to 9.7 by the precision control including the proportional control and the PID control. It is necessary to keep this in mind. That is, in the case of this precise control, it is desirable that the fluctuation range with respect to the set pH value is ± 0.05. Therefore, when the pH is set to 9.5, the control fluctuation range is pH 9.45 to 9.55. In the first and second embodiments, the control fluctuation range is controlled within a range of ± 0.05 of the set value.

It is a flowchart which shows one Example of this invention. It is a flowchart which shows the other Example of this invention.

Explanation of symbols

1 Seawater tank 2 Pretreatment device 3 First stage reverse osmosis membrane device 4 Decarbonation treatment device 4
5 Intermediate tank 6 Alkaline liquid tank 7 Second stage reverse osmosis membrane device 8 Scale separator tank

Claims (3)

  In a method of desalinating seawater using a reverse osmosis membrane device connected in two stages in a series direction, the scale is dispersed in the permeate of the first-stage reverse osmosis membrane device upstream of the second-stage reverse osmosis membrane device. A desalination treatment method for seawater, characterized by performing an agent injection treatment and / or a decarboxylation treatment, and an alkali injection treatment such that the pH of the permeate is 9.2 to 9.7.   The desalination treatment method according to claim 1, wherein the alkali injection amount is controlled by an alkali proportional injection control method or a PID control method in the alkali injection treatment.   In a device for desalinating seawater with a reverse osmosis membrane device connected in two stages in a series direction, on the downstream side of the first-stage reverse osmosis membrane device and on the upstream side of the second-stage reverse osmosis membrane device, A scale dispersant injection processing device and / or a decarboxylation processing device is provided, and either of the devices is followed by an alkali injection device, or a scale generation prevention device is provided following the alkali injection device. A seawater desalination apparatus characterized by that.

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