JPH0252088A - Apparatus for making desalted water - Google Patents

Abstract

PURPOSE:To prevent a lowering of water quality by combining a membrane degassing apparatus equipped with a water repellent membrane for degassing dissolved gas with a two-step type reverse osmosis membrane apparatus. CONSTITUTION:Pretreated raw water is supplied to a membrane degassing apparatus 6 through a raw water inflow pipe 12. The acid such as hydrochloric acid in an acid storage tank 1 is injected in the raw water before reaching the membrane degassing apparatus 6 by driving an acid injection pump 2 to be mixed therewith by a line mixer 3 to adjust the pH of the raw water after mixing to 5.5 or less. After a part of a carbonic acid ion present in the raw water is changed to carbon dioxide, the raw water is treated with the membrane degassing apparatus 6. The membrane degassing apparatus 6 is divided by a water repellent membrane 7 and the raw water containing dissolved gas such as carbon dioxide is passed through one chamber and the dissolved gas in the raw water is passed through the water repellent membrane 7 to perform degassing. By this method, high purity desalted water is obtained stably.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a desalinated water production device that combines a membrane degassing device and a two-stage reverse osmosis membrane device.

<Prior art> There are distillation methods, ion exchange membrane methods, ion exchange resin methods, reverse osmosis membrane methods, etc. as means for removing salts contained in water. When targeting raw water with a total cation content of around 300 mg/l (CaC○3 equivalent) or lower, energy costs are relatively low, and organic matter and fine particles that coexist in the water can be removed along with salts at the same time. Therefore, reverse osmosis membrane method is often adopted.

In addition, reverse osmosis membrane equipment is indispensable for the production of ultra-pure water, such as semiconductor cleaning water and pharmaceutical water, which requires removing salts and as much organic matter and fine particles as possible from the water. To produce pure water, raw water is first treated with a reverse osmosis membrane device, which reduces the total amount of cations to 10p.
The mainstream is a system that combines a reverse osmosis membrane device and an ion exchange device, in which permeated water of around pm or less is obtained, and then the permeated water is treated with an ion exchange device.

In such a system that combines a reverse osmosis membrane device and an ion exchange device, it is better to remove salts as much as possible with the reverse osmosis membrane device in the first stage to reduce the burden on the ion exchange device in the second stage. As a result, two-stage reverse osmosis membrane devices have come into use.

That is, the raw water is supplied to the first reverse osmosis membrane device to obtain primary permeate water after PUT of 10 ppm or less in total cations, and the permeated water is further supplied to the second reverse osmosis membrane device to obtain ippm in total cations. Secondary permeated water is obtained after or below and is used as feed water for the subsequent ion exchange equipment.

In addition, in order to prevent the hardness components present in the raw water from adhering to the membrane surface of the reverse osmosis membrane, an acid is added to the raw water to lower the pH, and the raw water with the pH lowered is passed through the first reverse osmosis membrane. The equipment is being supplied.

By adding acid to the raw water, some of the bicarbonate ions that could normally be removed by the reverse osmosis membrane device become carbon dioxide, which cannot be removed by the reverse osmosis membrane device. It has also been proposed to install a degassing tower to remove carbon dioxide contained in the permeate water.

The specific flow of the conventionally proposed two-stage reverse osmosis membrane device described above is as follows.

That is, an acid is added to raw water that has been subjected to treatments such as coagulation sedimentation, filtration, and activated carbon filtration as necessary to adjust the pH to 4.5 to 5.5.
Then, the acid-added raw water is treated with a first reverse osmosis membrane device to obtain perfused water for the next week. Next, in order to remove dissolved gases such as carbon dioxide in the primary permeated water, the primary permeated water is aerated with nitrogen gas or treated with a vacuum degassing tower, and the degassed primary permeated water is transferred to the secondary permeated water.
Treated with reverse osmosis membrane equipment to obtain secondary permeate water. Note that since the concentrated water from the second reverse osmosis membrane device obtained at the same time does not contain so many ions, it is usually recycled and mixed with the raw water in the previous stage of the first blue osmosis membrane device for recovery.

The obtained secondary permeated water is then treated with a strongly acidic cation exchange resin and a strongly basic anion exchange resin.
It is treated with a bed type or mixed bed type ion exchange device to remove residual ions. Further, in order to obtain ultrapure water, the deionized water is often further subjected to ultraviolet irradiation treatment, ion exchange treatment, ultrafiltration treatment, reverse osmosis membrane treatment, etc.

However, the conventional two-stage reverse osmosis membrane device described above has the following drawbacks.

In other words, the raw water supplied to the first reverse osmosis membrane device has dissolved oxygen because it has not been deaerated, and as a result, microorganisms grow in the water stagnation area or on the membrane surface of the reverse osmosis membrane device.
There is a drawback that the flow rate of permeated water and the quality of permeated water are reduced. Furthermore, due to the presence of dissolved oxygen, when heavy metals coexist in water, the heavy metals act as oxidation catalysts, oxidizing the reverse osmosis membrane, and deteriorating its performance. Furthermore, carbon dioxide removal is insufficient, and the treated water of the two-stage reverse osmosis membrane device does not necessarily have a satisfactory value.

<Problems to be Solved by the Invention> The present invention solves the above-mentioned drawbacks of conventional desalination equipment, and provides a compact desalination device that does not breed microorganisms in the reverse osmosis membrane device and prevents oxidation of the reverse osmosis membrane. The purpose of this invention is to provide a salt water production device. Furthermore, it is an object of the present invention to provide a desalinated water production device that can obtain desalinated water of higher purity than treated water obtained from a conventional two-stage reverse osmosis membrane device.

<Means for Solving the Problems> The desalinated water production apparatus according to the present invention, which has been made to achieve the above object, is a method for producing carbonic acid to convert some or all of the bicarbonate ions present in raw water into carbon dioxide. Hydrogen ion decomposition means, a membrane deaerator equipped with a water-repellent membrane to degas carbon dioxide and other dissolved gases present in the treated water of the bicarbonate ion decomposition means, and It consists of a two-stage reverse osmosis membrane device in which the first reverse osmosis membrane device processes the permeated water to obtain primary permeated water, and then the second reverse osmosis membrane device processes the permeated water to obtain secondary permeated water. This is a characteristic feature.

<Effect> The present invention will be explained below based on the drawings.

FIG. 1 is an explanatory diagram of a flow showing an example of an embodiment of the present invention, in which 1 is an acid storage tank, 2 is an acid injection pump, 3 is a line mixer, 4 is a pH 11 moderator, and the acid storage tank 1 and acid injection pump are shown in FIG. The pump 2, line mixer 3, and pH controller 4 constitute a hydrogen carbonate ion decomposition means 5. 6 is a membrane deaerator equipped with a water-repellent membrane 7; 8 is a first reverse osmosis membrane device; 9 is a second reverse osmosis II cost device; the first reverse osmosis membrane device 8 and the second reverse osmosis The IIW device 9 constitutes a two-stage reverse osmosis membrane device 10.

Reference numeral 11 indicates a mixed bed pure water production apparatus filled with a mixed resin of a strongly acidic cation exchange resin and a strongly basic anion exchange resin.

Next, to explain the operation of the device of the present invention, raw water that has undergone pretreatment such as coagulation sedimentation, filtration, and activated carbon treatment as necessary is supplied to the membrane deaeration bag W6 via the raw water inflow pipe 12. The acid injection pump 2 is driven into the raw water before it reaches the device 6, and the acid storage tank 1 is
An acid such as hydrochloric acid is injected into the mixture and mixed using the line mixer 3, and the pH after mixing is adjusted to 5.5 or less, preferably around 4.0.

In addition, as shown in the figure, pHMm is installed after the line mixer 3.
Measure the pH of the raw water after the line mixer 3, and make sure that the pH is the preset pH (for example, 4).
．． It is preferable to adjust the amount of acid injection using the acid injection pump 2 linked to the pH controller 4 so that the pH value becomes 0).

The raw water before adding acid usually contains bicarbonate ions, and the pH of the raw water is determined by the abundance ratio of the bicarbonate ions and carbon dioxide originally present in the raw water. The pH is around 7, but by injecting an acid (for example, hydrochloric acid) into the raw water, depending on the amount of acid injected, the hydrogen carbonate ions in the raw water become carbon dioxide as shown in equation (1), and hydrogen carbonate The abundance ratio of ions and carbon dioxide changes, and the pH decreases.

HCCh-+HCj! -4CJ-+HzO+C0z-,
・(1) For example, if the pH after acid injection is 4.0, the third
As shown in the figure, the entire amount of bicarbonate ions in the system becomes carbon dioxide, which would normally be the case in the two-stage reverse osmosis device 1.
Since hydrogen carbonate ions, which result in an ion load of 0, can be removed by a membrane deaerator 6, which will be described later, this is advantageous.

After some or all of the bicarbonate ions present in the raw water are converted into carbon dioxide in this way, the raw water is treated with the membrane deaerator 6. The membrane deaerator 6 used in the present invention is I
Raw water containing dissolved gases such as carbon dioxide is passed through one side of the n-aqueous membrane 7, and the other side is evacuated, allowing the dissolved gas in the water to pass through the In aqueous membrane 7 for degassing. There are various types of aqueous membranes, such as flat membranes, spiral membranes, tubular membranes, and hollow fiber membranes. In addition, an element having a large number of hollow fiber membrane-like In aqueous membranes, in which raw water is passed through each hollow fiber membrane, and the outside of the hollow fiber membrane is evacuated, is preferable because it has good degassing performance. be.

Note that an ordinary vacuum pump may be used as the vacuum generator, but as shown in FIG. Therefore, an ejector 14 is attached to the secondary concentrated water pipe 13, and the primary concentrated water is used as the driving fluid, and the ejector 14 is
This is preferable because a vacuum pump can be omitted by generating a vacuum using the absorption power of . In addition, the ejector 14 is not limited to one stage, but can be used as needed. ! The degree of vacuum can also be increased in several stages.

Such deaeration treatment can remove carbon dioxide originally contained in the raw water and dissolved gases such as carbon dioxide and dissolved oxygen produced by changes in hydrogen carbonate ions in the raw water due to the addition of acids. Next, the degassed water is pressurized by a high pressure pump 16 through a degassed water pipe 15, and then transferred to the first reverse osmosis membrane device 8.
supply to.

Primary permeated water and primary concentrated water are obtained by the reverse osmosis membrane device 8, but the primary concentrated water is obtained from the ejector 1 as described above.
4 is used as a driving fluid, and is discharged to the outside of the system via a secondary concentrated water pipe 13 and an ejector 14.

On the other hand, primary permeated water from which approximately 95% of salts in water have been removed by the first reverse osmosis membrane device 8 is supplied to the second reverse osmosis membrane device 9 via the primary permeated water pipe 17.

Secondary permeated water and secondary concentrated water are also obtained in the second reverse osmosis membrane device 9, but the quality of the secondary concentrated water is relatively good.
It is preferable to collect the secondary concentrated water by mixing it with the treated water of the membrane deaerator 6, for example, through the secondary concentrated water pipe 18.

By recovering the secondary concentrated water into the system in this manner, the total amount of concentrated water discharged outside the system can be reduced and the recovery rate of desalted water can be increased.

On the other hand, the secondary permeate water can further remove about 95% of the salts in the primary permeate water, so the electrical resistivity of the treated water obtained with a conventional two-bed three-column ion exchanger is equal to or higher than that of the conventional two-bed three-column ion exchange equipment. It can be used as desalinated water for various purposes. Further, if necessary, residual ions can be further removed by the mixed bed type pure water production apparatus 11 via the secondary permeation pipe 19 as shown in the figure.

Note that the ion exchange device installed after the two-stage reverse osmosis membrane device 1O is not limited to the mixed bed type pure water production device 11, but may also be a double bed type pure water production device. Also, for example, if the processing capacity is relatively small, a non-regenerative mixed bed cartridge polisher can be used.

The hydrogen carbonate ion decomposition means 5 shown in FIG. 1 is composed of an acid storage tank 1, an acid injection pump 2, a line mixer 3, and a pH controller 4. Although it is based on a so-called simple acid addition method, the hydrogen carbonate ion decomposition means 5 used in the present invention
(For example, a dealkalization softener using an H-type weakly acidic cation exchange resin tower, or a part of the raw water passed through an H-type strongly acidic cation exchange resin tower and the treated water. A known dealkalization softening device using a cation exchange resin can be used, such as a dealkalization softening device in which water is passed through a Na form-forming acidic cation exchange resin column after mixing with other parts of the raw water.

FIG. 2 is an explanatory diagram showing the flow of another embodiment of the present invention. 21 and a pHlq moderation meter 22 are added, and the other configurations are the same as the flow shown in FIG.

As described above, the membrane deaerator 6 removes dissolved gases such as carbon dioxide and dissolved oxygen, but it is difficult to completely remove carbon dioxide.

Therefore, a trace amount of carbon dioxide remains in the treated water of the membrane deaerator 6. The carbon dioxide cannot be removed by the reverse osmosis membrane device and is ultimately contained in the secondary permeate water, but the presence of the carbon dioxide reduces the electrical resistivity of the secondary permeate water.

For example, if the presence of other inorganic ions is ignored, the electrical resistivity of water at 5 ppm carbon dioxide (calculated as Ca CCh) is around 0.3 MΩ-cm (at 25°C), and ipp
m (CaC○3 conversion) is around IMΩ-cm (25'c).

Therefore, in order to increase the electrical resistivity of the secondary permeated water, it is necessary to convert the remaining carbon dioxide into bicarbonate ions and carbonate ions that can be removed by a reverse osmosis membrane device.

As shown in Figure 3G, the pH of water containing carbon dioxide was adjusted to 8.
．． If it is around 5, most of the carbon dioxide can be changed into hydrogen carbonate ions.

Therefore, as shown in FIG. 2, the alkali (for example, sodium hydroxide) in the alkali storage tank 21 is injected into the treated water of the membrane deaerator 6 using the alkali injection pump 20.
The pH is raised to around 8.5 to convert carbon dioxide into bicarbonate ions.

Specifically, as shown in Figure 2, the pH of the treated water after addition of alkali is measured with the controller 22, and the pHg1 is adjusted so that the pH becomes a preset pH (for example, 8.5).
It is preferable to adjust the amount of alkali injection using the alkali injection pump 20 which is linked to the meter 22.

Note that the alkali that can be used is not limited to sodium hydroxide, and any alkali that can increase the pH of degassed water and change carbon dioxide into bicarbonate ions can be used. In some cases, ammonia gas may be used as the alkali.

By changing the carbon dioxide in the water supplied to the two-stage reverse osmosis membrane device 10 into bicarbonate ions in this way, the bicarbonate ions are converted to the first reverse osmosis membrane device 8 along with other ions.
, and the second reverse osmosis membrane device 9, it is possible to further improve the quality of the secondary permeated water.

<Effects> As explained above, the desalinated water production device of the present invention converts some or all of the hydrogen carbonate ions present in raw water into carbon dioxide, and removes this in advance with a membrane deaerator, so a two-stage reverse osmosis membrane is used. The ion load of the device can be reduced.

In addition, by adjusting the pH of the raw water to around 4.0 before treating it with a membrane deaerator, the ion load on the two-stage reverse osmosis membrane device can be reduced as much as possible, and the electrical specific resistance of the resulting desalinated water It has the effect of increasing Furthermore, since dissolved oxygen in the raw water is removed along with carbon dioxide at the front stage of the two-stage reverse osmosis membrane device, it is possible to prevent the growth of microorganisms in the stagnant water area and membrane surface of the reverse osmosis membrane device. Oxidation caused by dissolved oxygen can be effectively prevented.

Furthermore, since the membrane degassing device used in the present invention uses a water-repellent membrane, it does not need to be constructed as expensive as conventionally used vacuum degassing devices, and its structure can be made extremely compact. Further, by using an ejector using the primary concentrated water of the first reverse osmosis membrane device as a driving fluid as the vacuum generating device used in the membrane degassing device, the vacuum pump can be omitted and the device can be made more compact.

Furthermore, as shown in the embodiment shown in Fig. 2, a two-stage reverse osmosis membrane device is equipped with an alkali addition means for converting a small amount of carbon dioxide remaining in the treated water of the membrane deaerator into bicarbonate ions. The specific resistance of the resulting demineralized water can be further increased.

As described above, the desalted water production apparatus of the present invention is compact as a whole, does not require a large installation area, and can stably obtain highly purified desalinated water, so it brings great benefits to industry.

[Brief explanation of the drawing]

FIG. 1 is a flow explanatory diagram showing an example of an embodiment of the present invention, FIG. 2 is a flow explanatory diagram showing another embodiment of the present invention, and FIG. 3 is a flow diagram showing carbon dioxide and carbon dioxide at pH. FIG. 2 is an explanatory diagram showing the molar ratio of hydrogen ions and carbonate ions. 1... Acid storage tank 2... Acid injection pump 3.
...Line mixer 4...pH controller 5...bicarbonate ion decomposition means 6...membrane deaerator 7...↑Ω aqueous membrane 8...
・First reverse osmosis membrane device 9...Second reverse osmosis membrane device 10・
...Two-stage reverse osmosis membrane device 11...Mixed bed type pure water production device 12...Raw water inflow pipe 13...-Next concentration water pipe 1
4...Ejector-15...Deaerated water pipe 1G...High pressure pump 17...-Next week water pipe 18...Secondary concentrated water pipe 19...Secondary permeate water pipe 20...Alkali injection pump 21... Alkali storage tank 22... pH 311 meter 232... Alkali addition means Figure 3 H

Claims (1)

[Claims] 1. A hydrogen carbonate ion decomposition means for converting some or all of the hydrogen carbonate ions present in raw water into carbon dioxide, and carbon dioxide present in the treated water of the hydrogen carbonate ion decomposition means and others. A membrane deaerator equipped with a water-repellent membrane for degassing the dissolved gas of A desalinated water production device comprising a two-stage reverse osmosis membrane device for obtaining secondary permeated water through treatment with a second reverse osmosis membrane device. 2. The desalted water production apparatus according to claim 1, further comprising an alkali addition means for converting carbon dioxide remaining in the treated water of the membrane deaerator into bicarbonate ions. 3. Claim 1, in which an ion exchange device for removing residual ions in the secondary permeate water is installed downstream of the two-stage reverse osmosis membrane device.
Or the desalted water production apparatus according to claim 2.