JP2000051665A - Desalination method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To ensure the availability of water with stably high purity for a long time by deaerating the water to be treated to remove a dissolved oxygen and at the same time, reversely osmosing the water through an osmotic membrane to desalinate it and further circulating the water through an electric regeneration desalination device after adjusting the pH value above a specific level. SOLUTION: The deaeration treatment is to remove a dissolved oxygen from water to be treated using a deaeration device and the reverse osmotic(RO) membrane treatment is to separate and remove salts and an ionic substance using a reverse osmotic membrane action. The treatment of the water through the deaeration and the RO treatment can be started with the deaeration treatment and the RO treatment in that order or alternatively, can be started with the RO treatment and the deaeration treatment in that order. Next the pH value of the water to be introduced into an electric regeneration desalination device is adjusted to 9 or more. The dosage of an alkali agent is dominant by setting the pH value at 9 or more. Therefore the conductivity at the inlet of the electric regeneration desalination device can be made almost constant regardless of the qualitative variation of a water supply. Besides silica in the water becomes easily ionized and consequently tends to easily move from the treated water chamber of the electric regeneration desalination device to its concentrated water chamber.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for desalinating raw water by subjecting it to reverse osmosis membrane treatment and treating it with an electric regeneration type desalination apparatus, and in particular, can maintain stable water quality for a long time. It relates to a desalination method.

[0002]

2. Description of the Related Art Conventionally, raw water such as industrial water, city water, well water, etc.
As a device for obtaining demineralized water used for washing water for pharmaceuticals, semiconductors, lenses, liquid crystals, etc., an electric regeneration type desalination device is used. In this electric regeneration type desalination apparatus, a plurality of anion exchange membranes and cation exchange membranes are alternately arranged to form a concentrated water chamber and a treated water chamber alternately, and an anion exchange resin and a cation exchange resin are provided in the treated water chamber. It is a configuration that is mixed and filled, and does not require a regeneration step of periodically passing a regenerant through an ion-exchange resin, as in an ion-exchange pure water apparatus known as a desalination apparatus, and continuously. Since demineralized water can be obtained, it has been frequently used in recent years.

When water to be treated is introduced into the treated water chamber of this electric regeneration type desalination apparatus, ions in the treated water move through the ion exchange resin, decrease in the treated water chamber, and concentrate in the concentrated water chamber. You. For this reason, desalinated water is recovered from the treated water chamber.

[0004]

However, the desalination rate of this electric regeneration type desalination apparatus is not always sufficient.
In particular, if silica (SiO ₂ ) or carbonic acid (CO ₂ ) is large in the water to be treated introduced into the electric regeneration type desalination apparatus, the quality of the treated water is reduced. In order to improve the quality of treated water, it is conceivable to increase the amount of current supplied to the electric regeneration type desalination apparatus. However, an increase in the amount of current also causes water to be decomposed, resulting in waste of energy. Furthermore, the water supply quality fluctuates,
When the electrical conductivity at the entrance of the electric regeneration type desalination equipment changes,
There is also a problem that the treated water quality changes under a constant voltage (current). Therefore, with a constant current (voltage), there is a limit to the improvement of the quality of treated water.

It is known that silica (SiO ₂ ) is easily ionized under alkaline conditions. For this reason, the improvement of the silica removal rate by the electric regeneration type desalination apparatus can be expected, but it has been found that the treated water quality is not always stable in the long-term operation of the electric regeneration type desalination apparatus under alkaline conditions. When the cause of the instability was examined, when the water to be treated introduced into the electric regeneration type desalination apparatus was alkaline and dissolved oxygen (DO) was present, when the electric regeneration type desalination apparatus was operated for a long time, In addition, the ion-exchange membrane and ion-exchange resin (ion-exchange fiber) that make up the electric regeneration type desalination equipment deteriorate and produce eluted substances, and the value of the specific resistance gradually decreases, affecting the quality of treated water. It was estimated that. The present invention has been made based on such findings, and it is an object of the present invention to provide a desalination method capable of obtaining stable water quality for a long period of time and obtaining high-purity water quality. It is.

[0006]

According to the desalting method of the present invention, the water to be treated is degassed to remove dissolved oxygen, and is treated with a reverse osmosis membrane (RO) to desalinate and adjusted to pH 9 or more. Thereafter, the water is passed through an electric regeneration type desalination apparatus.

First, the deaeration process will be described. This deaeration treatment is to remove dissolved oxygen (DO) from the water to be treated by a deaeration device. When DO flows into an electric regeneration type desalination device described later, the ion exchange material is oxidized and deteriorated.
Remove in advance. As the deaerator, N ₂ deaerator,
An ordinary deaerator such as a vacuum deaerator or a membrane deaerator can be used, and it is desirable that the DO deaeration is reduced to 1 mg / l or less.

If the pH of the carbonate ions is in the range of 4 to 5.5, the carbonate ions are dissolved in water as carbon dioxide (CO ₂ : carbon dioxide gas).
Is kept in this range, carbonic acid is removed together with DO. On the other hand, carbonic acid is ionized when the pH is higher than the above range, and is dissolved in water as carbonate ions (bicarbonate ions). When carbonate ions flow into the electric regeneration type desalination device, it becomes a load on the electric regeneration type desalination device, and the quantitative balance of anions and cations is disrupted, so that more ions cannot be removed by the electric regeneration type desalination device. It remains and it becomes difficult to improve the quality of the treated water. Therefore, it is desirable that the carbonate ions be removed before the electric regeneration type desalination apparatus, and it is desirable that the carbonate ions be removed by this deaeration treatment. The pH may be adjusted arbitrarily by adding an acid (such as hydrochloric acid or sulfuric acid), or may be brought into contact with an H-type cation exchange resin instead of adding the acid. still,
Since the carbonate ions can be removed by a reverse osmosis membrane (RO) treatment described later, the removal rate is not high (removal rate of 60 to 90).
%) May be removed by RO processing.

Next, the reverse osmosis membrane (RO) treatment will be described. This RO treatment separates and removes salts and ionic substances by the action of a reverse osmosis membrane. Desalination (deionization) is also performed in an electric regeneration type desalination apparatus described later, but the desalination rate is limited, and the load on the electric regeneration type desalination apparatus is reduced by performing RO treatment in advance. The reverse osmosis membrane used for such RO treatment is not particularly limited as long as it has a NaCl removal rate of 98% or more. For example, a commercially available membrane such as a polyamide membrane, a cellulose acetate membrane, or an aramid-based membrane is used. be able to. The water recovery rate, operating pressure, and the like are not limited at all.

[0010] In addition, since the RO treatment also separates fine particles and organic matter in the water, it is possible to prevent contamination of the electric regeneration type desalination apparatus due to the inflow thereof, and to provide a current efficient and stable electric regeneration type desalination apparatus. Can drive. Further, even when carbonate ions are not removed by the deaeration treatment as described above, carbonate ions can be removed by this RO treatment. Further, the RO treatment can also remove a part of the silica, reduce the flow of silica into the electric regeneration type desalination apparatus, reduce the silica load of the electric regeneration type desalination apparatus, and treat the treated water quality of the electric regeneration type desalination apparatus. Becomes stable.

In particular, by performing the RO treatment in multiple stages (two or three stages), that is, by providing multiple RO units, the salt concentration and the silica concentration can be further reduced, and the load on the electric regeneration type desalination device is reduced. As a result, high-purity treated water is obtained.

In the treatment of the water to be treated by the degassing treatment and the RO treatment, the water to be treated may be treated in the order of degassing treatment and RO treatment, or may be treated in the order of RO treatment and degassing treatment. good. In these treatments, an acid (HCl or H ₂
SO ₄ or the like, or an H type (Na type may be mixed) weak or strong cation exchange resin tower (Bayer “C
NP-80 "," SP112 "etc.) to adjust the pH. When processing in the order of degassing processing and RO processing, p
It is desirable to adjust to H4 to 5.5. When the RO treatment and the deaeration treatment are performed in this order, it is desirable to adjust the pH to 6 to 7.5. At this pH most of the carbonic acid is (heavy)
Most of (bi) carbonate ions can be removed by RO treatment because they are carbonate ions. In the subsequent degassing treatment, DO is mainly removed, but carbonic acid (CO ₂ ) remaining without being dissociated at this pH is also removed.

Next, the pH adjustment will be described. This p
In the H adjustment, the pH of the water introduced into the electric regeneration type desalination apparatus is adjusted to 9 or more. By adjusting the pH to 9 or more, the amount of the alkaline agent becomes dominant, so that the electric conductivity at the inlet of the electric regeneration type desalination apparatus can be made substantially constant even when the quality of the feedwater fluctuates. In addition, the silica in the water is easily ionized, so that the silica is easily moved from the treated water chamber to the concentrated water chamber of the electric regeneration type desalination apparatus (the state is easily removed).

Preferably, the pH is 9-10. When the pH is 8 or less, the silica removal rate by the electric regeneration type desalination apparatus is about 70 to 80%. However, when the pH is 9 or more, the silica removal rate can be 90% or more.
If it is 9.5 or more, the silica removal rate can be improved to 95% or more. The silica removal rate is further improved by increasing the pH, but when the pH exceeds 10, the ion exchange material of the electric regeneration type desalination apparatus is likely to deteriorate.

In addition to the above-described processing, an arbitrary processing step can be added. For example, the following treatment may be performed before water is passed through the electric regeneration type desalination apparatus.

The water to be treated is first passed through a turbidity step (apparatus) to remove SS (suspended substances) and dissolved substances that can be insolubilized. The turbidity removal apparatus used in this clarification step includes coagulation sedimentation, filtration, and membrane separation for clarification (for example, microfiltration (MF) membrane and ultrafiltration (UF)
Membrane) can be used. By performing this turbidity removal step, the load on the subsequent electric regeneration type desalination apparatus can be reduced, and the quality of treated water can be improved.

Removal (softening) of hardness (Ca, Mg) components The hardness components are adsorbed and removed by contact with a cation exchange resin. The cation exchange resin may be weakly acidic or strongly acidic, and may be used in H type or Na type. C
Most of the a ions and Mg ions are removed by the above-mentioned RO treatment, but if they remain and flow into an alkaline electric regeneration type desalination apparatus, they may cause scale, so it is desirable to remove them in advance. .

TOC removal Water is passed through an ultraviolet oxidizer having a wavelength of 185 nm and irradiated with ultraviolet rays to decompose organic substances in the water.
TOC is decomposed to CO ₂ and removed, or is converted into a low molecular organic substance and removed by a RO device or the like at a later stage, so that the quality of treated water is improved. The ultraviolet irradiation may be performed after or before the addition of the alkali agent, but is preferably performed under alkaline conditions.

Thereafter, water is passed through an electric regeneration type desalination apparatus. As shown in FIG. 2, the electric regeneration type desalination apparatus includes a concentrated water chamber 23 and a cation exchange membrane 22 which are formed by an anion exchange membrane 21 and a cation exchange membrane 22. The treated water chamber 24 is separated from the anode chamber 27 and the cathode chamber 28, and the treated water chamber 24 or the treated water chamber 24 and the concentrated water chamber 23 are filled with a mixed bed of a cation exchange resin and an anion exchange resin. A commonly used commercially available electric regeneration type desalination apparatus can be used. An anion exchange fiber or a cation exchange fiber can be used instead of the anion exchange resin or the cation exchange resin. In this electric regeneration type desalination apparatus, ions in the water to be treated introduced into the treatment water chamber 24 move in the resin 20 in the direction of the gradient of the potential of the anode 25 and the cathode 26 based on affinity, concentration and mobility, Furthermore, it moves across the cation exchange membrane 22 or the anion exchange membrane 21 that separates the treated water chamber 24 and the concentrated water chamber 23, so that charge neutralization is maintained in all chambers. Then, the ions in the supply water are removed from the treated water chamber 24 by the semi-osmotic characteristics of the ion exchange membranes 21 and 22 and the directionality of the potential gradient.
, And is concentrated in the adjacent concentrated water chamber 23. Therefore, pure water (deionized water) is supplied from the treated water chamber 24.
Is collected. In addition, 27 is an anode room and 28 is a cathode room. In such an electric regeneration type desalination apparatus, water which removes DO and a part of silica and is adjusted to alkaline pH is introduced by the treatment up to the preceding stage. Therefore, the load on the electric regeneration type desalination apparatus is small, and the treated water quality does not change even under a constant voltage (current), and the water quality is stable over a long period (specific resistance, silica, etc.).
Can be maintained.

As described above, if the amount of the supplied current is increased to remove the silica in the electric regeneration type desalination apparatus, water is also decomposed and energy is wasted. Since the silica is easily ionized by appropriately adjusting to alkalinity, and it is easy to move from the treated water chamber to the concentrated water chamber of the electric regeneration type desalination apparatus, the amount of current supplied to the electric regeneration type desalination apparatus is increased. Even if a constant current (voltage) is not required, it can be efficiently removed.

Further, since the carbonic acid component is sufficiently removed by the deaeration treatment (and the RO treatment) before the electric regeneration type desalination apparatus, the inflow of the carbonic acid component into the electric regeneration type desalination apparatus is extremely small. . For this reason, there is no problem of a decrease in the water quality of the treated water of the electric regeneration type desalination device due to the above-mentioned CO ₂ , and a good water quality can be obtained stably.

[0022]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 2 is a system diagram showing four examples of the embodiment of the present invention.

In the present invention, the water to be treated can be water usually used as raw water for producing pure water, such as industrial water, city water, well water, etc., and can be used from a semiconductor production process using ultrapure water. Wastewater may be biologically treated to a TOC of 1 ppm or less according to a conventional method, and if necessary, may further be subjected to RO treatment and ion exchange treatment as treated water.

Then, the water to be treated was first subjected to the above-mentioned softening treatment to remove the hardness (Ca, Mg) component. Further, the pH was adjusted to ₄ to 5.5 by adding a mineral acid such as HCl and H ₂ SO _4, and the mixture was subjected to degassing treatment and RO treatment. Then, after performing the above-described turbidity treatment to remove turbidity and insolubilized dissolved substances in the water to be treated, the pH is adjusted to 6 to 7.5, and the resultant is subjected to RO treatment. It was subjected to air treatment. In, water was passed through a cation exchange resin tower and subjected to degassing and RO treatment. In the above, water was passed through a cation exchange resin tower, the pH was adjusted to 6 to 7.5 by adding an alkali, and the mixture was subjected to an RO treatment, and if necessary, an acid was added to perform a deaeration treatment.

In each of the above deaeration treatments, DO is removed from the water to be treated. Further, since the pH of the water is acidic at 5.5 or less, the (bi) carbonic acid component in the water is in a CO ₂ state, and the carbonic acid can be removed simultaneously with the DO. In each RO treatment, salts and ionic substances are separated and removed, and a part of silica is also removed. In each of the RO treatments in the above, most of the carbonic acid is (heavy)
Most of them can be removed because they are carbonate ions. Since almost neutral water having a pH of 6 to 7.5 is supplied, an RO film having a high NaCl removal rate of 98% or more, such as a polyamide film or a cellulose acetate film, is used. be able to. In the subsequent degassing treatment, DO is mainly removed, but the remaining carbonic acid (CO
₂ ) is also removed by adjusting the pH by adding an acid. When the pH after the RO treatment is 5.5 or less, the acid addition may be omitted.

Thereafter, an alkali is added to the mixture to adjust the pH.
Is adjusted to 9 or more, and water is passed through the electric regeneration type desalination apparatus. In the electric regeneration type desalination apparatus, desalination is performed according to the above-described principle, demineralized water (pure water) is obtained from the treated water chamber 24, and concentrated water is discharged from the concentrated water chamber 23. The permeated water of the RO device is supplied to the electrode chambers (the anode chamber 27 and the cathode chamber 28) of the electric regeneration type desalination apparatus. Pure water from the treated water chamber 24 is discharged out of the system and supplied to a place of use. On the other hand, the concentrated water from the concentrated water chamber 23 is returned to the preceding stage of the RO device and supplied to the RO device. Also, the effluent from the electrode chamber is returned to the preceding stage of the RO device.

In the above-mentioned items (1) and (2), DO is removed and a part of the silica is removed in the treatment up to the preceding stage of the electric regeneration type desalination apparatus, and water whose pH is adjusted to alkaline is introduced. For this reason, the treated water quality does not change even under a constant voltage (current) even when the supply water quality changes, and stable water quality (specific resistance, silica, etc.) can be maintained for a long time.

[0029]

The present invention will be described more specifically below with reference to examples and comparative examples.

[Example 1] Atsugi-shi water was treated with UF ("KU-1010" manufactured by Kurita Water Industries Ltd.). HCl with p
After adjusting to H5.0, the degassing membrane (Hoechst “Liqu
-Cell: 4 inches ") at 1.5 m ³ / hr. Degassed water is passed through RO (Nitto Denko “ES-20” 4-inch, 2-piece series) at 0.47 m ² / hr, operating pressure 6 to 6.5 kg / cm ² , water recovery rate 70% Driven by After adding NaOH to the RO treated water and adjusting the pH to 9.2, 1 of the 180 L / hr was introduced into the treated water chamber of the electric regeneration type desalination apparatus (Kurita Kogyo “Pure Ace P120”).
40 L / hr was supplied and 40 L / hr was supplied to the concentrated water chamber and the electrode chamber.
hr. The operation was performed at a voltage of 50 V and a current of 0.3 A. The quality of the obtained treated water was examined, and the results are shown in Table 1.

Comparative Example 1 The pH was not adjusted (p.
Except for H5.6), water was passed through the electric regeneration type desalination apparatus under the same conditions as in Example 1 above. The quality of the obtained treated water was examined, and the results are shown in Table 1.

Comparative Example 2 Water was passed through an electric regeneration type desalination apparatus under the same conditions as in Example 1 except that the degassing treatment was not performed. The quality of the obtained treated water was examined, and the results are shown in Table 1.

[0033]

[Table 1]

As is clear from Table 1, according to the present invention, stable water quality (specific resistance, silica, etc.) can be maintained for a long time.

[0035]

As described above in detail, according to the desalination method of the present invention, the oxygen regeneration (DO) is removed by the degassing treatment and the reverse osmosis membrane (RO) treatment in the electric regeneration type desalination apparatus. At the same time, part of the silica is removed.
Since water whose H is adjusted is introduced, stable water quality can be obtained for a long time, and high-purity water quality can be obtained. That is, in the present invention, the pH is previously adjusted to alkaline before the water is passed through the electric regeneration type desalination apparatus, so that the silica is easily ionized, and the treated water chamber of the electric regeneration type desalination apparatus is transferred to the concentrated water chamber. Since it is easy to move, a constant current (voltage) can be efficiently removed without increasing the amount of current supplied to the electric regeneration type desalination apparatus. Also,
By making the feed water alkaline, the electrical conductivity at the inlet becomes almost constant even if the feed water quality changes, and the treated water quality does not change even under a constant voltage (current). Further, since a part of the silica is removed by the RO treatment, the load on the electric regeneration type desalination apparatus is small. Further, since the carbonic acid component is sufficiently removed together with the DO by the deaeration treatment (and the RO treatment), the inflow of the carbonic acid component into the electric regeneration type desalination apparatus is extremely small. For this reason, there is no problem of a decrease in the water quality of the treated water of the electric regeneration type desalination device due to the above-mentioned CO ₂ , and a good water quality can be obtained stably.

[Brief description of the drawings]

FIG. 1 is a system diagram showing four examples of an embodiment of the present invention.

FIG. 2 is a schematic configuration diagram showing the structure of an electric regeneration type desalination apparatus.

[Explanation of symbols]

 Reference Signs List 20 ion exchange resin 21 anion exchange membrane 22 cation exchange membrane 23 concentrated water chamber 24 treated water chamber 25 anode 26 cathode 27 anode chamber 28 cathode chamber

 ──────────────────────────────────────────────────続 き Continued on the front page F-term (reference) 4D006 GA03 GA06 GA07 GA17 GA32 HA47 JA43Z JA44Z KA01 KA03 KA52 KA57 KA72 KB11 KB13 KB14 KB17 KB30 KD11 KD17 KE15Q MA03 MA13 MA14 MC18 MC54 MC55 PA01 PB02 PB23 PC01 PC37 AB42 AB0 BA23 BB09 CA03 CA04 CA14 CA15 4D061 AA01 AB07 AB13 AB15 AC06 AC18 BA02 BA09 BB01 BB04 BB13 BB14 BB18 BB19 CA03 CA08 CA09 CA11

Claims (1)

[Claims] The present invention is characterized in that the water to be treated is degassed to remove dissolved oxygen, desalted by reverse osmosis membrane treatment, adjusted to pH 9 or more, and then passed through an electric regeneration type desalination apparatus. Desalination method.