JP2001129563A - Ph control method of water - Google Patents

Abstract

PROBLEM TO BE SOLVED: To facilitate the pH control of water to be treated, to reduce apparatus cost and running cost and to prevent the discharge of carbon dioxide to the atmosphere in a pH control method of water for adding a slaked lime aqueous solution to water to be treated to raise the pH of water to be treated and subsequently adding carbonic acid to water to be treated to lower the pH thereof. SOLUTION: Carbonic acid is added to water to be treated by directly dissolving carbon dioxide in water to be treated through a gas permeable membrane. For example, carbon dioxide in a carbon dioxide cylinder 44 is introduced into a carbon dioxide dissolving apparatus using a gas permeable membrane through a carbon dioxide introducing pipe 46 and dissolved in the water to be treated in a carbon dioxide dissolving tank 40 through the gas permeable membrane by the carbon dioxide dissolving device 50.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for adjusting the pH of tap water, general industrial water, industrial water, waste water, and the like, and more particularly, to a method of adjusting the pH of water using slaked lime and carbonic acid.

[0002]

2. Description of the Related Art FIG. 3 is a flowchart showing an example of a conventional tap water production process. In the figure, 2 is a slow stirring tank, 4 is a coagulation sedimentation tank, 6 is a sand filtration tank, 8 is a carbonated water mixing tank, 10 is a treatment water tank, and 12 is a pipe between the sand filtration tank 6 and the carbonated water mixing tank 8. , A reference numeral 14 denotes a carbonated water mixing mechanism connected to the carbonated water mixing tank 8, and 16 denotes a carbonated water mixing tank 8.
2 shows a pH meter connected to a pipe between the water tank and the treatment water tank 10.

The slaked lime aqueous solution adding mechanism 12 includes a slaked lime aqueous solution storage tank 18, an injection pipe 20 and an injection pump 22.
By operating the injection pump 22, the slaked lime aqueous solution in the slaked lime aqueous solution storage tank 18 is added to the water to be treated flowing through the pipe between the carbonated water mixing tank 8 and the treated water tank 10 through the injection pipe 20.

[0004] The carbonated water addition mechanism 14 includes a carbon dioxide gas cylinder 2.
4. An injection valve 26, a mixer 28, a pressure water pipe 30 and an injection pipe 32 are provided. In the mixer 28, carbon dioxide from the carbon dioxide gas cylinder 24 is blown into the water from the pressure water pipe 30 under pressure to dissolve the carbon dioxide. Water is produced, and the carbonated water is added to the water to be treated in the carbonated water mixing tank 8 through the injection pipe 32. An open-to-atmosphere pipe 34 is connected to the carbonated water mixing tank 8.

[0005] The processing in the process of FIG. 3 is performed as follows. First, a flocculant is added and a rapid stirring tank (not shown)
The water to be treated, which has been stirred in, is introduced into the slow stirrer 2 and further stirred, and then sequentially treated in the coagulation sedimentation tank 4 and the sand filtration tank 6.

Then, an aqueous solution of slaked lime (calcium hydroxide) is added from the slaked lime aqueous solution adding mechanism 12 to the filtered water to be treated. The reason why the slaked lime aqueous solution is added to the water to be treated is that the pH of the water to be treated is based on drinking water (pH 5.8 to 8.
This is for adjusting to the range of 6) and for improving the Langeria index of the water to be treated. The Langeria index is an index of the corrosiveness of water.If the value of the Langeria index is positive, calcium carbonate precipitates out of water, if negative, calcium carbonate dissolves in water, and if it is zero, the precipitation also dissolves. It is said that the state is not present, and that the corrosion of the piping can be prevented by setting the water's Langelia index to -1 or more. As a method of improving the Langeria index of water, a method of sequentially adding a slaked lime aqueous solution and carbonated water to water is often used.

Thereafter, the water to be treated is introduced into the carbonated water mixing tank 8, where the carbonated water is added to the water to be treated from the carbonated water addition mechanism 14. The addition of carbonated water to the water to be treated is intended to lower the pH of the water to be treated when the pH of the water to be treated becomes too high due to the addition of the slaked lime aqueous solution, and to improve the Langerian index of the water to be treated. That's why. In other words, although the pH of the water to be treated rises due to the addition of the slaked lime aqueous solution, the degree of the rise in the pH greatly differs depending on the water quality,
In water quality with low buffering property, pH 8.6 which is a water quality standard of tap water
In such a case, an acid is injected and p
It is necessary to readjust H. It is conceivable to use hydrochloric acid or sulfuric acid as the acid, but such a strong acid tends to cause a sharp drop in pH, and is difficult to use as a reconditioning acid. On the other hand, since carbonic acid is a weak acid, it does not cause a sharp drop in pH, and is very suitable as an acid for readjustment. Also, slaked lime produces calcium bicarbonate by reaction with carbonic acid, but it is desirable that calcium bicarbonate is large in order to improve the Langerian index, and therefore slaked lime and carbonic acid are required, especially after adding slaked lime Addition of carbonic acid is preferred for the production of calcium bicarbonate. Further, the following effects can be obtained by adding carbonic acid. That is, calcium carbonate as a solid impurity may be contained in the slaked lime aqueous solution added to the water to be treated, but by converting this into soluble calcium bicarbonate by reaction with carbonic acid, the transparency of water is increased. be able to. In this case, the amount of carbonated water added to the water to be treated is
pH of effluent from carbonated water mixing tank 8 measured by pH meter 16
It is controlled based on H.

[0008]

As described above, in the process of producing tap water, the pH of the water to be treated is increased by adding an aqueous solution of slaked lime to the water to be treated, and then further adding carbonic acid to the water to be treated. Lowering the pH of the water is performed. Such an operation is also performed in pH adjustment of general industrial water, industrial water, wastewater, and the like. In this case, the addition of carbonic acid to the water to be treated is conventionally performed by blowing carbon dioxide gas into water to produce carbonated water as in the carbonated water addition mechanism shown in FIG. Although the method is performed by injection, such a method of adding carbonic acid has the following disadvantages.

In the method of dissolving carbon dioxide by blowing it into water, the efficiency of dissolving carbon dioxide in water is low, and it is difficult to dissolve a desired amount of carbon dioxide in water. Therefore, it is difficult to control the carbonic acid concentration of the carbonated water to be injected into the water to be treated to a predetermined value, and as a result, it becomes difficult to control the pH of the water to be treated. In the method of dissolving carbon dioxide by blowing carbon dioxide into water, it is necessary to perform an operation of dissolving carbon dioxide under pressure in order to improve the efficiency of dissolving carbon dioxide in water and increase the carbon dioxide concentration of carbonated water. Cost and running cost increase. Since a two-stage process is required, a process of dissolving carbon dioxide gas in water to produce carbonated water and a process of injecting the same into the water to be treated, the equipment cost and running cost also increase in this respect. When carbonated water produced under pressure is injected into the water to be treated under normal pressure, carbon dioxide is released into the atmosphere at the time of injection.

As a method of adding carbonic acid to the water to be treated, a method of injecting an aqueous solution of slaked lime into the water to be treated and then directly blowing carbon dioxide gas into the water to be treated is also performed. In addition, the efficiency of dissolving the carbon dioxide gas in the water to be treated is low, and it is difficult to dissolve a desired amount of carbon dioxide gas in the water to be treated, so that it is difficult to control the pH of the water to be treated. Further, similarly to the above, in order to improve the dissolving efficiency of the carbon dioxide gas in the water to be treated, it is necessary to perform the operation of dissolving the carbon dioxide gas under pressure, and there is a problem that the apparatus cost and the running cost increase. .

The present invention has been made in view of the above-mentioned circumstances, and after adding an aqueous solution of slaked lime to the water to be treated to raise the pH of the water to be treated, carbonic acid is added to the water to be treated to be treated. A method for adjusting the pH of water for lowering the pH of water, wherein the pH of the water to be treated is easily controlled, and the cost of the apparatus is reduced.
An object of the present invention is to provide a method for adjusting the pH of water, which has a low running cost and can prevent carbon dioxide from being released into the atmosphere.

[0012]

According to the present invention, in order to achieve the above object, an aqueous solution of slaked lime is added to the water to be treated to raise the pH of the water to be treated, and then carbonic acid is added to the water to be treated. In lowering the pH of the water to be treated, the addition of carbonic acid to the water to be treated is carried out by directly dissolving carbon dioxide gas in the water to be treated via a gas permeable membrane.
An H adjustment method is provided.

A gas permeable membrane is one in which a liquid is directly in contact with a liquid on one side of the membrane and a gas on the other side of the membrane in a molecular state through the membrane in a molecular state. In the present invention,
Since carbon dioxide is dissolved in the water to be treated through such a gas permeable membrane, the dissolving efficiency of the carbon dioxide in the water to be treated can be increased to almost 100%, unlike a normal aeration operation, and A desired amount of carbon dioxide can be dissolved in the water to be treated. Therefore, the pH of the water to be treated can be easily controlled. In addition, the operation of dissolving carbon dioxide gas may be performed under normal pressure, and it is not necessary to perform the operation under pressure. In addition, carbon dioxide is dissolved in water to be treated in a one-step process of dissolving carbon dioxide gas in water through a gas permeable membrane. Since it can be added, the equipment cost and the running cost can be reduced. Furthermore, since the dissolving efficiency of carbon dioxide in the water to be treated is almost 100%,
Carbon dioxide is not released into the atmosphere when carbon dioxide is dissolved.

[0014] Also, when adding slaked lime aqueous solution and carbonic acid to the water to be treated to improve the Langeria index, the desired amount of carbonic acid changes due to the change in water quality. Furthermore, it is necessary to control the pH of the water to be treated as well as the amount of added carbonic acid in improving the Langeria index. Even in such a case, the method of injecting the carbonated water into the water to be treated after injecting the carbon dioxide gas into the water and then injecting the carbonated water into the water to be treated is the same as described above in terms of the amount of carbon dioxide added to the water to be treated and the water to be treated. PH
However, the present invention in which carbon dioxide is dissolved in water to be treated via a gas permeable membrane can solve these problems.

The material of the gas permeable membrane used in the present invention is not limited, and any membrane can be used as long as it is permeable to carbon dioxide gas and not permeable to water to be treated. Membrane or non-porous membrane, for example, a membrane made of polyolefin such as polyethylene, polypropylene, polymethylpentene, a membrane made of fluororesin such as polytetrafluoroethylene, polyvinylidene fluoride (PVDF), a membrane made of polysulfone, silicone A rubber film or the like can be suitably used. The shape of the gas permeable membrane is not limited, and any shape such as a flat membrane, a tubular membrane, a spiral membrane, and a hollow fiber membrane can be used.

[0016]

Next, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to the following examples. FIG. 1 is a flowchart showing an example of a tap water production process using the present invention. The manufacturing process of FIG. 1 is different from the manufacturing process of FIG. 3 in that a carbon dioxide gas dissolving tank 40 is provided between the sand filtration tank 6 and the treated water tank 10 in place of the carbonated water mixing tank 8, and the carbon dioxide gas dissolving tank 40 A carbon dioxide dissolving mechanism 42 is connected in place of the carbonated water adding mechanism 14. In addition,
The other parts in FIG. 1 are the same as those in FIG. 3, and therefore, in FIG. 1, the parts having the same configuration as in FIG.

The carbon dioxide gas dissolving mechanism 42 includes a carbon dioxide gas cylinder 44, a carbon dioxide gas introducing pipe 46, an injection valve 48, and a carbon dioxide gas dissolving device 50 using a gas permeable membrane. In the water to be treated. The carbon dioxide dissolving device 50 is, specifically, shown in FIG.
As schematically shown in FIG. 3, a plurality of hollow fiber membrane-shaped gas permeable membranes 54 having both ends opened are fixed in a U-shape at a lower portion of a container 52 connected to a carbon dioxide gas introducing pipe 46.

The carbon dioxide gas dissolving mechanism 42 of the present embodiment introduces the carbon dioxide gas in the carbon dioxide gas cylinder 44 into the container 52 of the carbon dioxide gas dissolving device 50 through the carbon dioxide gas introducing pipe 46, and converts the carbon dioxide gas into a hollow fiber membrane. The gas flows into the gas permeable membrane 54, whereby the carbon dioxide gas is directly dissolved in the water to be treated in the carbon dioxide gas dissolving tank 40 via the gas permeable membrane 54. In this case, the pressure of the carbon dioxide gas in the gas permeable membrane 54 is usually 0.1 to 10 times higher than the pressure of the water to be treated (normal pressure).
The pressure may be as high as about 3.0 kg / cm ² (about 0.01 to 0.3 Pa). Further, by adjusting the pressure and the flow rate of the carbon dioxide gas flowing through the gas permeable membrane 54, the amount of carbon dioxide dissolved in the water to be treated can be controlled.

The processing in the step of FIG. 1 is performed as follows. Steps until the slaked lime aqueous solution is added to the water to be treated by the slaked lime aqueous solution adding mechanism 12 are the same as the steps in FIG. Next, in the step of FIG. 1, the water to be treated is introduced into the carbon dioxide gas dissolving tank 40, where the carbon dioxide gas is dissolved in the water to be treated via the gas permeable membrane 54 by the carbon dioxide gas dissolving mechanism 42 as described above. You. The addition of carbonic acid to the water to be treated is for the same purpose as described above. in this case,
The amount of carbon dioxide dissolved in the water to be treated is controlled based on the pH of the effluent from the carbon dioxide dissolving tank 40 measured by the pH meter 16.

The structure of the apparatus for dissolving carbon dioxide gas is not limited to the structure shown in FIG. 2, but may be any structure capable of directly dissolving carbon dioxide gas in the water to be treated via a gas permeable membrane. You may. Further, the carbon dioxide gas dissolving device may be directly installed in the pipe without providing a carbon dioxide gas dissolving tank. Further, a stirring means such as a stirrer may be provided in the carbon dioxide gas dissolving tank so that the reaction between slaked lime and carbonic acid can be performed more quickly.

The above-mentioned dissolution of carbon dioxide gas in the water to be treated is performed when the pH of the water to be treated is lowered when the pH of the water to be treated becomes too high due to the addition of the slaked lime aqueous solution.
Further, in the case of improving the Langeria index of the water to be treated, the same operation can be performed even if the amount of dissolved carbon dioxide is different.

[0022]

As described above, according to the method for adjusting the pH of water according to the present invention, the dissolving efficiency of carbon dioxide in the water to be treated can be made almost 100%, and the desired amount of carbon dioxide can be obtained. Since the gas can be dissolved in the water to be treated, the pH of the water to be treated can be easily controlled. In addition, the operation of dissolving the carbon dioxide gas may be performed under normal pressure, and the carbon dioxide can be added to the water to be treated in a one-stage process of dissolving the carbon dioxide gas in water through the gas permeable membrane.
Running costs can be reduced. Moreover, since the dissolving efficiency of carbon dioxide in the water to be treated is almost 100%, the carbon dioxide is not released into the atmosphere when dissolving the carbon dioxide. Also, when adding slaked lime aqueous solution and carbonic acid to the water to be treated for the purpose of improving the Langeria index, the amount of carbonic acid added to the water to be treated and the pH of the water to be treated can be easily controlled.

[Brief description of the drawings]

FIG. 1 is a flowchart showing an example of a tap water production process using the present invention.

FIG. 2 is a view schematically showing a carbon dioxide gas dissolving device having a carbon dioxide gas dissolving mechanism used in the step of FIG. 1;

FIG. 3 is a flowchart showing an example of a conventional tap water production process.

[Explanation of symbols]

 2 Slow stirring tank 4 Coagulation sedimentation tank 6 Sand filtration tank 8 Carbonated water mixing tank 10 Treatment water tank 12 Slaked lime aqueous solution addition mechanism 14 Carbonated water addition mechanism 16 pH meter 40 Carbon dioxide gas dissolution tank 42 Carbon dioxide gas dissolution mechanism 44 Carbon dioxide gas cylinder 46 Carbon dioxide gas Inlet pipe 48 Injection valve 50 Carbon dioxide gas dissolving device 52 Container 54 Gas permeable membrane

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Claims (1)

[Claims] 1. An aqueous solution of slaked lime is added to the water to be treated to raise the pH of the water to be treated, and then carbonic acid is added to the water to be treated to lower the pH of the water to be treated. Wherein the addition of carbonic acid is performed by directly dissolving carbon dioxide gas in the water to be treated via a gas permeable membrane.