JPH091156A - Method for oxidizing ferrous ion contained in underground water - Google Patents

Abstract

PURPOSE: To efficiently oxidize ferrous ions contained in underground water in a short time. CONSTITUTION: Underground water is aerated in an aeration tank 2 to reduce free carbonic acid present in the underground water and the pH value of the underground water is raised to a prescribed value. Next, an alkali agent is added to the underground water reduced free carbonic acid to further raise the pH value to a prescribed value, thus the ferrous ions contained in the underground water is efficiently oxidized in a short time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION This invention relates to the groundwater treatment of groundwater,
The present invention relates to a method for oxidizing ferrous ions contained in groundwater, which is performed as a pretreatment in the field of water treatment such as industrial water treatment and water distribution treatment.

[0002]

2. Description of the Related Art In the field of water treatment such as ground water treatment, industrial water treatment, and water distribution treatment, water purification treatment is performed by coagulation / sedimentation treatment and sand filtration or membrane filtration. There is.

However, groundwater generally contains a larger amount of iron than surface water, and most of the iron is
Since groundwater is anaerobic, it is reduced and exists as ferrous ions. Therefore, in a normal water treatment method, iron in groundwater cannot be removed, and ferrous ions present in water are oxidized and deposited, and then they must be removed by solid-liquid separation means. I won't.

The following method is known as a method for oxidizing the iron content existing in water. (1) Oxidation by aeration (2) Oxidation by chlorine injection (3) Oxidation by catalytic filtration (4) Oxidation by iron bacteria (5) Oxidation by injection of potassium permanganate (6) Oxidation by ozone injection

Among the above-mentioned methods, the method which is generally performed from the viewpoint of cost is (1) oxidation by aeration,
The methods are (2) oxidation by chlorine injection and (3) oxidation by catalytic filtration.

[0006]

Since iron in water is instantly oxidized by chlorine, it is the easiest method by the chlorine injection of (2) above. However, in this method, when the concentration of organic substances present in water is high, by injecting chlorine, organic chlorine compounds such as trihalomethane harmful to the human body are generated, and when the iron concentration fluctuates significantly, There are problems such as difficulty in controlling the chlorine injection rate.

[0007] The oxidation method by contact filtration of the above (3) is excellent in that it can not only oxidize iron but also remove it, but it has a slow filtration rate and is not suitable for membrane filtration. There are problems in terms of water quality and equipment as compared with treatment methods with high solid-liquid separation ability.

The oxidation method by aeration described in (1) above is not so much affected by the fluctuation of iron concentration in groundwater,
Depending on the quality of the water, there is a problem that a large amount of air for aeration is required and groundwater must be retained in the aeration tank for a long time.

Due to the presence of free carbonic acid in groundwater, its pH value is generally low. Therefore, when purifying groundwater, remove the free carbonic acid present in water and adjust its pH.
It is necessary to increase the value. Aeration treatment and addition treatment of an alkaline agent are known as means for removing free carbon dioxide in groundwater.

As means for removing free carbon dioxide by aeration, a packed tower system in which water to be treated is passed through a packed tower filled with a granular contact substance, the inside of the tower is treated from a diffuser pipe provided at the bottom of the tower. A bubble column system and a spray column system in which air is blown into water to be sprayed are known.

However, in the case of the packed tower system and the spray tower system, a sufficient time for removing the free carbonic acid,
When groundwater cannot be retained in the tower, and the clogging is likely to occur in the tower, and the concentration of ferrous ions or suspended matter in the water is high, iron oxide precipitates in the tower. As a result, there are problems such as difficulty in maintenance. In the case of the bubble column method, there is a problem that the pH value does not rise sufficiently and the oxidation rate is slow, but there is an advantage that the maintenance is easy and the treated water can stay for a sufficient time to remove free carbonic acid. is there.

[0012] Therefore, the object of the present invention is to solve the above-mentioned problems, and to remove the ferrous ions contained in groundwater,
It is intended to provide a method for oxidizing ferrous ions contained in groundwater that can be efficiently oxidized and removed in a short time.

[0013]

According to the method of the present invention, ground water is aerated in an aeration tank,
By decreasing the free carbonic acid present in the groundwater, the pH value of the groundwater is raised to a predetermined value, then,
By adding an alkaline agent to the groundwater in which the free carbonic acid is reduced, and further increasing the pH value to a predetermined value,
It is characterized by efficiently oxidizing the ferrous ions contained in the groundwater in a short time. The increase in pH value due to aeration of the groundwater is 7 to 7.
It is desirable that the range is up to 5, and the increase in pH value due to the addition of the alkaline agent is within the range of 8 to 8.5.

[0014]

[Function] The pH value of groundwater is usually 6 to 6.5, which is low.
Therefore, ferrous ions contained in groundwater cannot be efficiently oxidized unless such a low pH value is raised. The reason for the low pH value of groundwater is that there is an excess of free carbonic acid in the water. That is, the free carbonic acid content of groundwater is generally 50 mg / l or more. Therefore,
If this free carbonic acid is removed, the pH value of groundwater will rise and the ferrous ion can be efficiently oxidized.

The oxidation reaction rate of ferrous ions (divalent iron ions) is represented by the following equation. Over d [Fe (II)] / dt = k [Fe (II)], [O _2], [OH ^{-] 2} (Note) Source: "iron removal, removing manganese processing of water" (published
Industrial Water Research Committee) As is clear from the above reaction formula, the decrease of ferrous iron (divalent iron ion) is proportional to the square of [OH ⁻ ].
Therefore, in order to reduce ferrous ions,
It is important to raise the pH value.

In the case of general water quality, ferrous ions in water are easily oxidized to trivalent iron by aeration to be precipitated as iron oxide, which is likely to occur in sand filtration or membrane filtration in the latter stage. This can be removed in various solid-liquid separation steps. However, when the concentration of ferrous ions is high like groundwater, or when divalent iron forms a complex or the like and is difficult to be oxidized, the oxidation reaction rate of ferrous ions due to aeration is slow. . Therefore, it is necessary to raise the pH value of groundwater to accelerate the oxidation reaction rate of ferrous ions.

The efficiency of raising the pH value by aeration is
It is due to the removal rate of free carbon dioxide from groundwater,
Depends on free carbon dioxide concentration in groundwater. That is, as the concentration of free carbon dioxide in groundwater decreases, the rate of increase in pH value due to aeration becomes slower, and the removal efficiency of free carbon dioxide decreases.

FIG. 2 shows two types of groundwater A, which have different water qualities.
It is a graph which shows the relationship between the retention time in the aeration tank of B, and its pH value. As is clear from FIG. 2, the ground water A containing a large amount of free carbon dioxide has a pH value of about 7 due to the aeration process in the aeration tank.
Until it falls below, and the ground water B has a pH value of 7.5.
It is possible to raise the pH value in a relatively short time until it falls below the lower limit. However, when the pH value of groundwater A is 7 or more and the pH value of groundwater B is 7.5 or more, the content of free carbon dioxide in groundwater is reduced to about 10 mg / l or less, and as a result, it is left in the aeration tank for a long time. PH even when retained
The efficiency of increasing the value is low.

On the other hand, the pH due to the addition of an alkaline agent to groundwater
The efficiency of increasing the value does not deteriorate due to the decrease in the concentration of free carbon dioxide in groundwater. Therefore, if free carbonic acid in groundwater is removed as much as possible by aeration and the pH value is raised to a predetermined value and then the alkali agent is added, the pH value can be raised efficiently with a small amount of the alkali agent. . In addition, when an alkaline agent is added to groundwater before performing aeration, the alkaline agent is consumed for neutralizing free carbonic acid present in high concentration in groundwater, resulting in an increase in pH value. Requires a huge amount of alkaline agent.

FIG. 3 shows an alkaline agent (NaO) for groundwater.
3 is a graph showing the relationship between the H) addition rate and its pH value.
In FIG. 3, the solid line shows the case where the alkaline agent (NaOH) was added to the groundwater by the method of the present invention after the aeration in the aeration tank, and the dotted line shows the case where the alkaline agent was directly added to the groundwater without performing the aeration. NaOH) is added. As is clear from FIG. 3, when the alkaline agent (NaOH) was added directly to the groundwater without performing aeration, the pH value increasing efficiency was poor even if a large amount of the alkaline agent was added.

FIG. 4 shows groundwater obtained by subjecting groundwater to aeration treatment in an aeration tank, and then adding an alkali agent and allowing the groundwater to stay.
It is a graph which shows the relationship between pH value and the total residence time of groundwater in an aeration tank and a retention tank required to oxidize ferrous ions in groundwater to 0.01 mg / l or less.
In FIG. 4, 43 minutes of the total retention time is the retention time of the aeration tank, and the remaining time is the retention time of the retention tank. From FIG. 4, it can be seen that the pH value of groundwater must be finally raised to 8 or more in order to shorten the total residence time to about 70 minutes or less.

The higher the pH value of groundwater, the faster the oxidation reaction rate of groundwater in the retention tank after the addition of the alkali agent, and the shorter the retention time. However, the upper limit of the pH value is 8.
5 is preferable. The pH value may be adjusted to around 8 by adding an acid after the pH value is increased to more than 8.5 to complete the reaction. When the pH value is raised to 10 or more, ferrous ions are precipitated as Fe (OH) ² , but this is not desirable because it is not the oxidation of ferrous ions.

From the above viewpoint, in the present invention,
By aeration of groundwater, its pH value can be increased to 7
It is necessary to raise the pH value to a range of 8 to 8.5 by adding an alkaline agent to the ground water whose pH value has risen in this way. Is.

If the pH value of the groundwater is less than 7 due to aeration, the pH value cannot be efficiently increased even if the alkaline agent is added thereafter, while the pH value of the groundwater increases to 7.5. If it exceeds, it takes a long time to raise the pH value by aeration to that value, and the efficiency of raising the pH value decreases.

When the pH value rise by addition of an alkaline agent is less than 8, the total residence time required to oxidize ferrous ions in groundwater to 0.01 mg / l or less becomes long and the ferrous ions are efficiently oxidized. I can't let you do it. On the other hand, if the pH value increase due to the addition of the alkaline agent exceeds 8.5, it will be against the water quality standard of the tap water, and it will not be possible to use it as drinking water.

FIG. 1 is a schematic process diagram showing one embodiment of the method of the present invention. In FIG. 1, 2 is an aeration tank and 9 is a retention tank, and the aeration tank 2 and the retention tank 9 are connected by a conduit 10. The interior of the aeration tank 2 is divided into three aeration chambers 2a, 2b, 2c by two vertical partition plates 5a, 5b which are attached at a predetermined interval in the width direction. First partition plate 5a
A predetermined gap is provided between the lower end of the tank and the bottom of the tank, and between the upper end of the second partition plate 5b and the upper surface of the tank.

An air diffuser 4 is provided at the bottom of each of the aeration chambers 2a, 2b, 2c. The groundwater supplied to the upper portion of the aeration chamber 2a by the conduit 1 sequentially flows from the aeration chamber 2a to the aeration chambers 2b and 2c and is discharged from the lower portion of the aeration chamber 2c, while being provided at the bottom of each aeration chamber. Aeration is performed by the air jetted from the air diffuser 4.

The inside of the aeration tank 2 may be one chamber without being divided into a plurality of aeration chambers as described above, but division into a plurality of chambers allows efficient removal of free carbon dioxide. You can It should be noted that it is sufficient that the number of rooms when divided is 3 to 5. In this case, it is not necessary to perform aeration in all of the divided chambers, and it is not necessary to perform aeration in the chamber near the outlet side of the treated water.

A branch pipe 11 is attached in the middle of a conduit 10 connecting the aeration tank 2 and the retention tank 9, and the branch pipe 11 is connected to an alkaline agent storage tank 6 and a pump 7. On the downstream side of the connecting portion of the branch pipe 11 of the conduit 10,
The mixer 8 is connected.

The groundwater to be treated is the aeration tank 2
Is supplied from above the inlet side into the aeration chamber 2a of
On the other hand, air is blown by the blower 3 from the diffuser pipes 4 provided at the bottom of each of the aeration chambers 2a, 2b, 2c,
Aeration is applied to the groundwater flowing in the aeration chambers 2a, 2b, 2c.

Aerated groundwater is discharged from the aeration tank 2, passes through the conduit 10, and stays in the retention tank 9
In the middle of the conduit 10, the alkaline agent is added from the branch pipe 11 to the groundwater passing through the conduit 10, and the mixer 8
Mixed by. The groundwater to which the alkaline agent has been added and mixed in this way is guided to the retention tank 9, and the ferrous ions are oxidized while retained in the retention tank 9 for a predetermined time.

The aeration tank 2 is used to add the alkaline agent.
It may be carried out at any position of the conduit 10 on the outlet side of the tank, or may be carried out directly in the groundwater introduced into the retention tank 9. In addition, it is desirable that the groundwater to which the alkaline agent is added is sufficiently stirred by a mixer or the like.

As the alkaline agent, soda ash (NaCo ₃ ), caustic soda (NaOH), slaked lime (Ca (OH) ₂ ), etc. which are generally used in water treatment are used, but not limited to these. However, any one may be used as long as it has an effect of increasing pH.

[0034]

【Example】

[Embodiment 1] Next, the present invention will be further described with reference to embodiments. Using the apparatus shown in FIG. 1, the ferrous ion contained in groundwater was oxidized under the following conditions by the method of the present invention. Concentration of ferrous iron in ground water (raw water): 21 mg / l Flow rate: 8 l / min Residence time in aeration tank: 30 minutes Blow-in air amount in aeration tank: 120 l / min NaOH addition amount: 16 mg / l (raw water) Residence Residence time in tank: 30 minutes

As a result, ferrous ions in groundwater (raw water) were oxidized to less than 0.01 mg / l. The pH of the raw water at this time is 6.7, and the pH of the aeration-treated water is
Was 7.1 and the pH of the final treated water was 8.0.

[Embodiment 2] Next, the apparatus shown in FIG. 1 was also used to oxidize ferrous ions contained in groundwater under the following conditions by the method of the present invention. Concentration of ferrous iron in ground water (raw water): 21 mg / l Flow rate: 8 l / min Residence time in aeration tank: 43 minutes Blow-in air amount in aeration tank: 120 l / min NaOH addition amount: 16 mg / l (raw water) Residence Residence time in tank: 10 minutes

As a result, ferrous ions in ground water (raw water) were oxidized to less than 0.01 mg / l. The pH of the raw water at this time is 6.7, and the pH of the aeration-treated water is
Was 7.4 and the pH of the final treated water was 8.3.

[Comparative Example] Using the apparatus shown in FIG. 6, NaOH was directly added to the groundwater in the aeration tank 2 to oxidize ferrous ions contained in the groundwater under the following conditions. It was Ferrous ion concentration in raw water: 21 mg / l Flow rate: 8 l / min Residence time in aeration tank: 100 min Blow-in air amount in aeration tank: 120 l / min NaOH addition amount: 16 mg / l (raw water)

As a result, it took 100 minutes to oxidize the ferrous ion concentration in the raw water to less than 0.01 mg / l.

In the above-described Examples 1 and 2 and Comparative Example, when the treatment was performed while changing the addition amount of NaOH,
The residence time required for the ferrous ions to be oxidized to less than 0.01 mg / l was investigated, and the results are shown in FIG. As is clear from FIG. 5, Examples 1 and 2 of the present invention are
Compared with the comparative example, the residence time of groundwater in the tank was remarkably short, and the ferrous ions contained in the groundwater could be efficiently oxidized.

[0041]

Industrial Applicability As described above, according to the method of the present invention, ferrous ions contained in groundwater can be efficiently oxidized and removed in a short time, which is industrially useful. The effect is brought.

[Brief description of the drawings]

FIG. 1 is a schematic process drawing showing an embodiment of the method of the present invention.

FIG. 2 is a graph showing the relationship between the residence time of groundwater in the aeration tank and its pH value.

FIG. 3 is a graph showing the relationship between the addition rate of an alkaline agent (NaOH) to groundwater and its pH value.

[Figure 4] pH of groundwater when alkaline agent was added to the groundwater and allowed to stay in the retention tank after aeration treatment
It is a graph which shows the relationship between a value and the total residence time required to oxidize ferrous ions in groundwater to a predetermined value.

FIG. 5 is a graph showing the relationship between the amount of NaOH added and the residence time required for ferrous ions to be oxidized to a predetermined value in Examples and Comparative Examples.

FIG. 6 is a schematic process diagram showing a conventional method.

[Explanation of symbols]

 1 conduit 2 aeration tank 2a, 2b, 2c aeration chamber 3 blower 4 diffuser pipe 5a, 5b partition plate 6 alkaline agent storage tank 7 pump 8 mixer 9 retention tank 10 conduit 11 branch pipe

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshihide Kageyama 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd.

Claims (2)

[Claims] 1. A groundwater is aerated in an aeration tank to reduce free carbonic acid existing in the groundwater, thereby raising the pH value of the groundwater to a predetermined value, and then, the groundwater in which the free carbonic acid is reduced. An alkaline agent is added to the solution to further raise the pH value to a predetermined value, whereby the ferrous ions contained in the groundwater are efficiently oxidized in a short time. A method for oxidizing ferrous ions contained in the material. 2. The ground water is aerated to raise its pH value to the range of 7 to 7.5, and then the alkaline agent is added to further raise the pH value to 8 to 8.5. The method of claim 1, wherein the method is raised to a range.