JP2003326250A - Water treatment method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a water treatment method which enables the rising of pH of treated water subjected to the pH drop due to flocculation and separation using an acid-type inorganic flocculant conveniently without using chemicals. <P>SOLUTION: The flocculation and separation of water to be treated using the acid-type flocculant is performed, thereafter, decarbonation of the water treated by the flocculation and separation is performed, and thereby, the pH of the water treated by the flocculation and separation is made to rise. For example, an aluminium-base or iron-base flocculant 4 is charged into water to be treated, the flocculation and separation of the water to be treated is performed in a rapid agitation tank 2, a slow agitation tank 7 and a sedimentation tank 9, and thereafter, the treatment of aerating the air from a diffuser pipe 14 to the water treated by the flocculation and separation is performed in a clarifying tank 12 as the decarbonation. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a water treatment method in the field of water treatment for performing coagulation separation treatment, for example, in the fields of water supply, general industrial water, wastewater treatment, and the like. P by the coagulation separation treatment used
The present invention relates to a method for increasing the pH of treated water in which H is lowered.

[0002]

2. Description of the Related Art Acid-based inorganic coagulants, such as iron-based and aluminum-based coagulants, are widely used in the coagulation and separation treatment of water. Such an acid-based inorganic coagulant consumes the alkali content contained in the water to be treated and lowers the pH of the treated water.

The above-mentioned decrease in pH of the coagulation / separation-treated water is caused by
It may promote the corrosion of equipment such as pumps and pipes that send treated water, and may cause the leaching of harmful heavy metals such as lead from the equipment. Also, as a result of elution of harmful heavy metals, tap water may have a water quality that does not reach the water quality standard.

Therefore, conventionally, in order to raise the pH of the coagulated separation-treated water whose pH has decreased, a chemical agent (alkali agent) such as sodium hydroxide or calcium hydroxide is added to the coagulated separation-treated water. ing.

[0005]

However, as described above, when a chemical such as sodium hydroxide or calcium hydroxide is added to the coagulation / separation-treated water having a lowered pH to increase the pH, the consumption of the chemical is reduced. The large amount leads to an increase in processing cost.

The present invention has been made in view of the above-mentioned circumstances, and easily raises the pH of treated water whose pH has been lowered by the coagulation separation treatment using an acid-based inorganic coagulant without using a chemical. It aims at providing the water treatment method which can be performed.

[0007]

Means for Solving the Problems As a result of various studies to achieve the above-mentioned object, the present inventor has obtained the following findings. That is, the iron-based or aluminum-based coagulant used in the coagulation separation treatment shows acidity as a chemical solution,
In the agglutination reaction, hydroxides are produced by the following reactions, and the hydroxides are flocs. Al ³⁺ + 3OH ⁻ = Al (OH) ₃ (1) Fe ³⁺ + 3OH ⁻ = Fe (OH) ₃ (2)

The (OH) ⁻ consumed in the above reaction is supplied from bicarbonate ion HCO ₃ ⁻ contained in water. That is, HCO ₃ ⁻ releases (OH) ⁻ to generate carbon dioxide as described below. Released (OH) ^- is to react with aluminum ions, iron ions, acids, carbon dioxide C
This results in O ₂ remaining.

[0009]

[Chemical 1]

[0010] and acid components contained in the coagulant, also added acid with the addition of flocculant in order to emerge retain optimum condition for agglutination reaction, neutralization reaction (OH) ^- reacted with (3 ) Promote the reaction of the equation. Part of the generated carbon dioxide in water becomes carbonic acid by the following reaction, has a property as a weak acid, and lowers pH.

[0011]

[Chemical 2]

The above-mentioned carbon dioxide concentration in water is lowered by lowering the carbon dioxide concentration, and when the carbon dioxide concentration is lowered, the amount of H ⁺ which lowers the pH is also reduced. Therefore, if the carbon dioxide gas generated in the reaction of the equation (3) is removed, the reactions of the equations (4) and (5) shift from right to left, and the once lowered pH rises.

As a result of further investigation based on the above matters, the present inventor can increase the pH of the treated water whose pH has once decreased by the coagulation separation treatment by performing the decarbonation treatment of the coagulation separation treatment water. It is possible to effectively remove carbon dioxide gas when a treatment for aerating air is adopted as the decarbonation treatment. Therefore, as a method of increasing the pH of the treated water whose pH has been lowered by the coagulation separation treatment, decarbonation is It has been found that the treatment, particularly air aeration, is simple and effective.

The present inventor has also found that the dehalogenation treatment of the coagulation / separation-treated water as described above reduces the amount of trihalomethanes contained in the coagulation / separation-treated water.

The present invention has been made on the basis of the above-mentioned findings. After the coagulation / separation treatment of the water to be treated is carried out using an acid type inorganic coagulant, the coagulation / separation treatment water is subjected to a decarbonation treatment to coagulate. Provided is a water treatment method characterized by increasing the pH of separated treated water.

In the present invention, examples of the acid-based inorganic coagulant used in the coagulation separation treatment include aluminum-based coagulants such as polyaluminum chloride and aluminum sulfate, ferric chloride, ferrous sulfate and ferric sulfate. An iron-based coagulant such as iron can be used.

In the present invention, the decarbonation treatment of the coagulation / separation-treated water means the conversion of the coagulation / separation-treated water into carbon dioxide gas (carbon dioxide).
Is the process of removing. As the decarbonation treatment, a treatment of aerating the coagulation / separation-treated water with air can be preferably adopted, and thereby the pH of the coagulation / separation-treated water can be easily and effectively increased. In this case, the process of aerating the coagulation-separation-treated water with air refers to all the processes of bringing air into contact with the coagulation-separation-treated water. When air aeration is used as the decarbonation treatment, it is possible to control the pH rise of the coagulation / separation-treated water by the aeration amount and the aeration time.

Further, in the present invention, as shown in the embodiment described later, acid sludge can be used as a part or all of the acid-based inorganic coagulant, whereby the amount of coagulant used can be reduced. it can. Here, the acid sludge refers to a mixture of sludge generated by coagulation separation treatment and acid.

Further, in the present invention, as shown in the embodiment described later, when the coagulation separation treatment of the water to be treated is carried out using the acid type inorganic coagulant, an acid can be added to the water to be treated, As a result, the pH at the time of aggregation can be adjusted to the optimum pH for the aggregating agent used, and the quality of treated water can be improved.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the drawings.

(Embodiment 1) FIG. 1 is a flow chart showing an example of a water treatment apparatus used for implementing the present invention. In the apparatus of FIG. 1, 1 is a raw water inflow pipe, 2 is a rapid stirring tank, 3 is a rapid stirrer, 4 is a coagulant storage tank, 5 is a coagulant injection pipe,
6 is a flocculant injection pump, 7 is a slow stirring tank, 8 is a slow stirring machine, 9 is a sedimentation tank, 10 is a sludge sludge drainage pipe, 11 is a filtration tank, 12 is a water purification tank, 13 is an aeration blower, and 14 is Air diffuser,
Reference numeral 15 is an exhaust pipe, 16 is a water distribution pipe, and 17 is a water distribution pump.

In this apparatus, raw water and a flocculant (acid-based inorganic flocculant) are introduced into the rapid stirring tank 2 from the raw water inflow pipe 1 and the flocculant storage tank 4, respectively. Rapid mixing and stirring with the agent is performed. In the subsequent stage of the rapid stirring tank 2, a slow stirring tank 7 for accelerating the growth reaction of flocs generated by the agglutination reaction and a settling tank 9 for settling the flocs of agglomerates are installed.
Further, in the water purification treatment in waterworks in which the coagulation sedimentation treatment is widely performed, a filtration tank 11 for further improving the water quality is installed after the sedimentation tank 9. This filtration tank 11
The water purification tank 12 is installed in the latter stage, and water is further supplied to the demand destination by the water distribution pipe 16.

In this apparatus, the pH of the treated water lowered by the coagulating sedimentation treatment may be increased before the filtration treatment. However, when air aeration is performed as a means of increasing the pH before the filtration treatment, residual flocs that could not be completely removed by the precipitation treatment are destroyed, which lowers the capture efficiency in the filtration treatment and increases the pH. Since it is expected that the electric adhesion between the residual flocs and the filter material will be weakened and the trapping efficiency will be lowered, it is preferable to employ a means for increasing the pH after the filtration treatment. Therefore, in the present apparatus, the water purifying tank 12 is provided with an air diffuser 14 for performing air aeration by the aeration blower 13, and the pH of the treated water is increased by blowing air into the water in the water purifying tank 12 for aeration. ing. The blown air releases carbon dioxide gas to the air side by contacting with water, and this carbon dioxide gas is removed from the water and the pH of the water rises. The air containing carbon dioxide is discharged to the atmosphere through the exhaust pipe 15.

The first embodiment shows a case where an acidic coagulant having a low pH is used alone. As the coagulant, polyaluminum chloride or aluminum sulfate is used in the aluminum system, and iron chloride or iron sulfate is used in the iron system. These coagulants have strong acidity, resulting in a significant decrease in pH of the treated water, but by performing coagulation-precipitation treatment and further aeration by blowing air into the treated water in a water purification tank after the filtration treatment,
The pH of the treated water can be recovered to a pH slightly lower than that of the raw water.

(Embodiment 2) FIG. 2 is a flow chart showing another example of the water treatment apparatus used for implementing the present invention. In the apparatus of FIG. 2, 20 is an acid storage tank, 21 is an acid injection pipe,
22 is an acid injection pump. In this device, raw water inflow pipe 1
Acid is added from the acid storage tank 20 to the raw water flowing through the tank. Since the other parts of the device of FIG. 2 are the same as those of the device of FIG. 1, the same parts in FIG. 2 as those in FIG. 1 are designated by the same reference numerals and the description thereof will be omitted.

The second embodiment is provided with an acid injection facility for the water to be treated in order to make the pH of the water to be treated weakly acidic prior to the addition of the coagulant. That is, in the case of an aluminum-based coagulant such as polyaluminum chloride or a sulfuric acid band, it is possible to improve the quality of treated water by keeping the pH of the water to be treated at the time of coagulation slightly acidic. An acid is injected into the water to be treated to adjust the pH of the water to be treated to 6.0.
It is adjusted to about 7.2. In this example, since the acid is used, the pH after the coagulation treatment is likely to decrease,
By performing air aeration after the coagulation-sedimentation process,
The pH of the treated water can be restored.

(Embodiment 3) FIG. 3 is a flow chart showing still another example of the water treatment apparatus used for implementing the present invention. In the apparatus of FIG. 3, 30 is a sludge thickening tank, 31 is a sludge treatment drawing pipe, 32 is a sludge reuse drawing pipe, 33 is a sludge aeration tank, 34 is a sludge aeration blower, and 35 is a sludge diffuser pipe.
6 is a sludge injection pipe, 37 is a sludge injection pump, 38 is an acid storage tank, 39 is an acid injection pipe, 40 is an acid injection pump, 4
Reference numeral 1 denotes an acid sludge mixing / injection pipe. Since the other parts of the device of FIG. 3 are the same as those of the device of FIG. 1, the same parts in FIG. 3 as those in FIG. 1 are designated by the same reference numerals and the description thereof will be omitted.

The third embodiment is an example in which the amount of coagulant used is reduced by reusing sludge to improve the quality of treated water. By improving the quality of treated water and reducing the amount of coagulant,
This is an example of a process of suppressing the generation of aluminum hydroxide or iron hydroxide of the reaction formulas (1) and (2) from the aggregating agent.

In this apparatus, in order to reduce the coagulant,
Coagulated sediment sludge generated in the sedimentation tank 9 is settled sludge drainage pipe 1
From 0 to the sludge thickening tank 30, after performing sedimentation and concentration operation, sludge is drawn from the sludge reuse drawing pipe 32 to the sludge storage aeration tank 33, and air is aerated by the sludge diffuser pipe 35 to improve the spoilage sludge. To do. After that, sludge injection pipe 36
And acid injection pipe 39 are connected, and acid sludge mixing and injection pipe 4
In step 1, sludge and acid are mixed in a pipe to prepare acid sludge. As the acid, for example, sulfuric acid, hydrochloric acid or the like is used. The acid sludge mixing / injection pipe 41 is connected to the raw water inflow pipe 1, and the acid sludge is added to the raw water flowing in the raw water inflow pipe 1. afterwards,
The same processing as that of the first embodiment is performed.

As described above, in the third embodiment, the sludge generated by the coagulation-sedimentation treatment is used, and in order to reduce the amount of the coagulant used and improve the coagulation treatment, a sludge circulation line is provided and the sludge is It is intended to reuse the flocculant by adding an acid to and dissolving the hydroxide (aluminum hydroxide or iron hydroxide) produced by the flocculation reaction. In this example, since the acid is used to reuse the sludge, the pH of the treated water is lowered after the coagulation-sedimentation treatment due to the consumption of the alkali component of the coagulant used together with the used amount of the acid. In this case, since the acid is used, the pH after the coagulation treatment is likely to decrease, but the pH of the treated water can be recovered by performing aeration with air after the coagulation-precipitation treatment as in the first embodiment.

In the above embodiment, a method of blowing air into the treated water in the water tank is adopted as the contact means with the air for recovering the pH of the treated water, but the contact means is not limited to this. Instead, for example, a method of contacting the treated water with air in the packed tower, a method of contacting the treated water with air in the pipe, a method of contacting the treated water with air by stirring the surface of the treated water, etc. May be adopted. Further, when the filtration treatment is not required, the aeration of the treated water may be performed immediately after the precipitation treatment.

[0032]

[Example] Using the apparatus of the third embodiment, the pH rise result of the coagulation / separation treated water by aeration was examined. In this case, the pH of the treated water (raw water) in the coagulation separation treatment was about 8, and the pH of the coagulation separation treatment water was 7.05. The amount of air aerated in the water purification tank 12 was 1.56 L / min, and the water temperature was 12.8 ° C. The results are shown in Fig. 4. From FIG. 4, it can be seen that the pH of the treated water, which has been lowered to 7.05 by the coagulation sedimentation treatment, has recovered to 7.65 after 30 minutes of air aeration.

Using the apparatus of the third embodiment, the results of reducing the concentration of trihalomethane in the coagulated and separated treated water by air aeration were examined. In this case, the pH of the aggregated separation-treated water was 6.99 and the total trihalomethane concentration was 13 μg / L. Further, the air aeration rate in the water purification tank 12 is 0.84 L / min, and the water temperature is 2
It was 0.3 ° C. As a result, the pH before starting air aeration
The treated water having a concentration of 6.99 and a total trihalomethane concentration of 13 μg / L had a pH of 7.32 at an aeration time of 20 minutes, a pH of 13 μg / L at a total trihalomethane concentration, and a pH of 30 minutes at an aeration time.
7.48, total trihalomethane concentration 12 μg / L, pH 7.65 at aeration time of 40 minutes, total trihalomethane concentration 10
At ag / L and aeration time of 120 minutes, the pH was 8.11, and the total trihalomethane concentration was 5 μg / L. It was confirmed that the aeration of air reduced the trihalomethane concentration of the coagulated and separated treated water.

[0034]

As described above, according to the water treatment method of the present invention, the pH of the treated water whose pH has been lowered by the coagulation separation treatment using the acid type inorganic coagulant can be easily carried out without using any chemical. Can be raised. Therefore, according to the present invention, it is possible to eliminate the need to add a chemical agent (alkali agent) to the coagulation / separation-treated water or to reduce the amount of the chemical agent added.

[Brief description of drawings]

FIG. 1 is a flow chart showing an example of a water treatment device used for carrying out the present invention.

FIG. 2 is a flowchart showing another example of the water treatment device used for implementing the present invention.

FIG. 3 is a flowchart showing still another example of the water treatment device used for carrying out the present invention.

FIG. 4 is a graph showing an example of a pH increase result of coagulation separation treatment water by aeration with air.

[Explanation of symbols]

1 Raw water inflow pipe 2 Rapid stirring tank 4 Flocculant storage tank 7 Slow stirring tank 9 settling tank 10 Settled sludge drainage pipe 11 filtration tank 12 water purification tank 14 Air diffuser 20 Acid storage tank 30 sludge thickening tank 33 Sludge storage aeration tank 36 Sludge injection piping 38 Acid Storage Tank 41 Acid sludge mixing and injection piping

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Akihiro Wakasugi             1-5-20 Kinosaki-cho, Oita-shi, Oita Oita             Municipal Waterworks Bureau (72) Inventor Keisuke Muratani             1-5-20 Kinosaki-cho, Oita-shi, Oita Oita             Municipal Waterworks Bureau (72) Inventor, Takeichiro Takahashi             1-5-20 Kinosaki-cho, Oita-shi, Oita Oita             Municipal Waterworks Bureau (72) Inventor Toru Sekiya             Olga 1-2-8 Shinsuna, Koto-ku, Tokyo             Within the corporation F-term (reference) 4D011 AA15 AD03                 4D015 BA15 BA23 BB05 CA14 DA04                       DA05 DA13 DA15 DC02 EA07                       EA13 EA16 EA32 FA02 FA25                 4D037 AA01 AA11 AB11 BA23 BB05                       CA06 CA08

Claims (4)

[Claims] 1. A method of performing coagulation separation treatment of water to be treated using an acid-based inorganic coagulant, and then performing decarboxylation treatment of the coagulation separation treatment water to raise the pH of the coagulation separation treatment water. Water treatment method. 2. The water treatment method according to claim 1, wherein the decarbonation treatment of the coagulation-separation-treated water is a treatment of aerating the coagulation-separation-treated water with air. 3. The water treatment method according to claim 1, wherein a part or all of the acid-based inorganic coagulant is acid sludge. 4. The acid is added to the water to be treated when the coagulation separation treatment of the water to be treated is carried out using an acid-based inorganic coagulant. Water treatment method.