JPH0839066A - Treatment of water - Google Patents

Abstract

PURPOSE:To markedly enhance the quality of treated water, in treating raw water by subjecting raw water to decarbonation treatment by adding an acid thereto and passing the treated water through reverse osmosis membrane separators arranged in two stages, by adding alkali to the water supplied to the reverse osmosis membrane separators to stagnate the water under an atmosphere cut-off condition. CONSTITUTION:At first, raw water obtained by applying pollutant and chlorine removing pretreatment to city water or recovered water is introduced into a decarbonation column 1 and subjected to decarbonation treatment by an acid added from piping 12. Next, the water is passed through a first RO membrane separator 2 from piping 13 equipped with a pump P and conc. water is drawn out of piping 14 while transmitted water drawn out of piping 15 is introduced into a stagnation tank 3 while alkali is added to the transmitted water to be stagnated for a predetermined time. Thereafter, the water is passed through a second RO membrane separator 4 and transmitted water is drawn out of piping 19 as treated water. As an acid to be added, sulfuric acid or hydrochloric acid is pref. and the addition amt. thereof is controlled so as to adjust the pH of water introduced into the decarbonation tower 1 to about 5.0-6.0.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating water, and in particular, after deoxidizing raw water by adding an acid, the water is sequentially passed through a reverse osmosis membrane (RO membrane) separation device arranged in two stages. The present invention relates to a method for treating treated water to improve the quality of treated water.

[0002]

2. Description of the Related Art Conventionally, as a method for producing pure water from city water, well water, industrial water, recovered water, and the like, after pretreatment (decontamination, dechlorination) of such water, acid is added to remove it. There is a method in which decarbonation treatment is carried out in a carbonic acid tower, decarbonation-treated water is successively passed through an RO membrane separation device arranged in two stages, and further RO membrane separation-treated water is treated by an ion exchange device.

Further, in the treatment by the RO membrane separators arranged in two stages as described above, in order to improve the quality of treated water, a hydroxylation is performed between the RO membrane separator in the former stage and the RO membrane separator in the latter stage. A method has been proposed in which sodium (NaOH) is injected to convert carbon dioxide gas in the permeate of the RO membrane separator in the former stage into hydrogen carbonate ions and to remove it in the RO membrane separator in the latter stage (Japanese Patent Laid-Open No. 61-4591). Issue).

[0004]

In the treatment using such a two-stage RO membrane separator, it is desired that the water quality of the permeate of the latter RO membrane separator has a resistivity of 2 MΩ · cm or more. The water quality of the permeated water of the RO membrane separator in the latter stage obtained by the conventional method has a resistivity of about 0.5 to 1 MΩ · cm, which is about ½ to ¼ of the expected value.

When the quality of the permeated water of the RO membrane separator in the latter stage is low as described above, the load of the ion exchange device in the subsequent step becomes large, and particularly when the non-regeneration type ion exchange device in the field is used, The replacement frequency becomes high, which is not preferable from the viewpoint of work and cost.

The present invention solves the above-mentioned problems of the prior art, and after adding acid to raw water for decarbonation, RO arranged in two stages
An object of the present invention is to improve the quality of treated water obtained in a method for treating water that is sequentially passed through a membrane separation device.

[0007]

The method of treating water according to the present invention is to treat water by adding acid to raw water to decarboxylate it and then sequentially passing the water through a reverse osmosis membrane separation device arranged in two stages. In the method described above, after adding alkali to the water supplied to the reverse osmosis membrane separation device in the first stage or the second stage and allowing the water to stay under the air-shielded condition,
Water is passed through the reverse osmosis membrane separation device.

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

1 and 2 are system diagrams showing an embodiment of a method for treating water according to the present invention.
1 is a decarbonation tower, 2 is an RO membrane separator at the previous stage (hereinafter referred to as “first
It is referred to as "RO membrane separation device". ) 3 is a retention tank, 4 is an RO membrane separation device in the latter stage (hereinafter referred to as "second RO membrane separation device"), and each reference numeral 11 to 19 represents piping. P is a pump.

In the method shown in FIG. 1, the first RO membrane separation device 2 is used.
Between the second RO membrane separator 4 and the second RO membrane separator, and retains the same. City water, industrial water, well water, recovered water, etc. may be subjected to pretreatment such as turbidity removal and chlorine removal as necessary. The raw water obtained by the treatment is first introduced into the decarbonation tower 1 through the pipe 11,
Decarboxylation treatment is carried out in the presence of the acid added from 2. In addition,
The acid added here is preferably sulfuric acid, hydrochloric acid or the like, and the addition amount thereof is preferably such that the pH of the water introduced into the decarbonation tower 1 is about 5.0 to 6.0. .

The water discharged from the decarbonation tower 1 is then pumped to the pump P.
Water is passed to the first RO membrane separation device 2 through a pipe 13 provided with. The concentrated water of the first RO membrane separation device 2 is extracted from the pipe 14, while the permeated water extracted from the pipe 15 is introduced into the retention tank 3 after adding alkali from the pipe 16 and retained in this tank for a predetermined time. Let Then, the water is passed through the pipe 17 to the second RO membrane separation device 4, and the permeated water is extracted through the pipe 19 as treated water. On the other hand, concentrated water is pipe 1
Pull out from 8.

According to this method, the first RO membrane separation device 2
The permeated water of the second RO is introduced into the second RO membrane separation device 4 after the carbonic acid component contained in the permeated water is sufficiently converted into hydrogen carbonate ions while staying in the retention tank 3 after addition of the alkali. By removing the hydrogencarbonate ions in the membrane separation device 4, treated water of extremely high quality can be obtained.

In the method shown in FIG. 1, the retention tank 3 is isolated from the outside air and is maintained under an atmospheric shutoff condition. As the retention tank 3, one having pressure resistance capable of maintaining the permeated water pressure (outlet pressure) from the first RO membrane separation device 2, that is, the inlet pressure of the second RO membrane separation device 4 is used. However, when a separate pump is provided between the second RO membrane separation device 4 and the retention tank 3 in addition to the inlet of the first RO membrane separation device 2, the retention tank 3 is required to have the above pressure resistance. do not do. Therefore, it is also possible to use a retention tank having a predetermined capacity, which is sealed from the outside air by providing a nitrogen seal or the like.

The method shown in FIG. 2 is applied to the first RO membrane separation device 2
The retention tank 3 is arranged in the preceding stage, and the outflow water of the decarbonation tower 1 stays in the retention tank 3 for a predetermined time after the alkali is added from the pipe 16, and then the pipe 17 and the first RO membrane separation Device 2, pipe 15, second RO membrane separation device 4
After being processed, the treated water is taken out from the pipe 19.
Also in this method, the effluent water of the decarbonation tower 1 is added to the alkali and, after it is retained in the retention tank 3, the carbonic acid components contained therein are sufficiently converted into hydrogen carbonate ions, and then the first R
Since it is introduced into the O membrane separator 2, the hydrogen carbonate ions are efficiently removed in the first RO membrane separator 2,
Sufficiently high quality treated water is obtained.

The method shown in FIGS. 1 and 2 is an embodiment method of the present invention, and the present invention is not limited to the illustrated method as long as the gist thereof is not exceeded.

For example, in the method shown in FIGS. 1 and 2, the alkali can be added directly to the retention tank 3.

When the alkali is added before the first RO membrane separator, the alkali is added to the lower water tank 1 of the decarbonation tower 1.
It is also possible to add to A. In this case, the amount of water held in the lower water tank 1A may be adjusted to an appropriate amount, the outside air may be shut off by floating balls on the water surface, and alkali may be injected into the lower water tank 1A. In this case, the retention tank Need not be provided separately.

Further, the carbon dioxide gas converted in the decarbonation tower may be exhausted with nitrogen gas instead of the air normally used to drive it out of water to shut off the outside air.

In the present invention, the alkali is NaO.
H or the like can be used, and the addition amount thereof is preferably such that the pH of the water after addition of the alkali is about 7.8 to 8.8. The residence time after addition of alkali is 1
0 seconds or more, preferably 10 seconds to 3 minutes, more preferably 1
It is preferable that the time is about 2 seconds to 1 minute.

[0020]

The present inventors investigated the water quality of the permeated water of the RO membrane separator in the latter stage in order to elucidate the reason why sufficient water quality of the treated water was not achieved in the conventional method. It was found to contain a carbonic acid component.

From the above, the inventors of the present invention, in the conventional method, inject NaOH into the permeate of the RO membrane separator of the preceding stage, but immediately pass this water to the RO membrane separator of the latter stage. It is assumed that the RO membrane separation treatment is performed before the carbonic acid components in the water are sufficiently converted to hydrogen carbonate ions, and as a result, the concentration of carbonic acid components in the resulting treated water is high and the water quality is low, which is sufficient. In order to secure the reaction time, the method of the present invention in which the reaction is carried out under an air-shielded condition was found.

In the present invention, the alkali-added water is allowed to stay for a predetermined period of time so that the carbonic acid component in the water is sufficiently converted to hydrogencarbonate ion by the alkali, and then the RO membrane separation treatment is performed, so that the carbonic acid component concentration is remarkably increased. It is possible to obtain low-quality treated water.

Moreover, since this retention is performed under the condition that the atmosphere is shut off and is isolated from the outside air, it is possible to prevent the mixture of carbonic acid components from the atmosphere, and yet to maintain the RO membrane separation treatment water pressure sufficiently. You can do it in the state you did.

According to the method of the present invention, the resistivity of 0.
It is possible to obtain high-quality treated water having a resistivity of 2 to 3 MΩ · cm or higher in the two-stage RO membrane separation treatment after decarboxylation treatment that could only obtain water quality of about 5 to 1 MΩ · cm. is there.

[0025]

EXAMPLES The present invention will be described more specifically with reference to Examples and Comparative Examples below.

Example 1 Water was treated according to the method shown in FIG. That is, sulfuric acid was added to the water of Atsugi City that had been subjected to activated carbon filtration as a pretreatment to adjust the pH to 5.5, and decarboxylation treatment was performed in the decarboxylation tower 1.
The decarbonated water had a pH of 6.0. This water was then supplied to the first RO membrane separator 2 (operating pressure 30 kg /
cm ² ). The permeated water of this first RO membrane separator is NaOH
Was added, the pH was adjusted, and this water was sent to a retention tank 3 isolated from the outside air, where it was retained for 1 minute, and then the second RO
Water was passed through the membrane separator 4. The first RO membrane separation device 2
For both the second RO membrane separation device 4 and the RO membrane separation device, "NTR-759-HR" manufactured by Nitto Denko Corporation was used.

P of feed water (inflow water) of the second RO membrane separator
The relationship between H and the resistivity of the obtained treated water (permeate of the second RO membrane separator) is shown in FIG.

COMPARATIVE EXAMPLE 1 No permeated water of the first RO membrane separator was replaced with N without a retention tank.
The same treatment as in Example 1 was carried out except that water was passed through the second RO membrane separator immediately after the addition of aOH.

P of feed water (inflow water) of the second RO membrane separator
The relationship between H and the resistivity of the obtained treated water (permeate of the second RO membrane separator) is shown in FIG.

From FIG. 3, after the addition of NaOH, the mixture is allowed to stay for a predetermined time under an atmosphere-shielded condition and then passed through an RO membrane separation device. According to the method of the present invention, the NaOH membrane is immediately passed through after the addition of NaOH. It is clear that a resistivity as high as about 3 times is obtained as compared with the case of using water and the quality of treated water is significantly improved.

[0031]

As described above in detail, according to the water treatment method of the present invention, acid is added to the raw water for decarbonation, and then the water is sequentially passed through the RO membrane separators arranged in two stages. In the treatment method, the quality of the treated water obtained can be made extremely high, the load on the subsequent steps can be reduced, and efficient treatment can be performed.

[Brief description of drawings]

FIG. 1 is a system diagram illustrating a method of an embodiment of a water treatment method of the present invention.

FIG. 2 is a system diagram illustrating another embodiment method of the water treatment method of the present invention.

FIG. 3 is a graph showing the measurement results of Example 1 and Comparative Example 1.

[Explanation of symbols]

 1 Decarbonation Tower 2 First RO Membrane Separation Device 3 Retention Tank 4 Second RO Membrane Separation Device

Claims (1)

[Claims] 1. A method of treating water by adding acid to raw water for decarboxylation and then sequentially passing water to a reverse osmosis membrane separation device arranged in two stages to treat water. A method for treating water, characterized in that an alkali is added to the water to be supplied to the reactor, the mixture is allowed to stay under an air cutoff condition, and then the water is passed through the reverse osmosis membrane separation device.