JP3227765B2 - Membrane separation device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a membrane separation apparatus, and more particularly to a reverse osmosis membrane (RO) which is effective for producing pure water or ultrapure water from tap water, fresh water or tap water obtained by purifying the same.
And a membrane separation device using the same.

[0002]

2. Description of the Related Art Conventionally, as a method for desalinating seawater, a two-stage RO in which an RO membrane separation device is provided in two stages and treated is known. In this method, the permeated water of the first-stage RO membrane is pressurized by a high-pressure pump to be used as raw water (water supply) of the second-stage RO membrane, so that a lower-concentration permeated water can be obtained.
In the desalination treatment of can water, high-purity permeated water can be obtained by using such a method. However, this method requires two high-pressure pumps, so that the cost of fresh water is high. There are drawbacks.

On the other hand, apart from the above method, R provided in two stages
Two-stage RO that supplies water to O membrane separation device with one high pressure pump
Has been put to practical use in fields such as ultrapure water production. This method uses a low-pressure RO membrane separator having an operation pressure of about 15 kgf / cm ² , and supplies permeated water from the first-stage RO membrane separator directly to the second-stage RO membrane separator. Therefore, the inlet pressure of the first stage RO membrane separation device is approximately 30
kgf / cm ² , the permeated water pressure of the first stage RO membrane separator, ie, the feed water pressure of the second stage RO membrane separator is 15 kgf / cm ² .
It is designed and operated to be kgf / cm ² .

[0004] If fresh water, for example, tap water, is treated in the latter two-stage RO, high-purity pure water is obtained, so that ultrapure water can be produced only with the RO membrane separation apparatus. Therefore, in the production of ultrapure water, if this two-stage RO is used, the load of salts on the ion exchange resin can be reduced, and continuous processing can be performed. That is, discontinuous treatment for regenerating the ion exchange resin can be significantly reduced.

[0005] By the way, ultrapure water is used in large quantities in semiconductor manufacturing plants. However, with the improvement of the degree of integration of semiconductors, it is presently required to purify ultrapure water of higher purity. In the above-mentioned two-stage RO, when the pH of raw water is as low as 6 to 7, In addition, the dissolved carbon gas cannot be removed from the RO membrane, and the quality of the permeated water in the second-stage RO membrane separation apparatus cannot be improved.

Therefore, conventionally, in the production of ultrapure water by a two-stage RO, caustic soda (NaOH) is added to the permeated water of the two-stage RO, that is, the permeated water of the first-stage RO membrane separation apparatus to adjust the pH to an alkaline level. There has been proposed a method of operating with the temperature raised to the side (pH 8 to 9). In this method, the dissociation of carbonic acid in the feed water of the second stage RO membrane separation apparatus is increased as much as possible by adding caustic soda in order to remove carbonic acid by the RO membrane as shown in the following equation.

[0007]

Embedded image

[0008] By increasing the removal of the dissociated carbonic acid as described above, it is possible to reduce the load of the non-regeneration type ion exchange resin installed after the two-stage RO.

However, in the middle of the two-stage RO, NaOH
Is to increase the salt concentration of the RO permeated water once desalinated in the first stage RO membrane separation device,
It has been difficult to further purify the permeated water of the RO membrane separator at the stage.

[0010] In order to control the pH of the feed water of the second-stage RO membrane separator, the pH of the first-stage RO membrane separator is increased by adding NaOH to the second-stage RO membrane separator. The carbonic acid removal efficiency can be increased without supplying an excessive salt to the RO membrane separation device.

The arrangement of the two-stage RO in this case is as shown in FIG.

In FIG. 1, reference numeral 1 denotes a first-stage RO membrane separation apparatus;
Reference numeral 2 denotes a second-stage RO membrane separation apparatus. Raw water is piping 11
In the process of being introduced into the first-stage RO membrane separation apparatus 1, alkali is added from the pipe 12, and the raw water whose pH has been adjusted by the addition of the alkali is supplied to the first-stage RO membrane separation apparatus 1, The permeated water is supplied from the pipe 13 to the second stage RO membrane separation device 2. This second stage RO membrane separation apparatus 2
Is discharged out of the system from the pipe 14. The concentrated water in the first-stage RO membrane separation device 1 and the second-stage RO membrane separation device 2 is discharged from pipes 15 and 16, respectively.

[0013]

By the way, the conventional RO
The membrane has a correlation between the salt concentration of the feed water and the removal rate.
As shown in the figure, the feedwater was low in salt concentration (5 ppm as Na
Cl), high salt concentration (100-1500 ppm)
(NaCl) as compared with the case of (NaCl).

On the other hand, as shown in FIG. 1, in a two-stage RO in which the pH is adjusted by adding an alkali to the feed water of the first-stage RO membrane separation device, the salt concentration of the feed water of the second-stage RO membrane separation device is adjusted. Is 5 ppm as NaCl or less and a low salt concentration.
That is, when no alkali is added to the raw water (water supplied to the first-stage RO membrane separation device), carbonic acid remains in the permeated water of the first-stage RO membrane separation device. Although the salt concentration of the feedwater of the membrane separation device is not lower than 5 ppm as NaCl, in the two-stage RO in which the alkali is added to the raw water to adjust the pH, as described above, the carbonic acid in the raw water is dissociated, and this is the first stage. Since the water is removed by the first RO membrane separator, the permeated water of the first stage RO membrane separator has a low salt concentration.

For this reason, in the case of using the conventional RO membrane, the salt removal rate is low in the second-stage RO membrane separation apparatus, and as a result, the quality of the permeated water of the second-stage RO membrane separation apparatus is reduced. Is poor, for example, city water (conductivity 180-
200 μs / cm), the resistivity is 2 to
There is a disadvantage that only about 3 MΩ · cm of treated water can be obtained.

The present invention solves the above-mentioned conventional problems, and in a two-stage RO in which alkali is added to raw water to adjust the pH to the alkali side, the salt removal rate in the second-stage RO membrane separation apparatus is increased. It is an object of the present invention to provide a membrane separation device capable of obtaining high-quality treated water.

[0017]

The membrane separation device of the present invention comprises:
A means for adjusting the pH to 7.5 to 9 by adding an alkali to the raw water, and a first means for passing the raw water adjusted to a pH of 7.5 to 9
In a membrane separation device comprising a reverse osmosis membrane separation device of the first stage and a second stage reverse osmosis membrane separation device for passing the permeated water of the reverse osmosis membrane separation device of the first stage , Add the acid
a means for adjusting the pH to 4-5, and a means for adjusting the pH to 4-5
Decarboxylation means for decarboxylation of raw water
It is provided in front of means for pH adjustment Te, the second-stage reverse osmosis membrane separation device, characterized in that it is a reverse osmosis membrane separation device having a high salt rejection at low salt concentrations.

Hereinafter, the present invention will be described in detail.

[0019] The membrane separation apparatus of the present invention comprises adding an acid to raw water.
After adjusting the pH to 4 to 5 by decarboxylation,
To a pH of 7.5 to 9 and two-stage RO treatment.
When the RO membrane of the second stage RO membrane separation device is used, an RO membrane with a low salt concentration (5 ppm as NaCl or less) and a high salt removal rate is used, and the permeated water of the second stage RO membrane separation device is used. Improve water quality.

As such an RO film, for example, an RO film of SU900 series manufactured by Toray Industries, Inc. or an RO film of NTR700 series manufactured by Nitto Denko Corporation can be used. For example, as shown in FIG. 2, these RO membranes have a high salt removal rate with respect to water supplied with a low salt concentration, and the salt removal rate decreases as the salt concentration of the supplied water increases.

The membrane of the present invention is generally used except that an RO membrane exhibiting a high salt removal rate with respect to the low-salt-concentration water supply is used as the RO membrane of the second-stage RO membrane separation apparatus. In the same manner as the two-stage RO.

In the membrane separation apparatus of the present invention, the alkali added for adjusting the pH of the raw water is NaO.
H or the like can be used, the addition of such an alkali, raw water, you adjust pH7.5~ 9.

In addition, before or after the addition of the alkali, by adding a scale dispersing agent to the feed water of the first-stage RO membrane separation device, the flux of the first-stage RO membrane separation device is reduced. This is preferable because the operation can be prevented and the operation can be stably continued for a long time.

[0024]

As described above, in the two-stage RO in which the alkali is added to the raw water to adjust the pH to the alkaline side, the salt concentration of the permeated water in the first stage is extremely low. In a conventional RO membrane using a conventional RO membrane, in which the salt removal rate is high in a place where the salt concentration is high and the salt removal rate is low when the salt concentration in the feedwater is low, sufficient salt removal is required in the second stage RO membrane separation apparatus. It cannot be removed.

On the other hand, in the present invention, the second stage R
For example, as shown in Table 1 below, as the RO membrane of the O membrane separation device, the one having a high salt removal rate in a place where the salt concentration of the feed water is low, the permeated water of the first stage RO membrane separation device is used. Thus, in the second stage RO membrane separation apparatus through which low salt concentration feedwater is passed, it is possible to perform the treatment at a high salt removal rate.

[0026]

[Table 1]

Incidentally, conventionally, the conductivity is 180 to
When city water of 200 μs / cm is treated, the second stage R
The water quality of the permeated water of the O membrane separator is 2-3MΩ · cm
On the other hand, according to the present invention, the resistivity is 5 MΩ · cm.
The above-mentioned high quality treated water can be obtained.

[0028]

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

Example 1 An acid was added to city water (conductivity: 180 to 200 μs / cm) to adjust the pH to 4 to 5, and carbon dioxide was removed to 8 ppm or less by a decarbonation tower, and then a scale dispersant (acrylamide polymer) was used. ) Was added at 10 ppm, and then an alkali agent (Na) was added.
OH) to adjust the pH to 8-9, followed by two-stage RO treatment. That is, a pressure of 25 kgf / cm was applied to the first stage RO membrane separation apparatus.
After feeding in ^2, and supplies the permeate to RO membrane separation apparatus of the second stage, by extracting the permeate as treated water.

The first-stage RO membrane separation apparatus has an RO membrane having a salt removal rate of 99.0% (as NaCl) with respect to the feed water having a salt concentration of 1500 ppm as NaCl (“NTR759HR” manufactured by Nitto Denko Corporation). And the second-stage RO membrane separation apparatus has a salt concentration of 1 ppm as Na
98.0% of salt removal rate (as Na
Cl) (RO film No. 2 in Table 1 above) (“SU900” manufactured by Toray Industries, Inc.). As a result, the resistivity of the obtained treated water (the permeated water of the second-stage RO membrane separation device) was remarkably high at 5 to 6 MΩ · cm.

Comparative Example 1 In Example 1, the same RO film as used in the first-stage RO membrane separation apparatus was used as the RO film of the second-stage RO membrane separation apparatus.
O membrane, that is, a salt removal rate of 99.0% (as NaCl) with respect to a water supply of 1500 ppm asNaCl.
However, except for using an RO membrane having a salt removal rate of 95% (as NaCl) with respect to feed water having a salt concentration of 1 ppm as NaCl, a two-stage RO treatment was performed in the same manner.

As a result, the obtained treated water (R in the second stage)
O 2 membrane separator) has a resistivity of 2-3 MΩ · cm
And the water quality was inferior.

[0033]

As described above, according to the membrane separation apparatus of the present invention, the pH is adjusted to 4 to 5 by adding an acid to the raw water .
Thereafter, decarboxylation treatment is performed, and then alkali is added to adjust the pH to 7.0 .
In the case of performing the second-stage RO treatment after adjusting to 5 to 9 , it is possible to increase the salt removal rate in the second-stage RO membrane separation apparatus and obtain high-quality treated water.

[0034] The membrane separation apparatus of the present invention has the following features.
It is industrially extremely useful as an ultrapure water production apparatus using tap water, fresh water or tap water obtained by purifying the same as raw water.

[Brief description of the drawings]

FIG. 1 is a system diagram showing a two-stage RO process.

FIG. 2 shows a second stage RO in the membrane separation apparatus of the present invention.
It is a graph which shows the salt removal rate with respect to the salt concentration of feed water of RO membrane used for a membrane separation apparatus.

FIG. 3 is a graph showing a salt removal rate with respect to a salt concentration of feed water of an RO membrane used in a second-stage RO membrane separation apparatus in a conventional membrane separation apparatus.

[Explanation of symbols]

 1 First-stage RO membrane separation device 2 Second-stage RO membrane separation device

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) B01D 61/00-65/10 C02F 1/44

Claims (1)

(57) [Claims] 1. An alkali is added to raw water to adjust pH.7.5-
To 9Adjusting means and pH7.5 to 9Regulated raw water
First-stage reverse osmosis membrane separation device through which water is passed;
Second stage reverse immersion through permeated water of reverse osmosis membrane separation device
A membrane separation device comprising a permeable membrane separation device,Means for adjusting the pH to 4 to 5 by adding an acid to the raw water;
Decarbonation means for decarbonating raw water adjusted to 4 to 5 is before
Provided before the means for adding the alkali to adjust the pH.
And  The second-stage reverse osmosis membrane separation device has a low salt concentration.
It is a reverse osmosis membrane separator with high salt removal rate
Membrane separation device.