JPH091141A - Method for operating reverse osmosis membrane device - Google Patents

Abstract

PURPOSE: To make it possible to easily restore the permeated flow flux of a reverse osmosis membrane by interposing a washing operation for washing the reverse osmosis membrane with washing water of a specific pH value during the course of water flow operation in a method for treating the water which is to be treated and contains a hardness component and silica with a reverse osmosis membrane device. CONSTITUTION: The water 1 to be treated is subjected to the reverse osmosis membrane treatment in a fore-stage reverse osmosis membrane device 2 and is separated to fore-stage concd. water and fore-stage permeated water. Further, the fore-stage concd. water is supplied to a post-stage reverse osmosis membrane 4 and is separated to the post-stage concd. water and the post-stage permeated water. At this time, the pH of the post-stage concd. water is detected by a pH sensor 10 and the detected signal is fed back to an acid supplying means 26 which controls the pH to <=6. The washing water is supplied to a water-to-be-treated supplying pipe 6 and the fore-stage concd. water pipe 8 and an alkaline is supplied to a washing water line 32 by an alkali supplying means 34 at the time of the washing operation in such reverse osmosis membrane device. Both are mixed by a line mixer 36 and is so adjusted that the pH attains >=9 on the inlet side of the device.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for operating a reverse osmosis membrane apparatus for producing desalinated water or the like by using recovered water containing industrial water, city water, biologically treated water, or mixed water thereof as raw water. .

[0002]

2. Description of the Related Art Conventionally, there is a reverse osmosis membrane device as a device useful for producing demineralized water and the like. The reverse osmosis membrane device separates salts with the reverse osmosis membrane, and separates the water to be treated into concentrated water having a high salt concentration and permeated water having a low salt concentration by being desalted. In this reverse osmosis membrane device, if the reverse osmosis membrane device is operated in a state where the recovery rate (ratio of permeated water to treated water) is increased, the salt concentration in the concentrated water also becomes high, and the salt concentration exceeds the solubility. In some cases, the salts precipitate and clog the reverse osmosis membrane, reducing the flux of permeate. For this reason, when setting the recovery rate, it is necessary to pay attention to the hardness components such as calcium that are likely to precipitate, and particularly the concentrations of silica, and to set these concentrations so as not to exceed the solubility.

Conventionally, there have been the following methods for operating a reverse osmosis membrane device in consideration of preventing the precipitation of silica. (A) The reverse osmosis membrane device is operated while the recovery rate of the reverse osmosis membrane device is sufficiently reduced so that the concentration of silica in the concentrated water does not exceed its solubility. (B) A reverse osmosis membrane device is operated by adding a chemical (dispersant) for preventing silica precipitation to the water to be treated. (C) The pH of the water to be treated after softening the water to be treated in advance
Is adjusted to 8 or more to increase the solubility of silica, and the reverse osmosis membrane device equipped with the alkali-resistant reverse osmosis membrane is operated. In this case, the operation is stopped before the permeation flux of the reverse osmosis membrane is reduced to, for example, about 80% at the start of use, and the reverse flow is performed with washing water having a pH higher than that at the time of desalination. A regeneration method has been adopted in which the permeation membrane is washed to remove silica deposits. However, although the method (a) has relatively stable operation performance and desalination performance, it has a problem of low recovery rate. The method (b) has the advantage that the recovery rate can be increased, but it is necessary to add a dispersant depending on the amount of silica or hardness component present in the concentrated water, and the cost required for the dispersant is considerable. However, there is a problem that the water quality is deteriorated by adding a dispersant. or,
Although the method (c) can also increase the recovery rate, it has a problem that it is indispensable to provide a softening step in which the hardness component that precipitates due to alkalinity is removed in advance before the reverse osmosis membrane device.

[0004]

The present inventors have proposed a reverse osmosis membrane treatment method in which the pH of concentrated water is 6 or less when the water to be treated containing at least the hardness component and silica is subjected to the reverse osmosis membrane treatment. , Which was proposed prior to the present invention (Japanese Patent Application No. 5-31
5708). According to this method, as an example, the concentration of silica in the concentrated water corresponds to about 4 to 5 times its solubility.
00-500 mg / L (as SiO ₂ ) (The solubility of silica is about 100 mgas SiO ₂ at a water temperature of 25 ° C.)
/ L) can be performed for at least 2 months, and in this case, the permeate flux of the reverse osmosis membrane can be stably maintained at 85% or more of the permeate flux at the start of use. I confirmed that I was able to drive.

[0005] After that, when the present inventors subject the reverse osmosis membrane used for the operation of the reverse osmosis membrane device for the above two months to chemical cleaning, the following findings were obtained. That is, (1) the alkaline cleaning is overwhelmingly more effective than the acid cleaning for recovering the permeation flux of the permeated water of the reverse osmosis membrane. (2) Silica is a major component of the membrane fouling component that slowly decreases the permeation flux of the permeate of the reverse osmosis membrane over time under the operating conditions of the reverse osmosis membrane device in which the pH of the concentrated water is 6 or less. Hardness component is very small. (3) The degree of recovery of the permeation flux of the reverse osmosis membrane by acid cleaning is almost constant irrespective of the length of the cleaning time. It takes a long time depending on the degree.

From the above findings, in the reverse osmosis membrane treatment method for adjusting the pH of the concentrated water to 6 or less as proposed above, the alkali washing is particularly effective in recovering the permeation flux of the reverse osmosis membrane. Then, they became aware that the reverse osmosis membrane can be regenerated, and completed the present invention.

Therefore, the object of the present invention is to very easily recover the permeation flux of the reverse osmosis membrane, and further to improve the life of the reverse osmosis membrane. As a result, a high degree of operational stability can be secured for a long period of time. It is to provide a method of operating a reverse osmosis membrane device which can be performed.

[0008]

In order to achieve the above object, the present invention provides a method for separating water to be treated containing at least a hardness component and silica into permeated water and concentrated water by a reverse osmosis membrane device, In a method for operating a reverse osmosis membrane device, which performs a water-passing operation of a reverse osmosis membrane device while keeping the pH of concentrated water at 6 or less, the reverse osmosis membrane attached to the reverse osmosis membrane device is adjusted to pH 9 during the water passing operation. A method for operating a reverse osmosis membrane device, characterized by interposing the above-mentioned washing operation with washing water, including that the concentration of silica in the concentrated water is not less than the solubility of silica.

Further, in the present invention, water to be treated containing at least a hardness component and silica is separated into permeated water and concentrated water by a front-stage reverse osmosis membrane device, and the obtained concentrated water of the front-stage reverse osmosis membrane device is reverse-staged. The separation of the permeated water and the concentrated water by the osmosis membrane device is repeated, and the pH of the concentrated water in the last stage reverse osmosis membrane device is set to 6
In a method of operating a reverse osmosis membrane device for carrying out a water passage operation of a plurality of reverse osmosis membrane devices connected in series, the reverse osmosis attached to the last stage reverse osmosis membrane device during the water passage operation. A method for operating a reverse osmosis membrane device, characterized in that a washing operation of washing the membrane with washing water having a pH of 9 or more is interposed.
It includes that the concentration of silica in the concentrated water of the last stage reverse osmosis membrane device is equal to or higher than the solubility of silica.

[0010]

In the operating method of the reverse osmosis membrane device of the present invention,
It is intended to remove the contaminants adhering to the membrane surface during the water-passing operation by interposing alkali washing during the water-passing operation for passing the acid-treated water through the reverse osmosis membrane. It is desirable to carry out alkaline cleaning of the reverse osmosis membrane at the time of contamination, and this makes it possible to deposit silica on the surface of the reverse osmosis membrane, which is the main cause of lowering the permeation flux, when the amount of silica deposition is relatively small. This silica can be removed relatively easily. Therefore, (1) the permeation flux of the reverse osmosis membrane can be recovered by an extremely simple method, and therefore, the structure of the reverse osmosis membrane device is simplified. (2) Since the reverse osmosis membrane can be washed within a relatively short time, deterioration of the reverse osmosis membrane due to washing can be reduced, and therefore the life of the reverse osmosis membrane can be extended.
(3) Since it can be easily washed, the flux can be restored to the initial state by washing when the permeation flux has a very small decrease in the permeation flux. Therefore, a high degree of operational stability can be secured.

The method of the present invention having these advantages and the above-mentioned conventional methods (a) to (c) are compared as follows.

The conventional method (a) has a low recovery rate. In order to solve this problem, conventional methods (b) and (c) have been invented. However, in the conventional method (b), the hardness component accompanying the high recovery rate,
Concentration of silica and further precipitation occurs. Therefore, in order to avoid these precipitations, it is necessary to add a high-concentration dispersant of, for example, 50 to 100 ppm, which deteriorates the quality of the permeate obtained and complicates the treatment of wastewater (concentrated water). It becomes something. On the other hand, in the method of the present invention, the above problem for high recovery is solved by a simple method of adding an inorganic acid, and the dispersant is completely unnecessary, or even if it is added, the conventional method is used. A small amount is sufficient compared to. Therefore, the addition of the treatment process in the latter stage, the problem of the quality of drainage water, and the effect on the cost of chemicals are small.

In the conventional method (c), the chemicals to be added are extremely simple as in the method of the present invention, and the method for regenerating a reverse osmosis membrane is also very simple. The softening process in the first stage is indispensable. On the other hand, the method of the present invention does not require a softening treatment step, achieves a high recovery rate only by adding a simple chemical, and can regenerate the reverse osmosis membrane by a simple method.

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

FIG. 1 is a flow chart showing an embodiment of the reverse osmosis membrane device of the present invention. In FIG. 1, 2 is a front stage reverse osmosis membrane device, and 4 is a rear stage reverse osmosis membrane device. The water to be treated 1 is supplied to the upstream reverse osmosis membrane device 2 through the water to be treated 6 supply pipe,
Here, the former-stage concentrated water, which has been subjected to reverse osmosis membrane treatment to increase the salt concentration, and the former-stage permeated water, which has a reduced salt concentration, are separated.

As the water to be treated, industrial water, city water, recovered water (including biologically treated water), well water, river water, etc. can be used, and if necessary, these are subjected to membrane clarification, coagulation filtration, Supply after pretreatment such as activated carbon filtration, decarbonation, membrane degassing.

The pre-stage concentrated water is supplied to the post-stage reverse osmosis membrane device 4 through the pre-stage concentrated water pipe 8, where the post-stage concentrated water is subjected to reverse osmosis membrane treatment to further increase the salt concentration, and the salt concentration is lowered. It is separated into the latter-stage permeated water. Second-stage concentrated water is p
It is released to the outside of the system through the latter-stage concentrated water pipe 12 with the H sensor 10 interposed.

The front-stage permeated water passes through the front-stage permeated water pipe 14, while the rear-stage permeated water is taken out through the rear-stage permeated water pipe 16 and merges with the permeated water take-out pipe 18 to be used for a desired purpose. Alternatively, only the front-stage permeated water having good water quality is used for a desired purpose, and the rear-stage permeated water having relatively poor water quality is returned to the upstream side of the front-stage reverse osmosis membrane device 2 and supplied to the reverse osmosis membrane device 2. It is reused as water. In addition, 2
0, 22, and 24 are the first-stage permeate water pipe 14 and the second-stage permeate water pipe 1.
6, a front stage permeated water flow rate sensor, a rear stage permeated water flow rate sensor, and a permeated water take out pipe flow rate sensor which are respectively installed in the take-out pipe 18.

In the present invention, the pH of the latter-stage concentrated water is adjusted to 6
The reverse osmosis membrane treatment is controlled as follows.
H is detected, and this detection signal is fed back to the acid supply means 26.

The acid such as hydrochloric acid supplied from the acid supply means 26 is supplied to the pre-stage acid supply pipe 28 and / or the post-stage acid supply pipe 30.
And is supplied to the water to be treated and / or the concentrated water of the first stage through which the reverse osmosis membrane treatment is carried out while the pH of the concentrated water of the latter stage is 6 or less.

In the present embodiment, as the water to be treated, the concentrated water of the former stage reverse osmosis membrane device 2 is further supplied to the latter stage reverse osmosis membrane device 4 for reverse osmosis membrane treatment, thereby increasing the recovery rate of the treated water. At the same time, silica is continuously increased by the previous and subsequent reverse osmosis membrane treatments, and even if the concentration of the hardness component exceeds the solubility thereof, it continues continuously for a long time without lowering the flux of the reverse osmosis membrane. You can drive.

Examples of hardness components include Ca and Mg. Also, the pH of the latter-stage concentrated water is 6 or less, particularly 4.0 to
It is preferably set to 5.5.

The flow rate of permeated water through the reverse osmosis membrane during the water flow operation of the reverse osmosis membrane device is measured by the flow rate sensors 20, 22, 24.
Monitored by The flow rate is monitored for each reverse osmosis membrane or for the entire reverse osmosis membrane, and the permeation flux per unit membrane surface (for example, m ³ / m) is calculated from the measured permeated water flow rate (for example, m ³ / H). ² · H · kgf / cm ² )
When the value falls below a predetermined ratio compared to the start of use, the flow of water in the reverse osmosis membrane device in which the flux of permeated water is reduced is stopped and the reverse osmosis membrane in which the flux is reduced is washed. Is.

The specific value of the predetermined ratio varies depending on the water quality of the water to be treated, the type of reverse osmosis membrane, etc., but is generally 70% or more, and particularly preferably 80 to 90%.

The washing operation is performed as follows. That is, after stopping the reverse osmosis membrane treatment of the treated water, first, the washing water is supplied to the treated water supply pipe 6 and / or the concentrated water pipe 8 of the upstream reverse osmosis membrane device 2 through the cleaning water line 32. Then, the concentrated water side of both the reverse osmosis membrane device 2 and the reverse osmosis membrane device 4 or only the concentrated water side of the reverse osmosis membrane device 4 is washed.
The cleaning water supplied in this case is intended to discharge the concentrated water remaining on the concentrated water side in the reverse osmosis membrane device to the outside of the system, and the quality of the water is silica and precipitation of hardness components occurs. It is preferable that the water quality is not so high, that is, the water quality is the same as the permeated water of the reverse osmosis membrane device.

Next, alkali such as an aqueous solution of sodium hydroxide is supplied to the washing water line 32 by the alkali supply means 34 and mixed by the line mixer 36, and the pH becomes 9 or more at the inlet side of the reverse osmosis membrane device. Thus, it is supplied to the reverse osmosis membrane device. In addition, 38 is a pH sensor provided in the washing water line.

The reverse osmosis membrane is brought into contact with the supplied alkaline washing water to dissolve and remove the deposit mainly composed of silica deposited on the membrane surface of the reverse osmosis membrane on the concentrated water side. Is regenerated. PH of alkaline wash water
Is 9 or more, and particularly preferably 9.5 to 12.0. The contact time with the alkaline washing water varies depending on the state of the reduced flux of the reverse osmosis membrane, but is preferably 2 to 10 hours.

The washing operation may be carried out by enclosing the alkaline washing water in the reverse osmosis membrane apparatus, immersing it in the apparatus and letting it stand.
It is preferable to circulate while adjusting H to a desired value.

When carrying out the circulation cleaning, (a) a method of circulating the alkaline cleaning water only on the concentrated water side of the reverse osmosis membrane and circulating it, or (b) while permeating the alkaline cleaning water through the reverse osmosis membrane. Any method of washing and mixing the permeated water obtained and the concentrated water and circulating it may be used.

Further, since the reverse osmosis membrane in the latter stage of the reverse osmosis membrane treatment of concentrated water containing a high concentration of silica and hardness component is easily contaminated, the former stage and the latter stage can be separated and washed separately. An extra drainage path for the pre-stage concentrated water for operating the pre-stage reverse osmosis membrane device during the cleaning of the reverse osmosis membrane device may be additionally provided.

In the above description, the case where the reverse osmosis membrane devices are arranged in two stages has been described, but the present invention is not limited to this, and the reverse osmosis membrane device may have a single stage. It may be an arbitrary series of three or more stages. Further, in the above description, it has been described that the alkaline washing is performed when the permeation flux of the reverse osmosis membrane is reduced to a predetermined ratio or less during the water passing operation, but the present invention is not limited to this. The alkali cleaning may be carried out regularly regardless of the decrease of the permeation flux.

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

[0033]

【Example】

Example 1 The water to be treated was subjected to reverse osmosis membrane treatment using the reverse osmosis membrane device having the flow shown in FIG. The industrial water 52 was pretreated by the membrane turbidity device 54, and this was supplied as water to be treated 56 to the pre-stage reverse osmosis membrane device 58 at a flow rate of 5300 L / hr for reverse osmosis membrane treatment. The silica concentration in the water to be treated at this time is 10 to 1
It was in the range of 5 mg asSiO ₂ / L. NTR-7 manufactured by Nitto Denko Corporation is used for the reverse osmosis membrane of the former-stage reverse osmosis membrane device.
59HR was used. Front stage permeate flow rate is 4000 L / h
r, the flow rate of the pre-stage concentrated water was 1300 L / hr, and 200 L / hr of the pre-stage concentrated water of 1300 L / hr was stored in the storage tank 6
0, and was supplied to the latter-stage reverse osmosis membrane device 64 via the pump 62.

As the reverse osmosis membrane attached to the latter-stage reverse osmosis membrane device 64, six SU-706M manufactured by Toray Industries, Inc. were used in series. In the latter-stage reverse osmosis membrane treatment, the latter-stage permeated water of 160 L / hr and the latter-stage concentrated water of 40 L / hr were obtained.

The pH of the second-stage concentrated water is monitored by the pH sensor 66. Based on the signal of the sensor 66, hydrochloric acid 68 is sent to the storage tank 60 to constantly adjust the pH of the second-stage concentrated water to pH 5.5 to 6.0. Feedback control was performed so that it was within the range.

In this case, the water recovery rate in the entire system is 95.
%, The silica concentration in the latter-stage concentrated water is 200 to 300 mg.
It was in the range of / L. After continuous operation for 3 months, the permeation flux of the latter-stage reverse osmosis membrane dropped to 80% of the initial value, so that the latter-stage reverse osmosis membrane was washed with alkali. P for alkaline cleaning
After encapsulating H10 aqueous sodium hydroxide solution overnight and immersing it,
A circulation operation was carried out for one hour in which the alkaline washing water was allowed to flow only through the concentration side of the reverse osmosis membrane.

The results obtained are shown in Table 1. Comparative Example 1 The same apparatus as in Example 1 was used and operation was performed in the same manner as in Example 1, except that the pH of the second-stage concentrated water was not adjusted by supplying hydrochloric acid 68 to the storage tank 60. In this comparative example, since the permeation flux was drastically reduced, the continuous operation period was set to 3 days. The obtained results are also shown in Table 1.

[0038]

[Table 1] (Test Example) Reverse osmosis membrane treatment was carried out under the same conditions as in Example 1 and Comparative Example 1 above, and using each of the obtained reverse osmosis membranes, confirmation of the cleaning effect with hydrochloric acid and sodium hydroxide aqueous solution, and deposition The analyzed components were analyzed. The results are shown in Table 2. The cleaning was first carried out by acid cleaning with hydrochloric acid and then by alkali cleaning.

[0039]

[Table 2] The alkali cleaning was the same as that described in Example 1. The acid washing was carried out by circulating hydrochloric acid having a pH of 2 only on the concentrated side of the reverse osmosis membrane for 3 hours.

As is clear from Tables 1 and 2, the alkali cleaning effect is remarkable in Example 1, and the acid cleaning is almost ineffective. In addition, as in the present invention, pH 5.5-
By controlling the latter-stage concentrated water at 6.0, almost no precipitation of calcium and other hardness components was observed, the permeation flux of the latter-stage reverse osmosis membrane was only slightly reduced, and the permeation flow of silica was extremely slow. It can be seen that it only causes a drop in the bundle.

On the other hand, in Comparative Example 1 in which the pH of the second-stage concentrated water was not adjusted, the permeation flux was greatly reduced to 55% after only 3 days. Also, it was not completely regenerated by chemical cleaning.

From the above results, the silica (Example 1) which deposits when the pH of the concentrated water is controlled to 6 or less and the reverse osmosis membrane treatment is performed is different from the silica deposited in Comparative Example 1. Subtly differently, it is considered that the silica in Example 1 is easily dissolved by alkali cleaning, whereas the silica in Comparative Example 1 is in a form that is extremely difficult to dissolve. Although the present invention requires alkaline cleaning as described above, as can be seen from Table 2, not only silica but also hardness components such as calcium are deposited and adhered to the membrane surface of the reverse osmosis membrane to some extent. Therefore, it is preferable to carry out acid cleaning in addition to alkali cleaning in order to remove this hardness component.

[0043]

Since the present invention is configured as described above, the permeation flux of permeated water of the reverse osmosis membrane device is less decreased, and the permeation flux can be easily recovered, and the reverse osmosis membrane is prevented from being contaminated with the membrane. Since this can be reliably prevented, a high degree of operational stability of the reverse osmosis membrane device can be secured. Moreover, since a cleaning method that does not damage the reverse osmosis membrane can be adopted, the life of the reverse osmosis membrane can be extended as compared with the conventional case. Furthermore, the method of the present invention is simple in terms of equipment, and the chemicals used are inexpensive, which is advantageous in terms of cost.

[Brief description of the drawings]

FIG. 1 is a flowchart for explaining an embodiment of the present invention.

FIG. 2 is a flow chart showing an embodiment of the present invention.

[Explanation of symbols]

 1 treated water 2 front stage reverse osmosis membrane device 4 rear stage reverse osmosis membrane device 6 treated water supply pipe 8 front stage concentrated water pipe 10 pH sensor 12 rear stage concentrated water pipe 14 front stage permeated water pipe 16 post stage permeated water pipe 26 acid supply means 32 wash water line 36 Alkali supply means

Claims (4)

[Claims] 1. When separating water to be treated containing at least a hardness component and silica into permeated water and concentrated water by a reverse osmosis membrane device, the pH of the concentrated water is kept at 6 or less and In a method of operating a reverse osmosis membrane device that carries out a water passing operation, a washing operation for washing the reverse osmosis membrane attached to the reverse osmosis membrane device with washing water having a pH of 9 or more is interposed during the water passing operation. And operating method of reverse osmosis membrane device. 2. The method for operating a reverse osmosis membrane device according to claim 1, wherein the concentration of silica in the concentrated water is not less than the solubility of silica. 3. The water to be treated containing at least a hardness component and silica is separated into permeated water and concentrated water by a front-stage reverse osmosis membrane device, and the obtained concentrated water of the front-stage reverse osmosis membrane device is separated into a rear-stage reverse osmosis membrane. Reverse osmosis is carried out by repeating the separation of permeated water and concentrated water in the device and performing a water-flow operation of the reverse osmosis membrane device in which a plurality of consecutive reverse osmosis membrane devices are connected while keeping the pH of the concentrated water at 6 or less. In the operating method of the membrane device, the pH of the reverse osmosis membrane attached to the last-stage reverse osmosis membrane device is adjusted to pH during the water flow operation.
A method for operating a reverse osmosis membrane device, characterized in that a washing operation of washing with 9 or more washing water is interposed. 4. The method for operating a reverse osmosis membrane device according to claim 3, wherein the concentration of silica in the concentrated water of the last stage reverse osmosis membrane device is not less than the solubility of silica.