JPH0815595B2 - Desalination apparatus control method and apparatus - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for controlling seawater (or brackish water) desalination equipment equipped with a cellulose acetate reverse osmosis membrane module, and more specifically, a pH adjusting agent such as sulfuric acid which is a dangerous substance in raw water. Without adding, the residual chlorine thread oxidant concentration of seawater or brackish water (hereinafter referred to as raw water) at the inlet of the reverse osmosis membrane is controlled within a certain range smaller than the conventional value in accordance with the pH value of the raw water, The present invention relates to a method and apparatus for preventing oxidation of a reverse osmosis membrane and preventing slime (fine solid particles) from being generated, and maintaining a high desalination rate.

[0002]

2. Description of the Related Art Conventionally, cellulose acetate-based reverse osmosis membranes are
Oxidizing agent (sodium hypochlorite as sterilizer, liquid chlorine,
Since it has resistance to bleaching powder, ozone, etc.), it has the characteristic that it can be stably operated for a long period of time with the oxidant in contact with the film surface as a measure to prevent slime on the film surface. There is. However, when the pH of the raw water is 7 or higher, deterioration due to hydrolysis occurs, so the pH of the raw water at the membrane inlet is set to 6.
The pH is adjusted to 5 or less with sulfuric acid or the like. in this way,
Conventionally, it was a general method to adjust the pH of raw water to 6.5 or less with sulfuric acid or the like, and then add an oxidizer in an amount of about 0.5 to 1 ppm in terms of chlorine.

JP-A-63-69586 discloses a method of adding 0.1 to 5 ppm of phosphate to raw water in order to prevent oxidative deterioration of a reverse osmosis membrane and to prevent an increase in slime generation. Is listed. Also, JP-A-63-69
Japanese Patent No. 587 describes a method of adding 0.1 to 25 ppm of humic acids to raw water in order to prevent oxidative deterioration of a reverse osmosis membrane and prevent an increase in slime generation. As described above, conventionally, a method of adding sulfuric acid, phosphate, humic acid, etc. to raw water has been generally used.

[0004]

In the method of adding the oxidant after adjusting the pH with sulfuric acid or the like as described above, a remote island or the like having poor transportation convenience (the desalination device is often installed in the remote island or the like). It is difficult from the viewpoint of safety to transport sulfuric acid or the like as a pH adjusting agent to ()), and it is difficult to perform stable operation for a long period of time. In addition, analysis of long-term operation data showed that even when the pH was 6.5 or less, when the residual oxidant concentration reached 1 ppm in terms of chlorine, the membrane performance was significantly deteriorated, and the membrane deterioration was caused by the pH value and the residual oxidant concentration. It has been found that it is caused by a synergistic effect with.

That is, a pH adjusting agent such as sulfuric acid is not added in the pilot plant, and an oxidizing agent (in this case, NaCl is used).
O) alone was added at about 1 ppm and continuous operation was performed for about 1 year, and performance deterioration of the cellulose acetate reverse osmosis membrane used in the operation process was observed. . The membrane deterioration factor predicted at the design stage is high in raw water pH (pH 7.6 ~) because the process that does not use a pH adjuster such as sulfuric acid is adopted.
7.8). But,
Contrary to expectations, the results of the investigation revealed that oxidative deterioration due to the sterilizing agent (oxidizing agent) was the main factor. After that, when the residual sterilant concentration was controlled and controlled to about 0.1 to 0.2 ppm in terms of chlorine, the operation was performed, and the deterioration of the membrane performance was greatly improved. The present invention has been made in view of the above points, and by keeping the residual sterilant concentration of the membrane inlet raw water at a value within a certain range, which is significantly smaller than the value conventionally added, depending on the pH range of the raw water. It is an object of the present invention to provide a method and a device for preventing oxidation deterioration of a film and preventing slime generation and maintaining a high desalination rate.

[0006]

In order to achieve the above object, the method for controlling a desalination apparatus of the present invention is a permeated water obtained by allowing seawater or brackish water to pass through a cellulose acetate reverse osmosis membrane as raw water. In the desalination method in which the residual chlorine-based oxidant concentration at the inlet of the reverse osmosis membrane is
When the pH is 7.0-7.5, it is 0.2-0.4p in terms of chlorine.
pm or in terms of chlorine when the pH of raw water is 7.5 to 8.2
It is characterized in that the amount of the chlorine-based oxidant added to the raw water is controlled so as to fall within the range of 0.05 to 0.2 ppm. Specifically, the residual chlorine-based oxidizer concentration of the raw water at the membrane inlet is controlled as follows according to the pH value of the raw water at the inlet. pH 7.0 to 7.5: 0.2 to 0.4 ppm in terms of chlorine, preferably 0.2 to 0.3 ppm pH 7.5 to 8.2: 0.05 to 0.2 ppm in terms of chlorine, preferably 0.1 ~ 0.2ppm As a chlorine-based oxidizer, sodium hypochlorite (NaCl
O), liquid chlorine, bleaching powder (Ca (ClO) ₂ ) and the like are used. It is also possible to use ozone instead of the chlorine-based oxidizing agent. In this case, the residual ozone concentration is a value substantially corresponding to a chlorine conversion value of 0.05 to 0.4 ppm. If the residual chlorine-based oxidizing agent concentration at the inlet of the reverse osmosis membrane is less than 0.05 ppm in terms of chlorine, there is a disadvantage that the sterilizing effect of the residual chlorine-based oxidizing agent is weakened and slime is likely to occur on the surface of the reverse osmosis membrane. On the other hand, 0.4 ppm
If it exceeds, there is a disadvantage that the oxidative deterioration of the reverse osmosis membrane element due to the residual chlorine-based oxidizing agent becomes remarkable.

[0007] General seawater has stable properties and a substantially constant pH once the location of water intake is determined.
It is sufficient to periodically check the pH of the raw water with a pH meter and, if necessary, change the feedback oxidizer meter feedback control set point. On the other hand, in the case of brackish water, its properties may change drastically due to rainfall, etc., and it is necessary to install a pH meter and automatically change the set point of the feedback control of the residual oxidizer meter depending on the pH. There is also. In this case, the pH of the raw water and the residual chlorine-based oxidant concentration are detected, and the amount of chlorine-based oxidant added is controlled by these values.

As shown in FIG. 1, the controller of the desalination apparatus of the present invention comprises a pressure filter 10 for filtering seawater or brackish water.
A high pressure pump 12 for pressurizing the filtered raw water, a cellulose acetate-based reverse osmosis membrane module 14 for treating the pressurized raw water to separate it into permeate and concentrated water, and a chlorine-based oxidizer added to the raw water A desalination apparatus comprising an oxidant tank 16 for storing the oxidant, and an oxidant addition line 20 having an oxidant injection pump 18 for adding a chlorine-based oxidant to raw water,
A residual chlorine detector 24 is provided in the raw water line 22 between the pressure filter 10 and the reverse osmosis membrane module 14, and the detector 2
4 and the oxidant injection pump 18 depending on the pH value of the raw water
Change / control the feedback control set point
Is connected via a residual chlorine-based oxidizer controller 26. In FIG. 1, raw water is first introduced into an oxidizer mixer 30 by a raw water pump 28, where it is mixed with a chlorine-based oxidizer added upstream of the mixer 30. Next, the raw water is introduced into the pressure filter 10 to filter impurities, and then the safety filter 32 further filters fine powder particles. Then, the raw water is pressurized by a high pressure pump (for example, 55 kg / cm ² G to 65 kg / cm ²
It is pressurized by G) and introduced into the reverse osmosis membrane module 14 composed of a large number of cellulose acetate yarn reverse osmosis elements, and separated into permeated water (fresh water) and concentrated water (concentrated salt water).

Raw water line 22 upstream of the high-pressure pump 12
Is equipped with a residual chlorine detector 24, which detects residual chlorine in the raw water at the inlet of the reverse osmosis membrane module 14, and uses this value to inject the oxidant via the residual chlorine-based oxidant controller 26. Control the motor of the pump 18,
Adjust the amount of chlorine-based oxidizer added to the raw water. And
The residual chlorine-based oxidant concentration in the raw water at the inlet of the reverse osmosis membrane module 14 is controlled so as to fall within the range of 0.05 to 0.4 ppm, preferably 0.1 to 0.3 ppm in terms of chlorine.

When the raw water is brackish water whose pH is easily changed, as shown in FIG. 2, in addition to the apparatus shown in FIG.
A pH detector 34 is provided in the raw water line 22 between the pressure filter 10 and the reverse osmosis membrane module 14, and the pH detector 3 is provided.
4 and the residual chlorine-based oxidant controller 26, the operation unit 3
In some cases, the connection may be made via 6. In this case, the calculator 3
An output corresponding to the pH value detected in 6 is output and is input to the controller 26 as a set value.

[0011]

EXAMPLES Examples of the present invention, comparative examples, and reference examples will be described below. Comparative Example 1 A seawater desalination apparatus equipped with a cellulose acetate reverse osmosis membrane module was used to p-pure raw water (seawater) without injecting a pH adjuster.
When continuous operation was performed at H 7.6 to 7.8 and the concentration of residual oxidant (using NaClO) at the membrane inlet of 1.0 to 1.2 ppm, the water quality of the permeate was significantly deteriorated, and was the highest in 7 months after the start of operation. The desalination rate of the membrane element which deteriorated severely deteriorated to about 96% (see Table 1).

Example 1 When the membrane element of Comparative Example 1 which was severely deteriorated was decomposed and inspected, it was found that the oxidative deterioration due to the oxidant (using NaClO) was remarkable. Therefore, using the apparatus shown in FIG. When the residual oxidant concentration in the seawater of the part was controlled to 0.1 to 0.2 ppm in terms of chlorine and continuous operation was performed with a new membrane element, the desalination rate hardly changed even after 7 months of operation. (See Table 1).

Comparative Example 2 In a seawater desalination apparatus equipped with a cellulose acetate reverse osmosis membrane module, p of raw water (seawater) was injected without injecting a pH adjusting agent.
H7.0-7.2, the residual oxidant (using NaClO) concentration at the membrane inlet was continuously operated at 1.0-1.2ppm, and the water quality of the permeate was significantly deteriorated. The desalination rate of the membrane element with severe deterioration was 99.2% (see Table 2).

Example 2 From the results of Example 1, the existing seawater desalination facility was improved as shown in FIG. 1, and the residual chlorine concentration was 0.3 p at pH 7.0 to 7.2.
As a result of controlling to pm, the performance degradation 6 months after the change was significantly reduced compared to the performance degradation in the initial 6 months (Table 2
reference).

[0015]

[Table 1]

[0016]

[Table 2]

Reference Example 1 A conventional general process is shown. Using a seawater desalination apparatus equipped with a cellulose acetate reverse osmosis membrane module, dilute sulfuric acid was injected into raw water (seawater) having a pH of 8.0 to adjust the pH at the membrane inlet to 6.
Adjusted to 3 to 6.5, residual oxidizer (N
Use aClO) Concentration converted to chlorine is 0.5-1.0pp
Operating at m (without residual chlorine concentration control). With this device, the decrease in the desalination rate was small even after 7 months from the start of operation, and stable operation was continued (see Table 3). From this result, it can be seen that the above-mentioned embodiment of the present invention has the same effect as the conventional general process (Reference Example 1) without injecting the pH adjuster.

[0018]

[Table 3]

[0019]

Since the present invention is configured as described above, it has the following effects. (1) Since the desalting performance of the cellulose acetate reverse osmosis membrane can be maintained without using a pH adjuster such as sulfuric acid which is a dangerous substance, the operating cost is reduced and the maintenance is simplified. It (2) Since the amount of the oxidant added is reduced as compared with the conventional process, the operating cost is further reduced in combination with the effect of (1). (3) Since the cellulose acetate-based reverse osmosis membrane maintains the characteristic of resistance to an oxidizing agent, slime generation does not occur on the membrane surface. (4) Since the life of the film is extended, the conversion frequency of the film is reduced and the maintenance cost is reduced.

[Brief description of drawings]

FIG. 1 is a flow sheet showing an embodiment of a controller for a desalination apparatus according to the present invention.

FIG. 2 is a flow sheet showing another embodiment of the device of the present invention.

[Explanation of symbols]

 10 Pressure Filter 12 High Pressure Pump 14 Cellulose Acetate Reverse Osmosis Membrane Module 16 Oxidant Tank 18 Oxidant Injection Pump 20 Oxidant Addition Line 22 Raw Water Line 24 Residual Chlorine Detector 26 Residual Chlorine Oxidant Controller 34 pH Detector 36 Calculation vessel

Claims (4)

[Claims] 1. A desalination method comprising permeating seawater or brackish water as raw water through a cellulose acetate reverse osmosis membrane to separate the permeated water and concentrated water, wherein the residual chlorine-based oxidant concentration at the inlet of the reverse osmosis membrane is raw water. PH of
0.2 to 0.4 pp in terms of chlorine when 7.0 to 7.5
m, or when the pH of raw water is 7.5 to 8.2, converted to chlorine
A method for controlling a desalination apparatus, which comprises controlling the amount of chlorine-based oxidant added to raw water so as to fall within a range of 0.05 to 0.2 ppm. 2. The method for controlling a desalination apparatus according to claim 1, wherein the pH of the raw water and the concentration of the residual chlorine-based oxidant are detected, and the amount of the chlorine-based oxidant added is controlled by these values. 3. A pressure filter (10) for filtering seawater or brine, and a high-pressure pump (1) for pressurizing the filtered raw water.
2) and a cellulose acetate reverse osmosis membrane module (14) for treating pressurized raw water to separate it into permeated water and concentrated water
An oxidant tank (16) for storing a chlorine-based oxidant to be added to the raw water, and an oxidant addition line (20) having an oxidant injection pump (18) for adding the chlorine-based oxidant to the raw water. In the desalination apparatus, a residual chlorine detector (24) is provided in the raw water line (22) between the pressure filter (10) and the reverse osmosis membrane module (14), and the detector (24) and the oxidant injection pump are provided. (18) and set the feedback control according to the pH value of the raw water
A controller for a desalination apparatus, which is connected through a residual chlorine-based oxidant controller (26) for changing and controlling points . 4. A pH detector (34) is provided in the raw water line (22) between the pressure filter (10) and the reverse osmosis membrane module (14), and the pH detector (34) and residual chlorine-based oxidation are provided. The controller of the desalination apparatus according to claim 3, wherein the agent controller (26) is connected via a computing unit (36).