JP2928536B2 - Sterilization method of composite reverse osmosis membrane separation device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a desalination apparatus by a reverse osmosis method, and particularly to a desalination apparatus such as ultrapure water, canned water desalination, and wastewater reuse. .

[Prior Art] A membrane separation technology is used in a wide range of fields such as desalination of seawater and brackish water, industrial wastewater treatment, medical treatment, and production of industrial pure water. Among them, contamination of the membrane separation device by microorganisms may deteriorate the water quality of ultrapure water as fine particles in the liquid to be treated, or bacteria may grow on the membrane surface, and organic substances composed of microorganisms and their metabolites Therefore, sterilization of the membrane separation device is important because it causes problems such as the decrease in water permeability of the membrane. For this reason, various methods for disinfecting microorganisms in a membrane separation device have been proposed. As a general method, a method in which a disinfectant is constantly or intermittently brought into contact with a membrane is employed.

As a disinfectant, chlorine-based reagents have been widely used since ancient times, but high-performance composite reverse osmosis membranes have appeared in place of conventional asymmetric cellulose acetate membranes, and chlorine resistance to asymmetric cellulose acetate membranes has been demonstrated. Because of their inferiority, various new sterilization methods have been proposed to replace chlorine such as chloramines, hydrogen peroxide, peracetic acid, and sodium bisulfite. However, these disinfectants are more expensive than chlorine, have lower disinfecting power,
At present, there is a problem that handling is difficult, and operation utilizing the high performance of the composite reverse osmosis membrane has not been performed.

As a composite reverse osmosis membrane used industrially, a composite reverse osmosis membrane in which a polysulfone porous support membrane is coated with an active layer that substantially controls the membrane performance, particularly a polyamide-based material is used as the active layer material. The membrane has a high performance, and particularly has a certain degree of chlorine resistance and high performance.
No. 7106, JP-A-61-42302, JP-A-62-12160
A crosslinked aromatic polyamide-based film such as that disclosed in Japanese Patent Publication No. 3 has been noted.

[Problems to be Solved by the Invention] The present inventors, when performing water production by a membrane separation device using such a composite reverse osmosis membrane, the fluctuation of the membrane performance due to chlorine for sterilization, which is particularly problematic, Intensive study on the problem that the membrane performance is reduced by the sterilization process, especially when performing chlorine sterilization of a composite reverse osmosis membrane having high desalination performance and high water production at the same time, by adding polyphosphate, chlorine The present inventors have found that the performance degradation due to the above can be remarkably suppressed, and reached the present invention.

[Means for Solving the Problems] To achieve the above object, the present invention has the following constitution.

That is, "a method for sterilizing a composite reverse osmosis membrane device, comprising adding 5 to 10 ppm of a polyphosphate to the aqueous solution when the composite reverse osmosis membrane separation device is sterilized using a residual chlorine-containing aqueous solution. 』
About.

In the present invention, residual chlorine refers to the sum of free chlorine and bound chlorine, and the measurement of residual chlorine can be easily performed by an ortho-tolidine method or the like, and the method is JIS.
-K0101. For disinfection of a normal membrane separation device, about 0.1 to 10 ppm of residual chlorine is required in an aqueous solution. However, the required amount of residual chlorine for this sterilization should be changed depending on the properties of the liquid to be treated in the fresh water generator using the composite reverse osmosis membrane and the microorganism propagation state of the apparatus, and It is determined by the number of viable bacteria, the concentration of organic components, the rate of decrease in the amount of fresh water produced in the system by microorganisms, etc.

When performing sterilization with chlorine on the composite reverse osmosis membrane separator, it is necessary to determine the frequency of sterilization in consideration of the chlorine resistance of the composite reverse osmosis membrane and the microorganism propagation state of the separator, but usually it is necessary to determine the frequency of sterilization. It is often performed intermittently from about 1 hour a day to about 1 hour a week, and the required residual chlorine concentration is different from 0.1 to 10 ppm. For ordinary river water and well water, 0.1 to 1 ppm
Is often sufficient. In the present invention, a polyphosphate is a polymerized inorganic phosphate-based substance having two or more phosphorus atoms in a molecule represented by sodium hexametaphosphate and bonded to an alkali metal or an alkaline earth metal by an oxygen atom or the like. To tell. Representative polyphosphates include tetrasodium pyrophosphate, disodium pyrophosphate,
Sodium tripolyphosphate, sodium tetrapolyphosphate,
Sodium heptapolyphosphate, sodium decapolyphosphate,
Examples include sodium metaphosphate, sodium hexametaphosphate, and potassium salts thereof.

In the present invention, the composite reverse osmosis membrane separation device, fresh water,
A device for supplying the liquid to be treated to the composite reverse osmosis membrane module under pressure for the purpose of concentration and the like, and separating it into a permeate and a concentrate, usually a composite reverse osmosis membrane element, a pressure vessel,
It is composed of a pressure pump and the like.

As the composite reverse osmosis membrane, it is effective to apply the present invention to a membrane having a polyamide-based active layer, particularly a membrane having a crosslinked aromatic polyamide-based active layer.

In general, such a membrane is prepared by depositing an aromatic amine and a polyfunctional acid chloride on a porous support membrane such as polysulfone.
It is manufactured by forming an ultrathin film (active layer) of a cross-linked aromatic polyamide by reacting by a method utilizing an interfacial polycondensation reaction called a method. It is said that this type of membrane has relatively good chlorine resistance among composite reverse osmosis membranes, and that it can be contacted with, for example, 1 ppm of chlorine for about 1000 hours before the membrane performance is significantly reduced.

However, the present inventors have found that when these composite reverse osmosis membranes are actually incorporated into a separation apparatus and used, the membrane performance is reduced much faster than the expected chlorine contact time, and iron rust and the like are found. It is presumed that chlorine degradation of the film is catalyzed and accelerated by heavy metals, and it has been found that the addition of polyphosphate is effective in preventing this.

In the operation of reverse osmosis separation equipment, polyphosphate is used as a scale inhibitor such as calcium carbonate in seawater or hard water.
Further, it is a reagent used as a rust preventive for the metal of the piping material. In such a case, an addition amount of usually 10 ppm or more is necessary.

However, in the present invention, the polyphosphate acts as a heavy metal sequestering agent, a chlorine solution used for the liquid to be treated or sterilization, or acts on a heavy metal such as iron present on the film surface to form a chelate compound with the heavy metal. It is thought that it blocks the catalytic action of heavy metals. To perform normal chlorine sterilization,
It is sufficient to add about 5 to 10 ppm of the polyphosphate to the aqueous solution. When the addition amount is 5 ppm or less, the catalytic action of heavy metals cannot be sufficiently prevented, so that the performance of the composite reverse osmosis membrane deteriorates during sterilization with chlorine.

As the type of the composite reverse osmosis membrane element, a spiral type, a tubular type, a hollow type and the like are generally used, and the present invention can be applied to any type.

As the aqueous solution used for sterilization, it is possible to use usually irrigation water such as river water or groundwater or pure water obtained by a desalination system. The pH of the aqueous solution depends on the hydrolysis resistance of the membrane used,
The pH is determined in consideration of the corrosiveness of members such as piping, etc., but is generally in the range of pH 4 to 7, preferably 5 to 6.5.

[Examples] The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

In Examples, the exclusion (desalting) rate was determined by the following equation.

Example 1 Groundwater was pumped from a well into raw water, 1 ppm of chlorine and 7 ppm of sodium hexametaphosphate were added to the water, and the pH was adjusted to 6.7 with sulfuric acid.
Adjusted to ° C. The conductivity of the raw water at this time is about 200 μS / cm
Met. Raw water pressurized to 15 atm with a high-pressure pump is supplied to Toray's composite reverse osmosis membrane element SU-710, and a recovery rate of 10
% Of this reverse osmosis device was operated for 1000 hours. The amount of permeated water and the change in permeated water quality during operation are summarized in Table 1.

Example 2 To the raw water of Example 1, nickel, chromium and copper were further added.
0.1 ppm was added, and the operation was performed for 1000 hours under the same other conditions. Table 1 summarizes changes in permeated water quality during operation.

Comparative Example 1 The same operation as in Example 1 was performed except that sodium hexametaphosphate was not added and the other conditions were the same, and the operation was performed for 1000 hours. Table 1 summarizes changes in permeated water quality during operation.

Comparative Example 2 The same operation as in Example 2 was performed except that sodium hexametaphosphate was not added and the other conditions were the same, and the operation was performed for 1000 hours. Table 1 summarizes changes in permeated water quality during operation.

Comparative Example 3 In Example 2, 20 pp of sodium hexametaphosphate was used.
m, and the operation was performed for 1000 hours under the same other conditions. Table 1 summarizes changes in permeated water quality during operation.

[Effects of the Invention] According to the present invention, water production by a composite reverse osmosis membrane whose use range is limited due to the difficulty of sterilization while having high performance has been improved by adding a small amount of sodium hexametaphosphate, thereby improving economical efficiency. In addition, it has become possible to carry out with high reliability.

Claims (7)

(57) [Claims] (1) When sterilizing a composite reverse osmosis membrane separation device using a residual chlorine-containing aqueous solution, a polyphosphate is added to the aqueous solution in an amount of 5 to 50%.
A method for sterilizing a composite reverse osmosis membrane device, comprising adding 10 ppm. 2. The method according to claim 1, wherein an aqueous solution containing 0.1 to 10 ppm of residual chlorine and 5 to 10 ppm of polyphosphate is used. 3. The method according to claim 1, wherein the polyphosphate is hexametaphosphate. 4. The sterilization method according to claim 1, wherein the pH of the aqueous solution is 4-7. 5. The sterilization method according to claim 1, wherein the pH of the aqueous solution is 5 to 6.5. 6. The sterilization method according to claim 1, wherein the composite reverse osmosis membrane is a polyamide-based membrane. 7. The sterilization method according to claim 1, wherein the composite reverse osmosis membrane is an aromatic polyamide-based membrane.