JP3729260B2 - Water treatment method using reverse osmosis membrane - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a water treatment method using reverse osmosis membrane technology. Specifically, the present invention relates to a technique for treating, recovering and reusing water to be treated such as circulating cooling water in a cooling water system without deteriorating the membrane function.
[0002]
[Prior art]
In a circulating cooling water system, water in the system is discharged as blow water from the cooling tower to prevent scale failure due to concentration of scale components in the system, and water corresponding to the amount of blow water is supplied as makeup water. are doing. This blow water is water in which hardness components and scale components such as silica have already been concentrated to the precipitation limit by a high concentration operation 6 to 8 times that of the circulating cooling water system.
[0003]
For this reason, it has been difficult to collect the blow water and reuse it as cooling water. However, in recent years, a method for desalting, collecting, and reusing the reverse osmosis membrane technology is disclosed in Japanese Patent Application Laid-Open No. 2002-2002. It was proposed in No. 18437.
[0004]
Here, in a cooling water system such as a circulating cooling water system, a necessary amount of hydrazine (N ₂ H ₂ ) may be added for the purpose of preventing the slime damage of the aqueous system. Conventionally, even if the water to be treated containing hydrazine (water introduced into the reverse osmosis membrane device from the aqueous system, the same applies hereinafter) is treated with a reverse osmosis membrane, the desalination performance of the membrane is not adversely affected. Since recognition has been regarded as common technical knowledge, water to be treated containing hydrazine has been subjected to reverse osmosis membrane treatment in the same manner as other water to be treated, and hydrazine has also been used as a membrane disinfectant.
[0005]
[Problems to be solved by the invention]
However, since it has been a problem that the desalination performance of the reverse osmosis membrane is reduced, the inventors of the present application have conducted extensive research. As a result, the water to be treated containing hydrazine contains heavy metals and has a pH of It was found that the desalting performance of the reverse osmosis membrane is unexpectedly lowered when it is acidic.
[0006]
Therefore, the main object of the present invention is to provide a method capable of treating a hydrazine-containing treated water obtained from an aqueous system with a reverse osmosis membrane treatment without reducing the desalting performance.
[0007]
[Means for Solving the Problems]
In order to achieve the above object and solve the above technical problem, the present invention provides the following method .
[0008]
First, the present invention provides a method for preventing deterioration of a reverse osmosis membrane by adding a chelating agent that forms a stable ionic form with the heavy metal into water to be treated in an acidic region containing at least heavy metal and hydrazine. . In addition, there is no restriction | limiting in particular in the kind of reverse osmosis membrane to which this method is applied, For example, it can select suitably according to the kind of to-be-processed water from a polyamide-type film | membrane, a cellulose acetate type | system | group film | membrane, etc.
[0009]
In this method, the water to be treated, particularly the circulating cooling water in the cooling water system, is adjusted to an acidic region by an appropriate pH adjusting means, specifically within the range of pH 3 to 6, particularly preferably pH 4.5 to 5. By adjusting within the range of 5, the treated water containing calcium and silica is treated with a reverse osmosis membrane, so that scale formation of calcium carbonate in the membrane treatment apparatus and scale failure due to silica are prevented, and stable over a long period of time. It is possible to perform the desalting treatment. In addition, by making the water to be treated acidic, microbial metabolites, fine particles, and colloidal substances in the water aggregate to form relatively large particles, which can be efficiently removed by membrane filtration.
[0010]
When the water to be treated is adjusted to the acidic region, there is an advantage that the action and effect as described in the previous paragraph can be obtained, but it is added to the water to be treated as a scale inhibitor for heavy metals and water or / and as a film disinfectant. The present inventors have newly found out that the desalination performance by the reverse osmosis membrane is reduced when both of the hydrazines produced are contained. Since hydrazine has a strong reducing action, it is thought that it reacts with heavy metal ions in the water to be treated to form unstable compounds, and these compounds degrade reverse osmosis membranes and cause deterioration in desalination performance. It is predicted.
[0011]
Therefore, in the water treatment method provided in the present application, the chelating agent is devised so as to be added to the water to be treated before introduction of the reverse osmosis membrane through an appropriate chemical injection device. The chelating agent chelate-bonds with heavy metal ions contained in the water to be treated, forms a very stable ionic form, and exerts the action of hindering the reaction between hydrazine and heavy metal ions. It is possible to prevent deterioration of film performance due to unstable compounds.
[0012]
As the chelating agent, any one that can form the ionic form can be adopted, for example, polyphosphates such as phosphonic acid and hexametaphosphoric acid, organic acid salts such as citric acid and ethylenediaminetetraacetic acid salt, polyacrylic acid and polymaleic acid. It can be suitably selected from water-soluble polymers such as acids.
[0013]
Next, in the method according to the present invention, it is devised that a suspended component containing heavy metals that have become SS in the water to be treated is removed before the reverse osmosis membrane treatment and then introduced into the reverse osmosis membrane .
[0014]
According to this device, in addition to the effect of preventing the deterioration of the membrane performance due to the addition of the chelating agent, clogging of the membrane is effectively prevented, so that the effect of performing the reverse osmosis membrane treatment step can be obtained.
[0015]
The above method is based on a method of preventing deterioration of the reverse osmosis membrane by adding a chelating agent to water to be treated which is acidic and contains at least heavy metal and hydrazine.
[0016]
As described above, the present invention is a water treatment method capable of treating hydrazine-containing treated water obtained from an aqueous system with a reverse osmosis membrane without reducing desalting performance, and deterioration of a reverse osmosis membrane when treating the treated water. It has the technical significance of providing a prevention method.
[0017]
Hereinafter, a preferred embodiment of the method according to the present invention will be described with reference to FIG. FIG. 1 is a system diagram schematically showing the main steps of the present method .
[0018]
Reference numeral 1 in FIG. 1 simply represents an aqueous system (for example, a cooling tower) such as a circulating cooling water system, and the water such as cooling water passed through the aqueous system is for the purpose of preventing slime of the aqueous system. The required amount of hydrazine 2 is added. Therefore, the hydrazine 2 is contained in the blow water 3 from the water system 1, and the blow water 3 contains heavy metals such as copper and iron derived from a heat exchanger or the like.
[0019]
The blow water 3 from the aqueous system 1 is dedusted by a strainer 4, and then added with a slime inhibitor 5 and an acid 6 such as hydrochloric acid for pH adjustment, and decarboxylated in a decarboxylation tower 8. Reference numeral 7 is a pH meter provided decarbonation tower inlet, V ₁ ~V ₅ represents the on-off valve provided in each occurrence.
[0020]
In the decarboxylation tower 8, the carbonate ions (CO ₃ ²⁻ ) contained in the blow water 3 are converted into carbon dioxide gas and efficiently removed, thereby resulting in the carbon dioxide component in the subsequent reverse osmosis membrane R. In addition to effectively preventing scale failure, the quality of the treated water T can be improved.
[0021]
The effluent water 9 from the decarbonation tower 8 is introduced by a pump P ₁ into a turbidity removal apparatus 10 having membrane filtration means such as an MF (microfiltration) membrane, a UF (ultrafiltration) membrane, and a cartridge filter. Remove SS (suspended material) in water (turbidity removing step). Thereby, since turbidity and colloidal components in the water that cause membrane contamination in the subsequent reverse osmosis membrane R are removed, water can be stably passed through the reverse osmosis membrane R.
[0022]
Here, the turbidity removing step can be omitted when the amount of SS in the water to be treated is small. However, in order to perform the reverse osmosis membrane treatment continuously and stably, the step of actively performing the step Is desirable. This turbidity removal step may be performed in the previous step of the reverse osmosis membrane treatment step, and is not limited to the arrangement shown in FIG. For example, it may be before the decarboxylation tower 8.
[0023]
Concentrated water 11 from the turbidizer 10 shown in FIG. 1 is returned again to the decarbonation tower 8, while a pH adjuster 13 and a chelating agent 14 are added to the permeated water 12 from the turbidizer 10. Later, it is introduced into the intermediate water tank 15. The pH adjusting agent 13 and the chelating agent 14 may be added directly to the intermediate water tank 15.
[0024]
Here, the pH adjustment of the water to be treated 16 once stored in the intermediate water tank 15 is pH 3 to 6, in order to prevent scale failure of the reverse osmosis membrane R due to calcium or silica contained in the water to be treated 16. Preferably, it is carried out so as to be within the range of pH 4.5 to 5.5. Since the pH of the water to be treated 16 that has been decarboxylated varies compared with that before the decarboxylation treatment, an acid such as hydrochloric acid, sulfuric acid, nitric acid, or an alkali such as NaOH or KOH is used as the pH adjuster 13. Adjust the pH to the above range. In addition, in this pH adjustment, if the pH is excessively low, it may cause corrosion of equipment and piping, so it is preferable to adjust to the above range.
[0025]
The chelating agent 14 is chelate-bonded with heavy metal ions such as copper and iron contained in the permeated water 12 to form a very stable ionic form, whereby a scale inhibitor or reverse osmosis membrane added to the aqueous system 1 It exhibits the effect of preventing the reaction between hydrazine added as a bactericidal agent such as R and the heavy metal ions. This action can effectively prevent the formation of unstable compounds due to the reaction between heavy metals and hydrazine, and thus can reliably prevent the deterioration of the reverse osmosis membrane R that is considered to be caused by the unstable compounds.
[0026]
The chelating agent 14 is appropriately selected from, for example, polyphosphates such as phosphonic acid and hexametaphosphoric acid, organic acid salts such as citric acid and ethylenediaminetetraacetate, and water-soluble polymers such as polyacrylic acid and polymaleic acid. Can do.
[0027]
In addition to the pH adjuster 13 and the chelating agent 14, a phosphonic acid-based, polyphosphoric acid-based, polyacrylic acid-based, polyacrylamide-based scale inhibitor is added to prevent the reverse osmosis membrane R from generating scale. May be. Since organic scale inhibitors may cause fouling in the reverse osmosis membrane R, phosphonic acid-based and polyphosphoric acid-based scale inhibitors can be suitably employed.
[0028]
pH is adjusted in the above range, and treated water 16 that the chelating agent 14 is added, is introduced from the intermediate supply water tank 15 to the reverse osmosis membrane R by the pump P _2, stable desalination process is performed continuously After that, it is discharged out of the system as concentrated water 17 and treated water (permeated water) T, respectively. In addition, the code | symbol 18 is the pH meter attached to the intermediate water tank 16, and the code | symbol 19 is a level switch.
[0029]
Here, the type of the reverse osmosis membrane R is not particularly limited and can be appropriately determined depending on the water quality of the water 16 to be treated. However, the desalination rate is 85% or more, particularly preferably 90% or more. It is desirable to adopt one. This is because if the desalination rate is lower than the above value, the deionization efficiency is poor and the treated water T having good water quality cannot be obtained.
[0030]
The water treatment method according to the present invention is not limited to the blow water 3 from the cooling water system, and for example, a part or all of the circulating cooling water is extracted from the circulation pipe of the circulating cooling water system and is treated by the method of the present invention. Then, it can be devised to return to the circulating cooling water system again. The method of the present invention can be applied not only to cooling water but also to natural water, river water, well water, city water, industrial water, and the like.
[0031]
【Example】
Examples of the present invention and comparative examples thereof will be described below.
[0032]
Hydrochloric acid is added to tap water, water adjusted to pH 4-6 is filtered through an ultrafiltration membrane, a copper sulfate solution is added to the filtered water, and the copper concentration of the water to be treated supplied to the reverse osmosis membrane is adjusted. It adjusted so that it might become 0.2-0.3 mg / L. Furthermore, after adding hydrazine to this water so that it might become 5 mg / L, it supplied to the reverse osmosis membrane. NTR759HR manufactured by Nitto Denko was adopted as the reverse osmosis membrane, and water was passed under conditions of an operating pressure of 0.6 MPa and a recovery rate of 67%.
[0033]
<Example>
3 mg / L of a chelating agent (sodium hexametaphosphate) was added to a water supply tank for water to be treated with respect to the reverse osmosis membrane apparatus, and the water to be treated supplied from the water tank was subjected to reverse osmosis membrane treatment. The result is shown in FIG.
[0034]
As shown in FIG. 2, in this example, the desalination rate was kept at a high level of 94 to 96%, and even after 200 days, the desalination rate was maintained at around 95%. Therefore, the water to be treated was stably passed through the reverse osmosis membrane. Moreover, the operation differential pressure was stable without changing from 0.6 MPa immediately after the start of water flow.
[0035]
<Comparative example>
A reverse osmosis membrane treatment was performed under the same conditions as in the above example except that no chelating agent was added. The result is shown in FIG.
[0036]
As shown in FIG. 3, the desalting rate in this comparative example was almost the same level as in the example immediately after the start of water flow, but the desalting rate began to decrease significantly after 80 days. On day 130, the desalination rate dropped to 30%.
[0037]
From the examples and comparative examples described above, when the water to be treated is acidic and contains heavy metals, the chelating agent can effectively act to prevent deterioration of the reverse osmosis membrane and maintain a high desalination rate over a long period of time. I understand.
[0038]
【The invention's effect】
According to the method of the present invention, even when a water to be treated containing calcium and silica is subjected to a reverse osmosis membrane treatment by adjusting the water to be treated to an acidic region within a predetermined range by an appropriate pH adjusting means. Since generation of scale of calcium carbonate in the apparatus and scale failure due to silica can be prevented, stable desalting treatment can be performed over a long period of time. In addition, by making the water to be treated acidic, microbial metabolites, fine particles, and colloidal substances in the water aggregate to form relatively large particles, which can be efficiently removed by membrane filtration.
[0039]
In particular, in the case that contains both heavy and hydrazine such as copper in the for-treatment water, the hydrazine reacts with the water to be treated of the heavy metal ions to form a labile compound, this instability compound reverse osmosis membrane the degrade, based on the new finding that causes a decrease in the desalination performance, and more it has devised so as to add a predetermined chelating agent to the treatment water before introduction reverse osmosis membrane, the chelating agent is a heavy metal ion In order to prevent the reaction between hydrazine and heavy metal ions by chelating with hydrazine to form a very stable ionic form, membrane performance degradation due to the unstable compound ( which is a reactive product of hydrazine and heavy metal ions) Can be prevented .
[0040]
Furthermore, in the method according to the present invention, by devising to provide a turbidity step before the reverse osmosis membrane treatment, in addition to the effect of preventing the deterioration of membrane performance by adding a chelating agent, membrane clogging and membrane contamination are effective. Can be prevented.
[Brief description of the drawings]
FIG. 1 is a system diagram schematically showing main steps of a method according to the present invention .
FIG. 2 is a diagram (graph) showing the results of Examples .
FIG. 3 is a diagram (graph) showing the results of a comparative example .

Claims (5)

A method of preventing deterioration of a reverse osmosis membrane by adding a chelating agent that forms a stable ionic form with the heavy metal into water to be treated in an acidic region containing at least heavy metal and hydrazine . The method of claim 1, wherein the heavy metal is copper . The method of claim 1, wherein the chelating agent is sodium hexametaphosphate . The method according to claim 1, wherein the suspended component of the water to be treated is removed and then introduced into the reverse osmosis membrane . The method according to any one of claims 1 to 4, wherein the water to be treated is a circulating cooling water of a cooling water system .

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