WO2018193907A1 - Water treatment method - Google Patents

Abstract

Provided is a water treatment method in which, after free chlorine is made present in water to be supplied to a water treatment device by generating chlorine by the addition of a chlorine oxidizing agent or electrolysis, the water is supplied to the water treatment device, said method adding a nitrogen compound to the water to be supplied as a chlorine stabilizer before the free chlorine is made to be present in the water to be supplied.

Description

Water treatment method

The present invention relates to a water treatment method, and more particularly to a water treatment method for suppressing biofouling in a water treatment device such as a reverse osmosis membrane (RO membrane) device for seawater desalination.

RO membrane devices are widely used as means for producing pure water by treating industrial water, city water, well water, sea water, river water, lake water, factory waste water, and the like. In RO membrane treatment, chlorine-based oxidizing agents such as chlorine, sodium hypochlorite, and sodium chlorite are added to the treated water (feed water) in order to suppress biofouling by microorganisms contained in the treated water. To do. In addition, chlorine is also generated by electrolysis.

When water containing free chlorine is added by the addition of chlorinated oxidant or electrolysis, the RO membrane undergoes oxidative degradation. In particular, polyamide-based RO membranes are susceptible to oxidative degradation.

Conventionally, an activated carbon tower is installed at the front stage of the RO membrane apparatus to remove residual oxidants such as chlorine (Patent Document 1), or a reducing agent such as sodium bisulfite (SBS) or sodium sulfite is installed at the front stage of the RO membrane apparatus. Addition to decompose and remove chlorine (Patent Document 2).

When an activated carbon tower is installed, biofouling may occur in the tower, which may contaminate subsequent equipment, and it may have initial costs. Has been removed.

2a and 2b are system diagrams showing an example of a conventional seawater desalination facility.

In FIG. 2a, a chlorine-based oxidant such as sodium hypochlorite (NaClO) is added in the process of supplying seawater to the raw water tank 1 by a water pump. Thereafter, an inorganic flocculant such as ferric chloride (FeCl ₃ ) is added in the course of being fed from the raw water tank 1 to the reaction tank 2, and agglomeration treatment is performed in the reaction tank 2. Next, after being filtered by the two-layer filter 3, the RO membrane treatment is performed by the RO membrane device 6 through the water supply tank 4 and the security filter 5. RO membrane permeated water is taken out as treated water. On the inlet side of the safety filter 5, a reducing agent such as sodium bisulfite (SBS) is added to decompose and remove the residual oxidant.

In FIG. 2b, seawater is electrolyzed by the electrolyzer 7, and seawater containing chlorine generated by electrolysis is aggregated and filtered in the same manner as in FIG. 2a, and then a reducing agent such as SBS is added. And processed by the RO membrane device 6.

In order to prevent biofouling from occurring in the RO membrane device as a result of the elimination of the free chlorine in the feed water by the addition of the reducing agent, after the addition of the reducing agent, the combined chlorine-based slime that has little deterioration effect on the RO membrane. A control agent (for example, “Kuriverter IK-110” manufactured by Kurita Kogyo Co., Ltd.) may be added.

Japanese Patent Laid-Open No. 10-337563 Japanese Unexamined Patent Publication No. 7-308671

The conventional seawater desalination RO membrane treatment has the following problems.
(1) Free chlorine produced by the addition of a chlorine-based oxidant such as sodium hypochlorite or the electrolysis of seawater is easily decomposed by organic substances, bromine and iodine in the seawater. There are many chemical additions and power consumption to make up for the biofouling suppression effect by supplementing the degradation.
(2) When a chlorine-based oxidant is added to seawater, harmful organic chlorine compounds such as trihalomethane are produced, which is problematic for drinking water applications.
(3) In order to prevent oxidative degradation of the RO membrane, it is necessary to add a reducing agent or install an activated carbon tower on the RO membrane inlet side, and the treatment is complicated.
(4) Since free chlorine is removed by addition of a reducing agent or activated carbon treatment before the RO membrane device, suppression of biofouling in the RO membrane device is insufficient.
(5) The problem of the above (4) can be solved by adding the combined chlorine-based slime control agent at the entrance of the RO membrane device, but in this case, the added chemicals increase.

This invention aims at providing the water treatment method which solves the subject of the above-mentioned conventional method.

Prior to the generation of chlorine by addition of a chlorine-based oxidant or electrolysis, the present inventor added a nitrogen compound as a chlorine stabilizer to the feed water to stabilize the chlorine in the feed water. In addition to preventing the generation of trihalomethane, etc., it is possible to prevent the oxidative degradation of the RO membrane by reducing the free chlorine concentration, making it unnecessary to add a reducing agent and install an activated carbon tower on the RO membrane inlet side. Thus, it has been found that biofouling of the RO membrane can be effectively suppressed.

The gist of the present invention is as follows.

[1] In a water treatment method for supplying free water to a water treatment apparatus after adding free chlorine by adding a chlorine-based oxidant to the water supplied to the water treatment apparatus or generating chlorine by electrolysis, A water treatment method characterized by adding a nitrogen compound as a chlorine stabilizer to the feed water prior to the presence of free chlorine in the feed water.

[2] The water treatment method according to [1], wherein the water treatment device is a reverse osmosis membrane device.

[3] The water treatment method according to [1] or [2], wherein the supplied water is seawater.

[4] The water treatment method according to any one of [1] to [3], wherein the nitrogen compound is a sulfamic acid compound and / or an organic nitrogen compound.

[5] The water treatment method according to any one of [1] to [4], wherein hypochlorite and / or dichloroisocyanuric acid is added to the feed water as the chlorinated oxidant.

[6] The water treatment method according to any one of [1] to [5], wherein the nitrogen compound is added to the feed water in an amount of 0.3 to 50 mg / L.

[7] In any one of [1] to [6], the residual chlorine concentration of the feed water after the presence of the free chlorine is maintained at 0.1 to 10 mg-Cl ₂ / L. Water treatment method.

According to the present invention, prior to the addition of a chlorine-based oxidant or generation of chlorine by electrolysis, a nitrogen compound is added to the feed water as a chlorine stabilizer, and then the free chlorine added or generated in the feed water is stabilized. By making them, decomposition of chlorine and generation of harmful substances such as trihalomethane can be prevented. In addition, the presence of chlorine as stabilized chlorine in the feed water and the reduction of free chlorine concentration reduces the RO membrane oxidation degradation of the feed water, so the addition of a reducing agent on the RO membrane inlet side and the activated carbon tower The installation of can be made unnecessary. As a result, the process can be simplified, and biofouling in the RO membrane device can be effectively suppressed by the stabilized chlorine.

1a and 1b are system diagrams of a seawater desalination facility showing an example of an embodiment of a water treatment method of the present invention. 2a and 2b are system diagrams of a conventional seawater desalination facility. 6 is a graph showing the results of Experimental Example 1. 10 is a graph showing the results of Experimental Example 2. 10 is a graph showing the results of Experimental Example 3. 10 is a graph showing the results of Experimental Example 4. 10 is a graph showing the results of Experimental Example 5. 10 is a graph showing the results of Experimental Example 6. 6 is a graph showing the results of Comparative Experimental Example 1.

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

1a and 1b are system diagrams showing an example of seawater desalination equipment to which the water treatment method of the present invention is applied, and are the same as members having the same functions as the seawater desalination equipment shown in FIGS. 2a and 2b, respectively. The code | symbol is attached | subjected.

As shown in FIGS. 1a and 1b, in the present invention, a nitrogen compound is added to the feed water as a chlorine stabilizer prior to the addition of a chlorine-based oxidizing agent such as sodium hypochlorite (NaClO) or the generation of chlorine by electrolysis. Add to stabilize chlorine in the feed water.

The nitrogen compound added for stabilizing the chlorine is not particularly limited as long as it can stabilize the chlorine in the supply water as stabilized bound chlorine or activated bound chlorine. Examples of nitrogen compounds include sulfamic acid, sulfamic acid sodium salts, potassium salts, calcium salts, ammonium salts, and other sulfamic acid compounds, glycine, taurine, threonine, ornithine (L-ornithine), alanine, phenylalanine (L-phenylalanine), and the like. Organic nitrogen compounds are mentioned. Only 1 type may be used for a nitrogen compound and it may use 2 or more types together.

The amount of nitrogen compound added to the feed water depends on the type of nitrogen compound used, the amount of chlorinated oxidant added in the subsequent stage, the amount of chlorine generated by electrolysis, the chlorine concentration to be maintained in the feed water, etc. However, it is preferably 0.3 to 50 mg / L, particularly about 0.3 to 20 mg / L. If the amount of nitrogen compound added is too small, the chlorine in the feed water cannot be sufficiently stabilized. Even if the amount of the nitrogen compound added is too large, the processing cost becomes high or it causes fouling and is inappropriate.

In the present invention, a nitrogen compound as a chlorine stabilizer is added at the above-mentioned preferable addition amount, and after the nitrogen compound is sufficiently uniformly diffused in the feed water, a chlorine-based oxidant is added or electrolysis is performed. It is preferable to generate chlorine. For example, when adding a chlorine-based oxidizer, as shown in FIG. 1a, a nitrogen compound as a chlorine stabilizer is added on the discharge side of the water pump and is homogenized while staying in the raw water tank 1. Thereafter, it is preferable to add a chlorine-based oxidizing agent such as NaClO together with an inorganic flocculant such as FeCl ₃ at the outlet side of the raw water tank 1. In the case of electrolysis, as shown in FIG. 1b, it is preferable to add a chlorine stabilizer on the discharge side of the water pump to make it uniform during electrolysis in the electrolyzer 7 and to generate chlorine.

As shown in FIG. 1a, when adding a chlorine-based oxidant after adding a chlorine stabilizer, any conventionally known one can be used as the chlorine-based oxidant. As the chlorinated oxidant, from the viewpoint of product safety, hypochlorite such as sodium hypochlorite or dichloroisocyanurate such as dichloroisocyanuric acid and sodium dichloroisocyanurate is preferable. It is preferable to use sodium chlorate or dichloroisocyanuric acid. Only one type of chlorine-based oxidizing agent may be used, or two or more types may be used in combination.

The addition amount of the chlorine-based oxidant varies depending on the tendency of biofouling to occur in the water system to be treated, but is usually about 0.1 to 1.0 mg / L, particularly about 0.3 to 0.7 mg / L. preferable. By adding a chlorine-based oxidizing agent within the above range after adding the nitrogen compound of the chlorine stabilizer, the residual chlorine concentration at the inlet of the RO membrane device is 0.3 to 1.0 mg-Cl ₂ / L, particularly 0.5. It is preferable to control it to be about 0.7 mg-Cl ₂ / L. If the residual chlorine concentration at the RO membrane device inlet is lower than the lower limit, a sufficient biofouling suppression effect may not be obtained. If the residual chlorine concentration at the RO membrane device inlet exceeds the above upper limit, there is a risk of RO membrane deterioration, depending on the proportion of free chlorine in the residual chlorine.

Even in the case of electrolysis, it is possible to control the electrolysis conditions so that the residual chlorine concentration at the entrance of the RO membrane device is within the above range due to the generation of chlorine by electrolysis after addition of the nitrogen compound of the chlorine stabilizer. preferable.

In particular, in the present invention, the residual chlorine concentration at the RO membrane apparatus inlet is in the above range by stabilizing chlorine by a chlorine-based oxidizing agent or chlorine generated by electrolysis with a nitrogen compound, and the free chlorine concentration is 0.1. By controlling so that it becomes 1 mg-Cl ₂ / L or less, especially 0.05 mg-Cl ₂ / L or less, it is possible to obtain a sufficient biofouling suppression effect while preventing oxidative degradation of the RO membrane due to free chlorine. Can do. For this reason, it is not necessary to add a reducing agent or install an activated carbon tower in the previous stage of the RO membrane device.

In particular, the proportion of bonded chlorine (total of activated bonded chlorine and stabilized bonded chlorine) in the residual chlorine at the entrance of the RO membrane device is preferably 90% or more, and more preferably 80% or more of stabilized bonded chlorine. It is preferable that By controlling the type and addition amount of the nitrogen compound as the chlorine stabilizer so as to be such a ratio, it is possible to obtain a favorable biofouling suppressing effect while preventing the RO membrane from being deteriorated.

Free chlorine, activated bound chlorine, and stabilized bound chlorine correspond to chlorine measured by the method described in the Examples section below. Residual chlorine is the sum of these free chlorine, activated bound chlorine and stabilized bound chlorine.

In the present invention, as described above, by controlling the residual chlorine concentration, it is not necessary to add a reducing agent at the entrance of the RO membrane device or install an activated carbon tower, but these operations and devices are excluded. Not what you want. If necessary, a small amount of a reducing agent may be added.

In FIGS. 1a and 1b, the case where the present invention is applied to a seawater desalination facility is illustrated. However, in the present invention, the water treatment apparatus is not limited to the RO membrane apparatus, and a chlorine-based oxidizing agent is used to suppress biofouling. Although addition or electrolysis is performed, it can also be applied to water treatment by a water treatment device such as an ion exchange device in which free chlorine needs to be removed on the inlet side in order to prevent deterioration of the device. Supply water is not limited to seawater, but can also be applied to river water, well water, lake water, various types of drainage, or the like. From the viewpoint of solving the problems such as decomposition of chlorine, generation of organic chlorine compounds such as trihalomethane, and RO membrane deterioration, the effect of the present invention is particularly effectively exhibited when applied to a desalination facility using a seawater RO membrane device.

Hereinafter, the present invention will be described more specifically by giving experimental examples instead of the examples.

In the following experimental examples, the chlorine concentration (mg-Cl ₂ / L) was measured using a pocket chlorine measuring device “HACH2470” manufactured by Toa DKK, and each chlorine concentration was determined by the following measurement method or calculation method.

Free chlorine concentration: Chlorine concentration measurement result (mg-Cl ₂ / L) after 5 to 30 seconds with DPD (Free) reagent, which is a reagent for measuring free chlorine
Activated bound chlorine concentration: From the chlorine concentration measurement result (mg-Cl ₂ / L) after 300 seconds by the DPD (Free) reagent, which is a reagent for measuring free chlorine, the above-mentioned free chlorine concentration (mg-Cl ₂ / L) Value obtained by subtracting measurement results Stabilized bound chlorine concentration: Reagent for free chlorine measurement from the chlorine concentration measurement result (mg-Cl ₂ / L) after 180 seconds using DPD (Total) reagent which is a reagent for total chlorine measurement The value obtained by subtracting the chlorine concentration measurement result (mg-Cl ₂ / L) after 300 seconds using the DPD (Free) reagent. Residual chlorine concentration: Sum of the above free chlorine concentration, activated bound chlorine concentration and stabilized bound chlorine concentration (mg -Cl ₂ / L)

大 Oarai seawater was used as seawater, and a beaker test was conducted.

[Experimental Example 1]
An experiment was conducted to investigate the effect of adding sulfamic acid as a chlorine stabilizer to seawater.

Add sulfamic acid 30mg / L to seawater and stir and mix uniformly for 1 minute, then add 1.5mg-Cl ₂ / L sodium hypochlorite and examine the chlorine concentration after 5 and 60 minutes. It was.

Separately, 1.5 mg-Cl ₂ / L of sodium hypochlorite was added to seawater, 30 mg / L of sulfamic acid was added after 1 minute, and each chlorine concentration after 5 minutes and 60 minutes was examined.

These results are shown in FIG.

In FIG. 3, “(after) sulfamic acid” indicates the case where sulfamic acid is added after the addition of sodium hypochlorite, and “(previous) sulfamic acid” indicates the case where sulfamic acid is added before the addition of sodium hypochlorite. Indicates. The same applies to Experimental Examples 2 to 6 below.

[Experimental Examples 2 to 6]
The effects of addition were examined in the same manner as in Experimental Example 1 except that 7.5 mg / L of the following was added instead of sulfamic acid 30 mg / L. The results are shown in FIGS. 4 to 8, respectively.
Example 2: Glycine Example 3: Phenylalanine Example 4: Threonine Example 5: Ornithine Example 6: Taurine

[Comparative Experiment Example 1]
Sodium hypochlorite (NaClO) was added to seawater at 1.5 mg-Cl ₂ / L or 2.5 mg-Cl ₂ / L, and each chlorine concentration was measured after 5 or 60 minutes. The results are shown in FIG.

The following can be seen from Figs.

In the case of adding only sodium hypochlorite, when 1.5 mg-Cl ₂ / L is added, the residual chlorine concentration decreases to a residual chlorine concentration of 0.4 mg-Cl ₂ / L after 60 minutes. resulting in less than _1.2mg-Cl 2 / L even after 60 minutes at high concentrations the addition of _5mg-Cl 2 / L (Fig. 9).

On the other hand, by adding a nitrogen compound as a chlorine stabilizer and then adding sodium hypochlorite, even if the free chlorine concentration is reduced, the activated bound chlorine and the stabilized bound chlorine concentration are increased. The residual chlorine concentration can be maintained higher than when only sodium chlorate is added, and by adding 1.5 mg-Cl ₂ / L of sodium hypochlorite, the residual chlorine concentration is reduced to 1.2 even after 60 minutes. It can be maintained at about 1.5 mg-Cl ₂ / L. However, even if the same nitrogen compound is added after addition of sodium hypochlorite, such an effect cannot be obtained (FIGS. 3 to 8).

From these results, the following can be understood.
According to the present invention, after adding a nitrogen compound as a chlorine stabilizer in advance, a chlorine-based oxidant such as sodium hypochlorite is added or free chlorine is generated by electrolysis, whereby chlorine in the supply water is reduced. It can be stabilized, and the following excellent effects can be obtained.
1) Since the decomposition and consumption of chlorine by organic matter, bromine and iodine in seawater can be suppressed, the amount of chlorinated oxidant added necessary to suppress biofouling is reduced or the power consumption of the electrolyzer is reduced. be able to.
2) By reducing the free chlorine concentration, it is possible to reduce the influence of RO membrane deterioration and the like. In some cases, biofouling of the RO membrane can be suppressed without performing decomposition treatment with a reducing agent or activated carbon tower before the RO membrane device.
3) By stabilizing chlorine, it is possible to suppress the generation of harmful substances such as trihalomethanes.

Since the effect of the present invention cannot be obtained by the post-addition of the nitrogen compound, the effect of the present invention is that the chlorine-based oxidant is simply stabilized with the nitrogen compound and added as a combined chlorine-based oxidant. It turns out that it is completely different.

Although the present invention has been described in detail using specific embodiments, it will be apparent to those skilled in the art that various modifications can be made without departing from the spirit and scope of the invention.
This application is based on Japanese Patent Application No. 2017-082942 filed on Apr. 19, 2017, which is incorporated by reference in its entirety.

DESCRIPTION OF SYMBOLS 1 Raw water tank 2 Reaction tank 3 Two-layer type filter 4 Water supply tank 5 Security filter 6 RO membrane apparatus 7 Electrolyzer

Claims (7)

In the water treatment method of supplying free water to the water treatment apparatus after adding free chlorine by adding a chlorine-based oxidizing agent to the water supplied to the water treatment apparatus or generating chlorine by electrolysis,
A water treatment method comprising adding a nitrogen compound as a chlorine stabilizer to the feed water prior to the presence of free chlorine in the feed water. The water treatment method according to claim 1, wherein the water treatment device is a reverse osmosis membrane device. 3. The water treatment method according to claim 1 or 2, wherein the supplied water is seawater. The water treatment method according to any one of claims 1 to 3, wherein the nitrogen compound is a sulfamic acid compound and / or an organic nitrogen compound. 5. The water treatment method according to any one of claims 1 to 4, wherein hypochlorite and / or dichloroisocyanuric acid is added as the chlorine-based oxidant to the feed water. 6. The water treatment method according to claim 1, wherein the nitrogen compound is added to the feed water in an amount of 0.3 to 50 mg / L. The water treatment method according to any one of claims 1 to 6, wherein the residual chlorine concentration of the feed water after the presence of the free chlorine is maintained at 0.1 to 10 mg-Cl ₂ / L. .

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