JP2017006829A - Water treatment method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a water treatment method for more effectively reducing chemical oxygen demand (COD), and to provide a water treatment method that can easily optimize the amount of oxidant to be added even when hypohalous acid is used as the oxidant.SOLUTION: A water treatment method comprises a step in which hypochlorous acid and bromide ion are coexist in water containing sulfur component which is detected as COD. The amount of oxidant to be added in the water is preferably controlled based on the measurement of oxidant-reduction potential.SELECTED DRAWING: None

Description

  The present invention relates to a water treatment method, and more particularly, to a treatment method for reducing chemical oxygen demand (COD) in waste water.

  Wastewater from various factories, such as rainwater drainage from a blast furnace slag yard at an ironworks, may have a high COD value due to the inclusion of sulfur components. As a method for reducing COD, for example, a method of adding various oxidizing agents to waste water is known.

  As a method of adding an oxidizing agent to wastewater, hypohalous acid such as hypochlorous acid or hypobromous acid is used as an oxidizing agent, COD, biochemical oxygen demand (BOD), total organic carbon (TOC), There is a technique for reducing ammonia nitrogen (Patent Document 1). As a method for reducing COD, for example, Patent Document 1 discloses a technique in which an aqueous solution in which hypochlorous acid or hypobromite is generated by electrolyzing water in the presence of sodium chloride or sodium bromide is used as an oxidizing agent. Is disclosed.

  On the other hand, in the water treatment using hypochlorous acid, it is known that the disinfection power by chlorine is weakened under conditions of high pH and conditions containing nitrogen compounds such as ammonia and amines (Non-patent Document 1). This is thought to be due to the weakening of the oxidizing power by hypochlorous acid.

JP 2010-094576 A

Mitsumi Kaneko, “Water Hygiene”, Gihodo Publishing Co., Ltd., June 20, 1996, p. 289-305

  In the technique of performing water treatment using hypochlorous acid disclosed in Patent Document 1, it is not always possible to obtain a satisfactory effect in treatment performance, particularly under conditions of high pH and conditions containing nitrogen compounds. there were. In addition, among the components detected as COD, there is a need for a water treatment method that has a higher effect of reducing sulfur components such as sulfide ions, thiosulfate ions, and sulfite ions.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a water treatment method having a higher effect of reducing COD.

  (1) A water treatment method comprising a step of coexisting hypochlorous acid and bromide ions in water containing a sulfur component detected as COD (chemical oxygen demand).

  (2) The method according to (1), wherein the amount of hypochlorous acid added in the treated water is managed based on the measured value of the oxidation-reduction potential of the treated water in which hypochlorous acid and bromide ions coexist.

  (3) When the pH of the water is 7 and the water temperature is 25 ° C., the amount of hypochlorous acid added is controlled so that the measured value of the oxidation-reduction potential of the treated water is 400 mV or more (2 ) The method described.

(4) Hypochlorous acid is dissolved in the water by adding an oxidizing agent represented by the general formula R ^I OCl, and bromide ions are added by adding a halogen compound represented by the general formula R ^II Br. The method according to any one of (1) to (3), wherein the R ^I atom and the R ^II atom are at least one selected from a hydrogen atom, an alkali metal atom, and an alkaline earth metal atom. .

  (5) The method according to (4), wherein hypochlorous acid or sodium hypochlorite is used as the oxidizing agent.

  (6) The method according to (4) or (5), wherein an alkali metal or alkaline earth metal bromide is used as the halogen compound.

  (7) The method according to any one of (4) to (6), wherein sodium bromide is used as the halogen compound.

  (8) The method according to any one of (4) to (7), wherein the oxidizing agent and the halogen compound are separately added to the water.

  According to the present invention, hypochlorous acid and bromide ions are allowed to coexist in water, so that the oxidizing power of hypochlorous acid can be increased under conditions such as high pH and conditions containing nitrogen compounds such as ammonia and amines. Even under weakening conditions, a compound having a higher oxidizing power than hypochlorous acid is produced, so that the effect of reducing COD can be further enhanced with respect to a wider range of water quality.

  In addition, according to the present invention, a compound having a high oxidizing power is produced simply by coexisting hypochlorous acid and bromide ions in water, so that the work surface is not subjected to an electrolysis process that increases the processing cost. Thus, a more efficient water treatment method can be provided.

  Hereinafter, although embodiment of this invention is described, this invention is not limited to these.

  The inventors of the present invention provide hypochlorous acid or a salt thereof produced when an oxidizing agent composed of hypochlorous acid or a salt thereof and a halogen compound having a bromine atom are used together. We focused on the fact that it is not easily affected by dissolved nitrogen compounds and has high oxidizing power. Here, the oxidizing agent composed of hypochlorous acid or a salt thereof generates hypochlorous acid (HOCl) in water, and the halogen compound having a bromine atom generates bromide ions in water. That is, the water treatment method according to the present invention reduces COD by allowing hypochlorous acid and bromide ions to coexist in water.

  The water to which the method of the present invention is applied contains sulfur components such as sulfide ions, thiosulfate ions, and sulfite ions as components detected as COD (hereinafter referred to as “COD components”). The substance produced by hypochlorous acid and bromide ions used in the water treatment method of the present invention may be water having a neutral or higher pH, or water containing nitrogen compounds such as ammonia or amine. Since it has a strong oxidizing action, it is also useful for the oxidation of sulfur components that have been relatively difficult to oxidize with oxidizing agents.

  In particular, the water to which the method of the present invention is applied may contain at least one selected from ammonia and amines. In the method of the present invention, the content of hypochlorous acid contained in water can be determined from the measured value of the redox potential of water. Therefore, even if a discontinuous change (ammonia break) occurs in the residual chlorine concentration in water due to the oxidation reaction of the COD component by hypochlorous acid and the decomposition reaction of molecules having ammoniacal nitrogen by hypochlorous acid. Since the addition amount of hypochlorous acid consumed in common in these reactions can be controlled, the addition amount of an oxidizing agent comprising hypochlorous acid or a salt thereof can be easily optimized.

  In the method of the present invention, hypochlorous acid (HOCl) and bromide ions are allowed to coexist in water, whereby the reaction shown in the following formula (I) occurs, and hypobromite (HOBr) is generated. Inferred.

HOCl + Br ⁻ → HOBr + Cl ⁻ (I)

Then, in addition to hypochlorous acid dissolved in water, hypobromite generated by the reaction shown in the above formula (I) oxidizes the COD component, so that the COD of water can be reduced. In particular, the presence of hypobromite reduces the effect on the oxidizing power of pH and nitrogen compounds, etc., but also enhances the oxidizing power, so the oxidation of sulfur components that were relatively difficult to oxidize with hypochlorous acid. Also useful.
Here, when the COD component is sodium thiosulfate, it is presumed that the oxidation of thiosulfate ions undergoes reactions shown in the following formulas (II) and (III).

Na ₂ S ₂ O ₃ + 4HOBr + H ₂ O → 2H ₂ SO ₄ + 2HBr + 2NaBr
... (II)
Na ₂ S ₂ O ₃ + 4HOCl + H ₂ O → 2H ₂ SO ₄ + 2HCl + 2NaCl
... (III)

  Here, the oxidizing power by hypobromite is hardly affected by the pH and the content of nitrogen compounds. For example, the presence ratio of hypobromite in water can maintain a high state even when the pH is neutral or higher. In addition, monobromamine and dibromamine produced by reaction of hypochlorous acid with ammonia have higher oxidizing power than chloramine produced by reaction of hypochlorous acid with ammonia. As a result, an oxidation reaction with hypobromous acid as shown in formula (II) is more likely to occur than an oxidation reaction with hypochlorous acid as shown in formula (III).

  Thus, in addition to the oxidation reaction by hypochlorous acid shown by Formula (III), the oxidation reaction by hypochlorous acid shown by Formula (II) occurs, whereby the decomposition of the COD component can be promoted. Furthermore, bromine compounds generated in the formula (II) (HBr and NaBr in the formula (II)) react with hypochlorous acid to generate hypobromous acid again, so that hypochlorous acid is converted into water. While remaining, the strong oxidizing power of hypobromite can be maintained.

In the method of the present invention, hypochlorous acid can be dissolved in water by adding, for example, an oxidizing agent comprising hypochlorous acid represented by the general formula R ¹ OCl or a salt thereof to water. R ^I atoms contained in the oxidizing agent, the cost and handleability, safety, considering the surface such as solubility in water, one or more selected from a hydrogen atom, an alkali metal atom or alkaline earth metal atom . More preferably, the R ^I atom is at least one selected from a hydrogen atom and a sodium atom, and more preferably, the R ^I atom is a sodium atom. Compound R ^I atom is sodium atom is sodium hypochlorite, can be used as sodium hypochlorite for example 12% sodium hypochlorite.

Further, bromide ions can be dissolved in water by adding, for example, a halogen compound composed of bromide represented by the general formula R ^II Br to water. The R ^II atom contained in the halogen compound is one selected from a hydrogen atom, an alkali metal atom, and an alkaline earth metal atom in consideration of cost, handleability, safety, and reactivity with the oxidizing agent R ^I OCl. The above is preferable. More preferably, the R ^II atom is one or more selected from an alkali metal atom and an alkaline earth metal atom, and more preferably, the R ^II atom is a sodium atom. The compound in which the R ^II atom is a sodium atom is sodium bromide, and for example, 40% sodium bromide can be used as the sodium bromide.

As the oxidizing agent and the halogen compound, it is preferable to use a compound containing the same element as the R ^I atom and the R ^II atom. This facilitates the simultaneous generation of hypochlorous acid from the oxidant and the generation of bromide ions by dissociation of the halogen compound, so that a strong oxidizing power by hypobromite can be easily obtained. In particular, as the oxidant and the halogen compound, it is more preferable to use a salt containing the same metal element as the R ^I atom and the R ^II atom. Thereby, the production | generation of hypochlorous acid by hydrolysis of a hypochlorite ion and the production | generation of bromide ion by dissociation of a halogen compound can be performed easily simultaneously.

  The method of coexisting hypochlorous acid and bromide ions in water is not particularly limited, and an oxidizing agent and a halogen compound may be directly added to a tank with a stirrer in which water to be treated is stored. Good. In addition, a line mixer is connected to the pipe through which water to be treated is passed, and the oxidizer and halogen compound are injected into the pipe in front of the line so that the oxidizer, halogen compound and water are mixed in the line mixer. May be.

The order of adding the oxidizing agent and halogen compound to water may be added to water immediately after mixing the oxidizing agent and halogen compound, but it is also preferable to add the oxidizing agent and halogen compound separately to water. . In particular, by adding these separately, one of the oxidant and the halogen compound and the COD component are mixed with water first, so hypobromite and COD component with poor chemical stability. It is easy to proceed with the reaction.
Here, adding an oxidizing agent and a halogen compound separately to water means adding at least a part to water without mixing. In particular, when the oxidant and the halogen compound are added to water without being substantially mixed, it is desirable that the addition of the halogen compound and the place and timing of adding the oxidant are made closer. This makes it possible to efficiently react hypochlorous acid dissolved in water by adding an oxidant and bromide ions dissolved in water by adding a halogen compound, and also manage the amount of oxidant added. Easy to do.

  In addition, when the water to be treated is in a closed system or when the supply of blown water to the water is small, one of the oxidizing agent and the halogen compound is added to the water in advance, and the other is converted to water later. By adding, an oxidizing agent and a halogen compound may coexist. Among these, it is more preferable to add the halogen compound to water in advance and add the oxide to water later so that the oxidizing agent and the halogen compound coexist. Thereby, since one chemical | medical agent added previously reacts with the other chemical | medical agent added afterwards, a water treatment is made, Therefore The water treatment to water can be performed in a shorter time. In particular, by adding a halogen compound to water in advance, consumption of the oxidizing agent until hypobromous acid is generated can be suppressed, so that a reduction in the water treatment efficiency of the oxidizing agent can be suppressed.

  On the other hand, if a mixture of an oxidant and a halogen compound is stored, the above-described hypobromite generated by the reaction of the oxidant and the halogen compound is poor in stability, so this is added to water. By the time the oxidant is consumed, thereby reducing the water treatment efficiency of the oxidant. Therefore, after mixing, the oxidizing agent and the halogen compound are preferably added to water within 30 minutes, and more preferably added to water within 10 minutes.

  Here, the order of addition of the oxidizing agent and the halogen compound in the present invention is not limited to a temporal one, but includes a local one. For example, the addition of the oxidizing agent after the halogen compound is added in advance means that the addition of the oxidizing agent is started (temporal) after the addition of the halogen compound is started, and the addition point of the halogen compound is oxidized. The embodiment also includes a (spatial) mode upstream of the water system from the point where the agent is added.

  The addition amount of the oxidizing agent is determined depending on the content of the COD component in water and the presence or absence of other components such as ammonia and amine. As an example, on the basis of the volume of the target water, it is preferably 0.5 mmol / L or more, more preferably 1.5 mmol / L or more, and even more preferably 2.5 mmol / L or more. May be within a range of 30 mmol / L or less, more preferably 25 mmol / L or less, and even more preferably 20 mmol / L or less. In determining the addition amount of the oxidizing agent, a treatment test using a target water sample may be performed in advance, and the addition amount may be obtained based on the result.

  The addition amount of the halogen compound is not particularly limited, but is preferably 0.05 mmol / L or more, more preferably 0.2 mmol / L or more, and further preferably 0.3 mmol / L or more, based on the volume of the target water. It can be used in the range. By increasing the amount of halogen compound added, when coexisting with hypochlorous acid in water, the content of hypochlorous acid produced by the reaction with hypochlorous acid increases, so the treatment rate for water is reduced. Enhanced. On the other hand, the upper limit of the addition amount of the halogen compound is preferably 3.0 mmol / L or less, more preferably 2.5 mmol / L or less, and still more preferably 2.0 mmol / L, from the viewpoint of suppressing excessive content of the halogen compound. It is good also as follows. It is also preferable to determine the amount of the halogen compound added based on the result of a treatment test using a target water sample in advance.

The content ratio of the oxidizing agent and the halogen compound is not particularly limited, but the ratio of (number of moles of chlorine atom based on Cl _{2 based on the} oxidizing agent): (number of moles of bromine atom contained in the halogen compound) Is preferably 50: 1 to 1: 1, and more preferably 20: 1 to 5: 1.

  Control of the amount of oxidant added to water is based on the measured value of the oxidation-reduction potential (ORP) of water containing hypochlorous acid and bromide ions (hereinafter sometimes referred to as “treated water”). It is preferable to carry out by controlling the contents of hypochlorous acid and hypobromite in the treated water. More specifically, a step of measuring the oxidation-reduction potential, a step of determining the excess or deficiency of the content of hypochlorous acid and hypobromite in the treated water based on the value of the measured oxidation-reduction potential, When it is determined that the content of hypochlorous acid and hypochlorous acid is insufficient, it is preferable to manage the addition amount of the oxidizing agent by repeating the step of further adding the oxidizing agent. In this method, the treatment water is strongly oxidized by hypobromous acid from the content of hypochlorous acid and hypobromite contained in the treated water, which is estimated based on the oxidation-reduction potential. It is determined whether or not it is in a state. Therefore, according to this method, it is possible to reliably reduce COD by coexisting hypochlorous acid and bromide ions, and to suppress excessive addition of the oxidizing agent, so that the amount of the oxidizing agent added can be optimized. Can be planned.

  Here, the oxidation-reduction potential, which is a reference for the oxidant content, is influenced by the pH, the water temperature, and the concentration of the oxidant, and therefore cannot be specified in general. However, as an example, when the pH of the water to be treated is 7 and the water temperature is 25 ° C., the oxidation contained in the treated water so that the measured value of the oxidation-reduction potential of the treated water is 400 mV or more. It is preferable to control the addition amount of the agent. At this time, the measured value of the oxidation-reduction potential is preferably managed at 400 mV or more, more preferably managed at 520 mV or more, and further preferably managed at 600 mV or more. By managing the redox potential at a high value, the amount of COD reduction can be further increased.

  Moreover, in the method of this invention, you may further use a corrosion inhibitor, the anticorrosive agent for copper, a scale inhibitor, an antifoamer, surfactant, etc. as needed.

As test water to be treated, waste water containing a sulfur component detected as COD was used. Here, water containing 40 mg / L of thiosulfuric acid and oxygen demand (COD _Mn ) by potassium permanganate at 100 ° C. of 38 mg / L is designated as test water 1, 84 mg / L of thiosulfuric acid is contained, and COD Water having _Mn of 68 mg / L was designated as test water 2. These test waters were adjusted using hydrochloric acid so that the pH was 7.

For test waters 1 and 2, sodium hypochlorite (12% NaOCl), hydrogen peroxide (35% H ₂ O ₂ ) and ferric chloride (38% FeCl ₃ ) were used as halogen compounds as oxidizing agents. Sodium bromide (40% NaBr) was added to each concentration shown in Table 1. Here, addition of sodium hypochlorite and sodium bromide to the test water was performed by immediately adding a mixture of sodium hypochlorite and sodium bromide to the test water.

The test water 1 was allowed to stand at room temperature for 10 minutes and the test water 2 for 60 minutes, and then neutralized with sodium sulfite and specified in JIS P 3801 [filter paper (for chemical analysis)]. COD _Mn was measured in test water filtered with 5 types A filter paper. COD _Mn is obtained by mixing potassium permanganate in test water to a concentration of 5 mmol / L in accordance with JIS K 0102 “Acid consumption by potassium permanganate at 100 ° C. (CODMn)”. It calculated | required from the consumption of the potassium permanganate when a liquid mixture was heated for 30 minutes with boiling water.

As is apparent from Table 1, the reduction rate of COD _Mn in the example in which sodium hypochlorite and sodium bromide were added to water (Example 1) was the same as that in Example 1, but only sodium hypochlorite. The COD _Mn reduction rate of the example added to water (Comparative Example A) was higher. In Example 1, the numerical value of the oxidation-reduction potential was also lower than that of Comparative Example A.
The reason why such a result was obtained is that by adding sodium hypochlorite and sodium bromide to water, hypochlorous acid and bromide ions coexist in water, and hypochlorous acid and bromide ions react. Thus, it is presumed that sodium hypobromite was formed, and that the oxidizing power was higher than that of sodium hypochlorite.

Further, in the example (Example 3) in which the conditions of the COD component concentration and the reaction time were changed from Example 1, the same COD _Mn reduction rate and oxidation-reduction potential as in Example 1 were obtained.

Of a mixture of sodium bromide sodium hypochlorite example added to water, the reduction rate of COD _Mn in Example 2 is lower than the COD _Mn of Example 1 was carried out in the same test water and reaction time Value. In Example 2, the value of the oxidation-reduction potential was also lower than that in Example 1.
The reason why the numerical value of the oxidation-reduction potential is low and the reduction rate of COD _Mn is low in Example 2 is contained in water because the content of sodium hypochlorite is as low as 1.0 mmol / L. It is presumed that the oxidant was consumed because the COD component could not be oxidized sufficiently.
From this, it is considered that the numerical value of the oxidation-reduction potential is useful as an index of the content of sodium hypochlorite when sodium hypochlorite and sodium bromide are added to water. In addition, since the numerical value of this oxidation-reduction potential is a numerical value that can be directly measured with respect to water to which sodium hypochlorite and sodium bromide are added, by controlling this numerical value, It may be possible to optimize.

On the other hand, in the examples using hydrogen peroxide as the oxidizing agent (Comparative Examples B and C), the reduction rate of COD _Mn is a constant value regardless of the amount of hydrogen peroxide added. It was a lower value than the example (Example 1) which added sodium oxyacid and sodium bromide, and the example (Comparative Example A) which added only sodium hypochlorite. In Comparative Examples B and C, the value of the oxidation-reduction potential was also lower than that in Example 1 and Comparative Example A, and was a constant value regardless of the amount of hydrogen peroxide added. Therefore, by adding sodium hypochlorite and sodium bromide, the method of the present invention in which hypochlorous acid and bromide ions coexist reduces COD compared to the method using hydrogen peroxide. It became clear that the effect of making it more effective.

In addition, in the example in which hydrogen peroxide and ferric chloride were added as oxidizing agents (Comparative Example D), the reduction rate of COD _{Mn was} also the example in which sodium hypochlorite and sodium bromide were added to the same test water (implementation) The value was lower than in Example 3). Further, in Comparative Example D, the numerical value of the oxidation-reduction potential was also lower than that in Example 3. From this, by adding sodium hypochlorite and sodium bromide, the method of the present invention in which hypochlorous acid and bromide ions coexist is compared with the method of adding hydrogen peroxide and ferric chloride. However, it has become clear that the effect of reducing COD can be further enhanced.

From the above, in the method of coexisting hypochlorous acid and bromide ions in water, the method using only sodium hypochlorite, the method using hydrogen peroxide, hydrogen peroxide and ferric chloride It has been clarified that the effect of reducing COD can be further enhanced as compared with the combined method.
In this method, the hypochlorous acid and bromide ions can coexist in water simply by adding an oxidizing agent and a halogen compound to water, thereby increasing the COD reduction effect. It was also found that the process of raising the temperature is not necessary, and that water treatment can be performed efficiently on the work surface.
It is also possible to optimize the amount of sodium hypochlorite added by managing the numerical value of the redox potential of water containing hypochlorous acid and bromide ions.

Claims (8)

  A water treatment method comprising a step of coexisting hypochlorous acid and bromide ions in water containing a sulfur component detected as COD (chemical oxygen demand).   The method according to claim 1, wherein the amount of hypochlorous acid added in the treated water is managed based on the measured value of the oxidation-reduction potential of the treated water in which hypochlorous acid and bromide ions coexist.   The hypochlorous acid addition amount is managed so that the measured value of the oxidation-reduction potential of the treated water is 400 mV or more when the pH of the water is 7 and the water temperature is 25 ° C. Method. Claims: Hypochlorous acid is dissolved by adding an oxidant represented by the general formula R ^I OCl to the water, and bromide ions are dissolved by adding a halogen compound represented by the general formula R ^II Br. Item 4. The method according to any one of Items 1 to 3, wherein the R ^I atom and the R ^II atom are at least one selected from a hydrogen atom, an alkali metal atom, and an alkaline earth metal atom.   The method of Claim 4 which uses hypochlorous acid or sodium hypochlorite as said oxidizing agent.   6. The method according to claim 4, wherein an alkali metal or alkaline earth metal bromide is used as the halogen compound.   The method according to any one of claims 4 to 6, wherein sodium bromide is used as the halogen compound.   The method according to claim 4, wherein the oxidizing agent and the halogen compound are added separately to the water.