Reduction treatment method for waste acidifying agent solution

Abstract

To provide a reduction treatment method for an oxidizer waste solution, capable of preventing generation of acidic gas when carrying out the reduction treatment.SOLUTION: According to the reduction treatment method, an alkali pH adjuster for adjusting pH of an oxidizer waste solution is fed into the oxidizer waste solution prior to feeding a reducing agent into the oxidizer waste solution. The alkali pH adjuster neutralizes an acid formed during the reduction reaction of the oxidizer and the reducing agent to maintain the oxidizer waste solution in an alkaline condition, and thereby to prevent the generation of acidic gas.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a reduction treatment method in which a reducing agent is added to and treated with a waste liquid containing an oxidizing agent such as sodium hypochlorite.

Background Art

[0002] In conventional water purification treatment, the sand filtration method has been used as a solid-liquid separation method for water to be treated containing suspended substances. In recent years, in anticipation of more advanced solid-liquid separation, the introduction of a low-pressure membrane filtration method using a microfiltration membrane (MF membrane) or an ultrafiltration membrane (UF membrane) has been progressing.

[0003] On the other hand, one of the problems in the membrane filtration method is that membrane fouling progresses due to organic substances in raw water and components derived from flocculants such as aluminum, and a predetermined membrane filtration flow rate cannot be maintained. In order to prevent serious membrane fouling, chemical cleaning of the membrane is carried out about several times a year. In chemical cleaning of the membrane, generally, a high-concentration oxidizing agent solution (for example, a sodium hypochlorite solution having a concentration of 1000 mg / L or more), which is a cleaning chemical for organic fouling of the membrane, is used. In chemical cleaning of the membrane with an oxidizing agent solution, the membrane to be cleaned is immersed in the oxidizing agent solution for about several hours to one day.

[0004] Here, the waste liquid of the high-concentration oxidizing agent solution (hereinafter sometimes referred to as "oxidizing agent waste liquid") after being used for chemical cleaning of the membrane is either disposed of as industrial waste or discharged into sewage after waste liquid treatment. As waste liquid treatment methods, a method using a catalytic reaction by adding activated carbon and a method using a reduction reaction by adding a reducing agent are known. In addition, in order to implement the treatment method of oxidizing agent waste liquid with activated carbon, dedicated equipment is required, which leads to an increase in treatment cost. Therefore, as the treatment of oxidizing agent waste liquid, reduction treatment with a reducing agent is generally performed.

[0005] In a method for determining the end point of a reduction reaction, i.e., the end point of the reduction treatment of an oxidant waste liquid, generally, a concentration meter capable of measuring the residual oxidant concentration (for example, a chlorine concentration meter) is used. However, with this type of concentration meter, it may be difficult to measure a high concentration of the residual oxide concentration, and furthermore, it is difficult to continuously monitor the residual oxide concentration.

[0006] Therefore, in Patent Document 1, during the reduction process of chlorine (oxide), the oxidation-reduction potential value (ORP value) is measured, and the residual chlorine concentration is determined from the change in the ORP value to control the end point of the reduction treatment and the inflow rate of the reducing agent.

[0007] In Patent Document 2, the residual chlorine concentration is controlled by the pH value and the ORP value. In this method for controlling the residual chlorine concentration, a reference for the ORP value in a predetermined pH range is set. While adjusting the pH value within the predetermined range, when the ORP value exceeds the reference value, a reducing agent is injected, and when the ORP value becomes below the reference value, the reduction treatment is automatically terminated.

[0008] Patent Document 3 describes an arithmetic control method for controlling the supply amount of a reducing agent based on the corrected ORP value by correcting the ORP value based on the ORP value and the pH value of the water to be treated as a control method for a residual chlorine removal system.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0010] When the oxidizing agent contained in the oxidizing agent waste liquid is reductively treated using a reducing agent, acidic gas may be generated from the oxidizing agent waste liquid. For example, when sodium thiosulfate is added to the waste liquid of a sodium hypochlorite solution to reductively treat sodium hypochlorite, chlorine gas is generated from the waste liquid. Acidic gases are generally harmful, and in order to protect the environment and ensure the safety of workers, it is necessary to suppress the generation of acidic gases.

[0011] However, in the conventional reductive treatment methods described in Patent Documents 1 to 3 above, only the control of the reductive treatment using the ORP value is considered, and no method for suppressing the generation of acidic gas during the reductive treatment has been studied at all.

[0012] Therefore, an object of the present invention is to provide a method for reductively treating an oxidizing agent waste liquid capable of suppressing the generation of acidic gas during the reductive treatment.

Means for Solving the Problems

[0013] In one aspect, there is provided a method for reductively treating an oxidizing agent waste liquid in which the oxidizing agent contained in the oxidizing agent waste liquid is reductively treated with a reducing agent, wherein before the reducing agent is added to the oxidizing agent waste liquid, an alkaline pH adjuster for adjusting the pH value of the oxidizing agent waste liquid is added to the oxidizing agent waste liquid, and the alkaline pH adjuster neutralizes the acid generated during the reduction reaction between the oxidizing agent and the reducing agent to maintain the liquid property of the oxidizing agent waste liquid alkaline, thereby suppressing the generation of acidic gas. A method for reductively treating an oxidizing agent waste liquid is provided.

[0014] In one aspect, during the reductive treatment, the oxidation-reduction potential value of the oxidizing agent waste liquid is measured, and the time point at which the oxidation-reduction potential value rapidly decreases is determined as the end point of the reduction reaction. In one aspect, the oxidizing agent is sodium hypochlorite, the reducing agent is sodium thiosulfate, and the alkaline pH adjuster is sodium hydroxide. In one aspect, sodium hydroxide is added to the oxidizing agent waste liquid such that the ratio of the amount of substance of sodium hydroxide to the amount of substance of sodium hypochlorite is 0.09 or more.

[0015] In one aspect, the oxidizing agent is an oxidizing agent used for cleaning the filtration membrane of the membrane filtration facility. In one aspect, the oxidizing agent waste liquid is put into a waste liquid tank, and the waste liquid tank is provided with a diffuser tube for stirring a mixed liquid of the oxidizing agent waste liquid, the alkaline pH adjuster, and the reducing agent. In one aspect, the pH value of the oxidizing agent waste liquid after the reduction treatment is measured, and an acid pH adjuster is added to the oxidizing agent waste liquid after the reduction treatment so that the pH value of the oxidizing agent waste liquid after the reduction treatment meets the drainage standard.

Advantages of the Invention

[0016] According to the present invention, the alkaline pH adjuster added to the oxidizing agent waste liquid before adding the reducing agent neutralizes the acid generated during the reduction treatment, and the liquid property of the oxidizing agent waste liquid can be maintained alkaline. As a result, the generation of acidic gases such as chlorine gas can be suppressed during the reduction treatment. Furthermore, the ORP value during the reduction treatment hardly changes until the end point of the reduction treatment. As a result, the end point of the reduction reaction, which is the point at which the ORP value rapidly decreases to a predetermined value or less, can be easily and surely confirmed.

Brief Description of the Drawings

[0017]

Figure 1

Figure 2

Figure 3A

Figure 3B

Figure 3C

Figure 4

[0018] Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram showing an example of a reduction treatment apparatus. The reduction treatment apparatus shown in FIG. 1 is an apparatus for reducing a waste liquid of an oxidizing agent solution used for cleaning a membrane such as a microfiltration membrane (MF membrane) or an ultrafiltration membrane (UF membrane), although it is not intended to limit the use. The oxidizing agent solution introduced into the reduction treatment apparatus may be an oxidizing agent solution used for cleaning components such as pipes and immersion tanks of a membrane filtration facility. An example of the oxidizing agent solution used for cleaning the membrane is a sodium hypochlorite solution. An example of the reducing agent is sodium thiosulfate. Hereinafter, an example of a reduction treatment will be mainly described in which the oxidizing agent used for cleaning the membrane is sodium hypochlorite and the reducing agent is sodium thiosulfate. Further, the waste liquid of a sodium hypochlorite solution, which is an example of the oxidizing agent waste liquid, may be referred to as a "sodium hypochlorite waste liquid".

[0019] The reduction treatment apparatus shown in FIG. 1 includes a waste liquid supply line 1 through which a sodium hypochlorite waste liquid (oxidizing agent waste liquid) flows, a waste liquid tank 2 to which the waste liquid supply line 1 is connected and which stores the sodium hypochlorite waste liquid, a discharge line 7 connected to the waste liquid tank 2, and an alkaline pH adjuster line 8 connected to the waste liquid supply line 1. The alkaline pH adjuster line 8 supplies an alkaline pH adjuster (hereinafter referred to as an "alkaline pH adjuster") to the waste liquid supply line 1. An example of the alkaline pH adjuster is sodium hydroxide. The mixed solution of the sodium hypochlorite waste liquid and the alkaline pH adjuster is supplied to the waste liquid tank 2 through the waste liquid supply line 1. When the reduction treatment described later is completed, the reduced waste liquid in the waste liquid tank 2 is discharged from the waste liquid tank 2 through the discharge line 7.

[0020] In one embodiment, the alkaline pH adjuster line 8 may be directly connected to the waste liquid tank 2. In this case, the alkaline pH adjuster is directly supplied to the waste liquid tank 2 without passing through the waste liquid supply line 1, and a mixed solution of sodium hypochlorite waste liquid and the alkaline pH adjuster is formed in the waste liquid tank 2.

[0021] Although not shown, an on / off valve, a flow regulator (e.g., a flow control valve or a mass flow controller), etc. are arranged in the waste liquid supply line 1, and it is configured to be able to control the timing and flow rate of supplying the sodium hypochlorite waste liquid to the waste liquid tank 2. Similarly, an on / off valve, a flow regulator (e.g., a flow control valve or a mass flow controller), etc. (not shown) are arranged in the alkaline pH adjuster line 8, and it is configured to be able to control the timing and flow rate of supplying the alkaline pH adjuster to the waste liquid tank 2.

[0022] Furthermore, the reduction treatment device includes a reducing agent supply line 9 connected to the waste liquid tank 2. Sodium thiosulfate (reducing agent) used for the reduction treatment described later is supplied to the waste liquid tank 2 via the reducing agent supply line 9. An on / off valve, a flow regulator (e.g., a flow control valve or a mass flow controller), etc. (not shown) are arranged in the reducing agent supply line 9, and it is configured to be able to control the timing and flow rate of supplying sodium thiosulfate to the waste liquid tank 2.

[0023] The reduction treatment device further includes a diffuser pipe 3 disposed in the waste liquid tank 2, a blower 11 that supplies gas (for example, air) to the diffuser pipe 3, and a gas supply line 12 that extends from a gas supply source (not shown) to the diffuser pipe 3. The blower 11 is disposed in the gas supply line 12. The gas pressurized by the blower 11 reaches the diffuser pipe 3 through the gas supply line 12 and is discharged from the diffuser pipe 3 into the waste liquid in the waste liquid tank 2 (that is, a mixed solution of sodium hypochlorite waste liquid and an alkaline pH adjuster, or a mixed solution of sodium hypochlorite, an alkaline pH adjuster, and a reducing agent). The waste liquid in the waste liquid tank 2 is efficiently stirred by the gas discharged from the diffuser pipe 3. In the embodiment shown in FIG. 1, the diffuser pipe 3, the blower 11, and the gas supply line 12 constitute a stirring device for stirring the waste liquid in the waste liquid tank 2. In one embodiment, the blower 11 may be omitted.

[0024] As long as it is possible to stir the waste liquid in the waste liquid tank 2, the stirring device is not limited to the combination of the diffuser pipe 3, the blower 11, and the gas supply line 12. For example, the stirring device may be a combination of a stirring blade and a drive machine that rotates the stirring blade.

[0025] The reduction treatment device further includes an ORP meter 15 for measuring the ORP value of the waste liquid. The ORP meter 15 shown in FIG. 1 includes an ORP electrode 4 and a meter body 5 that converts the measurement value obtained by the ORP electrode 4 into an ORP value. In the present embodiment, the ORP electrode 4 of the ORP meter 15 is immersed in the waste liquid in the waste liquid tank 2, but the type and configuration of the ORP meter 15 are arbitrary as long as the ORP value of the waste liquid in the waste liquid tank 2 can be measured.

[0026] A pH meter 6 is arranged in the discharge line 7 of the reduction treatment device, and the pH meter 6 measures the pH value of the reduced waste liquid flowing through the discharge line 7. Further, an acid pH adjuster line 10 is connected to the discharge line 7. The acid pH adjuster line 10 is a line for supplying a pH adjuster having acidity (hereinafter referred to as "acid pH adjuster") to the reduced waste liquid flowing through the discharge line 7 based on the measured value of the pH meter 6. Specifically, the acid pH adjuster is supplied from the acid pH adjuster line 10 to the discharge line 7 based on the measured value of the pH meter 6 so that the pH value of the reduced waste liquid falls within the pH range that can be discharged into sewage or a river. In one embodiment, the acid pH adjuster line 10 may be directly connected to the waste liquid tank 2 where the reduction treatment is completed.

[0027] Furthermore, in one embodiment, the measured value of the pH meter 6 may be used to determine whether an acidic gas is being generated and whether the input amount of the alkaline pH adjuster is sufficient. As will be described later, if the liquid property of the waste liquid in the waste liquid tank 2 changes from alkaline to acidic during the reduction treatment due to a problem such as insufficient amount of the alkaline pH adjuster in the waste liquid tank 2, an acidic gas will be generated. Therefore, by measuring the pH value of the waste liquid flowing through the discharge line 7, it is possible to determine whether an acidic gas is being generated and whether the input amount of the alkaline pH adjuster is sufficient.

[0028] Next, a method for reducing sodium hypochlorite waste liquid (oxidizing agent waste liquid) using the above-described reduction treatment device will be described.

[0029] FIG. 2 is a flowchart showing a method for reducing an oxidizing agent waste liquid according to an embodiment. As shown in FIG. 2, first, a predetermined amount of sodium hypochlorite waste liquid (oxidizing agent waste liquid) is introduced into the waste liquid tank 2 via the waste liquid supply line 1 (S101). Sodium hypochlorite contained in the sodium hypochlorite waste liquid is an oxidizing agent that is reduced by a reducing agent. Sodium hypochlorite is contained in the sodium hypochlorite waste liquid at a high concentration of, for example, 1000 mg / L or more.

[0030] Next, an alkaline pH adjuster (sodium hydroxide) is introduced into the waste liquid tank 2 (S102). The gas discharged from the air diffuser 3 stirs the waste liquid in the waste liquid tank 2 (that is, the mixed liquid of the sodium hypochlorite waste liquid and the alkaline pH adjuster) (S103). As a result, the alkaline pH adjuster is efficiently mixed with the sodium hypochlorite waste liquid. In the present embodiment, the alkaline pH adjuster is introduced into the waste liquid tank 2 from the alkaline pH adjuster line 8 via the waste liquid supply line 1. The alkaline pH adjuster may be introduced into the waste liquid tank 2 simultaneously with the sodium hypochlorite waste liquid, or may be introduced into the waste liquid tank 2 after the sodium hypochlorite waste liquid is introduced into the waste liquid tank 2. Further, as described above, the alkaline pH adjuster line 8 may be directly connected to the waste liquid tank 2 to directly introduce the alkaline pH adjuster into the waste liquid tank 2.

[0031] Thus, the alkaline pH adjuster is introduced into the sodium hypochlorite waste liquid before sodium thiosulfate, which is a reducing agent, is introduced into the sodium hypochlorite waste liquid. The reason for this will be explained below.

[0032] The reaction formula in the reduction treatment of sodium thiosulfate and sodium hypochlorite is as shown in the following formula (1). 4HClO+Na2S2O3+H2O → 2NaCl+2H2SO4+2HCl ···(1) Furthermore, a high-concentration sodium hypochlorite waste liquid containing 1000 mg / L or more of sodium hypochlorite has an ORP value of about 500 to 600 mV. When sodium thiosulfate, which is a reducing agent, is added to this sodium hypochlorite waste liquid, hydrogen ions derived from an acid are generated, the ORP value rises to 1000 mV or more, and then the pH value rapidly decreases, generating chlorine gas, which is an acidic gas. When chlorine gas is generated, the ORP value decreases to less than 400 mV.

[0033] Here, generally, the existence form of sodium hypochlorite in the sodium hypochlorite waste liquid differs depending on the pH value, and takes the forms described below.

Chemical formula

[0034] Therefore, by neutralizing the acid (see formula (1)) generated by the reduction reaction between sodium hypochlorite and sodium thiosulfate with the previously added alkaline pH adjuster, the liquor property of the sodium hypochlorite waste liquid is maintained alkaline. As a result, the generation of chlorine gas (acidic gas) generated by the reaction between sodium hypochlorite and hydrogen ions (H + ) can be suppressed.

[0035] Furthermore, when the inventors intensively studied the reduction treatment of the oxidant waste liquid, it was found that by adding an alkaline pH adjuster to the sodium hypochlorite waste liquid before adding sodium thiosulfate to the sodium hypochlorite waste liquid, the change in the ORP value during the reduction reaction between sodium hypochlorite and sodium thiosulfate can also be suppressed. That is, it was found that by adding an alkaline pH adjuster to the oxidant waste liquid before adding the reducing agent, the ORP value does not change significantly until the end point of the reduction reaction of the oxidant. Although this phenomenon will be described later, due to this phenomenon, the end point of the reduction reaction, which is the point when the ORP value decreases to a predetermined value (for example, 400 mV) or less, can be easily confirmed without relying on complicated controls such as correction calculations of the ORP value.

[0036] The amount of the alkaline pH adjuster to be added to the waste liquid tank 2 is determined so as to be able to sufficiently neutralize the acid generated by the reduction reaction between sodium hypochlorite and sodium thiosulfate. More specifically, the amount of the alkaline pH adjuster is determined to be such that even when sodium thiosulfate is added to the sodium hypochlorite waste liquid, the liquor property of the mixed liquid of the sodium hypochlorite waste liquid and sodium thiosulfate can be maintained alkaline. This amount of the alkaline pH adjuster may be theoretically calculated based on the amount of sodium hypochlorite contained in the sodium hypochlorite waste liquid added to the waste liquid tank 2, or may be determined by experiments and / or simulations performed in advance.

[0037] Furthermore, the amount of sodium hypochlorite contained in the sodium hypochlorite waste liquid may be calculated (estimated), for example, based on the concentration of sodium hypochlorite in the cleaning liquid before membrane cleaning and the amount of the sodium hypochlorite waste liquid introduced into the waste liquid tank 2. In one embodiment, a concentration meter (not shown) may be arranged in the waste liquid supply line 1 to measure the concentration of sodium hypochlorite in the waste liquid supply line 1. In this case, the amount of the alkaline pH adjuster is calculated based on the measured value of the concentration meter and the amount of the sodium hypochlorite waste liquid introduced into the waste liquid tank 2.

[0038] Thus, by introducing sodium hydroxide, which is an alkaline pH adjuster, into the sodium hypochlorite waste liquid before introducing sodium thiosulfate, which is a reducing agent, into the sodium hypochlorite waste liquid, which is an oxidizing agent waste liquid, the generation of chlorine gas, which is an acidic gas, can be suppressed. As a result, environmental pollution can be prevented and the safety of workers can be ensured. Furthermore, when sodium thiosulfate is introduced into the sodium hypochlorite waste liquid for reduction treatment, the ORP value hardly changes until the end point of the reduction treatment, and as a result, the end point of the reduction treatment, which is the point at which the ORP value rapidly decreases, can be easily and surely determined.

[0039] Returning to the flowchart shown in FIG. 2, sodium thiosulfate, which is a reducing agent, is introduced (S104) into the sodium hypochlorite waste liquid sufficiently mixed with the alkaline pH adjuster in S103, and the sodium hypochlorite is reduced by the sodium thiosulfate. Also during the reduction treatment, gas is released from the diffuser tube 3. Thereby, sodium thiosulfate is efficiently mixed with the sodium hypochlorite waste liquid, and the reduction reaction is promoted. As described above, by introducing the alkaline pH adjuster into the sodium hypochlorite waste liquid before introducing sodium thiosulfate, the generation of chlorine gas is suppressed.

[0040] The end point of the reduction treatment is determined based on the measurement result of the ORP meter 15 (S105). More specifically, the meter main body 5 of the ORP meter 15 monitors the measured value (ORP value) obtained by the ORP electrode 4 disposed in the waste liquid tank 2. When the ORP value becomes smaller than a predetermined value (for example, 350 mV) (No in S105), it is determined that the reduction treatment has ended, and the supply of sodium thiosulfate to the waste liquid tank 2 is stopped (S106). When the ORP value is equal to or greater than the predetermined value (Yes in S105), the meter main body 5 of the ORP meter 15 continues the supply of sodium thiosulfate to the waste liquid tank 2.

[0041] When the reduction treatment of sodium hypochlorite ends, the reduced waste liquid is discharged from the waste liquid tank 2 through the discharge line 7 (S107). At this time, the pH value of the reduced waste liquid flowing through the discharge line 7 is measured by the pH meter 6, and the pH value of the reduced waste liquid is compared with a predetermined value determined by the drainage standard for sewage (S108). If the pH value of the reduced waste liquid is equal to or greater than the predetermined value (for example, 9) (Yes in S108), an acid pH adjuster is introduced into the discharge line 7 or the waste liquid tank 2 through the acid pH adjuster line 10 so that the pH value of the reduced waste liquid becomes smaller than the predetermined value, and then it is discharged into the sewage (S110). If the pH value of the reduced waste liquid is smaller than the predetermined value (No in S108), the reduced waste liquid is directly discharged into the sewage (S110).

[0042] Next, a verification experiment for confirming the effect that occurs when an alkaline pH adjuster is added to the oxidant waste liquid before adding a reducing agent to the oxidant waste liquid will be described. In the verification experiment, sodium hypochlorite solution was aliquoted into a plurality of 1 L beakers, and the residual chlorine concentration, pH value, and ORP value of the sodium hypochlorite solution in each beaker were measured respectively. The diethyl paraphenylenediamine method (DPD method) was used for the measurement of the residual chlorine concentration. Next, an aqueous sodium hydroxide solution (alkaline pH adjuster) having a concentration of 1 mol / L was added to each beaker while changing the amount in the range of 0 to 50 mL, and the respective pH values and ORP values were measured again.

[0043] Next, a sodium thiosulfate solution with a concentration of 100 g / L was added to the sodium hypochlorite solution to which an alkaline pH adjuster had been added, in the range of 0.5 to 10 mL. The time point when the addition of the sodium thiosulfate solution started to be added to the sodium hypochlorite solution was set as the measurement start time point. One minute after the measurement start time point, a solution sample was taken from each beaker, and the residual chlorine concentration, pH value, and ORP value were measured. At this time, for the solution with a residual chlorine concentration of 1.0 mg / L or more, an arbitrary amount of sodium thiosulfate solution was added again 5 minutes after the measurement start time point, and the same measurement was performed. In these verification tests, the time point when the residual chlorine concentration of the solution in the beaker became less than 1.0 mg / L was set as the end point of the reduction reaction.

[0044] Table 1 shown below is a table representing the results of the verification experiment. Specifically, Table 1 shows the ratio of sodium hydroxide added to each beaker with respect to the sodium hypochlorite concentration of the sodium hypochlorite solution placed in each beaker, and the ratio of sodium thiosulfate added to each beaker with respect to the sodium hypochlorite concentration of the sodium hypochlorite solution placed in each beaker. Furthermore, Table 1 shows the pH value before the reduction treatment and the pH value after the reduction treatment of the sodium hypochlorite solution placed in each beaker in the verification experiment.

[0045]

Table 1

[0046] Furthermore, FIGS. 3A to 3C are graphs showing the results of verification experiments. More specifically, FIG. 3A is a graph showing the change in residual chlorine concentration with respect to the injection amount of sodium thiosulfate, FIG. 3B is a graph showing the change in pH value with respect to the injection amount of sodium thiosulfate, and FIG. 3C is a graph showing the change in ORP value with respect to the injection amount of sodium thiosulfate. In FIG. 3A, the vertical axis represents the residual chlorine concentration, and the horizontal axis represents the injection amount of sodium thiosulfate. In FIG. 3B, the vertical axis represents the pH value, and the horizontal axis represents the injection amount of sodium thiosulfate. In FIG. 3C, the vertical axis represents the ORP value, and the horizontal axis represents the injection amount of sodium thiosulfate.

[0047] In Tables 1 and FIGS. 3A to 3C, Comparative Example 1 corresponds to a conventional reduction treatment method that does not consider the generation of acidic gas during the reduction treatment at all. Specifically, Comparative Example 1 is an example in which a sodium thiosulfate solution is added to a sodium hypochlorite solution without adding a sodium hydroxide solution, which is an alkaline pH adjuster, to a beaker. Comparative Example 2 is an example in which a sodium hydroxide solution is added to a beaker, but acidic gas is considered to have been generated during the reduction treatment.

[0048] In Comparative Example 1, when the injection amount of sodium thiosulfate was 2.5 g / L, the residual chlorine concentration decreased significantly (see Table 1 and FIG. 3A), and the pH value of the solution in the beaker changed rapidly from 11.7 to 2.5 (see Table 1 and FIG. 3B). Furthermore, when the injection amount of sodium thiosulfate was 2.5 g / L, the ORP value increased from 540 mV to 1190 mV (see FIG. 3C). Also in Comparative Example 2, when the injection amount of sodium thiosulfate was 4.2 g / L, the residual chlorine concentration and the pH value decreased significantly, respectively. These phenomena are considered to have occurred because an acid was generated according to the reaction shown in the above formula (1), resulting in a decrease in the pH value of the sodium hypochlorite solution, and chlorine gas (acidic gas) was generated due to the decrease in the pH value.

[0049] In the case of Comparative Example 1 and Comparative Example 2 where, like the conventional reduction treatment method, the liquid property of the sodium hypochlorite solution (corresponding to the oxidizing agent waste liquid) cannot be maintained alkaline during the reduction treatment, the ORP value of the sodium hypochlorite solution changes rapidly, and chlorine gas is generated. The rapid change in the ORP value during the reduction treatment is difficult to predict. When the ORP value changes significantly, the generated chlorine gas (acidic gas) will be released into the air. In this case, the surrounding environment will be polluted by the acidic gas, and the operator will be at risk.

[0050] On the other hand, in Examples 1 to 4, it was confirmed that the residual chlorine concentration and pH value decreased gradually, respectively. Also, the ORP value gradually decreased as the injection amount of sodium thiosulfate increased, or decreased rapidly to about 200 mV at the end point of the reduction reaction treatment where the residual chlorine concentration was less than 0.2 mg / L with little change (see FIGS. 3A and 3C).

[0051] From the results of the above verification experiments, by adding sodium hydroxide (alkaline pH adjuster) to the sodium hypochlorite waste liquid before adding sodium thiosulfate (reducing agent) to the sodium hypochlorite waste liquid (oxidizing agent waste liquid), the ORP value hardly changes until the end point of the reduction reaction. As a result, it was found that the end point of the reduction treatment can be easily and surely judged. Furthermore, it was found that by adding sodium hydroxide to the sodium hypochlorite waste liquid so that the ratio of the amount of substance of sodium hydroxide to the amount of substance of sodium hypochlorite (NaOH / NaClO) is 0.09 or more, the reduction treatment of sodium hypochlorite can be carried out without generating chlorine gas.

[0052] FIG. 4 is a graph showing the relationship between the ratio of the amount of substance of sodium thiosulfate to the amount of substance of sodium hypochlorite and the ratio of the amount of substance of sodium hydroxide to the amount of substance of sodium hypochlorite. In FIG. 4, the vertical axis represents the ratio of the amount of substance of sodium thiosulfate to the amount of substance of sodium hypochlorite, and the horizontal axis represents the ratio of the amount of substance of sodium hydroxide to the amount of substance of sodium hypochlorite.

[0053] As is clear from FIG. 4, as the amount of sodium hydroxide added to the sodium hypochlorite waste liquid increases, the injection amount of sodium thiosulfate decreases. The reason for this phenomenon is considered to be that the acid generated according to the reaction formula of formula (1) is neutralized by sodium hydroxide, accelerating the reduction treatment of sodium hypochlorite. Therefore, it is preferable that the amount of the alkaline pH adjuster added to the sodium hypochlorite waste liquid is introduced into the sodium hypochlorite waste liquid in an amount exceeding the amount that can just neutralize the acid generated by the reduction reaction between sodium hypochlorite and sodium thiosulfate.

[0054] The above-described embodiments are described for the purpose of enabling those having ordinary knowledge in the technical field to which the present invention pertains to implement the present invention. Various modifications of the above embodiments are naturally possible for those skilled in the art, and the technical idea of the present invention can also be applied to other embodiments. Therefore, the present invention is not limited to the described embodiments, but is interpreted in the broadest scope in accordance with the technical idea defined by the claims.

Explanation of Signs

[0055] 1 Waste liquid supply line 2 Waste liquid tank 3 Diffuser pipe 4 ORP electrode 5 Meter body 6 pH meter 7 Discharge line 8 Alkaline pH adjuster line 9 Reducing agent supply line 10 Acid pH adjuster line 11 Blower 12 Gas supply line 15 ORP meter

Claims

1. A method for reducing an oxidizing agent waste liquid by reducing the oxidizing agent contained in the oxidizing agent waste liquid with a reducing agent, comprising: before introducing the reducing agent into the oxidizing agent waste liquid, introducing an alkaline pH adjuster for adjusting the pH value of the oxidizing agent waste liquid into the oxidizing agent waste liquid; the alkaline pH adjuster neutralizes the acid generated during the reduction reaction between the oxidizing agent and the reducing agent, maintaining the liquid property of the oxidizing agent waste liquid alkaline, thereby suppressing the generation of acid gas; the oxidizing agent is sodium hypochlorite; the reducing agent is sodium thiosulfate; the alkaline pH adjuster is sodium hydroxide; sodium hydroxide is introduced into the oxidizing agent waste liquid such that the ratio of the amount of substance of sodium hydroxide to the amount of substance of sodium hypochlorite is 0.09 or more; the concentration of sodium hydroxide is in the range of 0.4 to 0.6 g / L, and the concentration of sodium thiosulfate is in the range of 4.7 to 5.1 g / L. A method for reducing an oxidizing agent waste liquid, characterized in that.

2. The method for reducing an oxidizing agent waste liquid according to claim 1, wherein the oxidizing agent is an oxidizing agent used for cleaning a filtration membrane of a membrane filtration facility.

3. The oxidizing agent waste liquid is introduced into a waste liquid tank, The waste liquid tank is provided with a diffuser tube for stirring a mixed liquid of the oxidizing agent waste liquid, the alkaline pH adjuster, and the reducing agent. A method for reducing an oxidizing agent waste liquid according to claim 1, characterized in that.

4. measuring the pH value of the oxidizing agent waste liquid after the reduction treatment; An acid pH adjuster is introduced into the oxidizing agent waste liquid after the reduction treatment such that the pH value of the oxidizing agent waste liquid after the reduction treatment meets the drainage standard. A method for reducing an oxidizing agent waste liquid according to claim 1, characterized in that.

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