CN115367903A

CN115367903A - Waste liquid treatment method and waste liquid treatment apparatus

Info

Publication number: CN115367903A
Application number: CN202210409479.9A
Authority: CN
Inventors: 黑泽裕贵; 丹羽将基; 金泽康平; 高家大
Original assignee: Toyota Motor Corp
Current assignee: Toyota Motor Corp
Priority date: 2021-05-18
Filing date: 2022-04-19
Publication date: 2022-11-22
Also published as: JP7494790B2; JP2022177569A

Abstract

The present invention relates to a waste liquid treatment method and a waste liquid treatment apparatus. The waste liquid treatment method comprises: a first separation step of taking out first treated water from a waste liquid containing a coating material; an oxidative decomposition step of oxidatively decomposing organic matter contained in the first treated water; and a second separation step of taking out the second treated water, wherein the oxidative decomposition step comprises: a first treatment step of performing a first treatment until the temperature of the first treated water reaches a first temperature; a cooling step of cooling the first treated water; and a second treatment step of performing a second treatment under a condition that the temperature of the first treated water is brought to a second temperature lower than the first temperature by the cooling step, wherein the first treatment and the second treatment are each a treatment of oxidatively decomposing an organic substance contained in the first treated water subjected to the first separation step by a hydroxyl radical generated by a fenton reaction using an iron catalyst and hydrogen peroxide.

Description

Waste liquid treatment method and waste liquid treatment apparatus

Technical Field

The present disclosure relates to a technique of a waste liquid treatment method and a waste liquid treatment apparatus.

Background

Conventionally, a technique for treating waste water containing organic substances such as coating waste water has been known (Japanese patent laid-open No. 2007-29825). In this technique, waste water containing organic substances is subjected to coagulation treatment to remove solid components, and then biologically degradable organic substances are decomposed by an activated sludge process using microorganisms. Then, by performing oxidation promoting treatment using an oxidizing agent such as ozone or hydrogen peroxide, the organic matter which is hardly decomposed by the living organism is decomposed. This reduces the BOD (biochemical oxygen demand) and COD (chemical oxygen demand) of the wastewater containing organic matter.

Disclosure of Invention

In the prior art, when the BOD and COD of the waste liquid containing the coating material are high, the burden on the activated sludge is large, and the waste liquid containing the coating material may not be continuously treated due to the death and extinction of microorganisms. In addition, when the BOD and COD of the coating material-containing waste liquid are high, the treatment time with the activated sludge may be longer than the treatment time with the chemical reaction. Therefore, a technique is desired in which the BOD and COD of the coating material-containing waste liquid are continuously reduced to desired values in a short time by a treatment different from the treatment with activated sludge.

The present disclosure can be implemented in the following manner.

(1) According to one embodiment of the present disclosure, a waste liquid treatment method is provided. The waste liquid treatment method comprises: a first separation step of separating a first solid component contained in a coating material-containing waste liquid from the coating material-containing waste liquid and taking out first treated water; an oxidative decomposition step of oxidatively decomposing organic matter contained in the first treated water after the first separation step; and a second separation step of separating a second solid component containing an iron compound among components contained in the first treated water from the first treated water and taking out second treated water after the oxidative decomposition step, the oxidative decomposition step including: a first treatment step of subjecting the first treated water subjected to the first separation step to a first treatment until the temperature of the first treated water reaches a first temperature; a cooling step of cooling the first treated water after the first treatment step; and a second treatment step of subjecting the first treated water subjected to the first treatment to a second treatment under a condition that the temperature of the first treated water is brought to a second temperature lower than the first temperature in the cooling step, wherein the first treatment and the second treatment are each a treatment of oxidatively decomposing the organic matter contained in the first treated water subjected to the first separation step by a hydroxyl radical generated by a fenton reaction using an iron catalyst and hydrogen peroxide. According to this aspect, the organic matter contained in the coating material-containing waste liquid can be oxidatively decomposed in a short time and continuously by a treatment using a chemical reaction, which is different from an activated sludge treatment using a biological decomposition reaction of microorganisms. In addition, according to this aspect, by providing the cooling step between the first treatment step and the second treatment step, an excessive increase in the temperature of the first treated water accompanying the fenton reaction can be suppressed. This can suppress inhibition of oxidative decomposition due to an excessive increase in the temperature of the first treated water, and thus can reduce the BOD and COD of the coating material-containing waste liquid to desired values.

(2) In the above aspect, the first treatment step may further include a temperature adjustment step of adjusting the temperature of the first treated water subjected to the first separation step to a third temperature lower than the second temperature before the start of the fenton reaction. According to this mode, the first treatment is performed after the temperature of the first treated water is made lower than the second temperature. This can suppress an excessive increase in the temperature of the first treated water accompanying the fenton reaction.

(3) In the foregoing manner, the first processing may include: a first addition step of adding the iron catalyst and the hydrogen peroxide to the first treated water subjected to the first separation step; and a first stirring step of stirring the first treated water after the first addition step, and the second treatment may include: a second addition step of adding the hydrogen peroxide to the first treated water having undergone the first treatment, the hydrogen peroxide being one of the iron catalyst and the hydrogen peroxide; and a second stirring step of stirring the first treated water after the second addition step. According to this aspect, in the second addition step, hydrogen peroxide, among the iron catalyst and hydrogen peroxide, is added to the first treated water. This can reduce the amount of the chemical to be added in the second addition step. In addition, according to this aspect, the fenton reaction using the iron catalyst and the hydrogen peroxide can be performed by stirring the first treated water in the first stirring step and the second stirring step.

(4) In the above aspect, the first separation process may include: (1a) A first coagulation treatment in which a coagulant is added to the coating material-containing waste liquid; (1b) A first separation process in which the coating material-containing waste liquid subjected to the first coagulation process is allowed to stand to thereby settle a first coagulated product produced by the first coagulation process, and the first coagulated product is separated from a first supernatant liquid as a supernatant liquid; and (1 c) a first filtration process of separating a first filtrate as a filtrate obtained by filtering the first aggregate with a filter from a first residue as a residue, wherein the first treated water subjected to the oxidative decomposition process may be composed of the first supernatant obtained by the first separation process and the first filtrate obtained by the first filtration process, and the second separation process may include: (2a) A second coagulation treatment in which the coagulant is added to the first treated water subjected to the second treatment; (2b) A second separation process of leaving the first treated water subjected to the second coagulation process to stand to settle a second aggregate produced by the second coagulation process, and separating the second aggregate from a second supernatant as a supernatant; and (2 c) a second filtration process of separating a second filtrate as a filtrate obtained by filtering the second aggregate with the filter from a second residue as a residue, wherein the second treated water may be composed of the second supernatant obtained in the second separation process and the second filtrate obtained by the second filtration process. According to this aspect, in the first separation process, after the first supernatant liquid as the first treated water contained in the paint-containing waste liquid is taken out, the first filtrate as the first treated water contained in the first aggregate is taken out by the first filtration process. Further, according to this aspect, in the second separation treatment, after the second supernatant liquid as the second treated water contained in the first treated water is taken out, the second filtrate as the second treated water contained in the second aggregate is taken out by the second filtration treatment. This can improve the recovery rate of each of the first treated water and the second treated water.

(5) In the above aspect, the iron catalyst may be ferrous sulfate, and the first addition step may include a step of adding the ferrous sulfate to the first treated water in an amount determined using the following formula (2) obtained by correcting the following formula (1).

y = ax + b formula (1)

y = ax + c type (2)

In the above formula (1), x is an addition amount (mL/L) of the ferrous sulfate to 1 liter of the first treated water in the first addition step, y is a generation ratio (%) of the second residue per 1 liter of the first treated water subjected to the first treatment and the second treatment, a and b are constants, respectively, and the above formula (1) is a formula obtained by approximating a plurality of measured data indicating the generation ratio with respect to the addition amount by a least square method. In the above formula (2), the constant c is a value obtained by adding a predetermined numerical value to the constant b in the above formula (1). According to this aspect, the amount of ferrous sulfate added to the generation ratio of the target second residue can be easily determined using the formula (2).

(6) According to another aspect of the present disclosure, a waste liquid treatment apparatus is provided. The waste liquid treatment device comprises: a coagulation tank in which a first separation treatment is performed to separate a first solid component contained in a paint-containing waste liquid from the paint-containing waste liquid and to take out first treated water; a Fenton tank for performing oxidative decomposition treatment and second separation treatment after the first separation treatment; and a control unit that controls the oxidative decomposition treatment according to the temperature of the first treated water, the oxidative decomposition treatment including: a first treatment in which the first treated water subjected to the first separation treatment is subjected to a first treatment until the temperature of the first treated water reaches a first temperature; a cooling treatment in which the first treated water is cooled after the first treatment; and a second treatment of subjecting the first treated water subjected to the first treatment to a second treatment under a condition that a temperature of the first treated water is brought to a second temperature lower than the first temperature by the cooling treatment, the first treatment and the second treatment being treatments of oxidatively decomposing organic matter contained in the first treated water subjected to the first separation treatment by using a hydroxyl radical generated by a fenton reaction using an iron catalyst and hydrogen peroxide, the second separation treatment being a treatment of separating a second solid component containing an iron compound among components contained in the first treated water subjected to the second treatment from the first treated water and taking out a second treated water. According to this aspect, the organic matter contained in the coating-containing waste liquid can be oxidatively decomposed in a short time and continuously by a treatment using a chemical reaction, which is different from the activated sludge treatment using a biological decomposition reaction of microorganisms. Further, according to this aspect, by providing the cooling process between the first process and the second process, it is possible to suppress an excessive increase in the temperature of the first treated water accompanying the fenton reaction. This can suppress the inhibition of the oxidative decomposition due to an excessive increase in the temperature of the first treated water, and thus can reduce the BOD and COD of the coating material-containing waste liquid to desired values.

The present disclosure can be achieved in various ways other than the above-described waste liquid treatment method. For example, the present invention can be realized as a method for manufacturing a waste liquid treatment apparatus, a method for controlling a waste liquid treatment apparatus, a computer program for realizing the control method, a non-transitory recording medium recorded in the computer program, and the like.

Drawings

Features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will be described below with reference to the accompanying drawings, in which like symbols represent like elements, and wherein:

FIG. 1 is a schematic configuration diagram of a waste liquid treatment apparatus according to a first embodiment.

FIG. 2 is a flowchart showing a waste liquid treatment method according to a first embodiment.

Fig. 3 is a flowchart showing details of the first separation step of the first embodiment.

FIG. 4 is a flowchart showing details of a first processing step of the first embodiment.

FIG. 5 is a flowchart showing details of the second processing step of the first embodiment.

Fig. 6 is a flowchart showing details of the second separation step of the first embodiment.

FIG. 7 is a flowchart showing details of the inspection step.

FIG. 8 is a diagram showing changes in COD in each step.

FIG. 9 is a view for explaining the amount of aluminum sulfate added as a primary flocculant.

FIG. 10 is a diagram for explaining an appropriate amount of hydrogen peroxide to be added.

FIG. 11 is data showing the correlation between the temperature of the first treated water and the amount of hydrogen peroxide added.

FIG. 12 is a graph showing the change with time in the temperature of the first treated water under the conditions shown in FIG. 11.

Fig. 13 is a diagram for explaining timings of performing the first addition and the second addition.

FIG. 14 is a diagram for explaining a method of selecting an appropriate amount of ferrous sulfate to be added.

Detailed Description

A. The first embodiment:

a-1: constitution of waste liquid treatment apparatus

Fig. 1 is a diagram showing a schematic configuration of a waste liquid treatment apparatus 1 for executing the waste liquid treatment method of the first embodiment. Fig. 1 schematically illustrates a part of the components of the waste liquid treatment apparatus 1. Hereinafter, the direction of gravity is set to the lower side, and the direction opposite to the direction of gravity is set to the upper side. The waste liquid treatment apparatus 1 of the present embodiment is an apparatus for treating a paint-containing waste liquid (hereinafter referred to as a paint-containing waste liquid W) generated in a coating process or the like. Specifically, the waste liquid treatment apparatus 1 performs a waste liquid treatment method to decompose organic substances contained in the paint-containing waste liquid W, thereby reducing biochemical oxygen demand (hereinafter referred to as BOD) and chemical oxygen demand (hereinafter referred to as COD). The higher the BOD and COD, the higher the amount of organic matter contained in the liquid to be treated, and the higher the degree of contamination. That is, the higher the values of BOD and COD, the greater the environmental load when the raw materials are discharged.

The paint-containing waste liquid W is, for example, a waste liquid when a paint pipe as a flow passage of a paint is cleaned with an organic solvent or the like in a coating process performed in a process of manufacturing an automobile. Examples of the paint include water-based paint and oil-based paint. In the present embodiment, the coating material is an aqueous coating material. The aqueous coating comprises water, pigments, dyes, resins, surfactants, crosslinking agents, and other organic substances. Examples of the organic solvent used for cleaning the coating pipe include paint thinner and aqueous diluent (mineral turpentine). In the present embodiment, the organic solvent used for cleaning the paint pipe is an aqueous diluent. Since the coating material-containing waste liquid W contains a hardly biodegradable component, COD measured by a chemical reaction (redox reaction using an oxidizing agent) is more suitable than BOD measured by a microorganism in order to measure the amount of organic matter in the coating material-containing waste liquid W. Therefore, hereinafter, the amount of organic matter contained in the coating material-containing waste liquid W is expressed using COD. The unit of COD in the present embodiment is a unit in which the amount of an oxidizing agent consumed when oxidizing an oxidizable substance contained in 1L of the paint-containing waste liquid W under a certain condition using the oxidizing agent is converted into an oxygen amount (mg). In this embodiment, the COD of the waste liquid W containing the coating material without any treatment is about 15000mg/L to about 19000mg/L.

The waste liquid treatment apparatus 1 includes a waste liquid storage tank 10, a chemical tank 11, a coagulation tank 13, a Fenton tank 15, an inspection tank 17, two

filtration devices

60 and 60a, and a controller 19. The waste liquid treatment apparatus 1 carries the waste liquid W containing the paint in the order of the waste liquid storage tank 10, the coagulation tank 13, the fenton tank 15, and the inspection tank 17, and executes a waste liquid treatment method.

The waste liquid storage tank 10 stores a paint-containing waste liquid W that is an object of performing the waste liquid treatment method according to the present embodiment. A pump 50 is provided in a pipe connecting the waste liquid storage tank 10 and the coagulation tank 13. Thereby, the waste liquid storage tank 10 discharges the paint-containing waste liquid W to the coagulation tank 13 under the control of the controller 19.

The chemical tank 11 stores chemicals such as hydrogen peroxide, a flocculant, and ferrous sulfate as a catalyst, which are supplied to the coagulation tank 13 and the fenton tank 15, respectively. The chemical tank 11 supplies a predetermined chemical to each of the coagulation tank 13 and the fenton tank 15 under the control of the controller 19.

The coagulation tank 13 separates solid components and liquid components contained in the coating material-containing waste liquid W. A pump 51 is provided in a pipe connecting the coagulation tank 13 and the fenton tank 15. In the present embodiment, the capacity of the coagulation tank 13 is 5m ³ 。

The

tanks

13, 15, and 17 are provided with

inspection devices

20, 20a, and 20b and

agitators

40, 40a, and 40b, respectively. The

inspection devices

20, 20a, and 20b measure the pH, temperature, COD, and the like of the liquid contained in the

tanks

13, 15, and 17. The

inspection devices

20, 20a, and 20b each include a detection unit 210, a COD measuring device 25, a collection unit 250, and a display unit 28.

The detection unit 210 includes an electrode for measuring the pH of the liquid and a temperature sensor. The COD measuring device 25 measures COD using the liquid in each of the

tanks

13, 15, and 17 collected from the collecting unit 250. The collecting part 250 is provided in a state of being immersed in the liquid contained in each

tank

13, 15, 17. The display 28 displays the measured pH, temperature, and COD.

The

agitators

40, 40a, 40b agitate the liquid contained in the

tanks

13, 15, 17. The

agitators

40, 40a, 40b are constituted by

rotors

410, 410a, 410b having a plurality of blades and

shaft portions

420, 420a, 420b serving as rotation centers. The

rotors

410, 410a, 410b are controlled by the control unit 19 to rotate around the

shaft portions

420, 420a, 420 b.

The

filter device

60, 60a is a device for separating a solid component from a liquid component. The first filter device 60 has a first extraction pipe 610, a first filter 620, and a first filter chamber 630. The first filtration device 60 is connected to an opening provided in the lower surface of the coagulation tank 13 via a first extraction pipe 610.

The first extraction pipe 610 is a pipe for extracting the first condensate M1 flocculated in the flocculation tank 13 and feeding the first condensate M to the filter 620. A valve for opening and closing a flow passage of the pipe is provided in the middle of the pipe. The first condensate M1 is conveyed by gravity from the coagulation tank 13 to the first filter 620 through the first extraction pipe 610.

The first filter 620 is filter cloth for filtering the first condensate M1 drawn out from the coagulation tank 13. The first filter 620 is disposed at an upper end side of the first filter chamber 630. In the present embodiment, the first aggregate M1 is conveyed by gravity to the first filter 620, and natural filtration is performed to separate a solid component from a liquid component.

The first filtering chamber 630 stores the first treated water W1 passing through the first filter 620. The first filtering chamber 630 is connected to the fenton tank 15. The first treated water W1 stored in the first filtering chamber 630 is sent to the fenton tank 15 by the pump 52.

The fenton tank 15 is a tank for performing a step of chemically decomposing organic substances contained in the first treated water W1 by a fenton reaction. A pump 53 is provided in a pipe connecting the fenton tank 15 and the inspection tank 17. The second supernatant W21 after the fenton reaction is transferred to the test chamber 17. In the present embodiment, the capacity of the Fenton tank 15 is 5m ³ . The fenton tank 15 includes a second inspection device 20a, a stirrer 40a, a filter device 60a, and a temperature adjustment mechanism 70. The configurations and functions of the second inspection device 20a and the stirrer 40a in the fenton vessel 15 are the same as those of the first inspection device 20 and the stirrer 40 in the coagulation vessel 13, and therefore, the description thereof is omitted.

The second filter device 60a has a second extraction pipe 610a, a second filter 620a, and a second filter chamber 630a. The second filter device 60a is connected to an opening provided in the lower surface of the fenton tank 15 via a second extraction pipe 610 a.

The second extraction pipe 610a is a pipe for extracting the second condensate M2 condensed in the fenton vessel 15 and sending the second condensate to the second filter 620 a. In the present embodiment, the second condensate M2 is conveyed from the fenton tank 15 to the second filter 620a through the second extraction pipe 610a by gravity.

The second filter 620a is filter cloth for filtering the second condensate M2 drawn out from the fenton vessel 15. The second filter 620a is disposed at an upper end side of the second filtering chamber 630a. In the present embodiment, the second aggregate M2 is conveyed by gravity to the second filter 620a, and natural filtration is performed to separate the solid component and the liquid component.

The second filtering chamber 630a stores the second treated water W2 passed through the second filter 620 a. The second filtering chamber 630a is connected to the inspection tank 17. The second treated water W2 stored in the second filtering chamber 630a is sent to the inspection tank 17 by the pump 54.

The temperature adjusting mechanism 70 has a function of adjusting the temperature of the liquid contained in the fenton tank 15. The temperature adjustment mechanism 70 is constituted by, for example, an indirect heat exchange mechanism using industrial water via a heat exchanger. The temperature adjustment mechanism 70 has a heating mechanism using a heater or the like in the case of temperature increase, and the temperature adjustment mechanism 70 has a cooling mechanism using a cooling machine or the like by heat transfer of industrial water in the case of cooling.

The inspection tank 17 is a tank for measuring the pH and COD of the second treated water W2. In the present embodiment, the capacity of the inspection tank 17 is 5m ³ . The inspection tank 17 has a third inspection device 20b and a stirrer 40b. Due to the third inspection in the inspection tank 17The structure and function of the apparatus 20b and the stirrer 40b are the same as those of the first inspection apparatus 20 and the stirrer 40 in the coagulation tank 13, and therefore, the description thereof is omitted.

The controller 19 controls the operations of the respective components of the waste liquid treatment apparatus 1 described above. The controller 19 controls the progress of the fenton reaction in the fenton tank 15, for example, based on the temperature of the first treated water W1. In fig. 1, a part of the components of the waste liquid treatment apparatus 1 controlled by the control unit 19 is schematically illustrated as a representative in an electrical connection manner.

A-2: waste liquid treatment method

Fig. 2 is a flowchart showing a waste liquid treatment method according to the first embodiment. In the present embodiment, the waste liquid W containing the paint is processed by executing steps S1 to S10 shown in fig. 2. In the present embodiment, as pretreatment for performing biological treatment using activated sludge, a waste liquid treatment method is performed. The COD target value of the second treated water W2 as the coating material-containing waste liquid W after the execution of the waste liquid treatment method of the present embodiment is about 1000mg/L.

In the waste liquid treatment method of the present embodiment, first, the first separation step is performed (step S1). When the first separation step (step S1) is started, the waste liquid W containing the paint stored in the waste liquid storage tank 10 is sent to the coagulation tank 13 (fig. 1). In the first separation step (step S1), the first solid content contained in the paint-containing waste liquid W is separated from the paint-containing waste liquid W, and the first treated water W1 is taken out. Specifically, the first solid content contained in the paint-containing waste liquid W is removed by adding a flocculant to the paint-containing waste liquid W. The first solid component mentioned here is, for example, a pigment and a part of a resin contained in the paint. The treated water taken out of the coagulation tank 13 (fig. 1) is sent to the fenton tank 15 (fig. 1) as first treated water W1.

Next, an oxidative decomposition step (step S4 to step S8) of oxidatively decomposing organic substances contained in the first treated water W1 taken out in the first separation step (step S1) is performed. In the present embodiment, the first processing step (step S4) of performing the first processing and the second processing step of performing the second processing are performedAnd a treatment step (step S8) of oxidatively decomposing organic substances contained in the first treated water W1 taken out in the first separation step (step S1). The first treatment and the second treatment are treatments for generating hydroxyl radicals by a fenton reaction using an iron catalyst and hydrogen peroxide and oxidatively decomposing organic substances contained in the first treated water W1 after the first separation step (step S1). The Fenton reaction is carried out under acidic conditions by passing hydrogen peroxide (H) ₂ O ₂ ) And ferrous ion (Fe) ²⁺ ) The reaction produces hydroxyl radicals (. OH) having a strong oxidizing ability. At this time, hydrogen peroxide functions as an oxidizing agent. The iron catalyst containing ferrous ions has a catalytic function for hydrogen peroxide. Therefore, the fenton reaction can also be said to be a reaction for generating hydroxyl radicals by a redox reaction. The fenton reaction is represented by the following formula (3). The mode of decomposing the organic substances contained in the first treated water W1 by the generated hydroxyl radicals is represented by the following formula (4).

Fe ²⁺ +H ₂ O ₂ →Fe ³⁺ +OH ^- OH formula (3)

OH + organics → CO ₂ (inorganic component) formula (4)

The temperature of the first treated water W1 increases due to the reaction heat in the fenton reaction. Therefore, a step of performing the temperature of the first treated water W1, i.e., a cooling step (step S6), is provided between the first treatment step (step S4) and the second treatment step (step S8). In the cooling step (step S6), the temperature of the first treated water W1 subjected to the first treatment step (step S4) is cooled to the second temperature or lower. In the present embodiment, the first treated water W1 is cooled by the temperature adjustment mechanism 70 (fig. 1). The second temperature is a temperature of the first treated water W1 for appropriately performing the subsequent second treatment process (step S8). In this embodiment, the second temperature is 45 ℃ (celsius). In this manner, by providing the cooling step (step S6) between the first treatment step (step S4) and the second treatment step (step S8), the fenton reaction is performed in two steps, thereby suppressing an excessive increase in the temperature of the first treated water W1. This suppresses the inhibition of oxidative decomposition associated with an excessive increase in the temperature of the first treated water W1.

After the second processing step (step S8), a second separation step (step S9) is performed. In the second separation step (step S9), the second solid content contained in the first treated water W1 subjected to the second treatment step (step S8) is separated from the first treated water W1, and the second treated water W2 is taken out. The second solid component referred to herein is composed of an iron compound derived from ferrous sulfate. The treated water taken out of the fenton tank 15 (fig. 1) is sent to the inspection tank 17 (fig. 1) as second treated water W2.

Next, an inspection process is performed on the second treated water W2 (step S10). In the inspection step (step S10), the pH and COD of the second treated water W2 are measured by the third inspection apparatus 20b (fig. 1). The inspection step (step S10) is a step of confirming how much solid and organic matter has been removed by the processing of the above-described steps S1 to S9. Therefore, the inspection step (step S10) can be said to be a step of confirming the quality of the second treated water W2. The waste liquid treatment method of the present embodiment is completed by executing the steps up to the inspection step (step S10). The measurement items of the second treated water W2 in the inspection step (step S10) are not limited to these items.

Fig. 3 is a flowchart showing details of the first separation step (step S1) of the first embodiment. In the first separation step (step S1), first, in step S110, pH adjustment of the waste liquid W containing the coating material is performed. In step S110, the paint-containing waste liquid W is adjusted to a pH suitable for the primary coagulation step (step S120) performed in the first coagulation treatment (step S120 to step S130) as the subsequent step. In step S110, the pH of the paint-containing waste liquid W is adjusted to 6.5 to 7.5 by adding at least one of sulfuric acid and sodium hydroxide contained in the chemical tank 11 (fig. 1) to the paint-containing waste liquid W.

After step S110, the first coagulation treatment is performed. In the present embodiment, the first coagulation treatment includes a primary coagulation step (step S120), pH adjustment (step S122), and a secondary coagulation step (step S130).

In the primary aggregation step (step 120), the first solid content present in the paint-containing waste liquid W in a state of being dispersed in the paint-containing waste liquid W is aggregated among the organic substances contained in the paint-containing waste liquid W. The first solid component is present as a water-insoluble component in a state of being dispersed in the coating material-containing waste liquid W.

In general, the surfaces of solid components present in a state of being dispersed in a liquid are negatively charged and repel each other. Therefore, the solid component does not coagulate in the liquid. In contrast, in the primary coagulation step (step S120), a primary coagulant having a positive charge is added to the coating material-containing waste liquid W. Thereby, the charges on the surface of the negatively charged solid component (here, the first solid component) are neutralized by the first coagulant and coagulated. By the coagulation of the first solid component, a basic floc as an aggregate of the first solid component is formed.

The primary flocculant is a flocculant for flocculating the first solid component, and is an inorganic flocculant made of a metal hydroxide or the like. In the case where the pH is close to neutral, the inorganic coagulant has a positive charge. Therefore, in the step (step S110) before the primary coagulation step (step S120) is started, the pH is adjusted to 6.5 to 7.5. Examples of the inorganic coagulant include aluminum coagulants and iron coagulants. In the present embodiment, aluminum sulfate, which is an aluminum-based flocculant, is used as a primary flocculant so as not to affect the amount of an iron catalyst to be added in a subsequent step of producing a fenton reaction.

In the present embodiment, 17.5mL of aluminum sulfate (a product having an alumina content of 8%) is added to 1L of the paint-containing waste liquid W in the primary coagulation step (step S120). At this time, the waste liquid W containing the paint was stirred by the stirrer 40 (FIG. 1) at a rotation speed of 300 rpm.

As shown in fig. 3, after step S120, pH adjustment of the waste liquid W containing the paint is performed in step S122. In step S122, the pH of the coating material-containing waste liquid W is adjusted to 6.5 to 7.5 by adding at least one of sulfuric acid and sodium hydroxide contained in the chemical tank 11 (fig. 1).

After step S122, a secondary aggregation step is performed (step S130). In the secondary flocculation step (step S130), a secondary flocculant is added to the coating-containing waste liquid W to further flocculate the basic flocs, which are aggregates of the first solid components. Specifically, at the start of the secondary flocculation step (step S130), a secondary flocculant having the property of adsorbing the basic flocs is added to the coating-containing waste liquid W and stirred. As a result, the basic flocs are brought into contact with each other and adsorbed in the coating material-containing waste liquid W, thereby forming coarse aggregated flocs (hereinafter referred to as first aggregates M1). Here, in step S150 described later, the paint-containing waste liquid W is left to stand, and the first aggregate M1 is precipitated and separated, whereby solid-liquid separation is performed. In this case, the larger the size of the basic flocs contained in the paint-containing waste liquid W is, the more likely the basic flocs settle by their own weight. Therefore, in the present embodiment, the primary flocculent is formed by the inorganic coagulant and then the secondary flocculation step (step S130) is provided to form the first aggregates M1 as aggregates of the primary flocculent.

The secondary flocculant is a polymer flocculant for flocculating the basic flocs. Examples of the polymer flocculant include anionic polymer flocculants, cationic polymer flocculants, nonionic polymer flocculants, and the like. Examples of the anionic polymer flocculant include acrylamide polymers. When selecting the type of the secondary flocculant, it is preferable to use a polymer flocculant that can be used between the primary flocculation step (step S120) and the secondary flocculation step (step S130) without greatly changing the pH of the coating material-containing waste liquid W. In the present embodiment, in step S110, the pH of the paint-containing waste liquid W is adjusted to 6.5 to 7.5 (around neutral). Therefore, as the polymer flocculant used in the secondary flocculation step (step S130), a polymer flocculant suitable for the treatment of a solution in the vicinity of neutrality is suitable. In the present embodiment, an anionic polymer flocculant which is suitable for treating neutral to alkaline solutions and has good flocculation property in the coating material-containing waste liquid W is used as the secondary flocculant.

In the present embodiment, in the secondary coagulation step (step S130), 6mL of the anionic polymer flocculant is added to 1L of the paint-containing waste liquid W. At this time, the waste liquid W containing the dope is stirred by the stirrer 40 (FIG. 1) at a rotation speed of 180 rpm.

In step S150, the first aggregate M1 contained in the coating material-containing waste liquid W is separated from the coating material-containing waste liquid W, and the first treated water W1 is taken out. In step S150, the first separation process and the first filtering process are sequentially performed.

In the first separation process, the first aggregate M1 contained in the coating material-containing waste liquid W subjected to the first coagulation process (steps S120 to S130) is separated from the first supernatant W11 as a supernatant by using the coagulation tank 13 (fig. 1). Specifically, the driving of the stirrer 40 (fig. 1) is stopped, and the waste liquid W containing the coating material is allowed to stand, whereby the first aggregates M1 are settled down to the lower surface of the aggregation tank 13 (fig. 1) by their own weight. Thereby, the first supernatant W11 was separated from the first aggregate M1.

In the first filtration process, the first condensate M1 and the first supernatant W11 conveyed into the first filtration apparatus 60 (fig. 1) together with the first condensate M1 are separated using the first filtration apparatus 60 (fig. 1). In the first filtration process, the first aggregate M1 and the first supernatant W11 conveyed from the coagulation tank 13 (fig. 1) are placed on the first filter 620 (fig. 1). In this state, when left standing for a predetermined time, the first residue M11 remains on the first filter 620 (fig. 1). The first residue M11 is incinerated in step S165.

On the other hand, the first filtrate W110 as a filtrate passes through the first filter 620 (fig. 1) and is stored in the first filter chamber 630 (fig. 1). Here, the first supernatant W11 obtained in the first separation process and the first filtrate W110 obtained in the first filtration process are collectively referred to as first treated water W1. The first filtrate W110 as the first treated water W1 is sent to the fenton tank 15 (fig. 1) in step S175. Note that the first filtering process may be omitted.

The first separation step (step S1) of the present embodiment is ended by the execution of steps S110 to S175. In the present embodiment, the time required for the first separation step (step S1) is about 6 hours. The COD of the first treated water W1 subjected to the first separation step (step S1) is about 5000mg/L to about 6000mg/L. The types and amounts of the primary flocculant and the secondary flocculant added in the first separation step (step S1), the pH of the coating-containing waste liquid W, the rotation speed of the stirrer 40 (fig. 1), the manner of coagulation, and other conditions are not limited to these.

Fig. 4 is a flowchart showing details of the first processing step (step S4) of the first embodiment. In the first treatment step (step S4), first, the pH of the first treated water W1 is adjusted in step S410. In step S410, the first treated water W1 is adjusted to a pH suitable for the fenton reaction performed in the first treatment (step S430 and step S450) as a subsequent step. In step S410, sulfuric acid contained in the chemical tank 11 (fig. 1) is added to the first treated water W1, thereby adjusting the pH of the first treated water W1 to 2.5 to 3.5.

After step S410, a temperature adjustment process is performed (step S420). The temperature adjustment step (step S420) is a step of adjusting the temperature of the first treated water W1 to a predetermined third temperature. In the present embodiment, the third temperature is 30 ℃. In the present embodiment, the third temperature is preferably set to the second temperature (45 ℃ in the present embodiment) or lower. This can suppress an excessive increase in the temperature of the first treated water W1 subjected to the first treatment accompanying the fenton reaction. The temperature adjustment process (step S420) may be omitted.

After step S420, a first adding process is performed (step S430). The first addition step (step S430) is a step of adding chemicals necessary for the fenton reaction to the first treated water W1 in the fenton tank 15 (fig. 1). In the first addition step (step S430), an antifoaming agent, an iron catalyst, and hydrogen peroxide as an oxidizing agent are added.

In the first addition step (step S430), first, an antifoaming agent is added to the first treated water W1. The defoaming agent is a chemical composed of, for example, polysiloxanes or polyethers. In the present embodiment, the concentration of the defoaming agent with respect to the first treated water W1 is 100ppm or more. Next, an iron catalyst is added to the first treated water W1. In this embodiment, the iron catalyst is ferrous sulfate containing ferrous ions. In the present embodiment, 17mL of ferrous sulfate is added to 1L of the first treated water W1. Subsequently, hydrogen peroxide is added to the first treated water W1. In the present embodiment, 110mL of 35 wt% hydrogen peroxide was added to 1L of the first treated water W1.

After step S430, a first stirring step (step S450) is performed. In the first stirring step (step S450), the fenton reaction is performed by stirring the first treated water W1. Therefore, the fenton reaction is performed in the first treated water W1 by sequentially performing the first addition step (step S430) and the first stirring step (step S450). In the first stirring step (step S450), the first treated water W1 is stirred by the stirrer 40a (fig. 1) at a rotation speed of 300 rpm. Hereinafter, the first adding step (step S430) and the first stirring step (step S450) are collectively referred to as a first process.

In the first treatment, the temperature of the first treated water W1 increases by the reaction heat to reach a maximum value (peak temperature). Then, the temperature of the first treated water W1 decreases. In the present embodiment, when the temperature of the first treated water W1 becomes 80 ℃, the first stirring step is terminated (step S450), and the first treatment is terminated. Hereinafter, the temperature of the first treated water W1 at the end of the first treatment is referred to as a first temperature.

Fig. 5 is a flowchart showing details of the second processing step (step S8) of the first embodiment. In the second treatment step (step S8), the fenton reaction is performed in the same manner as in the first treatment step (step S4), and therefore the pH of the first treated water W1 needs to be on the acidic side. At this time, the pH of the first treated water W1 is maintained in an acidic state without being greatly changed after the pH adjustment in step S410 (fig. 4). Therefore, the pH of the first treated water W1 does not need to be adjusted at the start of the second treatment step (step S8).

In the second processing step (step S8), first, the second adding step (step S830) is performed. The second addition step (step S830) is a step of adding hydrogen peroxide, which is one of the iron catalyst and hydrogen peroxide, again to the first treated water W1. In the present embodiment, in the second addition step (step S830), 110mL of 35 wt% hydrogen peroxide was added to the first treated water W1. Thereby, the fenton reaction is started.

Since the ferrous sulfate functions as a catalyst for the fenton reaction, the residual amount of ferrous sulfate in the first treated water W1 does not increase or decrease significantly at the start time of the first treatment and the start time of the second treatment. Therefore, it is not necessary to add the hydrogen peroxide in a plurality of portions as in the case of hydrogen peroxide. In the present embodiment, the addition of ferrous sulfate to the first treated water W1 is performed only in the first addition step (step S430 in fig. 4) without being performed in the second addition step (step S830). This can reduce the amount of the chemical to be added in the second addition step (step S830). The ferrous sulfate may be added in two steps, namely, the first addition step (step S430) and the second addition step (step S830). In the second addition step (step S830), the antifoaming agent may be added again to the first treated water W1.

After step S830, a second stirring process is performed (step S850). In the second stirring step (step S850), the fenton reaction is performed by stirring the first treated water W1. Therefore, the fenton reaction is performed in the first treated water W1 by sequentially performing the second addition step (step S830) and the second stirring step (step S850). In the second stirring step (step S850), the first treated water W1 is stirred by the stirrer 40a (fig. 1) at a rotation speed of 300 rpm. Hereinafter, the second adding step (step S830) and the second stirring step (step S850) are collectively referred to as a second process.

In the second treatment, the temperature of the first treated water W1 increases by the reaction heat to reach a maximum value (peak temperature). Then, the temperature of the first treated water W1 decreases. Therefore, in the present embodiment, the second stirring step is terminated when the temperature of the first treated water W1 reaches 80 ℃.

Fig. 6 is a flowchart showing details of the second separation step (step S9) of the first embodiment. In the second separation step (step S9), first, in step S910, the pH of the first treated water W1 is adjusted. In step S910, the first treated water W1 is adjusted to a pH suitable for the second coagulation treatment (step S920) as a subsequent step. In step S910, sodium hydroxide contained in the chemical tank 11 (fig. 1) is added to the first treated water W1, thereby adjusting the pH of the first treated water W1 to 6.5 to 7.5.

After step S910, a second coagulation process is performed (step S920). In the second coagulation treatment (step S920), a coagulant is added to the first treated water W1 subjected to the second treatment step (step S8) to remove the second solid component contained in the first treated water W1. The second solid component exists as a water-insoluble component in a state of being dispersed in the first treated water W1.

In the second coagulation treatment (step S920), the coagulant added to the first treated water W1 is a polymer coagulant. In the present embodiment, in order to aggregate the second solid component containing an iron compound having a positive charge on the surface, an anionic polymer aggregating agent capable of neutralizing the positive charge and aggregating the same is used. In the present embodiment, in the second coagulation treatment (step S920), 4mL of the anionic polymer flocculant is added to 1L of the first treated water W1. At this time, the first treated water W1 was stirred by the stirrer 40a (FIG. 1) at a rotational speed of 180 rpm.

In step S950, the second aggregates M2 contained in the first treated water W1 are separated from the first treated water W1, and the second treated water W2 is taken out. In step S950, the second separation process and the second filtering process are sequentially performed.

In the second separation process, the second aggregate M2 contained in the first treated water W1 subjected to the second coagulation process (step S920) is separated from the second supernatant W21 as a supernatant using the fenton tank 15 (fig. 1). Specifically, the driving of the agitator 40a (fig. 1) is stopped, and the first treated water W1 is allowed to stand, whereby the second aggregates M2 are settled down to the lower surface of the fenton vessel 15 (fig. 1) by their own weight. Thereby, the second supernatant W21 and the second aggregate M2 were separated.

In the second filtration process, the second coagulated product M2 is separated using the second filtration apparatus 60a (fig. 1) from the second supernatant W21 that is conveyed to the second filtration apparatus 60a (fig. 1) together with the second coagulated product M2. In the second filtering process, the second aggregate M2 and the second supernatant W21 transferred from the fenton tank 15 (fig. 1) are placed on the second filter 620a (fig. 1). In this state, when left standing for a predetermined time, the second residue M21 as a residue remains on the second filter 620a (fig. 1). The second residue M21 is incinerated in step S965.

On the other hand, the second filtrate W210 as a filtrate passes through the second filter 620a (fig. 1) and is stored in the second filter chamber 630a (fig. 1). Here, the second supernatant W21 obtained in the second separation process and the second filtrate W210 obtained in the second filtration process are collectively referred to as second treated water W2. The second filtrate W210 as the second treated water W2 is sent to the inspection tank 17 (fig. 1) in step S975. The second filtering process may be omitted.

The second separation step (step S9) of the present embodiment is ended by the execution of steps S910 to S975. In the present embodiment, the time required from the start time of the first processing step (step S4) to the end time of the second processing step (step S8) is about 9 hours.

After the second separation process (step S9), an inspection process (step S10) is performed. Fig. 7 is a flowchart showing details of the inspection step (step S10). In step S1100, it is determined whether the COD of the second treated water W2 is equal to or less than the target value. The COD of the second treated water W2 is measured by the third inspection device 20b (fig. 1). The controller 19 obtains the COD of the second treated water W2 from the third inspection device 20b (fig. 1), and executes step S1100.

If it is determined as "yes" in step S1100, all the steps of the waste liquid treatment method of the present embodiment are ended. If it is determined as no in step S1100, the re-fenton step is executed (step S1200).

The re-fenton step (step S1200) is a step of re-executing the first treatment and the second treatment so that the COD of the second treated water W2 becomes a target value or less. The conditions for adding hydrogen peroxide in the re-fenton step (step S1200) may be determined according to the COD of the second treated water W2 before the re-fenton step (step S1200) is performed. For example, when the difference between the COD of the second treated water W2 and the target value is smaller than the predetermined value, hydrogen peroxide may be added in an amount smaller than in the first addition step (step S430) and the second addition step (step S830) to perform the fenton reaction only once. The re-fenton step (step S1200) may be performed in a tank different from the fenton tank 15 (fig. 1).

In step S1300, it is determined whether or not the COD of the second treated water W2 after the execution of the re-fenton step (step S1200) is equal to or less than the target value. If it is determined as "yes" in step S1300, all the steps of the waste liquid treatment method of the present embodiment are ended. If it is determined as no in step S1300, the re-fenton process is executed again (step S1200).

A-3: change of COD caused by execution of each step

Fig. 8 is a graph showing the change in COD in each step. Fig. 8 shows the paint-containing waste liquid W that has not been subjected to any treatment, the first treated water W1 after the first separation step (step S1) shown in fig. 2, and the COD of the second treated water W2 after the second separation step (step S9) shown in fig. 2, in this order. Fig. 8 also shows COD in the case where the biological treatment step using activated sludge is performed on the second treated water W2 after the waste liquid treatment method of the present embodiment at the position where pretreatment as biological treatment is performed.

The COD of the first treated water W1 after the first separation step (step S1) is about 5000mg/L to about 6000mg/L. That is, in the first separation step (step S1), the COD of the coating material-containing waste liquid W can be reduced to about 5000mg/L to about 6000mg/L by removing the first solid matter from the coating material-containing waste liquid W.

The COD of the second treated water W2 after the second separation step (step S9) is about 800mg/L to about 1000mg/L. That is, the organic matter contained in the first treated water W1 is oxidatively decomposed by the first treatment (fig. 4) and the second treatment (fig. 5), and then the second solid matter is removed in the second separation step (step S9), whereby the COD of the coating material-containing waste liquid W can be reduced to about 800mg/L to about 1000mg/L. Therefore, the second treated water W2 after the waste liquid treatment method of the present embodiment is executed can be biologically treated with the load on the activated sludge reduced. In addition, since the waste liquid treatment method of the present embodiment is a treatment using a chemical reaction, the amount of hardly biodegradable substances contained in the second treated water W2 is reduced. Therefore, the biodegradability in the case of using activated sludge can be improved. In addition, when the organic matter contained in the second treated water W2 after the execution of the waste liquid treatment method of the present embodiment is biologically treated using activated sludge, the COD of the second treated water W2 can be reduced to 600mg/L or less.

According to the above embodiment, the COD of the coating material-containing waste liquid W can be reduced to a desired value by combining the step of adding the flocculant to separate the first solid component and the second solid component and the step of oxidatively decomposing the organic matter by fenton reaction. That is, the BOD and COD of the coating material-containing waste liquid W can be reduced to desired values by treatment using a chemical reaction, which is different from the activated sludge treatment using a biological decomposition reaction of microorganisms. This makes it possible to perform biological treatment while reducing the load on the activated sludge. Therefore, the possibility of death or the like of microorganisms constituting the activated sludge can be reduced. In addition, the treatment time using the activated sludge can be shortened. This enables the coating material-containing waste liquid W to be continuously treated in a short time.

Further, according to the above embodiment, by providing the cooling step (step S6) between the first treatment step (step S4) and the second treatment step (step S8), an excessive increase in the temperature of the first treated water W1 accompanying the fenton reaction can be suppressed. This can suppress inhibition of oxidative decomposition due to an excessive increase in the temperature of the first treated water W1, and thus can reduce the BOD and COD of the coating material-containing waste liquid W to desired values.

In addition, according to the above embodiment, after the BOD, COD of the coating material-containing waste liquid W is reduced to a desired value by the treatment using the chemical reaction, the biological treatment is performed. At this time, the amount of the hardly biodegradable substance contained in the second treated water W2 is reduced by the treatment using the chemical reaction. Therefore, the biodegradability in the case of using activated sludge can be improved.

In addition, according to the above embodiment, after the BOD, COD of the coating material-containing waste liquid W is reduced to a desired value by the treatment using the chemical reaction, the biological treatment is performed. Thereby, the amount of excess sludge produced by the biological treatment can be reduced. That is, the amount of waste in the treatment of the coating material-containing waste liquid W can be reduced.

In addition, according to the above embodiment, the step of oxidatively decomposing the organic substance by fenton reaction is performed in two times. Thus, the COD of the coating material-containing waste liquid W can be reduced to a desired value, that is, about 1000mg/L, by performing the Fenton reaction before the temperature of the first treated water W1 is excessively increased.

Further, according to the above embodiment, the COD of the coating material-containing waste liquid W is reduced to about 1000mg/L by combining the separation step of adding the flocculant to separate the solid component and the oxidative decomposition step of oxidatively decomposing the organic substance by fenton reaction. In this case, the time required for the separation step is about 6 hours, and the time required for the oxidative decomposition step is about 9 hours. Therefore, the coating material-containing waste liquid W can be continuously treated every 1 day.

Further, according to the above embodiment, the first treatment step (step S4) includes a temperature adjustment step (step S420) of adjusting the temperature of the first treated water W1 to a third temperature lower than the second temperature. This can suppress an excessive increase in the temperature of the first treated water W1 associated with the fenton reaction.

In addition, according to the above embodiment, the fenton reaction using the iron catalyst and the hydrogen peroxide can be performed by stirring the first treated water W1 in the first stirring step (step S450) and the second stirring step (step S850).

Further, according to the above embodiment, the first supernatant W11 as the first treated water W1 contained in the paint-containing waste liquid W is taken out in the first separation treatment, and then the first filtrate W110 as the first treated water W1 contained in the first aggregate M1 is taken out by the first filtration treatment. Further, according to this embodiment, the second supernatant W21 contained in the first treated water W1 is taken out as the second treated water W2 in the second separation treatment, and then the second filtrate W210 contained in the second aggregate M2 is taken out as the second treated water W2 by the second filtration treatment. This can improve the recovery rate of each of the first treated water W1 and the second treated water W2.

In addition, according to the above embodiment, the main components of the waste liquid treatment apparatus 1 are the chemical tank 11, the coagulation tank 13, the fenton tank 15, and the

filtration apparatuses

60 and 60a. That is, the waste liquid treatment method of the present embodiment can be executed with a simple apparatus configuration. This enables the waste liquid treatment apparatus 1 to be constructed at low cost.

B. Other embodiments are as follows:

b-1. Other embodiment 1:

in the above embodiment, the COD of the second treated water W2 was reduced to about 1000mg/L by adding a total of 220 (mL/1L of waste liquid) of 35 wt% hydrogen peroxide to 1L of the first treated water W1. However, the present invention is not limited thereto. By increasing the amount of hydrogen peroxide added to the first treated water W1, the COD of the first treated water W1 can be further reduced. For example, the amounts of hydrogen peroxide to be added to the first treated water W1 may be increased in the first addition step (step S430) of fig. 4 and the second addition step (step S830) of fig. 5, respectively. The number of fenton reactions performed may be increased to 3 or more while keeping the amount of hydrogen peroxide added per time constant (e.g., 110 mL/1L of waste liquid).

In this manner, the COD of the second treated water W2 can be reduced to 100mg/L. Therefore, the COD of the coating material-containing waste liquid W can be reduced to a desired value only by the treatment using the chemical reaction without performing the biological treatment using the biological decomposition reaction.

B-2: other embodiment mode 2:

in the above embodiment, in the cooling step (step S6) shown in fig. 2, the temperature adjustment mechanism 70 (fig. 1) is used to perform cooling until the temperature of the first treated water W1 becomes the second temperature or lower. However, the present disclosure is not limited thereto. In the cooling step (step S6), natural air cooling may be performed to cool the first treated water W1 by heat exchange with the atmosphere, for example. Even in this manner, the temperature of the first treated water W1 subjected to the first treatment step (step S4) shown in fig. 4 can be cooled to the second temperature or lower.

C. Preferred conditions of addition

C-1. Primary coagulant addition amount

Fig. 9 is a diagram for explaining an appropriate amount of aluminum sulfate to be added as a primary flocculant. In this measurement, the removal rate of organic substances in the paint-containing waste liquid W was measured while changing the amount of aluminum sulfate added to the paint-containing waste liquid W. In this measurement, in order to confirm the direct effect of aluminum sulfate, aluminum sulfate was added to the coating material-containing waste liquid W, and then the coating material-containing waste liquid W was allowed to stand to settle and separate the basic flocs, and the COD of the obtained supernatant was measured.

The horizontal axis of fig. 9 represents the amount of aluminum sulfate added to 1L of the coating material-containing waste liquid W. In FIG. 9, the unit of the amount of aluminum sulfate added is set to (mL/waste liquid 1L). The vertical axis of fig. 9 represents the COD removal rate. The COD removal rate referred to herein is a value representing by percentage how much the COD of the supernatant of this measurement was reduced relative to the COD of the coating material-containing waste liquid W before the addition of aluminum sulfate. That is, the first solid component was coagulated by adding aluminum sulfate in an amount corresponding to the COD removal rate of the supernatant liquid shown by the vertical axis of fig. 9.

Fig. 9 shows data obtained when the amount of aluminum sulfate added to 1L of the paint-containing waste liquid W was measured while varying the amount. The other conditions are the same as those in the first separation step (step S1). In this measurement, the COD of the waste liquid W containing the paint was examined to be about 15000mg/L to about 19000mg/L. In the present measurement, as shown in fig. 9, the following tendency was observed in the case where the amount of aluminum sulfate added to the paint-containing waste liquid W was increased stepwise. When the amount of aluminum sulfate added was increased from 5.0mL to 17.5mL, the removal rate of COD in the supernatant was increased by 25%. On the other hand, even when the amount of aluminum sulfate added was increased from 17.5mL to 35.0mL, the COD removal rate of the supernatant was increased by only 3%, and no significant increase in the COD removal rate of the supernatant was observed. That is, in the present measurement, when the first solid content contained in the paint-containing waste liquid W is coagulated by adding aluminum sulfate, no significant effect can be obtained even if more than 17.5mL of aluminum sulfate is added to 1L of the paint-containing waste liquid W. From the above, it is understood that the upper limit of the amount of aluminum sulfate added to 1L of the coating material-containing waste liquid W is preferably 17.5mL or less. This makes it possible to obtain the effect of aggregating the first solid component while suppressing the amount of aluminum sulfate added. In the above embodiment, the amount of aluminum sulfate added was set to 17mL.

C-2. Preferred amount of hydrogen peroxide to be added for Fenton reaction

Fig. 10 is a diagram for explaining an appropriate amount of hydrogen peroxide to be added. In this measurement, when the amount of hydrogen peroxide added to the first treated water W1 was changed, the COD of each of the first treated water W1 was measured, and the appropriate amount of hydrogen peroxide was examined. In this measurement, the COD of the first treated water W1 was investigated to be 6000mg/L.

The horizontal axis of fig. 10 represents the amount of hydrogen peroxide added to 1L of the first treated water W1. In FIG. 10, the unit of the amount of hydrogen peroxide added is set to (mL/waste liquid 1L). In the present measurement, 35 wt% of hydrogen peroxide was used. The vertical axis of fig. 10 shows the decomposition rate of organic matter contained in the first treated water W1 as the COD decomposition rate. The COD decomposition rate referred to herein is a value that represents by percentage how much the COD of the second treated water W2 from which iron compounds have been removed by the same method as in the second separation step (step S9) after the fenton reaction has occurred is reduced with respect to the COD of the first treated water W1 before the fenton reaction has occurred. That is, the hydroxyl radicals decompose the organic substances contained in the first treated water W1 at the COD decomposition rate shown in the vertical axis of fig. 10.

Fig. 10 shows data measured by changing only the amount of hydrogen peroxide added to 1L of the first treated water W1. The other conditions are the same as those in the first processing step (step S4) of the first embodiment. In the present measurement, as shown in fig. 10, when the amount of hydrogen peroxide added to the first treated water W1 is increased stepwise, the COD decomposition rate tends to increase as the amount of hydrogen peroxide added increases. That is, it is observed that the oxidative decomposition of the organic substance by the hydroxyl radical tends to be accelerated in proportion to the amount of hydrogen peroxide added.

In the above embodiment, the COD of the second treated water W2 is reduced to about 1000mg/L. In the example shown in FIG. 10, the COD of the second treated water W2 was 960mg/L at a COD decomposition rate of 84%, and the amount of hydrogen peroxide added was 220mL at this time. Therefore, the oxidation decomposition of the organic substances contained in the first treated water W1 having a COD of about 6000mg/L by the Fenton reaction and the reduction of the COD of the second treated water W2 to about 1000mg/L can be carried out as follows. That is, 220mL of 35 wt% hydrogen peroxide may be added to 1L of the first treated water W1. Therefore, in the above embodiment, the total amount of 35 wt% hydrogen peroxide added to 1L of the first treated water W1 was set to 220mL.

Here, hydrogen peroxide has a property of being decomposed into water and oxygen by a specific element. The oxygen produced here is gaseous oxygen. The decomposition reaction of hydrogen peroxide is shown in the following formula (5). Specific factors causing the decomposition of hydrogen peroxide are, for example, concentration, temperature, and the like. The hydrogen peroxide is more easily decomposed as the concentration of hydrogen peroxide in the solution, in this case, the first treated water W1, is higher. Further, the higher the temperature of the first treated water W1, the more easily the hydrogen peroxide is decomposed. Specifically, when the temperature of the solution containing hydrogen peroxide is 60 ℃ or higher, the decomposition reaction represented by formula (5) actively proceeds.

2H ₂ O ₂ →2H ₂ O+O ₂ Formula (5)

In the process (the above formula (4)) of oxidatively decomposing the organic matter of the first treated water W1 by the hydroxyl radical, reaction heat is generated. Further, as the amount of hydrogen peroxide added to the first treated water W1 increases, the oxidative decomposition represented by the formula (4) is promoted. That is, the reaction heat increases as the amount of hydrogen peroxide added increases. For example, when 198mL of hydrogen peroxide is added to 1L of the first treated water W1 having an initial temperature of 30 ℃, the temperature of the first treated water W1 becomes 80 ℃ or higher. When the temperature of the first treated water W1 becomes 60 ℃ or higher due to the heat of reaction, the hydrogen peroxide contained in the first treated water W1 is likely to cause the decomposition reaction of the formula (5) in an unreacted state.

Oxygen gas generated by the self-decomposition of hydrogen peroxide is retained in the fenton tank 15 (fig. 1). This oxygen gas causes rapid expansion of the first treated water W1 and an increase in the internal pressure of the fenton tank 15 (fig. 1). Therefore, in order to safely carry out the fenton reaction, it is preferable to minimize the decomposition reaction of hydrogen peroxide represented by formula (5). Next, suitable conditions for adding hydrogen peroxide when 220mL of hydrogen peroxide is added to 1L of the first treated water W1 will be described with reference to fig. 11 to 13.

Fig. 11 is data showing the correlation between the temperature of the first treated water and the amount of hydrogen peroxide added. Fig. 11 shows the conditions and the results of the present measurement. Fig. 11 shows the time required for the first treated water W1 to reach the peak temperature along with the fenton reaction, the peak temperature, the amount of oxygen generated, and the presence or absence of expansion of the first treated water W1 under the respective conditions X1 to X9. By using the measurement results of fig. 11, it is possible to determine the addition conditions for minimizing the decomposition reaction of hydrogen peroxide. In the conditions X1 to X9, the initial temperature of the first treated water W1 and the amount of 35 wt% hydrogen peroxide added to the first treated water W1 were different. The "initial temperature" in fig. 11 indicates the temperature of the first treated water W1 after step S420. The "time required to reach the peak temperature" in fig. 11 represents the time, in minutes, at which the temperature of the first treated water W1 reaches the maximum value. The "peak temperature" in fig. 11 indicates a value at which the temperature of the first treated water W1 reaches the maximum value. The "oxygen generation amount" in fig. 11 indicates the amount of oxygen generated when the fenton reaction is performed on 1L of the first treated water W1 under the conditions X1 to X9. The unit of oxygen production is (L/waste liquid 1L). In the present measurement, 17mL of ferrous sulfate was added to 1L of the first treated water W1.

Fig. 12 is a graph showing the change with time in the temperature of the first treated water W1 under the conditions shown in fig. 11. In fig. 12, the time required to reach the peak temperature and the peak temperature of fig. 11 are visualized under each of the conditions X1 to X9. In fig. 12, symbols corresponding to conditions X1 to X9 in fig. 11 are given. Fig. 12 shows the relationship between the elapsed time from the start of the fenton reaction and the temperature of the first treated water W1 for each of the conditions X1 to X9. The horizontal axis of fig. 12 represents the elapsed time from the start of the fenton reaction by the addition of hydrogen peroxide. In fig. 12, the elapsed time is expressed in units of 30 minutes. The vertical axis of fig. 12 represents the temperature of the first treated water W1. The peak temperature in fig. 11 corresponds to the temperature of the first treated water W1 at the vertex of each curve according to the conditions X1 to X9 in fig. 12. The time required to reach the peak temperature in fig. 11 coincides with the elapsed time at the vertex of each curve relating to conditions X1 to X9 in fig. 12.

The conditions X1, X4, and X7 are the conditions closest to the amount of hydrogen peroxide added (220 mL) in the first embodiment. Under the conditions X1, X4, and X7, as shown in fig. 11 and 12, a significant increase in the temperature of the first treated water W1 was observed under any of the conditions. Under the conditions X1, X4 having the initial temperature higher than the condition X7, as shown in fig. 11, the swelling of the first treated water W1 was also observed.

On the other hand, under condition X7, as shown in fig. 11, swelling of the first treated water W1 was not observed. When compared with other conditions X1, X2, X4, and X5 under which the swelling of the first treated water W1 is observed, as shown in fig. 12, the time required for the condition X7 to reach the peak temperature is longer than that under the other conditions X1, X2, X4, and X5. Further, with respect to the oxygen generation amount, in the case of comparing the condition X5 with the condition X7, although the condition X7 is more than the condition X5, no swelling of the first treated water W1 is observed under the condition X7. From this, it can be said that in order to perform the fenton reaction more safely, when the peak temperatures are about the same, a condition that can further extend the time required to reach the peak temperatures is preferable.

The conditions X2, X5, and X8 are conditions in which the amount of hydrogen peroxide added is reduced as compared with the conditions X1, X4, and X7. Under the conditions X2, X5 in which the initial temperature was made 30 ℃ or higher, as shown in fig. 11 and 12, a significant increase in the temperature of the first treated water W1 was observed. Under the conditions X2 and X5, as shown in fig. 11, swelling of the first treated water W1 was also observed. In contrast, under the condition X8 in which the initial temperature was set to 25 ℃, as shown in fig. 11 and 12, the peak temperature was lower than the conditions X1 to X7. Under the condition X8, as shown in fig. 11, the first treated water W1 was not swollen. From this, it can be said that a condition capable of further lowering the peak temperature is preferable for more safely performing the fenton reaction.

The conditions X3, X6, and X9 are conditions in which the amount of hydrogen peroxide added is further reduced compared to the conditions X2, X5, and X8. The amount of hydrogen peroxide added under conditions X3, X6, and X9 was half the amount of hydrogen peroxide added (220 mL) in the first embodiment. Under the condition X3 in which the initial temperature was set to 35 ℃, although no swelling of the first treated water W1 was observed, a significant increase in the temperature of the first treated water W1 was observed.

On the other hand, under the conditions X6 and X9 in which the initial temperature is set to 30 ℃ or lower, the peak temperature is in the vicinity of 70 ℃, and the temperature increase of the first treated water W1 is suppressed as compared with the other conditions X1 to X5 and X7. In the conditions X6 and X9, the time required to reach the peak temperature is longer than in the conditions X1 to X5. In addition, under the conditions X6 and X9, the amount of oxygen generation was small compared to the other conditions X1, X2, X4, X5, X7, and X8, and no swelling of the first treated water W1 was observed.

Accordingly, in order to carry out the fenton reaction more safely, the following conditions (i) to (iii) are preferably satisfied.

(i) No swelling of the first treated water W1 was observed

(ii) Make the time required for reaching the peak temperature longer

(iii) Further reducing peak temperature

Among the conditions X1 to X9 shown in fig. 11 and 12, the conditions satisfying the conditions (i) to (iii) are the condition X6 and the condition X9.

Here, the higher the initial temperature of the first treated water W1 and the temperature of the first treated water W1 after the start of the fenton reaction, the higher the reaction rate of the fenton reaction. In this case, the larger the reaction rate of the fenton reaction, the shorter the time period for oxidative decomposition of the organic substances contained in the first treated water W1. Therefore, it is preferable to consider the reaction rate of the fenton reaction also when determining the conditions for adding hydrogen peroxide. Therefore, among the conditions X6 and X9 satisfying the conditions (i) to (iii) for more safely performing the fenton reaction, the condition X6 having a higher initial temperature is the optimum condition.

In the present embodiment, the total amount of 35 wt% hydrogen peroxide added to 1L of the first treated water W1 was 220mL in order to reduce the COD of the second treated water W2 to 1000mg/L or less. On the other hand, in order to carry out the fenton reaction in a state of being safer and maintaining a desired reaction rate, the following is preferable. The initial temperature of the first treated water W1 is preferably about 30 ℃ as shown in the optimal condition X6 (fig. 11 and 12). The conditions for adding hydrogen peroxide are preferably such that 110mL of 35 wt% hydrogen peroxide is sequentially added to 1L of the first treated water W1, as shown in the optimal condition X6 (FIGS. 11 and 12). Therefore, in the present embodiment, hydrogen peroxide is added to the first treated water W1 in two portions. Specifically, in the first addition step (step S430) of fig. 4, 110mL of 35 wt% hydrogen peroxide was added to 1L of the first treated water W1 (hereinafter referred to as first addition). In the second addition step (step S830) of fig. 5, 110mL of 35 wt% hydrogen peroxide was added to 1L of the first treated water W1 (hereinafter referred to as second addition).

Fig. 13 is a diagram for explaining timings of performing the first addition and the second addition. Fig. 13 shows that 2m, which is smaller in capacity than the fenton tank 15 (fig. 1) of the above embodiment, is used to predict the temperature change of the first treated water W1 when the fenton reaction is performed in the fenton tank 15 (fig. 1) of the above embodiment ³ The results were determined experimentally in a simple cell. Fig. 13 shows the temperature change of the first treated water W1 when the first treatment step (step S4) and the second treatment step (step S8) are performed in the simple tank under the same conditions as in the first embodiment. In fig. 13, 3 measurements were performed under the same conditions. In order to distinguish the measurement results, labels of X6 (1), X6 (2), and X6 (3) are attached to fig. 13.

The horizontal axis of fig. 13 represents the elapsed time from the time when the first addition was made. Fig. 13 shows elapsed time in units of 15 minutes. The vertical axis of fig. 13 represents the temperature of the first treated water W1. In this measurement, the progress of the fenton reaction is read from the change in the temperature of the first treated water W1, as in the measurement of fig. 11 and 12.

In this measurement, as shown in fig. 13, the same tendency was exhibited in both cases. Specifically, after the first addition, the temperature of the first treated water W1 is gradually increased. Then, when the elapsed time reaches from about 2 hours and 30 minutes to about 3 hours, the temperature of the first treated water W1 reaches the maximum value (peak temperature). In any case, the peak temperature of the first treated water W1 in this measurement was around 80 ℃. Then, the temperature of the first treated water W1 decreases.

The timing to perform the second addition may be determined according to the temperature of the first treated water W1. For example, the second addition is performed in a state where the temperature of the first treated water W1 is not sufficiently lowered. In this case, at the time of performing the second addition, the fenton reaction is started again in a state where the temperature of the first treated water W1 has become higher than the desired temperature. This makes it possible to easily cause the decomposition reaction of hydrogen peroxide represented by formula (5). Therefore, it is preferable to determine the timing of performing the second addition so that the peak temperature of the first treated water W1 is the same in the first addition and the second addition. This can suppress an excessive increase in the temperature of the first treated water W1 even in the second treatment. This can prevent the oxidative decomposition from being inhibited as the temperature of the first treated water W1 increases excessively. It is not essential that the peak temperature of the first treated water W1 is the same between the first addition and the second addition.

As shown in fig. 13, when the second addition is performed at the time when the temperature of the first treated water W1 reaches 45 ℃, the peak temperature after the second addition is made is around 80 ℃. Thus, the timing for performing the second addition is preferably set to a timing at which the temperature of the first treated water W1 reaches 45 ℃ or lower after the temperature of the first treated water W1 reaches the peak temperature by performing the first addition. The initial temperature (third temperature in the first embodiment) needs to be set to a temperature (second temperature in the first embodiment) of the first treated water W1 or lower for appropriately performing the subsequent second treatment step (step S8).

Based on the conditions determined above, in the first addition step (step S430) of fig. 4, the antifoaming agent, the ferrous sulfate, and the hydrogen peroxide are added to the first treated water W1 (first addition). This starts the fenton reaction in the first treatment. In the second addition step (step S830) of fig. 5, hydrogen peroxide (second addition) is added to the first treated water W1. This starts the fenton reaction in the second treatment.

According to the above-described aspect, after the initial temperature of the first treated water W1 and the conditions for adding hydrogen peroxide are selected, the first treatment and the second treatment accompanied by the fenton reaction are performed. The third temperature, which is the initial temperature, is set to be equal to or lower than the second temperature, which is the temperature of the first treated water W1 at the start of the second treatment. This enables the fenton reaction to be performed more safely.

C-3 preferable amount of ferrous sulfate as Fenton reaction catalyst

As the amount of the added ferrous sulfate in the first addition step (step S430) of fig. 4 increases, the amount of the second residue M21 generated increases. Since the second residue M21 is to be incinerated, it is necessary to minimize the amount of second residue M21 generated. On the other hand, the reaction rate of the fenton reaction in the first treatment and the second treatment is increased as the amount of added ferrous sulfate is increased. Therefore, the relationship between the amount of ferrous sulfate added and the amount of second residue M21 generated was examined, and a method of selecting the minimum amount of ferrous sulfate added was examined.

Fig. 14 is a diagram for explaining a selected method of an appropriate addition amount of ferrous sulfate. Fig. 14 shows the correlation between the amount of added ferrous sulfate and the amount of generated second residue M21. The horizontal axis in fig. 14 represents the amount of ferrous sulfate added to 1L of the first treated water W1 having undergone the first separation step (step S1). In fig. 14, the unit of the amount of ferrous sulfate added is set to (mL/waste liquid 1L). The vertical axis of fig. 14 represents the generation ratio of the second residue M21 with respect to the first treated water W1 after the first treatment and the second treatment are performed. The generation ratio of the second residue M21 referred to herein is a numerical value in percentage representing the amount of the second residue M21 contained in 1L of the first treated water W1.

Fig. 14 shows data obtained when the amount of ferrous sulfate added to 1L of the first treated water W1 after the first treatment step (step S4) and the second treatment step (step S8) was measured while varying the amount of ferrous sulfate added. The other conditions are the same as those in the first embodiment.

First, the amount of ferrous sulfate added in the first addition step (step S430) was changed, and the generation ratio of the second residue M21 in the case where the first treatment and the second treatment were performed was measured. In the example shown in fig. 14, a plot is made using the plurality of measured data obtained here.

Next, a relational expression showing the relationship between the amount of added ferrous sulfate and the generation ratio of the second residue M21 was calculated using the plurality of measured data. Specifically, the relational expression is calculated by a so-called multiple regression analysis using x and y as variables. The relational expression referred to herein is an expression obtained by approximating, by the least square method, a plurality of actual measurement data (plot points in fig. 14) indicating the generation ratio of the second residue M21 with respect to the amount of ferrous sulfate added. Hereinafter, x is set as the amount of ferrous sulfate added to 1L of the first treated water W1 in the first addition step (step S430). In addition, y is hereinafter set as a generation ratio (%) of the second residue M21 to 1L of the first treated water W1 subjected to the first treatment and the second treatment. The relational expression calculated here is shown in the following formula (6). A and b are constants, respectively.

y = ax + b formula (6)

In the example shown in FIG. 14, the relationship calculated using the plurality of measured data is y =0.123x +1.351.

Then, a correction relational expression is calculated from the relational expression shown in the expression (6). Specifically, the correction relational expression is calculated by so-called section estimation. In the example shown in fig. 14, as the correction relational expression, an upper limit of a 95% confidence interval, a lower limit of a 95% confidence interval, an upper limit of a 95% prediction interval, and a lower limit of a 95% prediction interval are calculated, respectively. In fig. 14, the 95% confidence interval refers to a region in which equation (6) can be considered to fall within the range with a probability of 95%. The upper 95% confidence interval limit represents the upper limit in the region of the 95% confidence interval. The lower 95% confidence interval limit represents the lower limit in the region of the 95% confidence interval. In fig. 14, the relation shown in equation (6) is a linear function. Therefore, the modified relation representing the upper limit of the 95% confidence interval and the lower limit of the 95% confidence interval is a linear function. The corrected relational expression indicating the upper limit of the 95% confidence interval is shown in the following formula (7.1). The corrected relational expression indicating the lower limit of the 95% confidence interval is shown in the following formula (7.2). The constant c1 is obtained by adding a predetermined numerical value to the constant b represented by the formula (6). The constant d1 is a value obtained by subtracting a predetermined numerical value from the constant b represented by the formula (6). In the example shown in fig. 14, the predetermined numerical magnitude is the same in the constant c1 and the constant d 1.

y = ax + c1 formula (7.1)

y = ax + d1 formula (7.2)

In fig. 14, the 95% prediction interval refers to a region that can be considered to fall within the 95% confidence interval with a 95% probability. The 95% prediction interval indicates a region that falls within the 95% confidence interval with a 95% probability under the same conditions as the present measurement in the case where the first process and the second process are performed later. The upper limit of the 95% prediction section represents an upper limit in the region of the 95% prediction section. The 95% prediction interval lower limit means a lower limit in the region of the 95% prediction interval. In fig. 14, the correction relational expressions shown by the expressions (7.1) and (7.2) are linear functions. Therefore, the correction relational expression indicating the upper limit of the 95% prediction section and the lower limit of the 95% prediction section becomes a linear function.

The correction relational expression indicating the upper limit of the 95% prediction section is shown in the following expression (7.1.1). The correction relational expression indicating the lower limit of the 95% prediction interval is shown in the following expression (7.2.1). The constant c1 is obtained by adding a predetermined numerical value to the constant b represented by the formula (6). The constant c2 is also a value obtained by adding a predetermined numerical value to the constant c1 represented by the formula (7.1). That is, in constants b, c1, and c2 represented by equation (6), equation (7.1), and equation (7.1.1), the relationship of b < c1 < c2 holds. The constant d1 is a value obtained by subtracting a predetermined numerical value from the constant b represented by the formula (6). The constant d2 is also a value obtained by subtracting a predetermined numerical value from the constant d1 represented by the formula (7.2). That is, in constants b, d1, and d2 represented by equation (6), equation (7.2), and equation (7.2.1), the relationship of d2 < d1 < b holds. In the example shown in fig. 14, the predetermined numerical magnitude is the same in the constant c2 and the constant d 2.

y = ax + c2 formula (7.1.1)

y = ax + d2 type (7.2.1)

Next, an appropriate amount of ferrous sulfate to be added is selected using the correction relational expressions shown by the expressions (7.1), (7.2), (7.1.1) and (7.2.1). At this time, a target value is determined in advance as to the generation ratio of the second residue M21 to the first treated water W1. Fig. 14 illustrates a case where the target value of the generation ratio of the second residue M21 is set to 9%. In order to select an appropriate amount of ferrous sulfate to be added, it is necessary to consider the generation ratio of the second residue M21 that may be generated when the first process and the second process are performed later, as data that can be observed in the future. Therefore, in the present embodiment, the appropriate amount of ferrous sulfate to be added is selected using the correction relational expression for the upper limit of the 95% prediction section shown in the expression (7.1.1). In the first embodiment, the correction relational expression for selecting an appropriate amount of ferrous sulfate to be added corresponds to expression (7.1.1).

Specifically, in the corrected relational expression of the upper limit of the 95% prediction section represented by the expression (7.1.1), the amount of ferrous sulfate added when the generation ratio of the second residue M21 to the first treated water W1 (vertical axis in fig. 14) becomes a target value is referred to. In the example shown in fig. 14, in the corrected relational expression of the upper limit of the 95% prediction section shown in the expression (7.1.1), the amount of added ferrous sulfate when the generation ratio of the second residue M21 became 9% was 17 (mL/waste liquid 1L). Therefore, in the first embodiment, 17mL of ferrous sulfate is added to the first treated water W1 in the first addition step (step S430) of fig. 4. In determining the amount of ferrous sulfate to be added, it is not essential to use the relational expressions shown in formula (6), formula (7.1), formula (7.1.1), formula (7.2), and formula (7.2.1) as modified relational expressions.

According to the above embodiment, the first treatment and the second treatment are performed after the amount of ferrous sulfate to be added is selected so as to minimize the generation ratio of the second residue M21. This can suppress the amount of second residues M21 to be incinerated. In the above embodiment, the amount of ferrous sulfate added to the target generation ratio of the second residue M21 can be easily determined using the formula (7.1.1).

The present disclosure is not limited to the above-described embodiments, and can be implemented in various configurations without departing from the scope of the present disclosure. For example, in order to solve part or all of the above-described problems or to achieve part or all of the above-described effects, the technical features of the embodiments corresponding to the technical features of the respective embodiments described in the summary section of the invention may be appropriately replaced or combined. In addition, if the technical features are not described as essential in the present specification, they can be deleted as appropriate.

Claims

1. A waste liquid treatment method, comprising:

a first separation step of separating a first solid component contained in a paint-containing waste liquid from the paint-containing waste liquid and taking out first treated water;

an oxidative decomposition step of oxidatively decomposing organic matter contained in the first treated water after the first separation step; and

a second separation step of separating a second solid component containing an iron compound among components contained in the first treated water from the first treated water and taking out second treated water after the oxidative decomposition step,

the oxidative decomposition step includes:

a first treatment step of performing a first treatment on the first treated water subjected to the first separation step until the temperature of the first treated water reaches a first temperature;

a cooling step of cooling the first treated water after the first treatment step;

a second treatment step of performing a second treatment on the first treated water subjected to the first treatment under a condition that the temperature of the first treated water is brought to a second temperature lower than the first temperature in the cooling step,

the first treatment and the second treatment are each a treatment for oxidatively decomposing the organic matter contained in the first treated water subjected to the first separation step by using a hydroxyl radical generated by a fenton reaction using an iron catalyst and hydrogen peroxide.

2. The liquid waste treatment method according to claim 1, wherein the first treatment step further comprises a temperature adjustment step of adjusting the temperature of the first treated water subjected to the first separation step to a third temperature lower than the second temperature before the start of the fenton reaction.

3. The method of treating a waste liquid according to claim 1 or claim 2,

the first processing includes:

a first addition step of adding the iron catalyst and the hydrogen peroxide to the first treated water subjected to the first separation step; and

a first stirring step of stirring the first treated water after the first addition step,

the second processing includes:

a second addition step of adding the hydrogen peroxide to the first treated water having undergone the first treatment, the hydrogen peroxide being one of the iron catalyst and the hydrogen peroxide; and

a second stirring step of stirring the first treated water after the second addition step.

4. The liquid waste treatment method according to claim 3,

the first separation process includes:

(1a) A first coagulation treatment in which a coagulant is added to the coating material-containing waste liquid;

(1b) A first separation treatment in which the coating material-containing waste liquid subjected to the first coagulation treatment is left to stand to settle first coagulates generated by the first coagulation treatment, and the first coagulates are separated from a first supernatant liquid as a supernatant liquid; and

(1c) A first filtration process of separating a first filtrate as a filtrate obtained by filtering the first aggregate with a filter from a first residue as a residue,

the first treated water subjected to the oxidative decomposition step is composed of the first supernatant obtained by the first separation treatment and the first filtrate obtained by the first filtration treatment,

the second separation process includes:

(2a) A second coagulation treatment in which the coagulant is added to the first treated water subjected to the second treatment;

(2b) A second separation process of leaving the first treated water subjected to the second coagulation process to stand to settle a second aggregate produced by the second coagulation process, and separating the second aggregate from a second supernatant as a supernatant; and

(2c) A second filtration process of separating a second filtrate as a filtrate obtained by filtering the second aggregate with the filter from a second residue as a residue,

the second treated water is composed of the second supernatant obtained in the second separation treatment and the second filtrate obtained by the second filtration treatment.

5. The method of treating waste liquid according to claim 4,

the iron catalyst is ferrous sulfate and is prepared by mixing iron,

the first addition step includes a step of adding the ferrous sulfate to the first treated water in an amount determined by the following formula (2) obtained by correcting the following formula (1),

y = ax + b formula (1)

y = ax + c type (2)

In the above formula (1), x is an addition amount (mL/L) of the ferrous sulfate to 1 liter of the first treated water in the first addition step, y is a generation ratio (%) of the second residue per 1 liter of the first treated water subjected to the first treatment and the second treatment, and a and b are constants, respectively,

the above formula (1) is a formula obtained by approximating a plurality of measured data indicating the generation ratio with respect to the addition amount by a least square method,

in the above expression (2), the constant c is a value obtained by adding a predetermined numerical value to the constant b in the above expression (1).

6. A waste liquid treatment apparatus, wherein the waste liquid treatment apparatus has:

a coagulation tank in which a first separation treatment is performed to separate a first solid component contained in a paint-containing waste liquid from the paint-containing waste liquid and to take out first treated water;

a Fenton tank for performing oxidative decomposition treatment and second separation treatment after the first separation treatment; and

a control unit for controlling the progress of the oxidative decomposition treatment based on the temperature of the first treated water,

the oxidative decomposition treatment includes:

a first treatment in which the first treated water subjected to the first separation treatment is subjected to a first treatment until the temperature of the first treated water reaches a first temperature;

a cooling treatment in which the first treated water is cooled after the first treatment; and

a second treatment of the first treated water subjected to the first treatment under a condition that the temperature of the first treated water is brought to a second temperature lower than the first temperature by the cooling treatment,

the first treatment and the second treatment are treatments for oxidatively decomposing organic substances contained in the first treated water subjected to the first separation treatment by hydroxyl radicals generated by a fenton reaction using an iron catalyst and hydrogen peroxide,

the second separation treatment is a treatment of separating a second solid component containing an iron compound among components contained in the first treated water subjected to the second treatment from the first treated water and taking out second treated water.