JPH10151470A - Treatment of waste water - Google Patents

Abstract

PROBLEM TO BE SOLVED: To remove selenium(Se) from waste water in reduced metal consumption quantity and to further remove both of an oxidative substance and selenium(Se) by one process. SOLUTION: This waste water treatment method is equipped with a first process composed of at least one of a process bringing at least one kind of a metal or at least one kind of a metal compd. selected from a group consisting of metals of Fe, Mn, Ni and Cu and low valency compds. of these metals into contact with waste water and a process mixing a soln. or slurry of said metal compd. with waste water, a second process adding a sulfite ion type reducing agent to the waste water from the first process and a third process forming a precipitate in the waste water from the second process to separate the waste water into a conc. soln. containing the precipitate and treated water of which the concn. of the precipitate is lower than that of the concn. soln. The first and second processes are respectively executed in first and second treatment tanks 104,106 and the third process is executed in a third treatment tank 108 and a sedimentation tank 110.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wastewater treatment method for removing selenium (Se) from wastewater, and when applied to the treatment of flue gas desulfurization wastewater, oxidizing substances are removed from the flue gas desulfurization wastewater. And selenium (Se) are simultaneously and efficiently removed.

[0002]

2. Description of the Related Art Wastewater containing various chemical substances is discharged from factories and the like that manufacture metal products and industrial products. Conventionally, such industrial wastewater has been discharged to rivers, the sea, and the like after being subjected to predetermined treatment in wastewater treatment equipment. By the way, as interest in environmental pollution increases, the concentration in wastewater is extremely low, so even if chemicals have been overlooked for their effects on pollution, the accumulation of such chemicals causes environmental pollution. As a result, there is an increasing tendency to regulate the concentration of such chemicals in treated effluents. A representative one of such chemical substances is selenium (Se), and the concentration of selenium and selenium compounds in wastewater has recently been increasing.
It is regulated to 0.1mg / l or less.

As a method for removing selenium and selenium compounds (hereinafter collectively referred to as selenium including selenium compounds) in wastewater, for example, as disclosed in Japanese Patent Application Laid-Open No. 7-2502, By contacting iron or iron-based metal (hereinafter, iron or iron-based metal is collectively referred to as iron), selenium is precipitated on the surface of the iron metal,
Methods have been proposed to reduce and remove selenium concentrations in wastewater.

[0004]

However, in the above-described method of removing selenium from wastewater, the consumption of ferrous metal and the amount of generated sludge are extremely large, so that the cost of removing selenium increases, and the wastewater treatment is difficult. There was a problem that it was difficult to apply from an economic point of view. In particular, like flue gas desulfurization effluent discharged from a wet flue gas desulfurization treatment device that processes exhaust gas discharged from a thermal power plant to remove sulfur oxides,
When treating large amounts of wastewater containing selenium, its application has been difficult. The flue gas desulfurization effluent is defined in detail in the last definition column.

Accordingly, an object of the present invention is to provide a method for removing selenium from wastewater with a small consumption of iron or iron-based metal and a small amount of sludge generation, and in particular, a method for removing selenium-containing wastewater as in the treatment of flue gas desulfurization wastewater. An object of the present invention is to provide a wastewater treatment method that can be applied to a large amount of treatment.

[0006]

The present inventors have noticed that a large amount of Fe (OH) ₃ is contained in sludge, and oxidized iron metal by a reaction represented by the following formula (1). The first reaction for reducing and releasing selenium by a reduction reaction and separating selenium from the wastewater and the second reaction shown in the formula (2) reduce the oxidized iron metal with a sulfite ion-based reducing agent, and again The idea was to suppress the consumption of iron metal by combining with the second reaction that contributes to the first reaction. Fe, Fe ²⁺ + SeO ₄ ^2- → Fe ²⁺ , Fe ³⁺ + SeO ₃ ^2- (1) Fe ³⁺ + HSO ₃ ⁻ → Fe ²⁺ + SO ₄ ^2- (2) And various experiments as described later And confirmed that the invented reaction had an effect, and completed the method of the present invention.

When selenium is removed from flue gas desulfurization wastewater, oxidizing substances in the flue gas desulfurization wastewater can be removed at the same time by the first reaction, and sulfite ion-based reduction can be performed by the second reaction. Experiments have shown that the agent completely removes the remaining oxidizing substances of the first reaction and that a sulfite-containing liquid from a wet flue gas desulfurization unit can be used as a sulfite ion reducing agent. The liquid containing sulfurous acid is defined in detail in the last definition column.

A method for treating wastewater containing selenium In order to achieve the above object, a method for treating wastewater according to the present invention comprises metals of Fe, Mn, Ni and Cu and low-valent compounds of these metals. A first step comprising at least one of a step of bringing at least one metal or one metal compound selected from the group into contact with wastewater and a step of mixing a solution or slurry of the metal compound with wastewater; A second step of adding a sulfite ion-based reducing agent to the wastewater that has passed through, and a precipitate that is formed in the wastewater that has passed through the second step, and a concentrated liquid that contains the precipitate and treated water in which the concentration of the precipitate is smaller than the concentrated liquid. And a third step of separating selenium (Se) from the wastewater.

First Step In the first step, at least one metal selected from the group consisting of metals of Fe, Mn, Ni and Cu and low-valent compounds of these metals or one metal compound (hereinafter referred to as “metal”) , The metal is generally referred to as a metal. In the last definition column, the low valence number is defined.), The SeO ₄ ion is converted to SeO ₃ ion and further to Se simple substance, thereby easily sedimenting. Become. In addition, the metal ion dissociates from the metal and precipitates as another metal compound, and Se precipitates in the precipitate.
Can be removed in the form of coprecipitation or the like. It has been found that the pH of the waste water decreases when the waste water comes into contact with the metal, and the removal rate decreases. Therefore, the pH is adjusted to a high value before the contact, or the pH is simultaneously adjusted with the metal or the metal compound. Suitably, in the first step, iron is used as the metal and an iron compound is used as the metal compound, and the pH of the waste water is 6.5 or more, preferably 7.
Adjust to 5 or more. By adjusting the pH to this range, selenium can be effectively removed. In order to remove Se, a low-valent metal compound is added to the first metal compound so as to have a specified molar concentration in the range of 2 to 10,000 times the Se concentration.
Add to wastewater in the process. Since a solid such as metallic iron can be used in circulation and added in a batch operation, the amount of addition cannot be expressed by a factor of the Se concentration like a dissolved metal compound. Therefore, practically, only the consumed amount is added.

Of the metals listed, metallic iron is practically preferred. This is because the cost is low, it is safe, and the removal of Fe and Fe compounds from the liquid is easy. It is also effective in the third step of removing Se contained in the solution by coprecipitation with iron hydroxide or iron oxide. The type of metallic iron to be used is not particularly limited, and may be any metallic iron that has not been subjected to surface treatment such as plating. For example, reduced-iron having high purity or waste iron material such as iron scrap may be used. If the surface is oxidized, the oxide film is removed. In the case of a fixed-bed type contact tank, the shape of metallic iron may be granular, powdery, or massive.In the case of a fluidized-bed type contact tank, as long as a fluidized bed can be formed, the form of metallic iron may be granular or powdery. It may be a shape. It is also possible to simply deposit massive metallic iron in flue gas desulfurization wastewater.

[0011]Second step In the second step, the sulfite ion-based reducing agent is, for example, SO 2_Three ^2-(HSO_Three ^-) + Fe³⁺→ SO_Four ^2-(HSO_Four
^-) + Fe²⁺ The metal oxidized in the first step is reduced and
Function as a base agent, resulting in a reduction in metal consumption
To As described above, the present inventors have found that sulfite ions
Does not reduce Se, but through metal
Sulfite ion apparently reduces Se and removes it.
We found that it would contribute. PH of the second step is 3 or more
, Preferably 4 or more and 8 or less. at pH 3 or below
Indicates that the sulfite ion-based reducing agent is volatilized and the sulfite ion concentration
With a sulfite ion-based reducing agent
This is because the reaction does not proceed. At pH 8 or higher, Fe
(OH)_TwoPrecipitates, which is not preferred. Sulfite ion system
As a base agent, of course, H_TwoSO_Three, Na_TwoSO_Three, NaH
SO _ThreeAnd other commercially available reducing agents.

Third Step In the third step, the pH is adjusted to, for example, 8 to 12, to produce a precipitate containing the metal compound of the metal used in the first step, and Se is attached to them. Or by precipitating in co-precipitation with them to remove Se,
The added / input metal and metal compound are also removed. For example, when the metal is Fe, it precipitates as Fe (OH) ₂ rather than Fe (OH) ₃ . The pH is 1
Although it may be 2 or more, when the pH is 12 or more, the precipitation rate of Se tends to be saturated, and the consumption of alkali increases and the cost increases, which is not practical.

A feature of the method of the present invention is that Se can be effectively removed with a small amount of metal consumption, as can be seen from a comparison between Experimental Example 1 and Comparative Example 1. That is, in the first step, selenium is reduced and removed with a metal or a metal compound. In the second step, the metal or metal compound oxidized in the first step is reduced by using a sulfite ion-based reducing agent, so that it can be used again as a reducing agent. Third
In the step, Se is attached to the metal precipitate used in the first step in the form of coprecipitation or the like, and is removed from the wastewater. Therefore, by removing Se while precipitating the metal or metal compound dissolved in the first step in the third step, the dissolved metal or metal compound can be used for removing selenium. Also,
In the method of the present invention, metal ions in the wastewater can be removed in the third step, so that the degree of metal contamination in the wastewater can be further reduced.

EXPERIMENTAL EXAMPLE 1 One liter of wastewater discharged from a metal smelting plant containing Se at a concentration of 1.6 mg / l (0.020 mmol / l) was used as a wastewater sample, and it was a 3 mm-diameter spherical and 99% pure. 100g of
Was adjusted to pH 7.5, and the contact state between the drainage and the iron was maintained for 60 minutes while stirring with vibration. Then
The pH was adjusted to 6.0, and as a sulfite ion-based reducing agent,
Na ₂ SO ₃ was added to the wastewater sample at a rate of 20 moles per mole of Se and held for 20 minutes with vibratory stirring. Subsequently, after the spherical iron particles were separated, NaOH was added to the wastewater sample to adjust the pH to 9.0, followed by filtration. The sludge of Experimental Example 1 was used as a residue, and the treated wastewater of Experimental Example 1 was used as a filtrate. A sample was obtained. When the Se concentration of the treated wastewater sample of Experimental Example 1 was measured, the Se concentration was reduced to 0.06 mg / L. Further, the sludge of Experimental Example 1 obtained at the time of filtration was dried and its weight was measured.
It was 6 g / l.

Comparative Example 1 Except that no sulfite-based reducing agent was added,
A treated wastewater sample of Comparative Example 1 was obtained in the same manner as in Experimental Example 1. When the Se concentration of the treated wastewater sample of Comparative Example 1 was measured, the Se concentration was 0.17 mg / L. Further, the treated wastewater sample was suction-filtered and dried, and the amount of sludge obtained was measured. As a result, the amount of sludge was 1.2 g / l.

In Experimental Example 1, Se was more effectively removed from the wastewater than in Comparative Example 1, and the amount of sludge was equivalent to the amount of iron consumed. Consumption is much lower.

Method for treating flue gas desulfurization wastewater containing selenium When the method of the present invention is applied to treatment of flue gas desulfurization wastewater,
Oxidizing substances and selenium in flue gas desulfurization wastewater can be removed at the same time. First Step In the first step, in addition to Se, the oxidizing substance can be reduced, decomposed and removed by a redox reaction of the metal with the oxidizing substance. The pH suitable for efficiently removing both the oxidizing substance and Se is 6.5 or more, preferably 7.5 from the viewpoint of the presence or absence of the formation of a precipitate and the rate of its formation. However, when the pH is 7.5 or more, the Se removal rate tends to be saturated. In the first step, the temperature is set at 50 ° C to 100
Raise the wastewater to a temperature in the range of ° C. Thereby, the oxidizing substance can be more efficiently removed. In order to remove the oxidizing substance, a low-valent metal compound is added to the wastewater in the first step so as to have a specified molar concentration in the range of 1 to 1000 times the oxidizing substance concentration. Therefore, in practice, a low-valent metal compound is added to wastewater so as to have a molar concentration capable of removing both the oxidizing substance and Se. When using a metal, a relatively excessive amount of the metal is dissolved in the wastewater so as to have a concentration according to the above concentration.

Second step In the first step, most of the oxidizing substances are removed, but the concentration of the oxidizing substances and Se fluctuates in the actual treatment of wastewater. And the content ratio thereof fluctuates, so that the content of oxidizing substances in flue gas desulfurization effluent fluctuates. As a result, the oxidizing substance often cannot remain in the flue gas desulfurization effluent without completely removing the oxidizing substance in the first step. In the second step,
The metal consumed in the first step is reduced to reduce the amount of metal consumption, and as can be seen from the comparison between Experimental Example 2 and Comparative Example 2, the remaining oxidizing substance is reduced by a sulfite ion-based reducing agent. It can be reduced and decomposed and removed.

Experimental Example 2 1 liter of a waste gas desulfurization effluent obtained by treating coal-fired exhaust gas with a soot-mixed wet-type flue gas desulfurization device using limestone as an absorbent was sampled for 1 liter. And used it as a wastewater sample. The wastewater sample had a pH of 5.8, a temperature of 45 ° C., and an oxidizing substance concentration of 10.0.
mg / l (residual chlorine equivalent). Then, 100 g of iron particles having a diameter of 3 mm and a purity of 99% were added to the drainage sample,
The pH was adjusted to 7.5, and the state of contact between the waste water and iron was maintained for 60 minutes while stirring with vibration. Next, the pH was adjusted to 6.0, and Na ₂ SO ₃ was added as a sulfite ion-based reducing agent to 3%.
0 mg / l was added to the wastewater sample, and stirred for 20 minutes.
Hold for minutes. Subsequently, after separating the iron particles, NaOH was added to the wastewater sample to adjust the pH to 9.0, followed by filtration to obtain a treated wastewater sample of Experimental Example 2. When the concentration of the oxidizing substance in the treated wastewater sample of Experimental Example 2 was measured, the concentration was found to be 0.1%.
It was reduced to 31 mg / l (residual chlorine equivalent).

Comparative Example 2 Except that no sulfite ion reducing agent was added,
A treated wastewater sample of Comparative Example 1 was obtained in the same manner as in Experimental Example 1. When the concentration of the oxidizing substance in the treated wastewater sample of Comparative Example 1 was measured, the concentration was 0.73 mg / l.

In Experimental Example 2, it can be seen that the concentration of the oxidizing substance in the treated wastewater sample is much lower than in Comparative Example 1.

Third Step In the third step, Se can be precipitated and removed as described above, and the added metal can be removed and recovered from the wastewater as a precipitate. By performing the above first, second and third steps, the oxidizing substance and the S
e can be removed from the flue gas desulfurization effluent.

In a preferred embodiment, a liquid is sprayed into an exhaust gas containing sulfur oxides to make preliminary gas-liquid contact.
Secondary gas-liquid contact in which, in the presence of an oxygen-containing gas, mainly sulfur oxides in the exhaust gas are removed by bringing the exhaust gas that has passed through the primary gas-liquid contact and subsequently the primary gas-liquid contact into gas-liquid contact To remove sulfur oxides in the exhaust gas, as shown in the following Experimental Examples 3 and 4, the sulfurous acid content obtained by separating from the exhaust gas that has passed through the first gas-liquid contact is obtained. The liquid may be a sulfite ion-based reducing agent. Examples of the sulfite-containing liquid are specifically described in the last definition column.

[0024]Experimental example 3 Soot separation type wet flue gas degassing using limestone as absorbent
When coal-fired exhaust gas is treated by a sulfurizing device,
Flue gas extracted, mixed and filtered from both
One liter of desulfurized wastewater was collected and used as a wastewater sample.
The wastewater sample has a pH of 6.8, a temperature of 45 ° C, and oxidizing
Substance concentration 0.18mg / l (residual chlorine equivalent), Se concentration
Was 1.6 mg / l. Then, 2000mg to the wastewater sample
Ferrous chloride (FeCl_Two) And adjust to pH 7.5.
And maintain the contact state between the waste water and iron for 60 minutes while stirring.
I carried it. Then, the pH was adjusted to 6.0 and the sulfite ion
As a system reducing agent, it was obtained from a downcomer 62 described later shown in FIG.
Add 150 ml of liquid containing sulfurous acid and stir for 20 minutes
Hold for a while. The sulfite-containing liquid is in the form of a slurry,
Sulfurous acid gas concentration is 190mg-SO _Three/ l, dust content rate
0.8 wt% and gypsum content was 4.7 wt%. Continued
NaOH was added to the wastewater sample to adjust the pH to 9.5.
And sludge of Experimental Example 3 as a residue, and the filtrate
And a treated wastewater sample of Experimental Example 3 was obtained. Experimental example 3
Analysis of the treated wastewater sample revealed that the Se concentration was 0.04.
mg / l and oxidizing substance concentration 0.01 mg / l (residual chlorine
Calculated). This allows the sulfite-containing liquid
It was confirmed that it was effective as a sulfate ion type reducing agent.
Was. Next, the sludge of Experimental Example 3 obtained at the time of filtration was dried.
After drying and measuring the weight, the sludge weight (gypsum)
Was 13.3 g / l. In addition, the above operations
Prepare another treated wastewater sample in the same manner as
Gypsum concentration obtained from the wet-type flue gas desulfurization unit
gypsum slurry of wt% was added, and the gypsum content in the sludge was 9
It was set to 0% and filtered by suction. Good filterability and sludge
Confirmed that (solid matter) can be used as a cement raw material
We were able to.

Experimental Example 4 A waste water sample was prepared in the same manner as in Example 3, and was brought into contact with ferrous chloride. Next, the pH was adjusted to 6.0, and as a sulfite ion-based reducing agent, 150 ml of a sulfite-containing liquid obtained from the below-described sulfite-containing liquid reservoir 82 shown in FIG. 3 was added, and the mixture was maintained for 20 minutes with stirring. . The sulfite-containing liquid is in the form of a slurry and has a sulfur dioxide gas concentration of 261 mg-
The content of SO ₃ / l, the dust content was 0.5 wt%, and the gypsum content was 3.8 wt%. Subsequently, NaOH was added to the wastewater sample to adjust the pH to 9.5, followed by filtration to obtain a treated wastewater sample of Experimental Example 4. When the treated wastewater sample of Experimental Example 4 was analyzed, it was confirmed that the Se concentration was below the regulation value and the oxidizing substance concentration was below the predetermined value.

Still another preferred embodiment has a solid-liquid separation step for removing solids from the wastewater before the first step, and the wastewater introduced into the solid-liquid separation step has a concentrated liquid separated in the third step. Mix the liquid. A solid-liquid separation step is provided to remove solids such as dust, gypsum, slaked lime, and unreacted limestone transferred from the exhaust gas from the flue gas desulfurization effluent, thereby facilitating the treatment according to the method of the present invention and the third step. Can be removed together. In addition, a solid that is difficult to separate can be easily separated by performing solid-liquid separation by combining the solid content and the precipitate. Further, by performing a necessary solid-liquid separation operation in the solid-liquid separation step after the third step, the steps and equipment can be simplified. Further, metals and metal compounds having a reducing action are dissolved from the concentrated liquid (sediment) separated in the third step, and can be reused.

[0027] A further preferred embodiment of the method of the present invention is a third embodiment.
A part of the concentrate containing the precipitate separated in the step is mixed with the wastewater being treated in the first step. Refluxing the precipitate and dissolving or dissociating metals or metal ions from the precipitate in the wastewater by changing the pH, so that they can be used again as metal ions during the redox reaction in the first step and in the third step. Can be used as a precipitate at the time of coprecipitation by metal ions in the above, so that the consumption of metal can be reduced.

In a further preferred embodiment of the method of the present invention, a flue gas desulfurization effluent is a wet type in which an absorbent using a calcium-based absorbent is brought into gas-liquid contact with an exhaust gas to mainly remove a sulfurous acid gas contained in the exhaust gas. This is a flue gas desulfurization wastewater after gypsum is separated from gypsum slurry sent from the flue gas treatment tank of the flue gas desulfurization equipment, and the gypsum slurry or solid-liquid separation is performed in the flue gas desulfurization wastewater introduced into the solid-liquid separation process. It is characterized by mixing mixed gypsum. The solids contained in the flue gas desulfurization effluent introduced into the first step, for example, residual gypsum and unreacted limestone are fine crystal grains and are relatively hard to separate into solid and liquid, and the precipitate separated in the third step The concentrated liquid containing the substance is also sludge-like, and hardly undergoes solid-liquid separation. Therefore, by mixing gypsum slurry or gypsum containing gypsum with large crystal grains sent from the exhaust gas treatment tank into the flue gas desulfurization effluent, the separation efficiency in the solid-liquid separation step can be increased. As a result, Se, which can be handled as a precipitate by coprecipitation or the like, can be surely separated into solid and liquid, so that Se can be efficiently removed as a whole.

[0029]

BEST MODE FOR CARRYING OUT THE INVENTION As an apparatus for carrying out the method of the present invention, a treatment tank for carrying out a first step is provided with a means for adjusting the pH by adding an acid such as hydrochloric acid or an alkali such as caustic soda and a stirring means. I have. For example, metallic iron may be deposited as a reducing agent in the treatment tank, or metallic iron may be added to form a fluidized bed in the flue gas desulfurization wastewater. Further, a fixed-bed processing tank provided with a packed layer of a metal and a metal compound may be used. Further, a circulation type packed bed may be used. The treatment tank for performing the second step is a normal mixing tank for mixing the wastewater that has passed through the first step with the sulfite ion-based reducing agent. The equipment for carrying out the third step is provided with a means for adjusting the pH by adding an acid or an alkali. For example, when iron is used as a metal,
A tank for producing an iron hydroxide such as (OH) ₂ and a sedimentation tank for co-precipitating these precipitates and Se to separate them from flue gas desulfurization wastewater are provided. Hereinafter, embodiments of the present invention will be described specifically and in detail with reference to the accompanying drawings and examples.

[0030]

EXAMPLE 1 This example is an example in which the basic process of the method of the present invention, which is a combination of any one of claims 4 and 7 and 8, is applied to the treatment of flue gas desulfurization wastewater. FIG. 4 is a flow sheet of a processing apparatus for implementing the present embodiment. The present processing apparatus 100
Is a solid-liquid separator 6 of the conventional gypsum separator 9 shown in FIG.
This is a device for treating flue gas desulfurization wastewater that has flowed out.
The flue gas desulfurization drain line 3 and the mother liquor line 5 returning to the flue gas desulfurization unit 8 upstream of the limestone powder injection position of the mother liquor line 5 or downstream of the solid-liquid separation unit 6 toward the waste water treatment unit 8.
And may be provided at any place of the upstream line 7 that branches off.

The apparatus 100 includes a solid-liquid separation apparatus 102 and a first treatment tank 10 for performing the first step, along the flow of wastewater.
4, a second processing tank 106 for performing the second step, a third processing tank 108 for adjusting the pH in the third step, and a sedimentation tank 110 for generating a precipitate in the third step.

An introduction pipe 112 for introducing waste water is connected to the solid-liquid separation device 102. The solid-liquid separation device 1
02 and the first processing tank 104 are connected by a line 114 to the first processing tank 104.
The processing tank 104 and the second processing tank 106 are connected by a line 116, and the second processing tank 106 and the third processing tank 108 are connected by a line 1.
The third treatment tank 108 and the sedimentation tank 110 are connected to each other by the line 18 so that drain water flows sequentially. Further, the settling tank 110 is connected to a delivery pipe 122 for sending out treated wastewater and a concentrated liquid pipe 124 for returning a concentrated liquid containing sediment from the tank bottom to the introduction pipe 112.
Each line may be provided with a pump to send water to the next tank, or may be made to flow depending on the height difference.

The solid-liquid separation device 102 is provided with a solid content existing in the wastewater introduced into the first step, for example, gypsum and unreacted limestone remaining after solid-liquid separation from gypsum slurry, dust and the like, and a sedimentation tank. This is a solid-liquid separation device that removes, as mixed sludge, precipitates present in the concentrated liquid refluxed from 110 to the wastewater. For this device, for example, a rotary drum type or filter cloth traveling type vacuum suction type filter or a centrifugal separator is used.

The first treatment tank 104 is a tank in which the first step of the method of the present invention is carried out. An acid such as hydrochloric acid or an alkali such as NaOH is added to adjust the pH of the waste water to 6.5 or more, preferably 7.5. A means 126 for adjusting as described above, a means 128 for adding a powder of iron compound such as metal iron or FeCl ₂ as a metal reducing agent, and a stirrer 130 are provided. In the first treatment tank 104, a fluidized bed can also be formed by stirring the added granular material such as metallic iron or the like with waste water in the stirrer 130. Instead of forming a fluidized bed of a granular material such as metallic iron by stirring by the stirrer 130, a fluidized bed of a granular material such as metallic iron can be formed by using agitation by a jet of drainage.

Further, as the first processing tank 104, a horizontal fixed-bed processing tank and a vertical fixed-bed processing tank can be used. In the fixed-bed processing tank, a fixed bed (filled layer) filled with metallic iron is formed in the tank, where the metallic iron and the wastewater are brought into contact. As the metallic iron, a large granular or massive iron material can be used. Instead of the packed bed, a massive iron material may be deposited in the wastewater. In addition, if a circulation type packed tower is used, the wastewater is circulated to bring metallic iron into contact with the wastewater,
The contact time can also be adjusted by adjusting the filling amount of metallic iron and the circulating amount of wastewater.

In order to carry out the second step of the method of the present invention, the second treatment tank 106 mixes a sulfite ion-based reducing agent and waste water, and performs a redox reaction between the sulfite ion and the oxidizing substance and a sulfite ion. This is a tank for causing an oxidation-reduction reaction between metal and metal ions. The second treatment tank 106 is provided with a means 134 for adding a sulfite ion-based reducing agent and, if necessary, a stirrer 136. It has a volume capable of retaining the waste water and the sulfite ion-based reducing agent for a time necessary for reducing and removing the oxidizing substance by an oxidation-reduction reaction between the waste water flowing at a predetermined flow rate and the sulfite ion-based reducing agent. The amount of the sulfite ion-based reducing agent can be adjusted by detecting the concentration of the remaining Se and the oxidizing substance, or detecting the oxidation-reduction potential (after the temperature and pH adjustment). Further, if necessary, a means (not shown) for adding NaOH / HCl for pH adjustment is provided.

The third processing tank 108 performs the third step.
For adjusting the pH of wastewater, such as caustic soda
Means 14 for adding an alkali or an acid such as hydrochloric acid
0 and stir the wastewater in the tank to homogenize caustic soda or hydrochloric acid
And a stirrer 142 for mixing the water. Third treatment tank 1
In 08, the pH was adjusted by adding caustic soda or hydrochloric acid to the wastewater.
By adjusting to 8 to 12, the precipitated iron hydroxide
Can be generated. Other than caustic soda as alkali
KOH, Ca (OH)_Two, Mg (OH) _TwoAlso use
it can. Mix and use these alkalis or use different
Lucari can be used in different stages of pH increase.
Wear. However, gypsum is generated, scaling and sludge amount
NaOH, which does not increase the cost and is relatively inexpensive,
Mg (OH)_TwoIs preferred.

The sedimentation tank 110 is an apparatus for performing the latter half of the third step. The sediment such as iron hydroxide generated in the third treatment tank 108 is co-precipitated with Se, and a concentrated liquid containing the sediment. And the treated wastewater having a concentration of the precipitate smaller than the concentrated liquid, and the treated wastewater is sent out of the system via the line 32. On the other hand, the concentrated liquid containing the precipitate is returned to the introduction pipe 112 for reuse of iron ions via the concentrated liquid pipe 124. As the sedimentation tank 110, a conventional sedimentation separation device, for example, a thickener can be used. When the concentrated liquid is not reused, a solid substance is separated using a solid-liquid separator such as a centrifuge or a filter cloth filter instead of the precipitation tank 110.

In this embodiment, using the above-described apparatus 100, first, a solid content is separated from waste water by a solid-liquid separation device 102, and the waste water containing no solid content is discharged into a first treatment tank 104.
And adjusting the pH to 6.5 or higher, preferably 7.5 or higher, and adding the ferrous chloride in the aqueous solution to the wastewater to a predetermined concentration. As a result, ferrous chloride reduces most of the oxidizing substances in the wastewater to convert, for example, sulfur peroxide to sulfate ions, and ferrous chloride itself becomes ferric chloride.
It is oxidized to iron, ie, divalent to trivalent iron ions, and dissociates in wastewater. When metallic iron is added, it is converted to ferrous hydroxide and further to ferric hydroxide by the same action, and iron ions are dissociated. Some of the iron ions precipitate as iron hydroxide with accompanying Se.

Next, the waste water is introduced into the second treatment tank 106. In the second treatment tank 106, the added sulfite ion-based reducing agent reduces and removes the remaining oxidizing substances in the wastewater by the reduction reaction, and reduces the iron ions oxidized in the first step to reduce the oxidizing substances again. Provides reducing ability to iron ions. H ₂ SO ₃ , Na ₂ SO ₃ , Na
HSO ₃ , a liquid containing sulfurous acid is used. Next, the drainage
The pH is adjusted to 8 to 12 by entering the third treatment tank 108, adding caustic soda, and stirring and mixing. Thereby, ferrous hydroxide and ferric hydroxide are formed.

In the sedimentation tank 110, the sediment of the iron compound formed in the third treatment tank 108 is co-precipitated with Se.
Is separated into a concentrated liquid containing a precipitate and a treated wastewater in which the concentration of the precipitate is smaller than the concentrated liquid. Discharge pipe 1 for treated wastewater
While being sent out of the system via 22, the concentrated solution containing the precipitate is returned to the introduction tube 112 via the concentrated solution tube 124 to reuse the iron ions. The precipitate containing Se in the concentrate is
Solid-liquid separation is performed as sludge by the solid-liquid separation device 102 together with the solid content in the wastewater, and the sludge is discharged out of the system.

In this embodiment, the first processing tank 104 for performing the first step and the second processing tank 1 for performing the second step are also described.
06, a solid-liquid separator may be provided, and the first step and the second step may be performed in the same processing tank. The same applies to the following embodiments.

Embodiment 2 This embodiment is an example in which the basic process of the method of the present invention combining any one of claims 5 and 7 and 8 is applied to the treatment of flue gas desulfurization wastewater. Reference numeral 5 denotes a flow sheet of a processing apparatus for implementing the present embodiment. The apparatus 150 for carrying out the method of the present embodiment includes a line 152 branched from the concentrate pipe 124 and connected to the first processing tank 104 in addition to the configuration of the apparatus 100 of the first embodiment. In this embodiment, since the concentrated liquid is introduced into the first treatment tank 104, a fluidized bed treatment tank is preferable. Concentrate tube 124 and line 1
The concentrate containing the precipitate is passed through the first treatment tank 104
To the high pH environment, that is, 8 to 1 of the third processing tank 108.
By shifting from a pH in the range of 2 to the first treatment tank 104 having a somewhat lower pH, iron ions can be dissociated from ferrous hydroxide and ferric hydroxide in the precipitate and reused. Therefore, in the present embodiment, the amount of iron added in the first treatment tank 104 can be reduced, so that the amount of sludge coming out of the solid-liquid separation device 102 is reduced, and the sludge disposal cost can be reduced.

Embodiment 3 This embodiment is most suitable for the treatment of flue gas desulfurization effluent in which the content of Se is relatively large as compared with the content of oxidizing substances. FIG. 6 is an example showing an embodiment of the method of the present invention in which any one of the methods is combined, and FIG. 6 is a flow sheet of a processing apparatus 160 for implementing the present example. In the present processing apparatus 160, in addition to the configuration of the processing apparatus 100 of the first embodiment, a dissolving tank 164 provided with a stirrer 162 is provided between the second processing tank 106 and the third processing tank 108, and the concentrated liquid A line 166 branched from the pipe 124 is connected to the dissolving tank 164. In this embodiment, the concentrate tube 12
4 and the concentrated liquid containing the precipitate via the line 166 is returned to the dissolving tank 164 and mixed and stirred in the dissolving tank 164, whereby part or all of the precipitate is re-dissolved in the wastewater to remove iron ions. Dissociate. If necessary, the pH is adjusted to 3 to 7.
To adjust to 5, an acid may be added. As a result, the iron once used can be reused for the formation of the iron compound in the next third processing tank 108. In this embodiment, the amount of iron added in the first processing tank 104 is reduced, Therefore, the amount of sludge discharged from the solid-liquid separation device 102 can be reduced, and the disposal cost can be reduced. In this embodiment, the concentrated liquid containing the precipitate was used as the first liquid in the same manner as in the second embodiment.
It may be returned to the processing tank 104.

Embodiment 4 This embodiment is an example showing an embodiment of the method of the present invention described in claim 9, and FIG. 7 is a flow sheet of a processing apparatus 170 for carrying out this embodiment. In the processing apparatus 170,
In addition to the configuration of the processing apparatus 100 of the first embodiment, the introduction pipe 11
2, a gypsum slurry pipe 172 for adding gypsum slurry to wastewater is provided.

In this embodiment, by adding gypsum slurry to the wastewater as a filter aid, solid-liquid separation can be enhanced and the amount of sludge can be reduced. Further, by adding gypsum slurry so that the gypsum component in the sludge is about 80%, the sludge can be changed to low-grade gypsum and used as a raw material for cement or the like.
Note that, instead of the gypsum slurry, gypsum separated by a solid-liquid separation device of a wet flue gas desulfurization device may be added. As a result, the amount of sludge that must be disposed of at the management type disposal site can be reduced, so that the disposal cost can be significantly reduced.

In the first to fourth embodiments, the first step is performed.
In the processing tank 104, when metal particles, metal powders, or the like having fluidity are used as the metal reducing agent, for example, when iron particles are used, excessively charged iron particles are used in the first processing tank 10
In order to avoid unnecessary consumption during the pH adjustment in the second or third step by moving from Step 4 or to escape from the system, before the pH adjustment operation after the first step, an experimental example As in 1 and 2, it is preferable to separate and collect the iron particles. Further, even when a metal compound such as FeCl ₂ is used, if there is a substance that does not dissolve, it is preferable to separate and collect the same as in the case of iron particles.

Hereinafter, the technical terms used in this specification will be collectively defined. Definition of wastewater and flue gas desulfurization wastewater In this specification, wastewater refers to all wastewater including industrial wastewater, and in particular, flue gas desulfurization wastewater, whether wet or dry, soot mixed or soot separated. A wastewater treatment device that is discharged from a flue gas desulfurization device (the wastewater treatment device may be a device provided as a part of a wet flue gas desulfurization device, or is provided independently of the wet flue gas desulfurization device. The wastewater is sent to the system and treated there.
Furthermore, the flue gas desulfurization wastewater is a concept including all the wastewater discharged from the flue gas desulfurization unit. For example, the wastewater having the same composition as the absorbing solution, that is, the sulfurous acid gas desulfurizing agent such as limestone and the sulfurous acid gas such as gypsum The wastewater from which the absorbent containing the immobilized product was discharged, the mother liquor after solid-liquid separation of the absorbent, the wastewater further discharged from the dust tower, the mother liquor after solid-liquid separation of the wastewater, and periodically The concept includes drainage during periodic inspections and various types of cleaning water. Therefore, the flue gas desulfurization wastewater may contain sulfurous acid and desulfurization aid.

Definition of low-valent metal compound The low-valent metal compound used in the first step is a metal compound which is oxidized and converted into a high-valent metal compound.
Taking iron compounds as an example, ferrous chloride (FeCl ₂ ) is a low-valent metal compound, and ferric chloride (FeCl ₃ )
Is the highest valence metal compound. Also, Fe, M
Examples of low-valent compounds of the metals n, Ni and Cu are Fe
²⁺ , Mn2 ⁺ , Ni ⁺ , Ni2 ⁺ , Ni3 ⁺ , Cu ⁺ , Cu2 ⁺
Oxides, hydroxides, chlorides, sulfates, carbonates, sulfates and the like.

[0050] Here the definition of oxidizing agent, the oxidizing agent means a substance having an oxidizing ability that is included in the waste water of flue gas desulfurization, sulfur peroxide therein, for example S ₂ 0 ₈ ^{2 -} are also included. Oxidizing substances are
JIS K0102 Industrial wastewater test method for diethyl-P
-A component which can be quantified by a chlorine-equivalent value based on colorimetry when a stable color-developing state is obtained, except that the coloring time is extended in the phenylenediamine colorimetric method. Hereinafter, this method is called a DPD method. Further, for example, by using ion chromatography, it is possible to quantify only sulfur peroxide among oxidizing substances. If the concentration of oxidizing substances in the wastewater is high, the growth of nitrifying bacteria and denitrifying bacteria used in the denitrification process of the flue gas desulfurization wastewater treatment equipment will be inhibited, and as a result, the treatment discharged from the wastewater treatment equipment The amount of nitrogen in the water increases. In addition, the organic substance-adsorbing resin used as an adsorbent for adsorbing COD in flue gas desulfurization wastewater is rapidly deteriorated. Further, the same phenomenon occurs with a resin that removes boron and fluorine in flue gas desulfurization wastewater. Therefore, it is important in the treatment of flue gas desulfurization wastewater to remove oxidizing substances from the flue gas desulfurization wastewater before sending the flue gas desulfurization wastewater to the treatment apparatus.

Definition of Sulfurous Acid-Containing Liquid In the present specification, the term "sulfurous acid-containing liquid" refers to a primary gas-liquid contact in which a liquid is sprayed into an exhaust gas containing a sulfur oxide to make preliminary gas-liquid contact, The exhaust gas that has passed through the first gas-liquid contact is brought into gas-liquid contact with the absorbing liquid to carry out a second gas-liquid contact that mainly removes sulfur oxides in the exhaust gas in the presence of the oxygen-containing gas. Refers to a liquid or slurry obtained by separating from exhaust gas that has passed through the first gas-liquid contact when removing sulfur oxides.

For example, the sulfurous acid-containing liquid is a liquid or a slurry that remains in the lower part of the first downcomer 62 of the jet bubbling tube shown in FIG. First, the configuration of the jet bubbling reaction tank will be described with reference to FIG. The jet bubbling reaction tank 10 (hereinafter, referred to as reaction tank 10) is a main part of a flue gas desulfurization apparatus that uses limestone as an absorbent to remove sulfur oxides in exhaust gas. The reaction tank 10 includes an exhaust gas outlet chamber 12 provided so as to cross the tank in order from the top.
And an exhaust gas inlet chamber 14 and a lower space for storing an absorbent containing limestone. The exhaust gas outlet chamber 12 and the exhaust gas inlet chamber 14 are separated by a first partition 16 extending horizontally across the tank, and the exhaust gas inlet chamber 14 and the lower space traverse the tank in the same manner as the first partition 16. The second partition 18 extends in the direction in which the second partition 18 extends. Second diaphragm 18
Is located above the liquid level of the absorbing liquid layer 20 and forms a space portion 22 for exhaust gas outflow therebetween.

The exhaust gas outlet chamber 12 is connected to an outlet duct 24, and the exhaust gas inlet chamber 14 therebelow is connected to an inlet duct 26. A plurality of pipe-shaped communication pipes 28 (only one is simply shown in FIG. 2) are used as a gas riser for discharging the treated exhaust gas in gas-liquid contact with the absorbing liquid from the space portion 22 to the exhaust gas outlet chamber 12. The space 22 and the exhaust gas outlet chamber 12 are communicated with each other through the chamber 14.

The exhaust gas dispersion pipe 30 communicates with the exhaust gas inlet chamber 14 at the upper end, and descends downward from the second partition plate 18 of the exhaust gas inlet chamber 14 so as to be immersed in the absorbent 20 at the lower end. An opening, for example, a large number of small openings is provided at the lower end thereof, and the exhaust gas is dispersed from these openings into the absorbing liquid layer 20 to form a jet bubbling layer (floss layer) A. The jet bubbling layer A is a liquid-continuous phase gas-liquid contact layer composed of bubbles of exhaust gas and an absorbent containing limestone. The lower part of the reaction tank 10 accommodates the absorbing liquid layer 20, and the lower part of the tank has a stirrer 32 for stirring the absorbing liquid and a supply of oxygen necessary for fixing the sulfurous acid gas to the gypsum. An air supply pipe 34 having an air nozzle for ejecting an oxygen-containing gas, for example, air, is provided.

In order to perform primary gas-liquid contact mainly for the purpose of dust removal and cooling of exhaust gas, coolant nozzles 36 and 38 are provided in the inlet duct 26 and the exhaust gas inlet chamber 14, respectively. An absorbing liquid is supplied from the lower part of the reaction tank 10 by the absorbing liquid pump 40 as a cooling liquid.
In spraying the absorbing liquid into the exhaust gas as a cooling liquid, the absorbing liquid and the exhaust gas are brought into gas-liquid contact in the first gas-liquid contact, thereby cooling the exhaust gas and simultaneously removing dust and adding sulfur oxides in the exhaust gas. Some removal of occurs. Further, an industrial water nozzle 42 for spraying industrial water into the exhaust gas for pre-cooling the exhaust gas is provided in the inlet duct 26 upstream of the coolant nozzle 36.

In order to supply the absorbent to the reaction tank 10, a discharge pipe 44, a discharge pump 46, a discharge pipe 54 for extracting an absorbent containing gypsum from the bottom of the reaction tank, a solid-liquid separation for separating gypsum from the absorbent. An apparatus 48 and an absorbent supply pipe 50 for supplying limestone powder to a part of the mother liquor from which the gypsum is separated and then supplying the limestone powder to the reaction tank 10 as an absorbent slurry are provided.
Further, a drain pipe 52 is provided to branch off from the absorbent supply pipe 50 in order to send a part of the mother liquor to the waste water treatment apparatus. FIG.
The discharge pump 46 and the solid-liquid separation device 48 correspond to the discharge pump 4 and the solid-liquid separation device 6 in FIG.

The reaction tank 10 further includes a first liquid downcomer 62
To extract slurry (hereinafter simply abbreviated as sulfurous acid-containing liquid) containing gypsum and having a high concentration of sulfurous gas dissolved therein, which stays in the second liquid descending pipe 64 and the first liquid descending pipe 62. Pump 66, a sulfite-containing liquid extraction pipe 68 connected to the suction side of the extraction pump 66, a sulfite-containing liquid delivery pipe 70 connected to the discharge side of the extraction pump 66, and other connection piping systems. It has.

In order to recover as much as possible the absorbent which is sprayed in the inlet duct 26 and makes contact with the exhaust gas in the primary gas-liquid state and in which the sulfurous acid gas is dissolved at a high concentration without oxidizing as much as possible,
The liquid downcomer 62 is provided near a connection port between the inlet duct 26 and the reaction tank 10. The first liquid downcomer 62 is usually
The lower end of the first liquid downcomer 62 is connected by a communication pipe (not shown) so as to communicate with each other. A withdrawal pipe 68 of a withdrawal pump 66 is inserted therein.

In the present apparatus, the absorbing liquid sprayed from the absorbing liquid nozzle 36 comes into gas-liquid contact with exhaust gas having a relatively high concentration of sulfurous acid gas to absorb and dissolve sulfurous acid gas at a high concentration to become a liquid containing sulfurous acid. Fly ash (dust) is also collected. The sulfurous acid-containing liquid is separated into gas and liquid mainly in the inlet duct space 26, and the first liquid is kept without contact with the exhaust gas.
The liquid descending pipe 62 flows down. Next, the extraction pump 66
Is extracted by

Another sulfurous acid-containing liquid is a liquid or a slurry received in a sulfurous acid-containing liquid reservoir provided in the jet bubbling reaction tank shown in FIG. Referring to FIG.
The jet bubbling reaction tank 80 (hereinafter, referred to as reaction tank 80) will be described. The reaction tank 80 is replaced with the downcomer pipe 62 of the reaction tank 10 of FIG.
A reservoir 82 is provided immediately before the gas enters the exhaust gas inlet chamber 14, and the extraction pipe 68 is connected to the reservoir 82. The sulfurous acid-containing liquid is withdrawn from the reservoir 82 via a withdrawal pipe 68 and is delivered by a withdrawal pump 66 via a delivery pipe 70. Other configurations are the same as those of the reaction tank 10.

The gypsum slurry extracted by the absorbent pump 40 is mainly sent to the absorbent nozzle 36, where it is brought into gas-liquid contact with the exhaust gas having a relatively high sulfur dioxide concentration to dissolve the sulfur dioxide at a high concentration. And the entrance duct space 2
The gas is separated into a sulfurous acid-containing liquid by the gas-liquid separation in the liquid 6, and enters the reservoir 82 without continuing the contact with the exhaust gas.

[0062]

According to the method of the present invention, in the first step, selenium in waste water is reduced and removed by a redox reaction of the specified metal or metal compound, and in the second step, the metal oxidized in the first step is removed. The first ion is oxidized to give the reducing ability again,
In the third step, Se is removed along with the precipitate while the metal ions generated in the first step are precipitated as a precipitate of the metal compound in the third step, while reducing the consumption of the metal or the metal compound in the step. Further, the amount of metal ions in the wastewater sent to the downstream wastewater treatment device can be reduced. Further, when applied to the treatment of flue gas desulfurization wastewater, the oxidizing substance and Se can be efficiently and simultaneously removed from the flue gas desulfurization wastewater with a small amount of metal consumption in one process. Further, in the present invention, the amount of the metal or the metal compound used is reduced by recycling the concentrated liquid containing the precipitate of the metal or the metal compound, and furthermore, the amount of the sludge generated from the metal or the like can be reduced. It is effective in preventing secondary environmental pollution.

[Brief description of the drawings]

FIG. 1 is a flow sheet of a conventional gypsum separation apparatus.

FIG. 2 is a configuration diagram of a jet bubbling tank.

FIG. 3 is a configuration diagram of another type of jet bubbling tank.

FIG. 4 is a flow sheet of a processing apparatus for performing the method according to the first embodiment of the present invention.

FIG. 5 is a flow sheet of a processing apparatus for performing a method according to a second embodiment of the present invention.

FIG. 6 is a flow sheet of a processing apparatus for performing a method according to a third embodiment of the present invention.

FIG. 7 is a flow sheet of a processing apparatus for performing a method according to a fourth embodiment of the present invention.

[Explanation of symbols]

 2 Reaction tank, etc. 4 Discharge pump 6 Solid-liquid separation device or gypsum dewatering machine 8 Wastewater treatment device 9 Conventional gypsum separation device 10 Jet bubbling reaction tank 12 Exhaust gas outlet room 14 Exhaust gas inlet room 16 First partition 18 Second partition 20 Absorbent liquid layer 22 Space portion 24 Outlet duct 26 Inlet duct 28 Communication pipe 30 Exhaust gas dispersion pipe 32 Stirrer 34 Air supply pipe 36, 38 Coolant nozzle 40 Absorbent pump 42 Industrial water nozzle 44 Discharge pipe 46 Discharge pump 48 Solid-liquid separator Reference Signs List 50 Absorbent supply pipe 52 Drain pipe 62 First liquid down pipe 64 Second liquid down pipe 66 Extraction pump 68 Extraction pipe 70 Delivery pipe 80 Another type of jet bubbling tank 82 Reservoir 100 Processing apparatus for carrying out the first embodiment method 102 solid-liquid separator 104 first processing tank 106 second processing tank 108 third processing tank 110 Precipitation tank 112 Introducing pipes 114, 116, 118, 120 Line 122 Outgoing pipe 124 Concentrated liquid pipe 126 Acid adding means 128 Metal adding means 130 Stirrer 134 Adding means of sulfite ion-based reducing agent 136 Stirrer 140 Caustic soda adding means 142 Stirrer 150 Example Processing device for performing the method 2 152 Line 160 Processing device for performing the method 3 Example 162 Stirrer 164 Melting tank 166 Line 170 Processing device for performing the method 4 Example 172 Gypsum slurry tube

Claims (10)

[Claims] 1. a step of contacting at least one metal or one metal compound selected from the group consisting of metals of Fe, Mn, Ni and Cu and low-valent compounds of these metals with wastewater; A first step comprising at least one of a step of mixing a solution or a slurry of the metal compound into the wastewater; a second step of adding a sulfite ion-based reducing agent to the wastewater passed through the first step; and a wastewater passing through the second step. And a third step of separating into a concentrated solution containing the precipitate and treated water having a concentration of the precipitate smaller than the concentrated solution, wherein selenium (Se) in the wastewater is removed. Wastewater treatment method. 2. The method according to claim 1, wherein the sulfite ion-based reducing agent is H ₂ SO ₃ , N
processing method of the waste water according to claim 1, characterized in that at least one of a ₂ SO ₃ and NaHSO _3. 3. In the first step, iron is used as a metal and an iron compound is used as a metal compound.
The wastewater treatment method according to claim 1 or 2, wherein the pressure is adjusted to 6.5 or more, preferably 7.5 or more. 4. A solid-liquid separation step for removing solids from wastewater before the first step, wherein the wastewater introduced into the solid-liquid separation step is mixed with the concentrated liquid separated in the third step. The method for treating wastewater according to any one of claims 1 to 3. 5. The method according to claim 1, wherein a part of the concentrated liquid separated in the third step is used in the first step.
The wastewater treatment method according to any one of claims 1 to 4, wherein the wastewater is mixed with wastewater being treated in the step. 6. The method according to claim 1, wherein a mixing step of mixing a part of the concentrated liquid separated in the third step with the waste water is provided between the second step and the third step. Any one
A method for treating wastewater according to the item. 7. The flue gas desulfurization waste water discharged from a flue gas desulfurization device for removing sulfur oxides in exhaust gas,
The method for treating wastewater according to any one of claims 1 to 7, wherein the oxidizing substance and selenium in the flue gas desulfurization wastewater are removed. 8. A first gas-liquid contact in which a liquid is sprayed into an exhaust gas containing sulfur oxides to make preliminary gas-liquid contact, and then the exhaust gas and the absorbent which have passed through the first gas-liquid contact. When performing secondary gas-liquid contact, which mainly removes sulfur oxides in the exhaust gas in the presence of the oxygen-containing gas by gas-liquid contact, to remove sulfur oxides in the exhaust gas, The method for treating wastewater according to claim 7, wherein the sulfite-containing liquid obtained by separating from the exhaust gas that has passed through the liquid contact is used as a sulfite ion-based reducing agent. 9. An exhaust gas treatment tank of a wet-type flue gas desulfurization device in which the flue gas desulfurization effluent makes gas-liquid contact between an absorbent using a calcium-based absorbent and exhaust gas, and mainly removes sulfurous acid gas contained in the exhaust gas. From the gypsum slurry sent out from the gypsum slurry after solid-liquid separation, characterized by mixing gypsum slurry or gypsum separated into solid and liquid into the flue gas desulfurization wastewater introduced into the solid-liquid separation process. The method for treating wastewater according to any one of claims 7 and 8. 10. The method according to claim 1, wherein the first step and the second step are performed simultaneously in one step.