WO2021153038A1 - Method for controlling chemical oxygen demand in waste water in wet exhaust gas desulfurization device and method for assessing sterilization of wet exhaust gas desulfurization device - Google Patents

Abstract

A method for controlling a chemical oxygen demand in waste water in a wet exhaust gas desulfurization device, the method comprising: step 1 for measuring the chemical oxygen demand in the waste water in the wet exhaust gas desulfurization device; step 2 for discharging the waste water if the measured chemical oxygen demand is not higher than a reference value, and detecting the microbiomass of sulfur-oxidizing bacteria in the waste water by a PCR method using a pair of primers for amplification of a known gene of sulfur-oxidizing bacteria if the measured chemical oxygen demand is higher than the reference value; step 3 for adjusting the amount of an acidic microbicide in accordance with the detected microbiomass so as to perform sterilization; step 4 for detecting the microbiomass in the waste water again after sterilization; and step 5 for discharging the waste water if the microbiomass detected again is not higher than the reference value, and repeating steps 3 and 4 if the microbiomass detected again is higher than the reference value.

Description

Control method of chemical oxygen demand in wastewater in wet exhaust gas desulfurization equipment and sterilization evaluation method of wet exhaust gas desulfurization equipment

The present disclosure relates to a method for controlling a chemical oxygen demand in waste water in a wet exhaust gas desulfurization apparatus and a method for evaluating sterilization of the wet exhaust gas desulfurization apparatus.
The present application claims priority based on Japanese Patent Application No. 2020-014063 filed in Japan on January 30, 2020, the contents of which are incorporated herein by reference.

Exhaust gas discharged from thermal power plants, etc. is released into the atmosphere after desulfurization by an exhaust gas desulfurization device from the viewpoint of suppressing the release of sulfur oxides into the atmosphere. The exhaust gas desulfurization apparatus includes a dry exhaust gas desulfurization apparatus and a wet exhaust gas desulfurization apparatus. Of these, the dry exhaust gas desulfurization apparatus removes sulfur oxides in the exhaust gas by bringing the exhaust gas into contact with a solid adsorbent such as activated carbon and adsorbing the sulfur oxides on the solid adsorbent. Further, the wet exhaust gas desulfurization apparatus removes sulfur oxides in the exhaust gas by bringing the exhaust gas into contact with the absorbing liquid containing an alkaline component and absorbing the sulfur oxides in the absorbing liquid.

A plurality of methods are known as desulfurization methods using a wet exhaust gas desulfurization apparatus, depending on the type of absorption liquid used. Specific examples thereof include a lime gypsum method, a caustic soda method, and a magnesium method. For example, the absorption liquid (slurry) used in the lime gypsum method contains limestone (calcium carbonate). When sulfur oxides (sulfur dioxide, etc.) in the exhaust gas are absorbed by the absorption liquid, sulfite ions are generated in the absorption liquid. Then, the sulfate ion is generated by oxidizing the sulfite ion in the absorption liquid. Sulfate ions easily react with limestone to produce calcium sulfate (gypsum). As a result, the sulfur content derived from the sulfur oxide can be removed from the absorption liquid as gypsum.

The inventors have clarified that the chemical oxygen demand (COD) may be high in the absorption liquid (used absorption liquid) that has come into contact with the exhaust gas and has absorbed sulfur oxides. According to the study by the inventors, it is considered that the reason for this is that sulfur-oxidizing bacteria (for example, bacteria belonging to the genus Thermithiobacillus), which are autotrophic bacteria, proliferated in the absorbing solution that absorbed sulfur oxides. Then, when the sulfur-oxidizing bacterium grows, a heterotrophic bacterium (for example, a bacterium belonging to the genus Pseudomonas) that grows by utilizing the product (organic matter) of the sulfur-oxidizing bacterium can grow. Since organic substances such as sugars are also produced in the grown heterotrophic bacteria, it is considered that the COD of the used absorption liquid increases with the production of organic substances by the sulfur-oxidizing bacteria.

If the COD is higher than the drainage standard, the drainage cannot be discharged to rivers, etc. Therefore, it is preferable to suppress the increase in COD. Therefore, in order to suppress the increase in COD, it is considered effective to suppress the growth of the above-mentioned sulfur-oxidizing bacteria. By suppressing the growth of sulfur-oxidizing bacteria, the amount of organic matter produced by sulfur-oxidizing bacteria can be reduced. In addition, by suppressing the growth of sulfur-oxidizing bacteria, the growth of heterotrophic bacteria is also suppressed, and the amount of organic matter calculated by autotrophic bacteria can be reduced. As a result, it is considered that the amount of organic matter produced can be suppressed and the increase in COD of the used absorption liquid can be suppressed.

On the other hand, sulfur-oxidizing bacteria are microorganisms whose growth can be suppressed by general antibacterial agents, and it is known that their growth is suppressed by metals (for example, nickel and the like). Utilizing this function, sulfur-oxidizing bacteria kneaded with metal to prevent sulfur-oxidizing bacteria from assimilating hydrogen sulfide and sulfur dioxide to generate sulfuric acid and deteriorating sewage pipes made of concrete. Proliferation-preventing concrete and the like have been commercialized (see, for example, Patent Document 1).

Further, Patent Document 2 discloses a wet exhaust gas desulfurization apparatus configured so that the absorbing liquid sprayed by the spraying apparatus and a massive antibacterial member containing an antibacterial metal having an antibacterial action against sulfur-oxidizing bacteria can come into contact with each other. ing.

However, in the conventional wet exhaust gas desulfurization apparatus, when a bactericidal agent is added to sterilize the sulfur-oxidizing bacteria in the apparatus, the sterilization efficiency cannot be quantified, and in order to exert a sufficient bactericidal effect, There is concern that the amount of disinfectant added will be excessive.

Japanese Patent Application Laid-Open No. 4-149053 Japanese Patent Application Laid-Open No. 2019-126674

The present disclosure has been made to solve the above problems, and an object of the present disclosure is to provide a method for controlling a chemical oxygen demand in wastewater in a wet exhaust gas desulfurization apparatus capable of quantifying sterilization efficiency. do.

In order to solve the above problems, the method for controlling the chemical oxygen demand in wastewater in the wet exhaust gas desulfurization apparatus according to the present disclosure is a method for controlling the chemical oxygen demand in wastewater in the wet exhaust gas desulfurization apparatus.
Step 1 to measure the chemical oxygen demand in the wastewater of the wet exhaust gas desulfurization equipment,
When the measured chemical oxygen demand is equal to or less than the reference value, the wastewater is discharged, while when the measured amount exceeds the reference value, the amount of sulfur-oxidizing bacteria in the wastewater is measured as the sulfur-oxidizing bacteria. Step 2 of detection by the PCR method using a primer pair for amplification of a known gene of
Step 3 of adjusting the amount of the oxidizing fungicide according to the detected amount of the sulfur-oxidizing bacteria and sterilizing the sulfur-oxidizing bacteria.
Step 4 of re-detecting the amount of sulfur-oxidizing bacteria in the wastewater after sterilization, and
When the amount of the sulfur-oxidizing bacteria detected again is equal to or less than the reference value, the cells are released, while when the amount exceeds the reference value, the steps 3 and 4 are repeated.
including.

The sterilization evaluation method of the wet exhaust gas desulfurization apparatus according to the present disclosure is described.
A step of detecting the amount of sulfur-oxidizing bacteria in the wastewater by a PCR method using a primer pair for amplifying a known gene of the sulfur-oxidizing bacteria.
A step of adjusting the amount of the oxidizing fungicide according to the detected amount of the sulfur-oxidizing bacteria and sterilizing the sulfur-oxidizing bacteria.
A step of re-detecting the amount of sulfur-oxidizing bacteria in the wastewater after sterilization, and
When the amount of the sulfur-oxidizing bacteria detected again is equal to or less than the reference value, the step of evaluating that the wet exhaust gas desulfurization apparatus is sufficiently sterilized, and the step of evaluating that the wet exhaust gas desulfurization apparatus is sufficiently sterilized.
including.

According to the method for controlling the chemical oxygen demand in wastewater in the wet exhaust gas desulfurization apparatus of the present disclosure, the sterilization efficiency can be quantified.

It is a figure which shows the flow of the control method which concerns on embodiment of this disclosure. It is a schematic diagram which shows the sterilized or unsterilized sulfur-oxidizing bacterium. It is a schematic diagram which shows the conventional sterilization treatment or the detection method (plate method) of untreated sulfur-oxidizing bacteria. It is a graph which shows the detection method (PCR method) of the sterilized or untreated sulfur-oxidizing bacteria in this disclosure. It is a schematic diagram of the wet exhaust gas desulfurization apparatus used in the control method which concerns on embodiment of this disclosure. It is a graph which shows the bacterial flora analysis result of the gypsum sample in Example 1. It is a graph which shows the relationship between the pH and the culture time of the sulfur-oxidizing bacterium which was subcultured (the 1st time, the 2nd time and the 3rd time) in Example 1. It is a graph which shows the detection result (amplification curve) by the PCR method in Example 2. FIG. It is an agarose gel electrophoresis image of the amplification product by the PCR method in Example 2. It is a graph which shows the detection result (amplification curve) by the PCR method in Example 3.

<Method of controlling chemical oxygen demand in wastewater in wet exhaust gas desulfurization equipment>
FIG. 1 is a flow chart of a method for controlling a chemical oxygen demand in waste water in the wet exhaust gas desulfurization apparatus according to the embodiment of the present disclosure (hereinafter, may be abbreviated as “control method according to the embodiment”). ..
In step 1, the chemical oxygen demand (COD) in the wastewater of the wet exhaust gas desulfurization apparatus is measured.
In step 2, when the measured chemical oxygen demand is less than or equal to the reference value, the wastewater is discharged, while when the measured amount exceeds the reference value, the amount of sulfur-oxidizing bacteria in the wastewater is determined. It is detected by a PCR method using a primer pair for amplification of a known gene of the sulfur-oxidizing bacterium.
In step 3, the amount of the oxidative fungicide is adjusted according to the amount of the detected sulfur-oxidizing bacteria, and the sterilization is performed.
In step 4, the amount of the sulfur-oxidizing bacteria in the wastewater after sterilization is detected again.
In step 5, when the amount of the sulfur-oxidizing bacteria detected again is equal to or less than the reference value, the cells are released, while when the amount exceeds the reference value, the steps 3 and 4 are repeated.

In the present specification, the wet exhaust gas desulfurization apparatus (hereinafter, may be simply abbreviated as "desulfurization apparatus") refers to sulfur in the exhaust gas by bringing the exhaust gas generated in a thermal power plant or the like into contact with an absorbing liquid. This is for absorbing oxides (sulfur dioxide, etc.) in the absorbing liquid and removing them from the exhaust gas.

The absorption liquid used is arbitrary as long as it can absorb sulfur oxides, for example, a slurry in which limestone is dissolved (dispersed) (lime gypsum method), an aqueous sodium hydroxide solution (caustic soda method), and hydroxide. A slurry (magnesium method) in which magnesium is dissolved (dispersed) can be applied. In the present specification, as an example, a slurry in which limestone is dissolved (dispersed) is used. Although the slurry is not strictly a liquid, it is treated as a liquid in the present specification for convenience.

In general, the sulfur-oxidizing bacteria, sulfide (S ^2-), elemental sulfur (SO), a bacterium to utilize thiosulfate, polythionic acid, the oxidation energy of the reduced inorganic sulfur compounds sulfite as energy for growth. Sulfur-oxidizing bacteria include, for example, bacteria that temporarily deposit sulfur in the body such as the genus Bergiatoa, the genus Thiothlix, and the genus Thiobacterium; Sexual bacteria; thermophilic bacteria such as Thermothrix; ultrahyperthermal bacteria such as Acidianus, Metallosphaera, Sulfolobus and the like. As shown in Examples described later, the bacterium identified as a sulfur-oxidizing bacterium present in the desulfurization apparatus is a bacterium belonging to the genus Thermithiobacillus. Therefore, in the control method according to the embodiment, the sulfur-oxidizing bacterium indicates a bacterium belonging to the genus Thermithiobacillus.

Next, with reference to FIG. 1, each step of the control method according to the embodiment will be described in detail.

[Step 1]
In step 1, the COD in the wastewater of the wet exhaust gas desulfurization apparatus is measured. The COD measurement method is not limited, for example, a method specified in JIS K0102 (factory wastewater test method), a method specified in JIS K1010 (industrial water test method), a method specified in sewage test method, and the like. Examples thereof include a two-wavelength absorbance measurement method using ultraviolet absorbance. The methods specified in JIS K1010 (Industrial Water Test Method) include a method for _{measuring oxygen consumption (COD Mn} ) by potassium permanganate at 100 ° C. and a method for measuring oxygen consumption (COD _Cr ) by potassium dichromate. , There are three methods for measuring oxygen consumption (COD _{OH) by alkaline potassium permanganate.} Examples of the method specified in the sewage test method include a high temperature method and a low temperature method using a potassium permanganate solution, a potassium dichromate method, and the like.

For example, when a two-wavelength absorbance measuring method using ultraviolet absorbance is used, the sample is irradiated with light having a wavelength of about 254 nm, and the absorbance is determined to measure the concentration of organic substances in the sample. In the case of a sample containing a turbidity component, the absorbance of visible light (VIS) of about 365 nm or more and 435 nm or less is increased because the ultraviolet rays are scattered by the turbidity and the transmitted light is reduced, so that the apparent absorbance is increased. By measuring and subtracting from the apparent absorbance of ultraviolet rays (UV), the influence of the turbidity component can be reduced.

[Step 2]
In step 2, when the COD measured in step 1 is equal to or less than the reference value, the waste water is discharged. On the other hand, when the COD exceeds the reference value, the amount of sulfur-oxidizing bacteria in the wastewater is detected by a PCR method using a primer pair for amplification of a known gene of sulfur-oxidizing bacteria.

As the standard value of COD in wastewater, it is possible to set the ministerial ordinance that sets the wastewater standard, the allowable limit of COD specified in the wastewater standard in each local government, or a value lower than the allowable limit. For example, since the permissible limit of COD specified in the ministry ordinance that sets the wastewater standard in Japan is 160 mg / L (daily average 120 mg / L), the standard value of COD in this process should be 160 mg / L. It can be 140 mg / L, 100 mg / L, 80 mg / L, and 60 mg / L. In addition, the permissible limit by the daily average defines the average pollution state of the discharged water in one day.

In this step, the amount of sulfur-oxidizing bacteria in the wastewater is detected as the amount of known genes of sulfur-oxidizing bacteria. The known gene may be a gene possessed by a sulfur-oxidizing bacterium and is unique to the sulfur-oxidizing bacterium, and for example, a gene involved in sulfur metabolism can be used. Paracoccus sulfur oxide passage (also referred to as PSO pathway or sox pathway) and S ₄ intermediate pathway (S ₄ pathway) exist in sulfur metabolism. The enzyme encoded by the sox gene cluster (soxXAYZBCD) plays a major role in the PSO pathway. Specific examples of the enzyme include soxYZ, soxXA, soxB, and soxCD. Among them, the soxB gene is preferable as the known gene. The soxB gene of a sulfur-oxidizing bacterium consists of, for example, the nucleotide sequence represented by SEQ ID NO: 1.
On the other hand, the genes involved in S ₄ route, for example, thiosulfate dehydrogenase gene, sulfite dehydrogenase gene, tetrathionate Hydra b hydrolase (4THase) gene and the like.

A primer pair for amplification of a known gene can be designed by using a known method in consideration of the base sequence, Tm value, GC content, etc. of the target gene.
For example, examples of the primer pair for amplification of the soxB gene, particularly the region consisting of the base sequence represented by SEQ ID NO: 1, include one or more primer pairs selected from the group consisting of 1) to 8) below. Be done.
1) A forward primer consisting of the base sequence represented by SEQ ID NO: 2 and a reverse primer consisting of the base sequence represented by SEQ ID NO: 3;
2) A forward primer consisting of the base sequence represented by SEQ ID NO: 4 and a reverse primer consisting of the base sequence represented by SEQ ID NO: 5;
3) A forward primer consisting of the base sequence represented by SEQ ID NO: 6 and a reverse primer consisting of the base sequence represented by SEQ ID NO: 7;
4) A forward primer consisting of the base sequence represented by SEQ ID NO: 8 and a reverse primer consisting of the base sequence represented by SEQ ID NO: 9;
5) A forward primer consisting of the base sequence represented by SEQ ID NO: 10 and a reverse primer consisting of the base sequence represented by SEQ ID NO: 11;
6) A forward primer consisting of the base sequence represented by SEQ ID NO: 12 and a reverse primer consisting of the base sequence represented by SEQ ID NO: 13;
7) A forward primer consisting of the base sequence represented by SEQ ID NO: 14 and a reverse primer consisting of the base sequence represented by SEQ ID NO: 15;
8) A forward primer consisting of the base sequence represented by SEQ ID NO: 16 and a reverse primer consisting of the base sequence represented by SEQ ID NO: 17.

The above primer pair is an example of a usable primer pair. Normally, the primer can be designed to have a length of about 15 bases or more and 30 bases or less. Therefore, in the above primer pair, the base length is set to, for example, 1 base, 2 bases, 3 bases, or 5 bases in consideration of the GC content and the Tm value. Similarly, those having an increase or decrease of about 10 bases can also be used.

By performing PCR targeting the above-mentioned known gene, the amount of the gene can be quantified.
FIG. 2A is a schematic diagram showing sterilized or unsterilized sulfur-oxidizing bacteria. As shown in FIG. 2A, the cell wall of the unsterilized sulfur-oxidizing bacterium is retained, but the cell wall is destroyed by the sterilized sulfur-oxidizing bacterium, and DNA and the like flow out of the cell. In FIG. 2A, it is assumed that the sterilization strength is sterilization treatment A <sterilization treatment B.
In the conventional plate method, the wastewater after the sterilization treatment is sown on an agar medium or the like having a composition capable of culturing sulfur-oxidizing bacteria and cultured (see FIG. 2B). However, although the plate method can determine the presence or absence of sulfur-oxidizing bacteria, it cannot measure the amount of sulfur-oxidizing bacteria present in the wastewater, and the difference in sterilization efficiency due to the difference in sterilization strength can be seen. It is difficult to evaluate. Further, in the plate method, since the culture time is required, it takes a period of about 1 week or more and 2 weeks or less for the determination.

On the other hand, in the PCR method, the amount of sulfur-oxidizing bacteria contained in wastewater can be quantified as the amount of known genes of sulfur-oxidizing bacteria (see FIG. 2C). Specifically, as shown in FIG. 2C, in the sterilization treatment B having a high sterilization strength, the rise of the amplification curve is slower than in the unsterilized and sterilization treatment A, and the sulfur-oxidizing bacteria contained in the wastewater subjected to the sterilization treatment B. It is estimated that the amount of cells (the amount of genes) in the above is smaller than that obtained by the sterilization treatment A. Thereby, the difference in sterilization efficiency can be evaluated. Further, a two-step operation of extracting DNA from sulfur-oxidizing bacteria contained in wastewater and then performing PCR using the obtained DNA extract as a template sample may be performed for a period of 3 hours or more and 1 day or less, that is, , The measurement result can be obtained in a much shorter time than the plate method.

Examples of methods for extracting DNA from sulfur-oxidizing bacteria include a method of heat-treating cells, a method of adding a surfactant, a phenol extraction method, an osmotic shock method, a freeze-thaw method, an enzyme digestion method, and a DNA extraction kit. Known methods such as use of, ultrasonic treatment method, French press method, use of homogenizer and the like can be mentioned. These methods may be performed alone or in combination of two or more.

The PCR method is preferably a quantitative PCR method. Examples of the quantitative PCR method include MPN-PCR method, competitive PCR method, real-time PCR method and the like, and among them, the real-time PCR method is preferable.

The real-time PCR method is a method of detecting the amplification factor of PCR in real time and quantifying the amount of a gene based on the amplification factor. Quantification is performed using a fluorescent dye, and there are an intercalation method and a hybridization method.

The amount of the gene can be calculated by measuring the Ct value (Threshhold Cycle). The Ct value means the number of PCR cycles that results in a constant amount of amplification product, and since the Ct value is inversely proportional to the initial amount of the target gene, the amount of the gene can be calculated by measuring the Ct value. Can be done. Specifically, for example, first, using a standard liquid train obtained by serially diluting a sample containing a known amount of DNA, amplification curves arranged at equal intervals according to the amount of initial DNA are obtained. A threshold value is set at the point where the amplification curve rises, and the point where the amplification curve intersects is obtained as the Ct value. Next, a straight calibration curve is created between the Ct value and the initial amount of DNA. Next, the Ct value of the sample containing the unknown DNA amount is measured in the same manner, and the initial DNA amount of the sample containing the unknown DNA amount is obtained by comparing with the calibration curve.

Alternatively, as a simple method, as shown in Examples described later, a linear portion is selected from the amplification curves obtained as a result of PCR, and the fluorescence intensity in the section calculated by extrapolating the portion is selected. Can also be used as the initial amount of DNA.

[Step 3]
In step 3, the amount of the oxidizing fungicide is adjusted according to the amount of sulfur-oxidizing bacteria detected in step 2 and sterilized.

The oxidative fungicide may be any one that can destroy and sterilize the cell wall of sulfur-oxidizing bacteria and the biofilm composed of aggregates of microorganisms containing sulfur-oxidizing bacteria. For example, halogen-based fungicides, ozone, and hydrogen peroxide. Examples include hydrogen. Examples of the halogen-based bactericide include hypochlorous acid (HClO) and salts thereof (for example, sodium hypochlorite, etc.), ammonia chloramine (NH ₂ Cl), bromchlordimethylhydantoin (Br, Cl-DMH), and the like. bromine sulfamate (BrNHSO ₃ H), bromine chloramine _(NH 4 Br + HClO), and the like. Among them, a halogen-based bactericide or ozone is preferable, hypochlorous acid and a salt thereof are more preferable, and sodium hypochlorite is further preferable, from the viewpoint of strong bactericidal activity.

In this step, the amount of the oxidizing fungicide used for sterilization can be appropriately adjusted according to the amount of sulfur-oxidizing bacteria detected in step 2. Further, it can be appropriately adjusted according to the type of oxidizing fungicide to be used. For example, when sodium hypochlorite is used as the oxidizing bactericide, the amount of sodium hypochlorite added may be such that the final concentration in the treated water is 1 volume ppm or more and 70 volume ppm or less. can.

The sterilization time with an oxidizing fungicide can be, for example, about 1 minute or more and 24 hours or less, preferably 1 minute or more and 12 hours or less, and more preferably 1 minute or more and 6 hours or less. By setting the sterilization time to the above lower limit value or more, a more sufficient sterilization effect can be obtained, while when it is not more than the above upper limit value, damage such as corrosion to the desulfurization apparatus can be reduced.

When sodium hypochlorite is used as the oxidizing disinfectant, the disinfection may be stopped by adding a neutralizing agent of sodium hypochlorite after the above sterilization time has elapsed. Examples of the neutralizing agent include sodium thiosulfate and the like. The amount of the neutralizing agent added can be about the same as the amount of the oxidizing fungicide added.

[Step 4]
In step 4, the amount of the sulfur-oxidizing bacteria in the wastewater after sterilization is detected again. The amount of sulfur-oxidizing bacteria can be determined by the same method as described in step 2 above.

In step 4, in addition to the amount of cells, the COD in the wastewater after sterilization may be measured again. As a result, in the subsequent step 5, it is possible to more reliably determine the discharge or re-sterilization of wastewater.

[Step 5]
In step 5, when the amount of the sulfur-oxidizing bacteria detected again is equal to or less than the reference value, the sulfur-oxidizing bacteria are released. On the other hand, when the amount of cells exceeds the reference value, the above steps 3 and 4 are repeated.

As the reference value of the amount of sulfur-oxidizing bacteria, the amount of cells can be set so as to be equal to or less than the reference value of COD. For example, by investigating the correlation between the amount of sulfur-oxidizing bacteria and COD in advance, it is possible to set the amount of cells so as to obtain a desired COD as a reference value.

If the amount of cells exceeds the reference value in step 5, it can be determined that the sterilization in the desulfurization apparatus is insufficient, and the steps 3 and 4 are repeated in order to sterilize again. conduct.

If the measured value of COD in the wastewater after sterilization is obtained in step 5, in addition to the amount of cells, the measured value of COD is compared with the reference value of COD to release the product. Alternatively, it can be determined whether the cells are sterilized again and the amount of cells is detected again (the steps 3 and 4 are repeated). As a result, a more reliable determination can be made.

[Wet exhaust gas desulfurization equipment]
FIG. 3 is a schematic view of a wet exhaust gas desulfurization apparatus used in the control method according to the embodiment. The configuration of the desulfurization apparatus shown in FIG. 3 is shown below.

The desulfurization device 101 includes a housing 10, an exhaust gas introduction port 11a for introducing exhaust gas into the housing 10, an exhaust gas discharge port 11b for discharging the exhaust gas to the outside of the housing 10, and a spraying device 26 ( It is a retention portion for retaining the absorbing liquid sprayed by (described later), and includes a retention portion 13 formed as a part of the housing 10. The absorbing liquid is retained in the retaining portion 13. The retained absorption liquid usually contains sulfite ions generated by absorbing sulfur oxides in exhaust gas and sulfate ions generated by oxidation of sulfite ions. The absorption liquid is replenished to the inside of the housing 10 by the pump 21 through the absorption liquid replenishment system 71 connected to the absorption liquid tank (not shown).

Further, the desulfurization device 101 includes a spraying device 26 for spraying the absorbing liquid inside the housing 10, and an absorbing liquid discharging port 14 for discharging the absorbing liquid of the retention portion 13 to the outside of the housing 10. .. The absorption liquid is sprayed by the spraying device 26, and the sprayed absorption liquid and the exhaust gas come into contact with each other in the internal space 15 of the housing 10, so that the sulfur oxides in the exhaust gas can be absorbed by the absorption liquid. As a result, the sulfur oxides in the exhaust gas are removed, and the purified gas, which is the purified exhaust gas, is exhausted to the outside of the desulfurization apparatus 101.

Also, in the absorption liquid, sulfurous acid ions are generated as described above by the absorption of sulfur oxides (sulfur dioxide). Then, the sulfite ion is oxidized by the air diffused by the air diffuser 22 (described later) to generate the sulfate ion. The generated sulfate ion reacts with calcium carbonate in the absorption liquid to generate calcium sulfate (gypsum) and carbon dioxide. The generated carbon dioxide is exhausted to the outside of the desulfurization apparatus 101 together with the purifying gas. Further, the produced calcium sulfate is separated and recovered by the solid-liquid separator 43 described later.

The exhaust gas introduced into the housing 10 is usually at a high temperature, and the water contained in the absorbing liquid evaporates. Therefore, in order to supplement the water that has evaporated and decreased, the make-up water is appropriately supplied through the make-up water nozzle 51. The make-up water nozzle 51 is connected to a make-up water tank (not shown) by a make-up water supply system 77. Then, by driving the pump 24 provided in the make-up water supply system 77, the make-up water is supplied through the make-up water nozzle 51.

Further, the desulfurization apparatus 101 is provided with an air diffuser pipe 22 for dissipating the oxidizing air with respect to the retained absorption liquid. The air diffuser 22 is open to the atmosphere via the air supply system 72. Then, the air in the atmosphere (air for oxidation) is supplied to the air diffuser pipe 22 through the pump 23 provided in the air supply system 72, and is dispersed in the absorbing liquid. As a result, the sulfite ion in the absorption liquid can be oxidized, and the sulfate ion can be generated.

In the desulfurization device 101, the spraying device 26 is configured to spray the absorbing liquid discharged to the outside of the housing 10 through the absorbing liquid discharging port 14 into the inside of the housing 10. That is, in the desulfurization apparatus 101, the absorbing liquid is repeatedly used. Specifically, the desulfurization apparatus 101 includes a circulation system 73 and a pump 41 for extracting the absorption liquid from the retention portion 13 through the circulation system 73. Of these, the circulation system 73 has a circulation pipe 73a through which the absorption liquid discharged to the outside of the housing 10 through the absorption liquid discharge port 14 flows, and an absorption liquid flowing through the circulation pipe 73a inside the housing 10. Specifically, it is provided with an absorption liquid recirculation port 73b for returning to the retention portion 13). Then, by driving the pump 41, the absorbing liquid circulates between the retention portion 13 and the spraying device 26.

Further, in the desulfurization apparatus 101, the absorption liquid is circulated also in another route. That is, the circulation system 73 is provided with a three-way valve 42, and a part of the absorbing liquid flowing through the circulation system 73 is supplied to the solid-liquid separator 43 through the extraction system 74. The solid-liquid separator 43 is, for example, a belt filter (belt press). The absorption liquid supplied to the solid-liquid separator 43 contains calcium sulfate (gypsum) produced as described above. Therefore, calcium sulfate is separated in the solid-liquid separator 43, whereby the sulfur content derived from the sulfur oxide in the exhaust gas stream is removed as a solid content.

On the other hand, the absorbed liquid after separating calcium sulfate in the solid-liquid separator 43 is drained to the outside through the drainage system 75 and the three-way valve 44. However, a part of the absorbing liquid flowing through the drainage system 75 is returned to the inside of the housing 10 through the three-way valve 44 and the return system 76 from the viewpoint of liquid balance.

As described above, since the absorption liquid circulates through the absorption liquid recirculation port 73b, the absorption liquid that is sprayed inside the housing 10 and absorbs sulfur oxides and stays in the retention portion 13 is re-used. , Can be used for the absorption of sulfur oxides. As a result, the amount of the newly used absorbent liquid can be reduced.

The desulfurization device 101 may be configured so that the absorbing liquid sprayed by the spraying device 26 and an antibacterial metal member containing an antibacterial metal having an antibacterial action against sulfur-oxidizing bacteria can be brought into contact with each other. Specific examples of the antibacterial metal member include those described in Patent Document 2 (Japanese Patent Laid-Open No. 2019-1267664). Since sulfur-oxidizing bacteria and the like easily grow inside the housing 10 (particularly inside the absorbing liquid), when the massive antibacterial metal member 61 comes into contact with the absorbing liquid, the absorbing liquid containing sulfur oxides stays there. In the retention portion 13, which is an environment in which sulfur-oxidizing bacteria and the like easily grow, the growth of sulfur-oxidizing bacteria and the like can be suppressed. As a result, it is possible to suppress an increase in COD of wastewater.

Next, a method of controlling COD in wastewater using the desulfurization apparatus 101 will be described.
In the control method according to the embodiment, the COD in the drainage discharged to the outside of the apparatus is measured through the drainage system 75 and the three-way valve 44. The pipe to the outside of the device provided in the three-way valve 44 may be provided with an ultraviolet absorbance measuring instrument (UV measuring instrument) (not shown). This makes it possible to continuously measure the COD in the wastewater.

Next, if the COD in the wastewater is below the standard value, the wastewater is discharged as it is. On the other hand, when the COD exceeds the reference value, a part of the wastewater is sampled without being discharged, and the amount of sulfur-oxidizing bacteria in the wastewater is detected. The method for detecting the amount of cells is as described in the above step 2.

Next, adjust the amount of oxidative fungicide according to the amount of bacterial cells detected. The oxidizing disinfectant is added to the make-up water, for example, and is supplied to the inside of the housing 10 via the make-up water nozzle 51 by driving the pump 24 provided in the make-up water supply system 77. Alternatively, it is supplied to the inside of the housing 10 through a pit (not shown) provided on the bottom surface of the housing 10. When the oxidizing fungicide is supplied by the former method, the oxidizing fungicide can be distributed throughout the inside of the housing 10. On the other hand, when the oxidizing fungicide is supplied by the latter method, the oxidizing fungicide can be effectively acted on the sulfur-oxidizing bacteria adhering to the gypsum dispersed in the liquid, and is oxidizing. It is possible to suppress the corrosion of members due to the disinfectant. The sterilization time can be the time described in step 3 above.

Next, after sterilization, the wastewater discharged to the outside of the device is sampled again through the drainage system 75 and the three-way valve 44, and the amount of sulfur-oxidizing bacteria in the wastewater is detected. The method for detecting the amount of cells is as described in the above step 2.

Next, if the sulfur-oxidizing bacteria detected again are below the standard value, the wastewater is discharged. On the other hand, if the sulfur-oxidizing bacteria exceed the standard value, they are not released and are sterilized again with an oxidizing fungicide. The method for adding the oxidizing fungicide is as described above. After sterilization, the wastewater discharged to the outside of the apparatus is sampled again through the drainage system 75 and the three-way valve 44, and the amount of sulfur-oxidizing bacteria in the wastewater is detected.

Next, the detected bacterial cell mass and the reference value are compared, and the discharge of wastewater is suspended until the bacterial cell mass falls below the standard value, and sterilization → detection of the bacterial cell mass → judgment by comparison with the reference value. Repeat the operation.

<Sterilization evaluation method for wet exhaust gas desulfurization equipment>
The sterilization evaluation method of the wet exhaust gas desulfurization apparatus according to the embodiment of the present disclosure (hereinafter, may be referred to as "sterilization evaluation method according to the embodiment") includes the following steps.
A step of detecting the amount of sulfur-oxidizing bacteria in wastewater by a PCR method using a primer pair for amplifying a known gene of the sulfur-oxidizing bacteria (hereinafter, may be abbreviated as "bacteria amount detection step"). );
A step of adjusting the amount of an oxidizing fungicide according to the detected amount of sulfur-oxidizing bacteria and sterilizing it (hereinafter, may be abbreviated as "sterilization step");
A step of re-detecting the amount of sulfur-oxidizing bacteria in the wastewater after sterilization (hereinafter, may be abbreviated as "cell amount re-detection step");
When the amount of the sulfur-oxidizing bacteria detected again is equal to or less than the reference value, the step of evaluating that the wet exhaust gas desulfurization apparatus is sufficiently sterilized (hereinafter, may be abbreviated as "evaluation step"). ).

The cell amount detection step is the same as the case where the COD in the above step 2 exceeds the reference value.
The sterilization step is the same as the above step 3.
The cell amount redetection step is the same as the above step 4.
Therefore, the description of the same process as the process in the control method according to these embodiments is omitted.

[Evaluation process]
In the evaluation step, when the cell amount of the sulfur-oxidizing bacteria detected in the cell amount redetection step is equal to or less than the reference value, it is evaluated that the desulfurization apparatus is sufficiently sterilized. On the other hand, when the amount of cells exceeds the standard value, it is evaluated that the sterilization of the desulfurization apparatus is insufficient. The reference value of the amount of cells may be smaller than the amount of cells in the wastewater before sterilization, for example, 80% or 50% of the amount of cells in the wastewater before sterilization. The amount can be 30% and 10%. Further, as specified in the above step 5, an amount such that the COD becomes a desired value may be used as a reference value for the amount of bacterial cells.

If it is evaluated that the desulfurization apparatus is insufficiently sterilized, the above sterilization step and the above cell amount redetection step can be repeated, and the evaluation step can be carried out again. The above sterilization step, the above cell amount redetection step, and the present evaluation step may be repeated until the amount of cells in the waste water after sterilization becomes equal to or less than the reference value.

According to the sterilization evaluation method according to the embodiment, the degree of sterilization (sterilization efficiency) of the wet exhaust gas desulfurization apparatus can be quantitatively evaluated.

Hereinafter, the present invention will be described with reference to Examples, but the present invention is not limited to the following Examples.

[Example 1]
(Analysis of gypsum sample flora and examination of culture conditions for sulfur-oxidizing bacteria)
1. 1. Gypsum sample bacterial flora analysis Since an increase in COD in the desulfurization equipment was confirmed, the gypsum sample collected from the desulfurization equipment was analyzed for the bacterial flora in order to identify the cause. The results are shown in FIG.

From FIG. 4, it was clarified that the bacterium of the genus Thermithiobacillus, which is a kind of sulfur-oxidizing bacterium, is the dominant microorganism occupying about 50%. Furthermore, Pseudomonas bacteria that feed on Thermithiobacillus spp. Also account for more than 20%, and the saccharides contained in the gypsum sample are Pseudomonas spp. It was suggested that it may be derived.

From the above, it was considered that the increase in COD in the desulfurization apparatus was due to the growth of sulfur-oxidizing bacteria, and it was considered that suppressing the growth of sulfur-oxidizing bacteria was effective in suppressing the increase in COD.

2. Examination of culture conditions of sulfur-oxidizing bacteria Next, the sulfur-oxidizing bacteria in the gypsum sample were subcultured, and the culture conditions were examined. Specifically, 2 cc of gypsum sample was cultured for 500 hours using 150 cc of liquid medium (first culture). Then, the cells were further cultured for 400 hours using 150 cc of the medium with respect to 5 cc of the culture solution (second culture). Then, the medium was cultured for another 600 hours using 150 cc of the medium with respect to 15 cc of the culture solution (third culture). The composition of the media used are as shown _{below; NH 4 Cl: 0.4g, MgSO} 4 · 7H 2 O: 0.8g, Trace element solution ( composition as described below): 10mL, _KH 2 PO ₄ : 4 g, K ₂ HPO ₄ : 4 g, K ₂ S ₄ O ₆ : 3 g, Bromocresol purple (saturated aqueous solution): 2 mL, distilled water: 1000 mL. To prepare the medium, first, the above components excluding potassium dihydrogen phosphate, potassium dihydrogen phosphate and dipotassium tetrathionate were dissolved in 900 mL of distilled water and sterilized by autoclaving. Then, potassium dihydrogen phosphate and potassium dihydrogen phosphate were dissolved in 100 mL of distilled water, individually autoclaved for sterilization, cooled, and then mixed with a solution containing other salts autoclaved. Dipotassium tetrathionate was sterilized by filtration and then mixed with a solution containing other salts that had been autoclaved. The pH of the medium was adjusted to 6.9. Composition of Trace element solution is as shown _{below; Na 2 -EDTA: 50g, ZnSO} 4 · 7H 2 O: 11g, CaCl 2 · 2H 2 O: 7.34g, MnCl 2 · 4H 2 O: 2. _{5g, CoCl 2 · 6H 2 O} : 0.5g, (NH 4) 6 Mo 7 O 24 · 4H 2 O: 0.5g, FeSO 4 · 7H 2 O: 5g, CuSO 4 · 5H 2 O: 0.2g , NaOH: 11 g, distilled water: 1000 mL. To prepare the Trace element solution, first, Na _2- EDTA was dissolved in 1000 mL of distilled water, and the pH was adjusted to 7.0 with NaOH. Then, after adding other components, the pH of the final solution was adjusted to 6.0 with NaOH. In each of the 1st to 3rd cultures, the pH of the medium was measured and recorded every 24 hours (excluding Saturdays and Sundays). The result is shown in FIG.

As shown in FIG. 5, it was clarified that the slope of the graph became smaller and the degree of decrease in pH became smaller each time the dilution culture (passage culture) was repeated. It was presumed that this was due to the decrease in the growth rate of sulfur-oxidizing bacteria due to the decrease in the amount of gypsum particles during passage.

From the above, it was suggested that sulfur-oxidizing bacteria cannot grow unless they adhere to solid substances such as gypsum particles. That is, since the presence of a solid substance that can adhere is an essential condition, the degree of sterilization (sterilization efficiency) in the desulfurization apparatus is evaluated by the conventional plate method and the most probable number method (Most probe number Method: MPN method). It is considered difficult to do.

[Example 2]
(Measurement of the amount of sulfur-oxidizing bacteria by the PCR method)
1. 1. Primer design A primer was designed to quantify the amount of DNA in sulfur-oxidizing bacteria. First, the genetic information of the bacterium of the genus Thermithiobacillus was obtained from the following site <https://biocyc.org/organism-summary?object=GCF_000423825>. Align the sequence obtained from the above site with the nucleotide sequence of the soxB gene among the known genes possessed by the sulfur-oxidizing bacterium, and the region consisting of the nucleotide sequence represented by SEQ ID NO: 1 is the soxB of the bacterium of the genus Thermithiobacillus. It was identified as the base sequence of the gene.

Next, eight types of primer pairs for amplifying the region consisting of the base sequence represented by SEQ ID NO: 1 were designed. The specific arrangement is shown in Table 1 below.

2. Quantification of bacterial cell mass by PCR method Extrap Soil DNA Kit Plus ver. Using No. 2 (manufactured by Nippon Steel Environment Co., Ltd.), DNA of sulfur-oxidizing bacteria was extracted and purified from a gypsum sample collected from a desulfurization apparatus according to a protocol. Next, using the purified sample as a template, the amount of the soxB gene was quantified by a real-time PCR method using primer pairs 5 to 8 out of the above eight types of primer pairs. The PCR reaction conditions are as follows: after performing the reaction at 98 ° C. for 2 minutes as initial denaturation, 40 cycles of "DNA denaturation reaction at 98 ° C. for 10 seconds, annealing at 55 ° C. for 10 seconds, extension reaction at 68 ° C. for 30 seconds" are performed. be. The results are shown in FIGS. 6A and 6B.

As shown in FIG. 6A, an amplification curve was drawn in the real-time PCR method using primer pairs 5 to 8, indicating that the amount of the soxB gene can be quantified. Further, as shown in FIG. 6B, it was confirmed that an amplification product of the target soxB gene region was obtained by a real-time PCR method using primer pairs 5 to 8.

[Example 3]
(Measurement of cell mass of sulfur-oxidizing bacteria by PCR method in gypsum sample after sterilization)
1. 1. Quantification of bacterial cell mass by PCR First, 5 cc of sodium hypochlorite solution (final concentration: 1 volume ppm, 8 volume ppm or 70 volume ppm) was added to a 10 cc gypsum sample, and then the mixture was stirred for 30 minutes. Then, 5 cc of sodium thiosulfate solution was added to neutralize sodium hypochlorite. Then, 0.5 mL of the neutralized slurry was taken out and centrifuged (14000 rpm, 10 minutes). The supernatant was discarded and the remaining material was used as a sample for DNA extraction after sterilization. In addition, as a negative control, a sample consisting only of water without containing a gypsum sample was used.

Of the primer pairs shown in Example 2, the amount of the soxB gene was quantified by the real-time PCR method under the same conditions as the PCR conditions shown in Example 2 except that the number of cycles was set to 45 using the primer pair 8. did. The results are shown in FIG. Table 2 shows the fluorescence intensity (initial DNA amount) in the section calculated by extrapolating the straight line portion of the amplification curve shown in FIG. 7.

As shown in FIG. 7, it was confirmed that the higher the concentration of sodium hypochlorite, the slower the rise of the amplification curve, and the decrease in the amount of sulfur-oxidizing bacteria after sterilization. Further, as shown in Table 2, the amount of cells was reduced to about 1/100 in the 8-volume ppm treatment and about 1/100 in the 70-volume ppm treatment as compared with the sample treated with 1 volume ppm sodium hypochlorite. It was confirmed that the amount was reduced to about 1/1000.

<Additional notes>
The method for controlling the chemical oxygen demand in wastewater in the wet exhaust gas desulfurization apparatus and the sterilization evaluation method for the wet exhaust gas desulfurization apparatus according to each embodiment are grasped as follows, for example.

(1) The method for controlling the chemical oxygen demand in wastewater in the wet exhaust gas desulfurization apparatus according to the first aspect is a method for controlling the chemical oxygen demand in waste water in the wet exhaust gas desulfurization apparatus.
Step 1 to measure the chemical oxygen demand in the wastewater of the wet exhaust gas desulfurization equipment,
When the measured chemical oxygen demand is equal to or less than the reference value, the wastewater is discharged, while when the measured amount exceeds the reference value, the amount of sulfur-oxidizing bacteria in the wastewater is measured as the sulfur-oxidizing bacteria. Step 2 of detection by the PCR method using a primer pair for amplification of a known gene of
Step 3 of adjusting the amount of the oxidizing fungicide according to the detected amount of the sulfur-oxidizing bacteria and sterilizing the sulfur-oxidizing bacteria.
Step 4 of re-detecting the amount of sulfur-oxidizing bacteria in the wastewater after sterilization, and
When the amount of the sulfur-oxidizing bacteria detected again is equal to or less than the reference value, the cells are released, while when the amount exceeds the reference value, the steps 3 and 4 are repeated.
including.

In the control method according to the first aspect, the amount of sulfur-oxidizing bacteria before and after sterilization is quantified in the above steps 2 and 4. This makes it possible to quantify the sterilization efficiency from the change in the amount of sulfur-oxidizing bacteria before and after sterilization. Further, in the above step 3, the amount of the oxidizing fungicide is adjusted according to the amount of the sulfur-oxidizing bacteria detected in the above step 2. This makes it possible to prevent the amount of the oxidizing fungicide from being used excessively.

(2) The control method of the second aspect is the control method of (1), wherein the oxidizing disinfectant is one or more selected from the group consisting of halogen-based disinfectants, ozone and hydrogen peroxide. May be good.

In the control method of the second aspect, the sulfur-oxidizing bacteria can be effectively sterilized by using the oxidizing bactericide exemplified above.

(3) The control method of the third aspect is the control method of (1) or (2), and the chemical oxygen demand in the wastewater after sterilization may be measured again in the step 4. ..

In the control method of the third aspect, by measuring the COD in the wastewater after sterilization in the above step 4, it is confirmed that the COD in the wastewater after sterilization is reduced and the sterilization efficiency is confirmed from the reduced amount. can do.

(4) The control method of the fourth aspect is the control method according to any one of (1) to (3), and the known gene may be a soxB gene.

In the control method of the fourth aspect, the amount of sulfur-oxidizing bacteria in the wastewater before and after sterilization can be quantified by detecting the soxB gene of sulfur-oxidizing bacteria by the PCR method.

(5) The control method of the fifth aspect is the control method according to any one of (1) to (4), wherein the known gene comprises a base sequence represented by SEQ ID NO: 1. May be good.

In the control method of the fifth aspect, the amount of sulfur-oxidizing bacteria in the wastewater before and after sterilization is quantified by detecting the region consisting of the nucleotide sequence represented by SEQ ID NO: 1 of the sulfur-oxidizing bacteria by the PCR method. be able to.

(6) The sterilization evaluation method of the wet exhaust gas desulfurization apparatus according to the sixth aspect is
A step of detecting the amount of sulfur-oxidizing bacteria in the wastewater by a PCR method using a primer pair for amplifying a known gene of the sulfur-oxidizing bacteria.
A step of adjusting the amount of the oxidizing fungicide according to the detected amount of the sulfur-oxidizing bacteria and sterilizing the sulfur-oxidizing bacteria.
A step of re-detecting the amount of sulfur-oxidizing bacteria in the wastewater after sterilization, and
When the amount of the sulfur-oxidizing bacteria detected again is equal to or less than the reference value, the step of evaluating that the wet exhaust gas desulfurization apparatus is sufficiently sterilized, and the step of evaluating that the wet exhaust gas desulfurization apparatus is sufficiently sterilized.
including.

In the sterilization evaluation method according to the sixth aspect, the amount of sulfur-oxidizing bacteria before and after sterilization is quantified. This makes it possible to quantitatively evaluate the degree of sterilization of the wet exhaust gas desulfurization apparatus from the change in the amount of sulfur-oxidizing bacteria before and after sterilization.

11a Exhaust gas introduction port 11b Exhaust gas discharge port 12 Retaining liquid 13 Retaining part 14 Absorbent liquid discharge port 15 Internal space 22 Diffusing pipe 23,24,41 Pump 26 Dispersing device 42,44 Three-way valve 43 Solid-liquid separator 51 Make-up water nozzle 61 Massive Antibacterial metal member 62 Filter 71 Absorbent liquid replenishment system 72 Air supply system 73 Circulation system 73a Circulation pipeline 73b Absorption liquid recirculation port 74 Extraction system 75 Drainage system 76 Return system 77 Replenishment water supply system 101 Desulfurization equipment (wet exhaust gas desulfurization equipment)

Claims (6)

1. A method for controlling the chemical oxygen demand in wastewater in a wet exhaust gas desulfurization device.
   Step 1 to measure the chemical oxygen demand in the wastewater of the wet exhaust gas desulfurization equipment,
   When the measured chemical oxygen demand is equal to or less than the reference value, the wastewater is discharged, while when the measured amount exceeds the reference value, the amount of sulfur-oxidizing bacteria in the wastewater is measured as the sulfur-oxidizing bacteria. Step 2 of detection by the PCR method using a primer pair for amplification of a known gene of
   Step 3 of adjusting the amount of the oxidizing fungicide according to the detected amount of the sulfur-oxidizing bacteria and sterilizing the sulfur-oxidizing bacteria.
   Step 4 of re-detecting the amount of sulfur-oxidizing bacteria in the wastewater after sterilization, and
   When the amount of the sulfur-oxidizing bacteria detected again is equal to or less than the reference value, the cells are released, while when the amount exceeds the reference value, the steps 3 and 4 are repeated.
   Including methods.
2. The method according to claim 1, wherein the oxidizing disinfectant is at least one selected from the group consisting of halogen-based disinfectants, ozone and hydrogen peroxide.
3. The method according to claim 1 or 2, wherein in the step 4, the chemical oxygen demand in the wastewater after sterilization is measured again.
4. The method according to any one of claims 1 to 3, wherein the known gene is a soxB gene.
5. The method according to any one of claims 1 to 4, wherein the known gene comprises a base sequence represented by SEQ ID NO: 1.
6. It is a sterilization evaluation method for wet exhaust gas desulfurization equipment.
   A step of detecting the amount of sulfur-oxidizing bacteria in the wastewater by a PCR method using a primer pair for amplifying a known gene of the sulfur-oxidizing bacteria.
   A step of adjusting the amount of the oxidizing fungicide according to the detected amount of the sulfur-oxidizing bacteria and sterilizing the sulfur-oxidizing bacteria.
   A step of re-detecting the amount of sulfur-oxidizing bacteria in the wastewater after sterilization, and
   When the amount of the sulfur-oxidizing bacteria detected again is equal to or less than the reference value, the step of evaluating that the wet exhaust gas desulfurization apparatus is sufficiently sterilized, and the step of evaluating that the wet exhaust gas desulfurization apparatus is sufficiently sterilized.
   Including methods.

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