JP4790243B2 - Method for treating wastewater containing sulfur compounds - Google Patents

Description

  The present invention relates to a water treatment method for efficiently and stably treating a sulfur compound contained in wastewater.

A typical example of wastewater containing sulfur compounds is flue gas desulfurization wastewater. Flue gas desulfurization wastewater is discharged from wet flue gas desulfurization equipment such as power plants and ironworks. Most of the sulfur compounds contained in the combustion exhaust gas are sulfur dioxide (SO ₂ ). Generally, in the exhaust gas wet processing is processing to absorb the sulfur dioxide in the exhaust gas in an alkaline slurry and an alkaline absorbent, this result, sulfite (SO ₃ ^2-) and bisulfite ^- reduction, such as (HSO ₃₎ So-called flue gas desulfurization waste water containing a reactive sulfur compound is generated.

For example, when the alkali absorbent of sodium hydroxide, by the following reaction, to produce sodium sulfite (Na ₂ SO ₃₎ and nitrite sulfate sodium hydrogen (NaHSO _3).

2NaOH + SO ₂ → Na ₂ SO ₃ + H ₂ O (1)
Na ₂ SO ₃ + H ₂ O + SO ₂ → 2NaHSO ₃ (2)

As a flue gas desulfurization method, a method using a slurry containing 5 to 10% of calcium hydroxide (Ca (OH) ₂ ) or magnesium hydroxide (Mg (OH) ₂ ), which is cheaper than NaOH, is widely adopted. If calcium hydroxide (Ca (OH) _2), during the desulfurization waste water, calcium sulfite (CaSO ₃₎ and nitrite sulfate Calcium hydrogen (CaHSO ₃₎ is produced. When magnesium hydroxide (Mg (OH) _2), magnesium sulfite (MgSO ₃₎ and nitrite sulfate magnesium hydrogen (MgHSO ₃₎ is produced. Magnesium sulfite (MgSO ₃₎ and nitrite sulfate magnesium hydrogen (MgHSO _3), since the solubility is relatively large, clogging of the piping or the like has advantages such as less likely to occur. However, the solubility of these substances decreases with an increase in pH, and scale tends to be generated. Therefore, it is considered that pH is preferably 5.5 to 6.0 (see Non-Patent Document 1).

Further, depending on the type of wastewater, thiosulfuric acid (S ₂ O ₃ ²⁻ ) and dithionic acid (S ₂ O ₆ ²⁻ ) may be included as sulfur compounds.

Such sulfur compounds in wastewater are measured as COD (Chemical Oxygen Demand; determined using chemical oxygen demand, potassium permanganate) regulated by wastewater standards. For example, in the case of SO ₃ ^2- , the theoretical COD is 16/80 = 0.2 g-COD / g-SO ₃ ^2- . Therefore, if the drainage standard cannot be satisfied, it cannot be discharged as it is.

Therefore, the sulfur compounds in the wastewater are oxidized to sulfate ions (SO ₄ ²⁻ ), reacted with Ca (OH) ₂ and recovered as gypsum (CaSO ₄ ), or harmless sulfuric acid after oxidation treatment. It must be released as ions (SO ₄ ^2- ).

Known oxidation methods for sulfur compounds include an oxidation method using air and a chemical oxidation method using an oxidizing agent. As chemical oxidants that oxidize reducing sulfur compounds, hydrogen peroxide, hypochlorous acid, ozone, potassium permanganate, and the like are widely known (see Non-Patent Document 2). Alternatively, some biooxidation methods using sulfur-oxidizing bacteria have also been implemented (see Non-Patent Document 3 and Patent Document 1).
JP-A-3-341757 (Table 1) Japanese Patent Laid-Open No. 7-265876 (first page, FIG. 1) 4th edition supervised by Ministry of International Trade and Industry, Environmental Location Bureau, "Pollution Prevention Technology and Regulations-Atmosphere", Maruzen, 1995, p105-117 “Physical and chemical water treatment technology and equipment (below)” published by the Society of Chemical Engineering, published by Baifukan, 1978, p70-125 "Acclimation of sulfur-oxidizing bacteria from activated sludge and removal characteristics of thiosulfate", Journal of Japan Society on Water Environment, Vol. 18, No. 3, p231-p239, 1995

However, such a method has the following problems.
First, the air oxidation method will be described. In the case of sulfurous acid (SO ₃ ²⁻ ) and bisulfite (HSO ₃ ⁻ ), the reaction formula is as follows.

SO ₃ ^2- + 1 / 2O ₂ → SO ₄ ^2- (3)
^{_{HSO 3 - + OH - + 1}} / 2O 2 → SO 4 2- + H 2 O (4)

In this case, the dissolved oxygen required to oxidize 1 mM (80 mg) sulfurous acid (SO ₃ ²⁻ ) is 0.5 mM (16 mg). Since the solubility of oxygen in water is at most about 9 mg / L (water temperature: 20 ° C), when the concentration of sulfurous acid (SO ₃ ^2- ) increases, the supply of dissolved oxygen becomes rate-limiting, and oxidation using air Considerable time is required. In addition, there is no real-time index of the end point of air oxidation, and operation is performed to know the degree of oxidation by measuring COD.

Some sulfur compounds are difficult to oxidize in the air. For example, thiosulfuric acid (S ₂ O ₃ ²⁻ ) and dithionic acid (S ₂ O ₆ ²⁻ ) are difficult to oxidize with air.

In addition, the COD value of wastewater can be easily estimated from equations (3) and (4). For example, in the case of sulfurous acid (SO ₃ ^2- ), the theoretical COD is 16/80 = 0.2 g-COD / g-SO ₃ ^2- .

  Next, a chemical oxidation method using a chemical oxidizing agent will be described.

As described above, hydrogen peroxide, hypochlorous acid, ozone, and the like are widely known as chemical oxidants. It can also be applied to thiosulfuric acid (S ₂ O ₃ ²⁻ ) and dithionic acid (S ₂ O ₆ ²⁻ ), which are difficult to be oxidized by air. These reaction formulas are as follows in the case of sulfurous acid (SO ₃ ^2- ).

SO ₃ ^2- + H ₂ O ₂ → SO ₄ ^2- + H ₂ O (5)
SO ₃ ^2- + NaOCl → SO ₄ ^2- + NaCl (6)
SO ₃ ^2- + O ₃ → SO ₄ ^2- + O ₂ (7)

Since the above reaction occurs easily when a chemical oxidant is used, it is effective for shortening the reaction time. However, when the amount of chemicals required to oxidize 1 g of SO ₃ ²⁻ is calculated from these reaction equations, the results shown in Table 1 are obtained. As is clear from these results, the chemical oxidant is used in a larger amount than O _2, and the running cost increases when the concentration of the reducing sulfur compound in the wastewater is high. In addition, if there is a reducing substance (organic matter, reduced iron, etc.) other than sulfur in the wastewater, chemicals are consumed for this oxidation, and the running cost increases.

  Furthermore, if the chemical oxidant remains in the treated water, the living organisms in the environment are adversely affected. Therefore, control (addition of a reducing agent) is required to prevent the oxidant from remaining in the treated water.

Finally, an oxidation method using microorganisms using sulfur-oxidizing bacteria will be described. Sulfur-oxidizing bacteria is a general term for a group of autotrophic (CO ₂ utilization) or heterotrophic (organic utilization) bacteria that synthesize cells using the energy produced when sulfur is oxidized. The method using sulfur-oxidizing bacteria is applicable to thiosulfuric acid (S ₂ O ₃ ²⁻ ) and dithionic acid (S ₂ O ₆ ²⁻ ), which are difficult to be oxidized by air (Non-patent Document 3). In addition, there is an advantage that the required air amount is not different from the air oxidation method. However, in the case of wastewater containing a large amount of sulfur compounds capable of air oxidation, such as sulfurous acid (SO ₃ ^2- ), oxygen necessary for microbial reaction is preferentially consumed by sulfurous acid (SO ₃ ^2- ) as air oxidation. End up. For this reason, wastewater containing a large amount of sulfurous acid (SO ₃ ^2- ) and thiosulfuric acid (S ₂ O ₃ ^2- ) at the same time can easily reduce the activity of sulfur-oxidizing bacteria. It is not preferable to apply the above.

The gist of the present invention is a method of oxidizing a sulfur compound using ORP (Oxidation Reduction Potential) as an index in an oxidation tank when treating a sulfur compound contained in wastewater. ) To ( 6 ).

(1) A sulfur compound contained in wastewater is a treatment method of wastewater that supplies air or oxidizes by adding nitrite ions and supplying air, and measuring the pH of the wastewater, While adding acid or alkali to the waste water and controlling the pH to be 7 or more and 8 or less, the ORP (redox potential, silver / silver chloride standard value) of the waste water is measured to determine the sulfur compound. A wastewater treatment method characterized by estimating an oxidation degree and controlling an air supply amount to the wastewater so that the ORP falls within a predetermined range.

( 2 ) A wastewater treatment method containing a sulfur compound, characterized by oxidizing a sulfur compound remaining in the wastewater with a sulfur-oxidizing bacterium after the wastewater treatment by the method according to ( 1) .

( 3 ) During oxidation by the sulfur-oxidizing bacteria, acid or alkali is added to the wastewater to control the pH of the wastewater to 6-8, and air is supplied to the wastewater to provide an ORP ( The method according to ( 2 ), wherein the oxidation-reduction potential and the silver / silver chloride reference value are controlled to be within a predetermined range.

( 4 ) The sulfur compound contained in the wastewater is one or more of sulfurous acid, bisulfurous acid, thiosulfuric acid, and dithionic acid, any one of (1) to ( 3 ) above The method described in 1.

( 5 ) The method according to any one of (1) to ( 4 ), wherein the wastewater is desulfurization wastewater from a steelworks or a power plant.

( 6 ) Any one of the above (1) to ( 5 ), wherein treated water containing nitrite ions generated by biological wastewater treatment of wastewater containing ammonia as nitrite ions or denitration wastewater from a power plant is used. The method according to claim 1.

  By this invention, it becomes possible to oxidize a sulfur compound efficiently and stably from the wastewater containing a sulfur compound.

  Hereinafter, the present invention will be described in detail. An example of the processing flow of the present invention is shown in FIG.

First, the sulfur compounds contained in the waste water, the treatment process of wastewater oxidation by supplying air, when oxidizing the sulfur compounds contained in the waste water (1) by oxidation tank (2), oxidation tank ( Measure the ORP of 2) with an ORP meter (8), and control the air supply amount by the blower (10) to the oxidation tank (2) so that the ORP of the oxidation tank (2) becomes a certain value or more. For example, the inventors have a clear relationship between the ORP in the oxidation tank (2) and the concentration of sulfurous acid (SO ₃ ^2- ) in the wastewater, and the oxidation rate of sulfurous acid (SO ₃ ^2- ) is determined by ORP. It was found that it can be easily estimated.

FIG. 2 shows the relationship between the ORP in the oxidation tank (2) and the sulfurous acid (SO ₃ ²⁻ ) concentration when the pH is 6, 7, or 8. From this result, for example, when the sulfurous acid (SO ₃ ²⁻ ) concentration of the treatment target is 100 mg / L (COD, 20 mg / L of the wastewater standard value), the ORP value of the control target is about −20 mV to +50 mV. I understand that.

  Actually, the ORP value of the control target of the oxidation tank (2) changes depending on the target drainage standard value (COD regulation value) and the pH value. It is desirable to set a value.

Furthermore, the control of the pH (9) of the oxidation tank (2) is extremely important. That is, according to the knowledge of the inventors, it was recognized that when the pH of the oxidation tank (2) is less than 6, highly toxic sulfurous acid gas is easily generated from the wastewater (1). For this reason, it was judged that it is desirable to control the pH (9) of the oxidation tank (2) to 6 or more. It was also found that if the pH (9) of the oxidation tank (2) exceeds 8, precipitates are likely to be generated, and scale failures such as piping blockage are likely to occur. Furthermore, it was found from FIG. 2 that the ORP of the oxidation tank (2) was greatly influenced by pH (9), and the concentration of sulfurous acid (SO ₃ ^2- ) tended to decrease as pH (9) decreased. . These results, pH of the oxidation tank, that controls the 7 to 8.

In addition, we, when oxidizing the sulfur compounds in the waste water (1), nitrite ion ^- the oxidation rate addition was newly discovered that to improve the oxidation tank (2) (NO ₂₎ .

Up to now, as a wastewater treatment method using nitrite ions (NO ₂ ⁻ ), in Patent Document 2, wastewater containing ammonia under the condition of pH = 8 to 12 under heating conditions / metal catalyst combination, An invention for oxidizing ammonia is disclosed. However, this method is a method in which a metal catalyst is used in combination under high temperature and high alkali.

  On the other hand, the present invention is a method for oxidizing sulfur compounds in wastewater under normal and neutral pH conditions, and is a new discovery with different contents.

Hereinafter, a sulfur compound, is described nitrous sulfuric acid (SO ₃ ^2-) is indicated as representative species. The following equation is estimated for the route of the oxidation-reduction reaction between nitrite ions (NO ₂ ⁻ ) and sulfite (SO ₃ ²⁻ ).

_{^{2NO 2 - + 3SO 3 2- +}} 2H + → 3SO 4 2- + N 2 + H 2 O (8)

In this case, the nitrite ion (NO ₂ ⁻ ) required to oxidize 1 g of sulfite (SO ₃ ²⁻ ) is calculated as 0.38 g−NO ₂ ⁻ / g−SO ₃ ²⁻ . For example, if sulfite (SO ₃ ^2- ) 1000 mg / L is to be oxidized with nitrite ions, 380 mg / L nitrite ions may be added.

This reaction is also highly pH dependent and is likely to proceed at low pH. However, when the pH of the oxidation tank (2) is lowered to 6 or less, nitrous acid gas and sulfurous acid gas are likely to be generated. Further, as described above, if the pH of the oxidation tank (2) is more than 8, precipitates are likely to be generated and scale failure is likely to occur. Also, the reaction rate tends to decrease. From these results, even when nitrite ions (NO ₂ ⁻ ) are used, it is considered desirable to control the pH of the oxidation tank to 6 or more and 8 or less, more specifically 6.5 or more and 7.5 or less.

Table 2 below shows the amount of nitrite ion (NO ₂ ⁻ ) required to oxidize 1 g of sulfite in comparison with other chemicals. Now, nitrite ion (NO ₂ ^-) is found that requires less amount as compared with other drugs. Further, since nitrite ions themselves are not oxidized by air, it is desirable from the viewpoint of cost to supply nitrite ions together with air to the oxidation tank (2).

Examples of chemicals that supply nitrite ions include sodium nitrite (NaNO ₂ ). Nitrite ions may be added to the oxidation tank (2) using a chemical pump (5).

  However, when water containing a high concentration of nitrite ion at a level of several hundred mg / L is in the vicinity, it may be used to oxidize sulfite. As wastewater containing high-concentration nitrite ions at a level of several hundred mg / L, activated sludge treated water such as smelter coke factory wastewater (generally referred to as "ansen") can be considered. Steelworks coke factory wastewater contains a high concentration of ammonia. When this ammonia is treated with activated sludge, ammonia is oxidised by ammonia-oxidizing bacteria and nitrite-oxidizing bacteria. Often, the formula oxidizes to nitrite ions, followed to nitrate ions. However, since the function of nitrite-oxidizing bacteria is lower than that of ammonia-oxidizing bacteria and the reaction is likely to stop with nitrite ions, a large amount of treated water containing nitrite ions is generated.

2NH ₄ ⁺ + 3O ₂ → 2NO ₂ ⁻ + 2H ₂ O + 4H ⁺ (9)
^{_{2NO 2 - + O 2 → 2NO}} 3 - (10)

  In such a case, desulfurization waste water and nitrite ion-containing treated water may be mixed in advance and then passed through the oxidation tank (2) for air oxidation. This method makes it possible to reduce the amount of air compared to the case of only air oxidation.

  In addition, as effluent containing nitrite ions, application of nitrite ions contained in denitration wastewater generated from wet denitration equipment of a power plant is conceivable.

2NO ₂ + 2NaOH → NaNO ₃ + NaNO ₂ + H ₂ O (11)

  Finally, in the process of oxidizing sulfur compounds contained in wastewater, a method of oxidizing the remaining sulfur compounds using microorganisms after oxidizing the sulfur compounds with air or air and nitrite ions will be described. .

Some types of wastewater may contain sulfur compounds that are extremely difficult to oxidize with air. For example, thiosulfuric acid (S ₂ O ₃ ^2- ) and dithionic acid (S ₂ O ₆ ^2- ) are difficult to oxidize in the air, and most of them are in the treated water even when oxidized in the oxidation tank (2). It remains and is measured as COD. Therefore, in the case of wastewater containing such sulfur oxides, treatment by air oxidation alone is difficult and further oxidation treatment is required, but the chemical oxidation method increases the cost.

In order to oxidize such thiosulfuric acid (S ₂ O ₃ ²⁻ ) and dithionic acid (S ₂ O ₆ ²⁻ ), a biological oxidation method using sulfur-oxidizing bacteria is effective.

Sulfur-oxidizing bacteria having an active pH of 6-8 are Thiobacillus thioparus, Thiobacillus neapolitanus, Thiobacillus tepidarius, Thiobacillus novelus (Thiobacillus novellus) Thiobacillus versutus), Thiobacillus denitrificans and the like are known. Using these sulfur oxidizing bacteria or activated sludge containing these sulfur bacteria, thiosulfuric acid (S ₂₎ O ₃ ^2- ) and dithionic acid (S ₂ O ₆ ^2- ) are oxidized. These sulfur-oxidizing bacteria can be easily grown using activated sludge from municipal sewage as seed sludge using thiosulfuric acid (S ₂ O ₃ ^2- ) or dithionic acid (S ₂ O ₆ ^2- ). . In addition, activated sludge that treats coke factory effluent (generally referred to as "ansui") at a steelworks is likely to contain sulfur-oxidizing bacteria because the coke factory effluent contains many sulfur compounds. Since there are many, it can also be used as it is.

S ₂ O ₃ ^2- + 2O ₂ + H ₂ O → 2SO ₄ ^2- + 2H ⁺ (12)
2S ₂ O ₆ ^2- + O ₂ + 2H ₂ O → 4SO ₄ ^2- + 4H ⁺ (13)

These reactions proceed quickly in the presence of sulfur-oxidizing bacteria and air. However, as is clear from the equations (11) and (12), the pH (9) of the microbial reactor (3) tends to decrease with the progress of the reaction. It is necessary to maintain the pH (9) of the reaction tank (3) at a pH at which the activity of the sulfur-oxidizing bacteria can be maintained. What is necessary is just to control pH to 6-8, using a dilute sulfuric acid or sodium hydroxide solution. If the pH is less than 6 or more than 8, the activity decreases greatly. It is also desirable to measure the ORP (8) of the microbial reactor (3) and perform aeration using the ORP as an index. If the ORP value is +100 mV or more, thiosulfuric acid (S ₂ O ₃ ²⁻ ) and dithionic acid (S ₂ O ₆ ²⁻ ) are almost completely oxidized. This treatment method is also effective when the waste water contains organic matter. For example, the microorganism reaction tank (9) may be divided into two tanks, and a system capable of coexisting organic matter-decomposing bacteria and sulfur-oxidizing bacteria can be used.

Example 1 : Examination of air oxidation of sulfurous acid (SO ₃ ^2- ) Using the flue gas desulfurization wastewater discharged from the wet flue gas desulfurization equipment of the steelworks, the oxidation performance of sulfurous acid in the wastewater was examined by a beaker test. . The concentration of sulfurous acid in the flue gas desulfurization waste water varied greatly from 3000 mg / L to 20000 mg / L with an average of 10000 mg / L.

500 mL of waste water was added to a 1 L beaker, the water temperature was controlled at 25 ° C. and the pH was kept constant at 7.0, and 0.5 L / min air was supplied by a blower. Thereafter, water was sampled every several hours, the ORP and sulfite concentrations were measured, and the relationship between the ORP value and the sulfite concentration was examined. Furthermore, the same experiment was conducted by controlling the pH at 6.0 or 8.0 constant, and the influence of pH on the ORP was examined.
The results are shown in FIG.

  A strong correlation was observed between ORP and sulfite concentration under constant pH conditions. Therefore, it was considered possible to estimate the oxidation degree of sulfurous acid in wastewater by ORP in the oxidation tank (2).

  Further, since the ORP value is strongly influenced by pH, it was considered important to grasp the pH value of the oxidation tank. In the case of this experiment, if the target sulfite concentration is 100 mg / L (20 mg / L as COD), -20 mV or more when pH is 6.0, pH is 7.0, ORP is 0 mV or more, and pH is 8.0 The air aeration should be performed with 50 mV as a guide. More strictly, it is considered desirable to control at 6.5 or more and 7.5 or less.

  However, since the correlation between pH and ORP increases as pH increases, it is desirable that pH = 7 or more for more strict management.

  In addition, although this experiment was implemented also on the conditions of pH = 8.5, a lot of precipitates produced | generated and ORP measurement was difficult.

Example 2 : Examination of sulfite (SO ₃ ^2- ) oxidation using nitrite ions and air Oxidation performance of sulfite in wastewater using flue gas desulfurization wastewater discharged from a wet flue gas desulfurization unit Were examined by a beaker test.

500 mL of waste water was added to a 1 L beaker, the water temperature was controlled at 25 ° C., the pH was kept constant at 8, and 0.5 L / min air was supplied by a blower. Thereafter, water was collected after 2.5 hours, and the concentration of sulfurous acid was measured.
On the other hand, 500 mL of waste water and 20 mM (920 mg / L) of nitrite ions were added to a 1 L beaker, and a similar experiment was performed. The results are shown in FIG.

From this result, it was estimated that oxidation of sulfite by nitrite ions occurred prior to oxidation by air, and 2400 mg / L was oxidized by nitrite ions. 920/2400 = 0.38 If now calculates the oxidation efficiency of sulfite, 0.38g- NO ₂ obtained from (6) ^- was almost the same as / g- SO ₃ ^2- calculated values.

If the addition of nitrite ions, the treatment time is 5 hours, phosphorous sulfuric acid, (20 mg / L as COD) which was equal to or less than the target of 100 mg / L. On the other hand, nitrous sulfuric acid 250 mg / L treatment time even 7.5 hours for no addition remained.

  Therefore, when nitrite ions are easily obtained, if nitrite ions are used in combination with air, the oxidation efficiency can be improved and the processing time can be shortened.

Example 3 Treatment of Waste Water Containing Sulfur Compounds by Combined Use of Air Oxidation and Microbial Oxidation Treatment experiments for waste water containing various types of sulfur compounds were carried out by the treatment process shown in FIG. In the experiment, an artificial wastewater containing 1000 mg / L of sulfurous acid, 200 mg / L of thiosulfuric acid, and 200 mg / L of dithionic acid was used as a sulfur compound.

  First, sulfurous acid was oxidized in the oxidation tank (2). While passing wastewater (1) through the oxidation tank (2), the pH (9) of the oxidation tank (2) is controlled within the range of 7.0-7.5, and the ORP (8) near the outlet of the oxidation tank (2) Air aeration was performed from 0 mV to +10 mV. As a result, the sulfurous acid contained in the waste water (1) became 100 mg / L or less under the condition that the residence time of the oxidation tank (2) was 2 hours.

  Subsequently, thiosulfuric acid and dithionic acid were oxidized in the microbial reactor (3). As the sulfur-oxidizing bacteria, 2000 mg / L of sulfur-oxidizing bacteria contained in the waterworks activated sludge in the steelworks was added to the microbial reactor (3) as seed sludge. Control the pH (9) of the microbial reactor (3) holding sulfur-oxidizing bacteria within the range of 7.0-8.0, and adjust the ORP (8) near the outlet of the microbial reactor (3) to +100 mV to +150 mV. Aeration was performed. As a result, thiosulfuric acid and dithionic acid contained in the wastewater (1) were below the detection limit under the condition of residence time of 2 hours. Sulfurous acid was also below the detection limit.

  Therefore, when the waste water contains sulfurous acid, thiosulfuric acid, and dithionic acid, the sulfur compound can be completely oxidized by using a biological treatment with sulfur-oxidizing bacteria in combination with air oxidation.

It is a processing flow of the wastewater containing the sulfur compound of the present invention. In the oxidation of sulfurous acid using the air of the present invention, it is a diagram showing the relationship between the ORP of the reaction tank (2) and the concentration of sulfurous acid. (PH = 6 or 7 or 8) It is a figure which shows the oxidation efficiency improvement of a sulfurous acid by nitrite ion addition of this invention.

Explanation of symbols

1 Waste water containing sulfur compounds
2 Oxidation tank
3 Microbial reactor
4 Nitrite tank
5 Chemical injection pump
6 Acid or alkali tank
7 Chemical injection pump
8 ORP meter
9 pH meter
10 Blower
11 treated water

Claims (6)

A wastewater treatment method for oxidizing sulfur compounds contained in wastewater by supplying air, or adding nitrite ions and supplying air, and measuring the pH of the wastewater, While adding acid or alkali to the solution and controlling the pH to be 7 or more and 8 or less, the ORP (redox potential, silver / silver chloride reference value) of the wastewater is measured to determine the degree of oxidation of the sulfur compound. A method for treating wastewater, characterized in that the amount of air supplied to the wastewater is controlled so that the ORP falls within a predetermined range. A method for treating a wastewater containing a sulfur compound, characterized by oxidizing a sulfur compound remaining in the wastewater with a sulfur-oxidizing bacterium after the wastewater treatment by the method according to claim 1 . During oxidation by the sulfur-oxidizing bacteria, acid or alkali is added to the wastewater to control the pH of the wastewater to 6-8, and air is supplied to the wastewater to provide an ORP (redox potential) of the wastewater. , silver / silver reference value chloride) characterized in that the control such that the predetermined range, the method according to claim 2. The method according to any one of claims 1 to 3 , wherein the sulfur compound contained in the wastewater is one or more of sulfurous acid, bisulfurous acid, thiosulfuric acid, and dithionic acid. It characterized in that the waste water is a desulfurization waste water SeiTetsusho or power plant, the method according to any one of claims 1-4. The nitrite ion is treated with nitrite ion-containing treated water generated by biological wastewater treatment of wastewater containing ammonia or denitration wastewater from a power plant, according to any one of claims 1 to 5. The method described.

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