JP2003136092A - Method and apparatus for treating exhaust gas desulfurization wastewater - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wastewater treatment method detecting the concentration of a toxic substance, especially persulfuric acid contained in exhaust gas desulfurization wastewater, and subjecting the toxic substance to a necessary treatment process corresponding to the detection result. SOLUTION: The exhaust gas desulfurization wastewater treatment method includes a process for detecting persulfuric acid in the exhaust gas desulfurization wastewater 1 by a biosensor 3 for measuring the concentration of dissolved oxygen changed in the presence of the toxic substance inhibiting the metabolism of nitrite bacteria, especially persulfuric acid, an activated carbon treatment process for removing persulfuric acid or organic matter contained in the wastewater by an activated carbon treatment device 6 and a nitrification/denitrification process for decomposing an ammonia nitrogen compound in the wastewater by a nitrificaticn/denitrification device 5 and the order of the activated carbon treatment process and the nitrification/denitrification process is changed corresponding to the detection result of the toxic substance due to the biosensor 3 to treat wastewater. The apparatus 8 for treating exhaust gas desulfurization wastewater using the exhaust gas desulfurization wastewater treatment method is also disclosed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating flue gas desulfurization wastewater. More specifically, the present invention relates to a method for treating flue gas desulfurization wastewater, which is characterized by continuously detecting toxic substances in wastewater using a biosensor.

[0002]

2. Description of the Related Art Wastewater discharged from a soot mixing type flue gas desulfurization device contains a large amount of ammoniacal nitrogen compounds. Therefore, the ammoniacal nitrogen compounds are nitrified and denitrified by microorganisms and decomposed into nitrogen. There is known a processing method of removing by removing.

However, in such waste water, in addition to the ammoniacal nitrogen compound, persulfuric acid (peroxosulfate S ₂
O ₈ ^2- ), COD substances, etc. are included. The COD substance is a chemical substance contained in water and is a substance that is oxidized and causes pollution of water. Among these substances, persulfuric acid is a strong oxidant, so there is a problem that it poisons the microorganisms in the nitrifying and denitrifying process of ammoniacal nitrogen compounds by the action of the microorganisms and deteriorates the COD adsorption resin. , Is an obstacle to wastewater treatment. Therefore, it is necessary to remove persulfuric acid in advance.

The concentration of persulfuric acid in the waste water often changes according to the operating conditions of the flue gas desulfurization device. For example, the concentration of persulfuric acid may be high during startup or shutdown of the flue gas desulfurization device, but low during normal operation. In such a case, the persulfuric acid in the wastewater should be continuously monitored, and the treatment process can be freely changed according to changes in operating conditions so that the persulfuric acid can be efficiently removed if it exists. Is required.

On the other hand, in the fields of water supply, sewerage, fermentation, food processing, etc., biosensors have been drawing attention in recent years as monitoring measures against inflow of harmful chemical substances (Yoshiharu Tanaka, Hiroaki Tanaka, "Water and Drainage "Vol 40, N
o.4, pp.306-309 (1998)). The biosensor detects the presence of a toxic substance by growing a microorganism in test water and detecting its activity state, for example, a change in consumption of dissolved oxygen. JP 6-222041 JP, JP 7-63725 JP,
Japanese Patent Publication No. 7-85072 and Japanese Unexamined Patent Publication No. 9-89839 disclose biosensors for detecting toxic substances such as phenol, potassium cyanide, and trichlorethylene in tap water.

[0006]

SUMMARY OF THE INVENTION The present invention detects the concentration of toxic substances, especially persulfuric acid, contained in flue gas desulfurization wastewater,
An object of the present invention is to provide a method and an apparatus for efficiently performing nitrification / denitrification treatment of wastewater by microorganisms by changing the wastewater treatment process according to the result.

[0007]

[Means for Solving the Problems] A step of detecting a toxic substance in flue gas desulfurization effluent by a biosensor for measuring a dissolved oxygen concentration which changes due to the presence of a toxic substance that inhibits microbial metabolism, and the toxic substance or An activated carbon treatment step of removing organic matter contained in the wastewater, and a nitrification denitrification step of decomposing ammoniacal nitrogen compounds in the wastewater are included, and the activated carbon treatment is carried out according to the detection result of the toxic substance by the biosensor. Provided is a method for treating flue gas desulfurization wastewater, which comprises treating the wastewater by changing the order of the steps and the nitrification / denitrification step. It is preferable that the toxic substance is persulfuric acid, the biosensor measures a dissolved oxygen concentration that changes due to the presence of a toxic substance that inhibits metabolism of a microorganism, and the microorganism is a nitrite. When the value of the dissolved oxygen concentration change measured by the biosensor is equal to or higher than the reference value, the activated carbon treatment step is performed after the nitrification denitrification step, and when the value of the dissolved oxygen concentration change is equal to or lower than the reference value. Provided is a method for treating flue gas desulfurization wastewater, which comprises performing the nitrification denitrification step after the activated carbon treatment step. Furthermore, the present invention includes a biosensor for detecting a toxic substance in flue gas desulfurization wastewater by measuring a dissolved oxygen concentration which changes due to the presence of a toxic substance that inhibits metabolism of a microorganism, and a biosensor included in the toxic substance or the wastewater. Provided is a flue gas desulfurization wastewater treatment apparatus for treating the wastewater by the above-mentioned method, which comprises an activated carbon treatment apparatus for removing organic matter and a nitrification denitrification apparatus for decomposing ammoniacal nitrogen compounds in the wastewater. To do. It is preferable that the apparatus further comprises one or more valves provided between the activated carbon treatment apparatus and the nitrification / denitrification apparatus, and means for controlling opening / closing of the valves. Here, the term "between the activated carbon treatment device and the nitrification denitrification device" includes any connection existing in the treatment device for flue gas desulfurization wastewater comprising the activated carbon treatment device and the nitrification denitrification device. However, it is not limited directly to the activated carbon treatment device and the nitrification denitrification device.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will now be described in detail with reference to the drawings. The following description does not limit the invention.

In the present invention, a biosensor for detecting persulfate using nitrite can be preferably used. The present inventors have found that the persulfuric acid contained in the wastewater of the flue gas desulfurizer is
It was found that it acts as a toxic substance against nitrifying bacteria, especially nitrite bacteria. So far, the only toxic substances that can be detected by biosensors are cyan compounds, phenols, and organic chlorine compounds, and examples of application to flue gas desulfurization wastewater treatment, especially persulfuric acid detection as a toxic substance are given below. There wasn't.

It is known that when nitrite bacteria contain no inhibitory substance, they consume dissolved oxygen in water at a constant rate by respiration to convert ammoniacal nitrogen compounds into nitrite. However, if persulfate is contained in the waste water, the metabolic activity of nitrite bacteria is inhibited, the respiratory rate becomes slow, and the consumption rate of dissolved oxygen in the waste water decreases. Therefore, when a change in the dissolved oxygen concentration in the wastewater when the inhibitor is present is measured by a biosensor, it is smaller than the change in the dissolved oxygen concentration in the wastewater when the inhibitor is not present.

Therefore, in the present invention, a biosensor is used to detect the presence of an inhibitor as follows. A part of the wastewater is passed through an immobilized microbial membrane of nitrite to measure the change in dissolved oxygen concentration. Here, when the value of the change in dissolved oxygen concentration proportional to the oxygen consumption rate of nitrite is greater than or equal to the reference value, the inhibition of metabolism of nitrite is small, and the test effluent contains an inhibitor such that activated carbon treatment is required. You can see that it is not included. The reference value is a value that is appropriately determined depending on operating conditions such as habitation of microorganisms and drainage temperature, and is a reference value for determining the presence or absence of an inhibitor. On the other hand, the presence of inhibitors significantly reduces the oxygen consumption rate of nitrite bacteria,
The value of the change in the dissolved oxygen concentration becomes small, and it becomes possible to detect the presence of the inhibitor.

Such a biosensor comprises an immobilized microbial membrane in which nitrite bacteria are immobilized on a matrix such as a porous membrane,
It can be produced using a known technique using an appropriate dissolved oxygen electrode or the like. Nitrite bacteria are microorganisms that have the property of oxidizing ammoniacal nitrogen compounds by metabolism to convert them into nitrite nitrogen compounds, and the reaction is represented by the following formula.

[Chemical 1]

The nitrite bacteria have scientific names such as Nitrosomonas europia, but the present invention is not limited thereto. In particular, in the biosensor of the present invention, it is preferable to use a known immobilized microbial membrane type sensor.

According to the present invention, subsequent processing steps can be changed according to the detection result of the toxic substance by the biosensor. FIG. 1 shows a flue gas desulfurization wastewater treatment apparatus 8 of the present invention using a biosensor, and the overall steps will be described. In the pretreatment device 2, fluorine, calcium compounds, etc. in the desulfurization effluent 1 containing the ammoniacal nitrogen compound discharged from the flue gas desulfurization device are removed. A small part of the wastewater from which these have been removed is then introduced into the biosensor 3. The presence / absence of a toxic substance in the wastewater is detected by measuring the dissolved oxygen concentration, which changes due to the metabolism of microorganisms, with the biosensor 3.

When a toxic substance is detected by the biosensor 3, the valve control means 4 such as a converter controls the opening and closing of the valve as described in "at the time of switching" in Table 1. When a toxic substance is detected, the valve 11 is closed, the valve 12 is opened, and the wastewater passing through the pretreatment device 2 is
It is introduced into the activated carbon treatment device 6, where persulfuric acid which is a toxic substance is decomposed and removed by an activated carbon catalyst or the like. The wastewater from which the persulfuric acid has been removed through the activated carbon treatment step has the valve 14 and the valve 15 closed and the valve 13 open, so that the nitrifying and denitrifying device 5 decomposes the contained ammoniacal nitrogen compound, The treated water 7 is discharged out of the system of the device of the present invention.

[0016]

[Table 1]

On the other hand, when no toxic substance is detected by the biosensor 3, the valve control means 4 opens and closes the valve provided between the activated carbon treatment device 6 and the nitrification denitrification device 5 in Table 1. It is controlled as shown in "normal time". At this time, the valve 11 is opened and the valve 12 is closed, and the wastewater that has passed through the pretreatment device 2 is decomposed into ammoniacal nitrogen compounds by the action of microorganisms in the nitrification denitrification device 5. The wastewater from which the contained ammoniacal nitrogen compound has been decomposed and removed opens the valve 14 and opens the valve 16
Since it is closed, it is introduced into the activated carbon treatment device 6 and residual organic substances derived from industrial water are removed by the activated carbon treatment. At this time, the activated carbon treatment device 6 is installed after the nitrification denitrification device 5 even in the nitrification denitrification process.
Since organic matter derived from such industrial water is slightly decomposed,
This is because it is possible to prevent the adsorption force of activated carbon from being lowered in the subsequent activated carbon treatment step. Since the valve 13 is closed and the valve 15 is opened, the wastewater that has passed through the activated carbon treatment device is discharged outside the system as treated water 7. Here, the arrangement and number of valves in the flue gas desulfurization wastewater treatment apparatus 8 shown in FIG. 1 are merely examples, and the present invention is not limited to such an embodiment.

As described above, it is a feature of the present invention that the biosensor 3 can detect an inhibitor of microbial metabolism, and the subsequent processing steps can be controlled according to the result. The control can be performed by changing the order of processing steps by opening and closing valves provided between the processing devices by the control means 4 such as an exchanger connected to the biosensor 3. Here, the term “between treatment devices” refers to individual devices such as the nitrification denitrification device 5 and the activated carbon treatment device 6 included in the flue gas desulfurization wastewater treatment device 8, but only those that directly connect specific devices. It is not limited to The valves 11 to 16 shown in FIG. 1 include all valves provided for the purpose of changing the order of the treatment process of wastewater to be treated. The process of treating wastewater according to the presence or absence of an inhibitor will be described in detail below.

[When Persulfate is Detected] When the change in the dissolved oxygen concentration in the sample water measured by the biosensor 3 is less than the reference value, it indicates that persulfate is present. At this time, the wastewater that has passed through the pretreatment step 2 is introduced into the activated carbon treatment device 6 in order to remove the persulfuric acid. In the activated carbon treatment device 6, persulfuric acid is decomposed and removed by a decomposition reaction represented by the following formula using a catalyst.

[0020]

[Chemical 2]

As the catalyst used in the activated carbon treatment device 6,
Activated carbon, ceramics carrying activated carbon, a reducing agent and the like are preferably used. The present invention is not limited to this, but it is required to have a function of catalyzing the decomposition of persulfuric acid and a function of adsorbing and removing organic substances and the like existing in the wastewater. When activated carbon is used, activated carbon having any shape and size can be used, but activated carbon having a particle size of about 0.1 to 2 mm is preferably used. The activated carbon adsorption can be carried out by either the treatment in the reaction tank or the contact with water. In case of water contact,
It is preferable to carry out the adsorption tower filled with the granular activated carbon of about 0.1 to 2 mm under the water flow condition of SV = ¹ to 20 hr ⁻¹ .

Wastewater that has passed through the activated carbon treatment device 6 is sent to the nitrification and denitrification device 5. In the nitrification / denitrification device 5, microbial treatment is performed, and finally it is decomposed into nitrogen gas by using nitrifying bacteria and denitrifying bacteria. Nitrifying bacteria function to oxidize ammoniacal nitrogen compounds to nitrates, which include nitrite bacteria that convert ammoniacal nitrogen compounds to nitrites as shown in formula (1) and formula (2) as shown in formula (2). There are nitric acid bacteria that turn nitrite into nitrate. It is desirable that such a nitrification step is carried out under aerobic conditions by adjusting the pH to weakly alkaline, for example, about 7 to 8 with an alkaline agent such as sodium hydroxide. The nitrification step is achieved, for example, by reacting in an nitrification tank in which activated sludge containing nitric acid bacteria and nitrite bacteria is attached to a carrier.

The denitrification step is a step of reducing the generated nitric acid or nitrous acid by a denitrifying bacterium and converting it into nitrogen gas.
The denitrifying bacterium converts nitrate into nitrogen gas as shown in the following formula (3).

[Chemical 3]

The denitrification step is preferably carried out under anaerobic conditions, and as organic carbon sources, pure substances such as methanol, acetic acid, ethanol and glucose, as well as BOD from other waste water and molasses can be mentioned. Methanol is particularly preferable because of its simple injection equipment and low operating cost. The denitrification step can be carried out by reacting it in a denitrification tank in which activated sludge containing denitrifying bacteria is attached to a carrier as in the nitrification step.

Such a nitrification / denitrification apparatus 5 is described in detail in JP-A-11-090485 and JP-A-10-296296, and it is possible to use such a conventionally used apparatus. Yes, but not limited to. In the present invention, the waste water introduced into the nitrification / denitrification device 5 is free from persulfuric acid. Therefore, the functions of microorganisms such as nitrifying bacteria are not hindered in the nitrification / denitrification device 5.

Wastewater that has passed through the nitrification / denitrification device 5 is treated water 7 from which persulfuric acid and ammoniacal nitrogen compounds have been removed.

[When Persulfate is not Detected] When the biosensor 3 detects that the value of the change in the dissolved oxygen concentration in the sample water is above the reference value, the persulfate does not exist above the predetermined concentration. Indicates that. Therefore, the wastewater is introduced into the nitrification denitrification device 5 without passing through the step of persulfate decomposition by the activated carbon treatment device 6, and the ammoniacal nitrogen compounds in the wastewater are decomposed and removed by microorganisms such as nitrifying bacteria and denitrifying bacteria. . The wastewater after the removal of the ammoniacal nitrogen compound is originally contained in the organic matter dissolved in the wastewater by the same activated carbon treatment device 6 used for removing the persulfuric acid, for example, industrial water that is the cause of the wastewater. The organic substances that had been removed are removed.

The concentration of persulfuric acid in the waste water changes depending on the operating conditions of the desulfurization smoke exhaust device. Therefore, in the device of the present invention, the concentration of persulfuric acid is continuously monitored during the operation of the desulfurization smoke exhaust device, and the process steps are controlled so that they can be switched as necessary. This allows one device to
It is possible to realize an effective wastewater treatment process that varies depending on the amount of persulfuric acid contained in the wastewater, so that wastewater treatment can be performed efficiently.

[0029]

EXAMPLES Next, the present invention will be described in more detail with reference to examples. The examples do not limit the invention.

Example 1 Wastewater of a flue gas desulfurization apparatus was treated by the treatment apparatus of the present invention and a conventional treatment apparatus, and the results were compared.

Waste water containing no persulfuric acid and containing nitrogen derived from an ammoniacal nitrogen compound at a concentration of 100 mg / L is shown in Table 1.
The processing was performed under the normal valve opening / closing control conditions shown in. The test conditions were SV = 10 L / h and LV = 3 m / h. The activated carbon used is Mitsubishi Kagaku Diamond Hope. The water after treated with the pretreatment device was subjected to nitrification denitrification treatment, and total nitrogen (TN) was measured. In the nitrification and denitrification process, 200 mg / L of methanol (C /
N = 2.5) was added. The operation result at this time is shown in Example 1 of Table 2. In Table 2, S ₂ O ₈ is persulfate ion or persulfate, NH ₃ -N is nitrogen derived from ammoniacal nitrogen compound, and TN is ammonia, nitrite-derived, or nitrate-nitrogen-derived compound. The total nitrogen, which is the amount of nitrogen, is shown. Clearly, it was found that the nitrogen derived from the ammoniacal nitrogen compound was removed in the nitrification denitrification step.

Example 2 Next, 100 persulfate ions were added.
mg / L, 100% nitrogen derived from ammoniacal nitrogen compounds
The wastewater containing at a concentration of mg / L was treated under the valve opening / closing control conditions at the time of switching shown in Table 1. Table 2 shows the operation results at this time.
Example 2 of Persulfuric acid was completely removed in the activated carbon treatment process before the desulfurization wastewater flowed into the nitrification and denitrification process.
Therefore, it was found that, since persulfate was not contained, there was no inhibition of microbial metabolism, and nitrogen was removed in the nitrification and denitrification step to the same extent as the wastewater of Example 1 containing no persulfate.

[Comparative Example] Waste water containing 100 mg / L of persulfuric acid and 100 mg / L of nitrogen derived from an ammoniacal nitrogen compound was subjected to the same flow as in the normal state even though persulfuric acid was detected. The operation results when treated are shown in the comparative example of Table 2. In this case, the nitrifying and denitrifying function was deteriorated in the nitrifying and denitrifying step, and eventually the nitrogen derived from the ammoniacal nitrogen compound in the desulfurization wastewater was not treated at all.

[0034]

[Table 2]

Based on these results, the wastewater treatment device 8 of the present invention detects the persulfuric acid which inhibits the nitrifying and denitrifying reaction of microorganisms by the biosensor 3 and removes it in advance, thereby efficiently treating the wastewater. It turns out that is possible.

[0036]

[Effects of the Invention] By using a biosensor to confirm the presence or absence of toxic substances in advance and select the process sequence, it is possible to reliably detect even the case where persulfuric acid is contained in the flue gas desulfurization wastewater. After removing persulfate, nitrogen compounds can be efficiently treated without adversely affecting the metabolism of microorganisms. Further, the process sequence can be easily changed only by operating the valve. This makes it possible to properly use the persulfuric acid removal function and the organic matter removal function of the activated carbon treatment device using a catalyst such as activated carbon, with respect to changes in the drainage property.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a flue gas desulfurization wastewater treatment apparatus according to the present invention.

[Explanation of symbols]

1 desulfurization drainage 2 Pretreatment equipment 3 biosensor 4 Control means 5 Nitrification and denitrification equipment 6 Activated carbon processing equipment 7 treated water 8 Flue gas desulfurization wastewater treatment equipment 11 valves 12 valves 13 valves 14 valves 15 valves 16 valves

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Claims (5)

[Claims] 1. A step of detecting a toxic substance in flue gas desulfurization effluent by a biosensor for measuring a dissolved oxygen concentration which changes depending on the presence of a toxic substance inhibiting microbial metabolism, and a step of detecting the toxic substance or the effluent in the effluent. The method includes an activated carbon treatment step of removing organic substances contained therein and a nitrification denitrification step of decomposing ammoniacal nitrogen compounds in the wastewater. The activated carbon treatment step and the nitrification step are performed according to the detection result of the toxic substance by the biosensor. A method for treating flue gas desulfurization wastewater, which comprises treating wastewater by changing the order of the denitrification process. 2. The toxic substance is persulfuric acid, the biosensor measures a dissolved oxygen concentration which changes due to the presence of a toxic substance that inhibits metabolism of a microorganism, and the microorganism is a nitrite. Item 1. A method for treating flue gas desulfurization wastewater according to Item 1. 3. When the value of change in dissolved oxygen concentration measured by the biosensor is equal to or higher than a reference value, the activated carbon treatment step is performed after the nitrification / denitrification step, and the value of change in dissolved oxygen concentration is equal to or lower than a reference value. When it is, the method for treating flue gas desulfurization wastewater according to claim 1 or 2, wherein the nitrification denitrification step is performed after the activated carbon treatment step. 4. A biosensor for detecting a toxic substance in flue gas desulfurization effluent by measuring a dissolved oxygen concentration which changes due to the presence of a toxic substance inhibiting microbial metabolism, and a toxic substance or a toxic substance contained in the effluent. 4. An activated carbon treatment device for removing organic matter to be removed, and a nitrification denitrification device for decomposing ammoniacal nitrogen compounds in the waste water.
An apparatus for treating flue gas desulfurization wastewater for treating the wastewater by the method according to any one of 1. 5. The flue gas according to claim 4, further comprising one or more valves provided between the activated carbon treatment device and the nitrification / denitrification device, and means for controlling opening / closing of the valve. Desulfurization wastewater treatment equipment.