JP2001170683A - Biological denitrification method for water - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a biological denitrification method for water of a gaseous hydrogen utilization system which does not entail an increase of a pH by accompanying a denitrification reaction even for raw water having a high concentration of nitrate nitrogen and does not degrade denitrification treatment performance. SOLUTION: Contact materials 12 holding denitrification bacteria are packed in a reaction chamber 10. The raw water containing the nitrate nitrogen is supplied to the reaction chamber 10 and the gaseous hydrogen in the gaseous mixture diffused from an air diffuser 18 is regulated to a specified concentration or above and the nitrate nitrogen in the raw water is biologically subjected to the denitrification treatment. At this time, carbon dioxide is intermittently or continuously supplied to the raw water so that the pH of effluent attains 8 to 9.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for biological denitrification of water, and more particularly to water containing nitrate nitrogen and / or nitrite nitrogen (both are collectively referred to as nitrate nitrogen). To a reaction tank holding denitrifying bacteria and biologically denitrifying using hydrogen gas.

[0002]

2. Description of the Related Art Conventional biological denitrification processes utilize hydrogen contained in organic matter in raw water as a hydrogen donor. Also, when the amount of hydrogen contained in the organic matter in the raw water is small,
In order to supply a hydrogen donor, a method of injecting an appropriate amount of an organic substance such as acetic acid or methanol into raw water and performing a denitrification treatment has been put to practical use. However, in order to completely remove nitrate nitrogen in raw water by such a method, it is necessary to supply an excess amount of organic substances that is two to three times the reaction equivalent required for nitrogen removal. Unreacted organic matter remains in the effluent from the reactor. Therefore, it is necessary to provide an aeration tank at the latter stage of the reaction tank to remove residual organic substances.

[0003] Raw water containing nitrate nitrogen and having a low organic substance concentration includes nitric acid washing wastewater discharged from metal surface treatment, ammonia-containing wastewater discharged from a power plant, and groundwater in a specific area. In particular, when groundwater is used for drinking, it is necessary to remove nitrate nitrogen contained therein, but there is great mental anxiety about adding the above-mentioned organic substances such as acetic acid and methanol during the water purification process. In addition, as a practical problem, there is a large economic burden required to reliably remove organic substances remaining after the denitrification treatment. For this reason, a method has been proposed in which a hydrogen gas is used as a hydrogen donor in a biological denitrification method, instead of the organic substance, in which there is no possibility that the organic substance remains.

[0004]

According to experiments by the present inventors, the method using hydrogen gas as the hydrogen donor is applied to raw water having a relatively high nitrate nitrogen concentration and a relatively low organic matter concentration. In that case, it was found that a new problem occurred. That is, it was found that when the concentration of nitrate nitrogen was high, the pH of the water in the reaction tank was increased, the activity of the denitrifying bacteria was reduced, and the denitrification treatment was insufficient. The increase in pH due to the denitrification reaction also changes depending on the pH buffering property of the raw water, but in the case of ordinary groundwater, when the concentration of nitrate nitrogen exceeds 15 mg / L, the pH of the water in the reaction tank is increased by the denitrification reaction. Becomes 10 or more, and the denitrification treatment performance rapidly decreases.

[0005] An object of the present invention is to solve the above-mentioned problems of the prior art, and even in raw water having a high concentration of nitrate nitrogen, there is no increase in pH due to the denitrification reaction, and the denitrification treatment performance does not decrease. An object of the present invention is to provide a method for biological denitrification of water using hydrogen gas.

[0006]

According to the method of biologically denitrifying water according to the present invention, water containing nitrate nitrogen is supplied to a reaction tank holding denitrifying bacteria, and the biological denitrification is carried out using hydrogen gas. In the method for denitrification, carbon dioxide gas is intermittently or continuously supplied to water in the reaction tank.

Further, in the method for biologically denitrifying water according to the present invention, the pH value of the effluent from the reaction tank is in the range of 7 to 9,
Preferably, the supply amount of the carbon dioxide gas is controlled so as to be in the range of 8 to 9.

[0008] Further, the biological denitrification method of water according to the present invention is characterized in that the carbon dioxide gas is supplied to the water in the reaction tank in a state of being mixed with the hydrogen gas.

The technical background up to the present invention will be described below. In the method using hydrogen gas as a hydrogen donor, most of nitrate nitrogen in water is reduced to nitrogen gas by the following formula (1) or (2). 2NO ₃ ⁻ + 5H ₂ → N ₂ + 4H ₂ O + 2OH ⁻ (1) 2NO ₂ ⁻ + 3H ₂ → N ₂ + 2H ₂ O + 2OH ⁻ (2) Denitrification as evident from equation (1) or (2) Since the reaction produces hydroxyl ions, the pH of the water in the reactor is
Rises. This increase in pH depends on the concentration of nitrate nitrogen in the raw water, and when the concentration of nitrate nitrogen is high, the pH of the water in the reaction tank also increases, resulting in a decrease in denitrification treatment performance as described above. On the other hand, in the conventional method using an organic substance as the hydrogen donor or when the concentration of the organic substance in the raw water is high, bicarbonate ions due to carbon constituting the organic substance are generated in addition to the reaction of the above formula (1) or (2). The generated reaction proceeds concurrently, for example, as shown in the following equations (3) and (4). CH ₃ OH + H ₂ O → CO ₂ + 3H ₂ ... (3) CO ₂ + 2H ₂ O → HCO ₃ ⁻ + H ₃ O ⁺ ... (4) The bicarbonate ion generated by the above formula (4) causes an increase in pH. On the other hand, since it exhibits strong buffering properties, it is considered that no rapid increase in pH occurs and no decrease in denitrification treatment performance occurs. For this reason, in the conventional biological denitrification method, an increase in pH and a decrease in denitrification treatment performance with the progress of the denitrification reaction have not been considered a problem.

The present invention has been made on the basis of the above findings, and has been focused on the fact that bicarbonate ions exhibit a strong buffer against an increase in pH. As means for positively generating ions, carbon dioxide gas is supplied to water in a reaction tank.

The carbon dioxide gas to be supplied is harmless, and there is no particular problem even if it is supplied in excess, but it is desirable to supply only a necessary amount from the viewpoint of economy. For this purpose, the pH value of the effluent from the reactor is in the range of 7 to 9, preferably 8 to 9.
The supply amount of the carbon dioxide gas is controlled so as to be in the range of 9.

Further, by supplying carbon dioxide gas to the water in the reaction tank in a state of being mixed with the hydrogen gas, the contact efficiency between these gases and water is improved, and the above formula (1) is used.
The reaction of formula (2) and the bicarbonate ion generation reaction of formula (4) by blowing carbon dioxide gas proceed simultaneously, thereby realizing an efficient denitrification treatment.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a system diagram showing an embodiment of the method of the present invention. The reaction tank 10 is filled with a contact material 12 holding denitrifying bacteria, and raw water containing nitrate nitrogen is supplied to a lower part of the reaction tank 10 through a pipe 14.
The raw water rises in the reaction tank 10, undergoes a biological denitrification treatment by a denitrifying bacterium in the course of passing through the layer of the contact material 12, and is then discharged as effluent from a pipe 16.

A gas diffuser 18 for diffusing hydrogen gas or carbon dioxide gas is provided at a lower portion in the reaction tank 10. That is, the gas diffuser 18 is connected to a gas circulation path 22 connected to the gas phase section 20 of the reaction tank 10, and the mixed gas from the gas phase section 20 is blown by a blower 24 provided in the gas circulation path 22. Is repeatedly diffused into the reaction tank 10 from the diffuser 18. The gas circulation path 22 is connected to a hydrogen gas supply pipe 26 and a carbon dioxide gas supply pipe 28.

An exhaust pipe 30 is connected to the gas phase section 20 of the reaction tank 10, and a hydrogen gas monitor 32 is provided. The hydrogen gas monitor 32 detects the concentration of hydrogen gas in the mixed gas in the gas phase section 20. For example, when the concentration of hydrogen gas becomes 10% or less, a discharge valve 34 provided in the pipe 30 is provided.
Is opened, and a part of the mixed gas in the gas phase section 20 is removed from the reaction tank 10.
Operates to exhaust to the outside. When the exhaust is completed, the exhaust valve 34 is closed, and then the hydrogen gas supply line 26 is closed.
Is opened, and hydrogen gas is injected into the gas circulation path 22. The injected hydrogen gas is mixed with the mixed gas in the gas circulation path 22,
From the reaction tank 10. Due to the injection of the hydrogen gas, the concentration of the hydrogen gas in the mixed gas in the gas phase section 20 rapidly increases. Therefore, when the hydrogen gas concentration in the mixed gas reaches, for example, 50%, the supply valve 36 is closed to stop the injection of the hydrogen gas. By the above operation, 10 to 50%
Mixed gas containing a relatively high concentration of hydrogen gas in the reaction tank 10
The hydrogen gas is used as a hydrogen donor for the denitrification reaction, and the denitrification reaction proceeds efficiently.

As shown in the above equations (1) and (2), the denitrification reaction is a reaction in which hydrogen gas is consumed and nitrogen gas is generated, so that the concentration of hydrogen gas in the mixed gas changes with time. Decreases gradually. Therefore, as described above, when the hydrogen gas concentration has reached 10% or less, the evacuation operation and the subsequent hydrogen gas injection operation are repeated. Since the hydrogen gas is evacuated to 10% or less, most of the injected hydrogen gas can be used for the denitrification reaction, and the hydrogen gas can be effectively used.

As shown in the above formulas (1) and (2), the denitrification reaction consumes 5 mol or 3 mol of hydrogen gas and generates 1 mol of nitrogen gas. With the progress of the nitrogen reaction, the absolute amount of the mixed gas in the gas phase section 20 which is a closed system decreases, and the pressure in the gas phase section 20 decreases.
Therefore, the pressure in the gas phase section 20 tends to be unstable only by controlling the exhaust gas and the hydrogen gas injection by the hydrogen gas monitor 32. For this reason, it is preferable that a pressure sensor is disposed in the gas phase section 20 in conjunction with the above control, and control is performed such that the pressure in the gas phase section 20 is maintained within a predetermined range.

Next, the operation of injecting carbon dioxide gas will be described. As described above, hydroxyl ions are generated by the denitrification reaction, and the pH of the water in the reaction tank 10 rises. The injection of carbon dioxide gas is performed for the purpose of suppressing the increase in pH. That is, the pH monitor 3 is connected to the line 16 of the effluent.
When the pH value of the effluent passing through the pipe 16 becomes 9 or more, the supply valve 40 provided in the pipe 28 for supplying carbon dioxide gas is opened, and the carbon dioxide gas is supplied to the gas circulation path 22. inject. The injected carbon dioxide gas is mixed with the mixed gas in the gas circulation path 22 and is diffused into the reaction tank 10 from the diffusion device 18 through the blower 24. The injection of the carbon dioxide gas generates bicarbonate ions in the water in the reaction tank 10, and the pH of the water in the reaction tank 10 is reduced by the pH buffering action of the bicarbonate ions.
H gradually decreases and the pH of the effluent decreases as well. When the pH value of the effluent becomes 8 or less, the supply valve 40 is closed to stop the injection of the carbon dioxide gas.

As described above, the carbon dioxide gas joins with the mixed gas in the gas circulation path 22 and is supplied to the water in the reaction tank 10 in a state of being mixed with the hydrogen gas. The efficiency is improved, and the reactions of the above equations (1) and (2) and the reaction of generating bicarbonate ions by blowing carbon dioxide gas proceed simultaneously, so that the pH value of the water in the reaction tank 10 is almost equal to that of the effluent water. Similarly, it is maintained in the range of 8-9. According to the findings of the inventor, when the pH value is in the range of 6 to 9.5, the activity of the denitrifying bacteria does not decrease, and the denitrification treatment proceeds efficiently.

The unused portion of the injected carbon dioxide gas is mixed with hydrogen gas or nitrogen gas from the gas phase section 20 through the gas circulation path 22 again from the gas diffuser 18 to the reaction tank 10. It is diffused inside. Therefore, most of the injected carbon dioxide can be used without waste. Further, even when the injection of carbon dioxide gas is stopped when the pH value of the effluent water becomes 8 or less as described above, the carbon dioxide gas remaining in the gas phase portion 20 contributes to the bicarbonate ion generation reaction. Therefore, the pH value of the effluent does not rise sharply.

In the above description, the method of intermittently supplying the carbon dioxide gas based on the pH value of the effluent has been described. However, when the concentration of nitrate nitrogen in the raw water is constantly high, the set amount serving as the base load is set. It is preferable to inject carbon dioxide gas continuously, and to increase the injection amount when the pH value of the effluent temporarily rises. In this case, if the frequency of the increase and decrease of the injection amount is detected and the control for changing the set amount is performed so that the frequency of the increase and decrease becomes appropriate, the processing can be further stabilized. Further, although the pH of the effluent was used as an index for controlling the supply of carbon dioxide, the supply of carbon dioxide may be controlled based on, for example, the pH of water in the center of the reaction tank 10. . Further, as another method of controlling the supply of carbon dioxide, based on the idea that the required amount of carbon dioxide to be injected is determined in accordance with the consumption of hydrogen gas, a certain ratio of carbon dioxide to hydrogen gas during the supply of hydrogen gas is used. A gas may be supplied. According to such a method, the control system can be simplified.

Further, in the above-described embodiment, as a means for holding denitrifying bacteria in the reaction tank 10, the contact material 12 is filled in the reaction tank 10, and the denitrifying bacteria are held in the contact material 12. . However, the present invention is not limited to this. For example, well-known means for maintaining the denitrifying bacteria in the reaction tank 10 at a predetermined concentration in a floating state may be used.

[0023]

[Preliminary Experimental Examples] Tap water or tap water to which sodium hydrogen carbonate was added at various concentrations as a pH buffer was used as a sample liquid. After adjusting the pH to 10.5 by adding sodium hydroxide to each of these sample solutions, titration was performed until nitric acid was added and the pH of the sample solution reached 7.0. The result is shown in FIG.

In FIG. 2, the abscissa indicates the initial M-alkalinity of each sample solution, and the ordinate indicates the titer of nitric acid described above in terms of nitrate nitrogen (NO ₃ ⁻ ) concentration. The converted nitrate nitrogen concentration substantially corresponds to the nitrate nitrogen concentration that can be removed when the raw water having a pH of 7.0 is advanced to a pH of 10.5 by the denitrification reaction of the above formula (1). Guessed. From this figure, when about 15 mg / L of nitrate nitrogen is removed from raw water containing about 20 mg-CaCO ₃ / L of M-alkalinity, which is about the same as ordinary tap water, the pH of the raw water falls from 7.0 to 10.5. It is estimated that the activity increases and the activity of the denitrifying bacteria starts to decrease. Further, when the M-alkalinity of the raw water is set to about 80 mg-CaCO ₃ / L, the pH of the raw water becomes 7.0 to 1 even if nitrate nitrogen is removed at about 20 mg / L.
It does not increase by 0.5, and it is estimated that the activity of the denitrifying bacteria does not decrease. From the above preliminary experiments, the effect of carbon dioxide gas injection according to the present invention was indirectly supported.

[0025]

[Experimental example] A contact material (material: anthracite) holding denitrifying bacteria was filled into a reaction tank (effective volume 20 L) at a filling rate of 50%, and raw water having a nitrate nitrogen concentration of 20 mg / L was supplied at a flow rate of 2%.
Water was continuously passed at 0 L / h (nitrate nitrogen load: 20 mg / L · h). In addition, hydrogen gas was intermittently supplied to the reaction tank from the hydrogen tank during continuous water supply, and the concentration of hydrogen gas in the mixed gas circulated to the reaction tank was adjusted to be 10 to 50%. After the operation became stable, no carbon dioxide gas was supplied to the reaction tank until the first 47 days of water flow, and from the second half day 48, the intermittent supply of the hydrogen gas was 5% of the supply amount of the hydrogen gas. An amount of carbon dioxide was supplied.

FIG. 3 shows the results of the experiment. As is clear from FIG. 3, in the first half when the carbon dioxide gas was not supplied, the pH was about 7
The pH of the treated water increased to 10 or more with respect to the raw water, and the removal rate of nitrate nitrogen was around 70%. On the other hand, in the latter half of the supply of carbon dioxide, the pH of the treated water was 8 or less with respect to the raw water having a pH of about 7, and the nitrate nitrogen removal rate showed a high value of 95% or more. Incidentally, M- alkalinity of treated water, in the first half is 10~20mg-CaCO ₃ / L, in the second half was 90~120mg-CaCO ₃ / L.

[0027]

According to the present invention, there is provided a method of supplying water containing nitrate nitrogen to a reaction tank holding denitrifying bacteria and performing a biological denitrification treatment using hydrogen gas. Since the carbon dioxide gas is supplied intermittently or continuously, hydrogen gas that does not increase in pH due to the denitrification reaction and does not lower the denitrification treatment performance even for raw water with a high concentration of nitrate nitrogen A water-based method for biological denitrification of water can be provided.

[Brief description of the drawings]

FIG. 1 is an apparatus system diagram showing an embodiment of the present invention.

FIG. 2 is a graph showing the results of a preliminary experimental example according to the present invention.

FIG. 3 is a graph showing the results of an experimental example of continuous water flow according to the present invention.

[Explanation of symbols]

 10 reaction tank 12 contact material 18 air diffuser 20 gas phase part 22 circulation path 32 hydrogen gas monitor 38 pH monitor

Claims (3)

[Claims] 1. A method for supplying water containing nitrate nitrogen to a reaction tank holding denitrifying bacteria and performing a biological denitrification treatment using hydrogen gas, wherein carbon dioxide gas is added to the water in the reaction tank. A method for biological denitrification of water, characterized in that it is supplied intermittently or continuously. 2. The pH value of the effluent from the reaction tank is 7-9.
The method for biologically denitrifying water according to claim 1, wherein the supply amount of the carbon dioxide gas is controlled so as to fall within the range of. 3. The method for biological denitrification of water according to claim 1, wherein the carbon dioxide gas is supplied to the water in the reaction tank in a state of being mixed with the hydrogen gas.