JPH11216496A - Device for removing oxidized nitrogen - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a device for removing the nitrate nitrogen and nitrite nitrogen dissolved in water. SOLUTION: An electrolytic denitrification tank 6 is divided by a hydrogen ion-permeable membrane 12 into the cathode compartment contg. a cathode 11 and the anode compartment 14 contg. an anode 10. The cathode compartment is divided by a water-permeable cathode plate 11 into a cathodic reaction chamber 15a and a denitrification chamber 15b. Plate-shaped protrusions 8 and 7 are alternately arranged respectively in the cathodic reaction chamber 15a and denitrification chamber 15b to narrow the pH distribution in the cathode compartment.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for reducing and removing oxidized nitrogen, such as nitrate nitrogen and nitrite nitrogen, dissolved in water using a hydrogen oxidizing denitrifying bacterium.

[0002]

2. Description of the Related Art Pollution by oxidized nitrogen, such as nitrate nitrogen and nitrite nitrogen, of treated water used as domestic water is becoming serious, and the development of these purification means is urgently required. As a method to remove nitrate nitrogen and nitrite nitrogen,
There is a biological denitrification method using a hydrogen oxidizing bacterium which is one of the autotrophic bacteria.
-39095, JP-A-8-224598 and JP-A-9-224598
-75996.

[0003] Japanese Patent Publication No. 6-104230 discloses fixing of hydrogen oxidizing and denitrifying bacteria on the surface of a cathode for electrolysis of water. Japanese Patent Application Laid-Open No. H8-224598 discloses that oxygen generated at a metal anode is isolated by providing a diaphragm for transmitting electrons between the cathode plate and the anode plate. JP 9-759A
No. 96 discloses that an anolyte is supplied to a cathode chamber after a cathode and an anode are separated by a diaphragm.

[0004]

In order to prevent the denitrification reaction due to oxygen generated at the metal anode in the electrolysis of water from being hindered, a method of separating the anode and the cathode by a membrane such as an ion exchange membrane is disclosed. This causes a problem that the pH in the cathode chamber is not suitable for biological treatment. For this reason, it is necessary to add an agent such as an acid or an alkali for neutralization, and there is a problem that the use of the denitrification-treated water as domestic water is limited.

[0005] An object of the present invention is to increase the reaction rate by efficiently adjusting the pH in the fixed layer of the hydrogen oxidizing and denitrifying bacterium while minimizing the amount of the neutralizing agent used, thereby increasing the reaction rate and efficiently oxidizing. An object of the present invention is to provide an apparatus for removing oxidized nitrogen dissolved in water capable of removing nitrogen.

[0006]

In order to solve the problems of the prior art, the present invention uses the following means.

More specifically, the apparatus for removing oxidized nitrogen dissolved in water according to the present invention uses a cathode plate, an anode plate, and an electrolysis tank containing hydrogen oxidizing and denitrifying bacteria to perform the electrolysis of the water to be treated, thereby forming a cathode. In a device for generating hydrogen on the surface and biologically reducing and removing oxidized nitrogen dissolved in the water to be treated using the hydrogen by using the hydrogen, a means for mixing the water to be treated is provided on the cathode plate surface. It is characterized by the following.

[0008] By applying a voltage to the cathode and the anode in the electrolyzer, water is electrolyzed, and hydrogen and hydroxide ions are generated at the cathode, and oxygen and hydrogen ions are generated at the anode. Essentially, the amount of hydroxyl ions generated at the cathode and the amount of hydrogen ions generated at the anode are equal, and if mixed, the pH does not change. However, in order to improve the volumetric efficiency, it is desirable that the gap between the cathode plate and the anode plate in the electrolysis tank be as narrow as possible, and the flow rate of the reaction solution cannot be increased.

For this reason, the mixing of the reaction solution becomes insufficient,
In the vicinity of the cathode, the concentration of hydroxyl ions increases to make the region alkaline, and in the vicinity of the anode, the concentration of hydrogen ions increases to become acidic. The optimal pH for the denitrification reaction of hydrogen oxidizing denitrifying bacteria is about 7
８8.5, and the pH distribution inside the electrolysis tank causes a decrease in the denitrification reaction rate. Therefore, in the present invention, the pH distribution is reduced particularly by providing a mixing means on the surface of the cathode plate.

The next apparatus according to the present invention is characterized in that a projection is provided on the electrode plate surface by using a narrow space between the cathode plate and the anode plate as a mixing means. In the vicinity of the protrusion provided on the electrode plate surface, the flow speed and the flow direction of the reaction liquid change, thereby causing the liquid to mix.

When the projections provided on the electrode plate surface are made of a conductive substance, the electrolysis current is concentrated on the projections, causing a local increase in pH and hydrogen generation.

Therefore, the next apparatus according to the present invention includes:
The projection is made of a non-conductive material.

When mixing the reaction solution with the projections provided on the cathode surface, it is desirable to provide similar projections not only on the cathode surface but also on the surface facing the cathode surface. In addition, the projection provided on the cathode surface and the surface opposite to the cathode surface,
It is preferable to install them at different positions in order to perform mixing efficiently.

Therefore, the next device according to the present invention includes:
The projections provided on the cathode plate surface and the projections provided on the surface facing the cathode plate are alternately arranged.

Since the denitrification reaction by the hydrogen oxidizing and denitrifying bacterium used in the present invention is an anaerobic reaction, it is desirable to dispose a diaphragm between the anode and the cathode to block oxygen generated at the anode. It is desired that the diaphragm is made of a material that blocks oxygen and allows hydrogen ions to pass therethrough.

Therefore, the next device according to the present invention includes:
The electrolysis tank is divided into a cathode chamber containing a cathode plate and a hydrogen oxidizing and denitrifying bacterium and an anode chamber containing an anode plate by a diaphragm which is arranged between the cathode plate and the anode plate and does not allow oxygen to permeate but permeates hydrogen ions. A surface facing the cathode plate is a diaphragm surface, and water to be treated is supplied to the cathode chamber.

In the present invention, it is necessary to increase the hydrogen supply rate, that is, the electrolysis current density, in order to increase the denitrification rate per volume of the electrolysis tank. However, when the current for electrolysis is increased, a phenomenon occurs in which the activity of the hydrogen oxidizing and denitrifying bacteria decreases. In particular, in the bioelectrode method in which a biofilm of oxidizing and denitrifying bacteria is formed on the surface of the cathode plate, the denitrification activity is significantly reduced. If the denitrifying bacteria are arranged at a place other than the surface where the electrolysis is performed, the current density for electrolysis can be increased without lowering the denitrifying activity.

In view of the above, the cathode is constituted by a member having a space through which water and ions can pass, and denitrifying bacteria are fixed also on the back of the cathode. In this case, in order to allow the reaction liquid to flow also on the back surface of the cathode, it is desirable to use plate-shaped projections arranged at right angles to the flow direction of the water to be treated.
The plate-shaped protrusions are alternately provided on the surface of the cathode plate facing the diaphragm and the back surface thereof, and the water to be treated can be alternately circulated between the surface of the cathode plate facing the diaphragm and the back surface. desirable.

Therefore, the next device according to the present invention includes:
The electrolysis tank is divided into a cathode chamber containing a cathode plate and a hydrogen oxidizing and denitrifying bacterium and an anode chamber containing an anode plate by a diaphragm which is arranged between the cathode plate and the anode plate and does not allow oxygen to permeate but permeates hydrogen ions. The cathode chamber is partitioned into a cathode reaction chamber and a denitrification chamber containing hydrogen oxidizing and denitrifying bacteria by a cathode plate provided with a gap through which water and ions can pass, and the cathode chamber is separated from the diaphragm by the cathode plate. The plate-like projections are alternately arranged on the surface to be treated and the surface facing the denitrification chamber on the back side, and the water to be treated is supplied to the cathode reaction chamber. The chamber and the denitrification chamber are alternately circulated through a cathode plate.

Since the flow rate of the water to be treated in the electrolysis tank is low, merely disposing the plate-like projections on the front and back surfaces of the cathode plate simply means that the cathode reaction chamber and the denitrification do not occur. It is difficult to circulate between rooms alternately. For this reason,
Close the flow path by contacting the plate-shaped projection with the opposite surface,
It is desirable to force the water to be treated to flow alternately between the cathode reaction chamber and the denitrification chamber.

Therefore, the next device according to the present invention includes:
The projections are brought into contact with opposing surfaces so that the cathode reaction chamber and the denitrification chamber are at least 2
It is characterized by being divided into more than one pieces.

The amount of hydrogen ions generated at the anode and the amount of hydroxyl ions generated at the cathode are equal, but the amount of hydrogen ions permeating into the cathode chamber due to the permeation resistance of the ion exchange membrane installed between the two electrodes Is less than the amount of hydroxyl ions. Also,
Since the hydroxyl ion is also generated by the denitrification reaction by the hydrogen oxidizing and denitrifying bacterium, the cathode compartment is alkalized.

For this reason, it is desirable to adjust the pH of the denitrifying bacteria fixed layer to a preferable pH environment for the denitrification reaction by previously dissolving carbon dioxide gas corresponding to the shortage of hydrogen ions in the water to be treated. Furthermore, by using a member having a void portion through which water and ions can pass as the cathode plate, the amount of hydrogen oxidizing and denitrifying bacteria fixed decreases, and the effect of improving the denitrifying rate cannot be obtained. Therefore, it is desirable to dispose a denitrifying bacteria fixed layer near the cathode. Therefore, as the next apparatus according to the present invention, a carbon dioxide dissolving tank for dissolving carbon dioxide in the water to be treated is added, and the hydrogen oxidizing and denitrifying bacteria are fixed on the surface near the cathode plate of the denitrification chamber. Provided with a denitrifying bacteria fixed layer having a gap through which water can pass, introducing the treated water in which carbon dioxide gas is dissolved into the cathode reaction chamber, and flowing the treated water between the cathode reaction chamber and the denitrification chamber. Are alternately passed through the cathode plate and the denitrifying bacteria fixed layer.

In the present invention, in order to prevent alkalinization in the cathode chamber, carbon dioxide gas corresponding to an insufficient amount of hydrogen ions is previously dissolved in the water to be treated, so that the denitrifying bacteria fixed layer is brought into a pH environment favorable for the denitrification reaction. Adjust. However, if the amount of electrolysis current is large or the concentration of oxidized nitrogen in the water to be treated is high, it is necessary to dissolve a large amount of carbon dioxide gas, causing a problem that the water to be treated is strongly acidified. In order to solve this problem, when the amount of electrolytic current is large or the concentration of oxidized nitrogen in the water to be treated is high, the denitrification treatment water flowing out of the cathode chamber should be returned to the carbon dioxide gas dissolving tank and circulated. Is desirable.

Therefore, the next device according to the present invention includes:
A flow path for returning the cathode water flowing out of the denitrification chamber to the carbon dioxide gas dissolving tank is provided, and at least a portion of the cathode water is returned to the carbon dioxide gas dissolving tank.

The amount of carbon dioxide to be dissolved in the water to be treated is determined by the concentration of oxidized nitrogen in the water to be treated and the denitrifying activity of the hydrogen oxidizing and denitrifying bacteria, and the amount of electrolysis current. The amount of carbon dioxide dissolved in the water to be treated is adjusted by controlling one or both of the amount of carbon dioxide supplied to the carbon dioxide gas dissolving tank and the amount of cathode water returned using the pH as an index.

Therefore, the next apparatus of the present invention is characterized in that the pH of the water to be treated for dissolving carbon dioxide gas flowing out of the carbon dioxide dissolving tank is adjusted by controlling the supply amount of carbon dioxide gas and / or the return amount of denitrification treatment water. And

Since unreacted hydrogen is dissolved in the cathode water flowing out of the cathode chamber, it is preferable to remove the unreacted hydrogen from the denitrification-treated water while making effective use of the hydrogen.

Therefore, in the next apparatus of the present invention, a denitrification tank in which hydrogen oxidizing and denitrifying bacteria are fixed is added, and the denitrification tank is not used through at least a part of the gas and cathode water flowing out from the denitrification chamber. And reducing the oxidized nitrogen in the cathode water to nitrogen using the hydrogen.

The denitrified water flowing out of the cathode water or the denitrification tank is
It is an anaerobic state without oxygen, is weakly alkaline, and contains carbonate, bicarbonate and carbonate ions. In order to utilize the denitrification water, it is desirable to dissolve and neutralize oxygen in the water and to reduce carbonic acid, bicarbonate ions and carbonate ions as much as possible.

Therefore, in the next apparatus of the present invention, a part of the anode water flowing out of the anode chamber under aeration is added to the cathode water or the denitrification water flowing out of the denitrification tank to adjust the pH to 5.8 to 8.5. , Preferably 6.5 to 7.5 treated water.

[0032]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the apparatus for removing nitrogen oxides according to the present invention will be described in detail with reference to FIG. The water to be treated 1 is supplied to an electrolytic denitrification tank 6. The electrolytic denitrification tank 6 is made of plastic having an inner volume of 500 ml, and includes a cathode 11 to which a DC voltage is applied by a hydrogen ion permeable membrane 12, a cathode chamber 15 for accommodating hydrogen oxidizing denitrifying bacteria, and an anode chamber 14 for accommodating the anode 10. It is configured. The cathode 11 is made of a stainless steel plate, and the anode 10 is made of a platinum-plated titanium plate.
Each area is 200 cm ² . On the surface of the cathode 11, a columnar projection 8 having a diameter of 5 mm and a height of 10 mm is provided. FIG. 3 shows an outline of the installation of the protrusion 8. Cathode room 1
The hydrogen-utilizing denitrifying bacteria in 5 are present on the surface of the cathode 11 and the denitrifying bacteria layer 19 formed on the inner surface of the cathode chamber. As the hydrogen ion permeable membrane 12, a monovalent ion selective cation exchange membrane was used.

The anode 10 and the cathode 11 are connected to a DC power supply 40, and a current flows by applying a DC voltage. As a result, electrolysis of water occurs. The reactions at the cathode and anode are represented by the following equations (1) and (2).

[0034]

Embedded image Electrode reaction (cathode) 10H ₂ O + 10e ⁻ → 5H ₂ + 10OH ⁻ (1)

[0035]

Embedded image Electrode Reaction (Anode) 5H ₂ O → 5 / 2O ₂ + 10H ⁺ + 10e ⁻ (2) In the electrolysis of water, theoretically, hydroxyl ions generated at the cathode and hydrogen ions generated at the anode are generated. The amount of generation is equal, and the hydrogen ion (H ⁺ ) generated at the anode is used for the neutralization reaction of the hydroxyl ion (OH ⁻ ) generated by the formula (1).
It must be quickly moved to the cathode compartment.

On the other hand, simultaneously generated oxygen inhibits the denitrification reaction of the hydrogen oxidizing denitrifying bacterium. For this reason, a hydrogen ion permeable membrane 12 having a function of permeating hydrogen ions without passing oxygen between the cathode and the anode is arranged to transfer oxygen to the anode water 22.
At the same time, the gas is exhausted from the anode chamber 14 and the intrusion of oxygen into the cathode chamber 15 is minimized.

The hydrogen-using denitrifying bacterium present in the cathode chamber 15 uses the hydrogen generated on the surface of the cathode 11 to reduce oxidized nitrogen such as nitrate nitrogen or nitrite nitrogen to nitrogen. The denitrification reaction of nitrate nitrogen by the hydrogen oxidizing denitrifying bacterium is represented by the following equation (3).

[0038]

Embedded image Denitrification reaction (microbial layer) 2NO ₃ ⁻ + 5H ₂ → N ₂ + 4H ₂ O + 2OH ⁻ (3) The denitrification amount is proportional to the amount of electrolytic current within the range of the capacity of the hydrogen oxidizing and denitrifying bacteria. .

From equations (1) and (3), 2.5 moles of hydrogen are required to denitrify 1 mole of nitrate ions, and the amount of electricity required to generate this hydrogen by electrolysis is theoretically About 48
0000 coulombs. A denitrification reaction is performed in accordance with the amount of hydrogen generated by the electrolysis, and oxidized nitrogen dissolved in the water to be treated is reduced to nitrogen, and separated and removed as a gas. The water to be treated from which the oxidized nitrogen has been removed by the hydrogen oxidizing and denitrifying bacteria is discharged from the cathode chamber 15 as treated water 50.

[0040]

[Table 1]

Table 1 shows the effects of the present invention using the apparatus of this embodiment. The water to be treated 1 has a nitrogen concentration of 20 pp
Dechlorinated tap water to which potassium nitrate was added so as to obtain m was used. The water to be treated is supplied at a constant flow rate of 300 ml / h,
The electrolytic current was increased stepwise by 5 mA every three days to gradually increase the denitrification rate. Treated water was sampled, the concentrations of nitrate nitrogen and nitrite nitrogen were quantified, and the nitrogen removal rate was determined from the amount reduced relative to non-treated water.

When the electrolysis current was increased to 15 mA, 3
The average daily nitrogen removal rate was 1.1 mg-N / h. The concentrations of nitrate nitrogen and nitrite nitrogen in the treated water are:
They were 7.4 mg / L and 8.8 mg / L, respectively. Thereafter, as a result, when the electrolytic current was increased to 20 mA, the nitrogen removal rate decreased. Therefore, it can be seen that the denitrification performance of this example is 1.1 mg-N / h at the maximum. Since the mixing was favorably performed by the projections provided on the cathode surface, an effect of improving the denitrification performance was obtained as compared with the denitrification performance of the comparative example described below.

As a comparative example, Table 1 shows the denitrification performance when the same experiment as above was performed using an apparatus using the cathode from which the projections 8 were removed in the oxidized nitrogen removing apparatus shown in FIG. Indicated. The nitrogen removal rate of this comparative example was 1.0 mg-
N / h.

FIG. 4 shows the shape of a cathode 11 provided with a projection 8 in another embodiment of the apparatus for removing oxidized nitrogen according to the present invention. In this embodiment, the width is 15 mm, the thickness is 2 mm, and the height is 10 mm.
A mm plastic plate was used as the projection 8. Table 1 shows the denitrification performance when the cathode according to the present example was used as the cathode of the denitrification apparatus shown in FIG. Compared to the columnar projections shown in FIG. 3, the plate-like projections shown in FIG. 4 have higher mixing capacity and improved denitrification performance.

FIG. 5 shows the shape of a cathode 11 provided with a projection 8 in another embodiment of the apparatus for removing oxidized nitrogen according to the present invention. In this embodiment, width 100 mm, thickness 2 mm, height 1
A 0 mm plastic plate was used as the projection 8. Table 1 shows the denitrification performance when the cathode according to the present example was used as the cathode of the denitrification apparatus shown in FIG. The denitrification performance equivalent to that of the columnar projection shown in FIG. 3 is obtained.

FIG. 6 shows a cathode 11 and protrusions 8a and 8b in another embodiment of the apparatus for removing nitrogen oxides according to the present invention.
And the arrangement and shape of the hydrogen ion permeable membrane 12. In this embodiment, a plastic plate having a width of 100 mm, a thickness of 2 mm and a height of 10 mm was used as the projections 8a and 8b. Projection 8
“a” has one end in contact with the cathode 11 and the other end facing the hydrogen ion permeable membrane 12 at intervals of 5 mm. One end of the projection 8b is in contact with the hydrogen ion permeable membrane 12, and the other end is opposed to the cathode 11 at an interval of 5 mm. The protrusions 8a and the protrusions 8b are alternately arranged at equal intervals, and the water to be treated injected into the cathode chamber 15 is dissolved while being in contact with the ion exchange membrane 12 and the cathode 11 alternately. Things are removed.

Table 1 shows the denitrification performance when the cathode and the projection shown in FIG. 6 according to the present embodiment are used as the cathode of the denitrification apparatus shown in FIG. Compared with the plate-like projections shown in FIG. 5, the present example has higher denitrification performance, and it can be seen that the mixing ability is greater when the plate-like projections shown in FIG. 6 are used.

FIG. 7 shows the shape and arrangement of the cathode 11 and the projections 7 and 8 in another embodiment of the apparatus for removing nitrogen oxides according to the present invention. In this embodiment, the cathode 11 has a diameter of 1 mm.
A perforated stainless steel plate provided with holes was used. Width 100
mm, 2 mm thick, 7 mm high plastic plate
And 8 were used. One end of the protrusion 7 is in contact with the cathode 11, and the other end is in contact with the inner wall of the electrolytic denitrification tank 6. One end of the projection 8 contacts the cathode 11, and the other end contacts the hydrogen ion permeable membrane 12.

The projections 7 and the projections 8 are alternately arranged at equal intervals, and the water to be treated injected into the cathode chamber 15 is dissolved while alternately contacting the ion exchange membrane 12 and the cathode 11. Nitrogen oxides are removed. Table 1 shows the denitrification performance when the cathode shown in FIG. 7 according to the present embodiment is used as the cathode of the denitrification apparatus shown in FIG. Compared to the plate-like projections shown in FIG. 6, the present example has a higher denitrification performance, and it can be seen that the mixing ability is greater when the plate-like projections shown in FIG. 6 are used. .

FIG. 1 shows another embodiment of the oxidized nitrogen removing apparatus according to the present invention. Water to be treated 1 in which oxidized nitrogen such as nitrate nitrogen or nitrite nitrogen is dissolved is supplied to a carbon dioxide gas dissolving tower 2, and in contact with bubbles 3 containing carbon dioxide in the dissolving tower 2, carbon dioxide is dissolved. Dissolve. The amount of dissolved carbon dioxide in the water to be treated 1 is controlled by adjusting the flow control valve 36 by the pH controller 45 based on the detection signal of the pH electrode 41. The set value of the dissolved amount of carbon dioxide is
It is determined based on the pH of the denitrification treatment water described later. The water to be treated with dissolving carbon dioxide gas is supplied to the electrolytic denitrification tank
Supplied to The electrolytic denitrification tank 8 is made of plastic having an internal volume of 1000 ml and contains an anode plate 10 and a cathode plate 11.

The anode plate 10 and the cathode plate 11 are connected to a DC power supply 40, and a DC voltage is applied. The area of each electrode is 200 cm ² . The electrolytic denitrification tank 8 is partitioned into a cathode chamber 15 and an anode chamber 14 by a diaphragm 12 provided between both electrodes and through which hydrogen ions can pass. The cathode chamber 15 is formed of a 30 mesh stainless steel mesh through which water to be treated is passed.
2 and a denitrification chamber 15b containing a denitrifying bacteria fixed layer 13 on which hydrogen oxidizing denitrifying bacteria are fixed, on the rear side thereof.
It is divided into and.

Plate-like projections 8 and 7 are provided alternately in the cathode reaction chamber 15a and the denitrification chamber 15b, respectively. The protrusions 8 and 7 are in contact with the cathode 11 and are in contact with the hydrogen ion passage membrane 12 or the wall surface of the denitrification chamber 15b, respectively. The denitrifying bacteria fixed layer 13 is also made of a porous plastic material so that the water to be treated does not hinder passage. The method for fixing the hydrogen oxidizing and denitrifying bacteria is not limited to this example, but may be a method for forming a biofilm on the surface of a ceramic or synthetic resin porous carrier.
Either a method of embedding in a gel substance or a method of encapsulating a substance having substance permeability in microcapsules may be used.

The water to be treated with dissolving carbon dioxide gas is supplied to the cathode reaction chamber 15.
a. The injected carbon dioxide dissolved water is
Hydrogen ions permeating through the hydrogen ion passage membrane 12 are dissolved and acidified. Next, it comes into contact with the cathode plate 11 and receives supply of hydrogen and hydroxyl ions generated by the electrode reaction on the surface thereof. In water electrolysis, theoretically, the amount of hydroxyl ions generated at the cathode and the amount of hydrogen ions generated at the anode are equal, and if both are mixed, the pH should not change.

However, the permeation resistance of the hydrogen ion exchange membrane,
When other cations are mixed, the amount of hydrogen ions actually permeating through the hydrogen ion exchange membrane and flowing into the cathode chamber is reduced. Therefore, in the vicinity of the cathode plate, the concentration of the hydroxide ion increases and the pH tends to change to the alkaline side. The pH suitable for the denitrification reaction of hydrogen oxidizing and denitrifying bacteria is 7 to 8.5.
If it is out of this range, the denitrification activity is greatly reduced.

Therefore, the present invention utilizes the fact that carbon dioxide is dissolved in the water to be treated in advance, and the carbon dioxide is dissociated as shown in the following formula (4) to generate hydrogen ions and bicarbonate ions (HCO ₃ ⁻ ). To buffer changes in pH.

[0056]

Embedded image Carbon dioxide gas dissociation reaction CO ₂ + H ₂ O → H ⁺ + HCO ₃ ⁻ (4) The water to be treated for dissolving carbon dioxide gas supplied with hydrogen generated by electrolysis when passing through the cathode 11 is then While passing through the denitrifying bacteria fixed layer 13, it comes into contact with the hydrogen oxidizing denitrifying bacteria and reduces the dissolved nitrate nitrogen and oxidized nitrogen such as nitrite nitrogen to nitrogen using hydrogen. Within the range of the capacity of hydrogen oxidizing and denitrifying bacteria, the amount of denitrification is proportional to the amount of electrolytic current, and the oxidized nitrogen dissolved in the water to be treated is converted to nitrogen in accordance with the amount of hydrogen generated by electrolysis. It is reduced and separated and removed as a gas.

By the way, from the equation (3), it can be seen that hydroxyl ions are also generated by the denitrification reaction, and the reaction solution is made alkaline. Therefore, the amount of dissolved carbon dioxide in the water to be treated is desirably set to an amount sufficient to neutralize the alkalinization caused by the denitrification reaction in addition to the shortage of hydrogen ions by the ion exchange membrane 12.

The cathode water 17 from which the oxidized nitrogen has been removed in the electrolytic denitrification tank 6 is discharged from a discharge port provided above the denitrification chamber 15b. At least a part of the discharged cathode water 17 is introduced into the denitrification tank 25. The denitrification tank 25 is provided with a denitrifying bacteria fixed layer 26 in which the hydrogen oxidizing denitrifying bacteria are fixed, and the unreacted hydrogen remaining in the discharged treated water is used to convert oxidized nitrogen in the cathode water into nitrogen. To be reduced to The method of fixing the hydrogen oxidizing and denitrifying bacteria includes a method of forming a biofilm on the surface of a porous carrier made of ceramics or synthetic resin, a method of embedding in a gel-like substance, and a method of encapsulating a substance having a material permeability in microcapsules. It is preferable to use any of the encapsulation methods.

A part of the denitrification treatment water after the denitrification in the denitrification tank 25 is returned to the carbon dioxide gas dissolving tank 2. A pH electrode 42 is provided in the return channel,
It is connected to a pH controller 46. pH controller 46 is p
Based on the measurement signal of the H electrode 42, it is determined whether or not the pH of the denitrification treatment liquid is within a predetermined control range, and the return pump 33 is adjusted by feedback control to control the return amount. Note that a method of controlling the flow control valve or the pump operating time may be used for controlling the return amount. When the supply amount of the water to be treated and the amount of current for electrolysis are constant, if the amount of the denitrified water returned is increased, the inflow into the electrolytic denitrification tank 6 and the denitrification tank 25 increases, and the electrolysis and denitrification reactions are correspondingly increased. , The concentration of hydrogen ions and hydroxyl ions in the liquid decreases, and the amount of change in pH can be reduced. Therefore, it is possible to reduce the amount of dissolved carbon dioxide necessary for suppressing the pH change.

In particular, when the current density for electrolysis per electrode area is large, or when the concentration of oxidized nitrogen in the water to be treated is high, it is effective to increase the amount of denitrified water returned.
When the amount of dissolved carbon dioxide is fixed, the pH can be controlled by adjusting the return pump 33. Usually, in consideration of the water quality such as the concentration of oxidized nitrogen in the water to be treated and the amount of current for electrolysis, the pH is controlled by using both the control of the amount of dissolved carbon dioxide and the control of the amount of returned denitrified water.

A part of the water to be treated 1 is supplied to the anode chamber by a pump 32 from an inlet provided at the lower part of the anode chamber. In the anode chamber, an anode 10 of a titanium plate coated with platinum is installed, and oxygen and hydrogen ions are generated by an electrolysis reaction of water. Oxygen that inhibits the denitrification reaction is prevented from penetrating into the cathode compartment by the hydrogen ion permeable membrane 12, and stays in the anode compartment. Most of the hydrogen ions pass through the hydrogen ion passage membrane 12 due to the potential gradient and move to the cathode chamber, but a part of the hydrogen ions stays in the anode water. For this reason, the hydrogen ion concentration of the anode water increases and the pH becomes about 2.5 to 4 acidic. A discharge port is provided in the upper part of the anode chamber 14, and the acidic anode water 22 containing oxygen is discharged.

A part of the denitrification treatment water discharged from the denitrification tank 25 and a part of the anode water 22 are guided to an oxygen dissolution tank 51 and mixed. Air is injected into the oxygen dissolving tank 51, and hydrogen and carbon dioxide dissolved in the denitrification-treated water are released into the bubbles by being brought into contact with the bubbles 52 generated in the diffuser 53, and Gas exchange is performed in which oxygen is dissolved in the denitrification water. Bicarbonate ions are dissolved in the denitrification water, and the water is alkalized by removing carbon dioxide gas.

On the other hand, the anodic water 22 is acidic, and is mixed with denitrification-treated water to produce neutralized treated water 50.
Get. The pH of the treated water 50 is adjusted based on the air injection amount controlled by adjusting the air flow control valve 37 and the detection signal of the pH electrode 43 provided in the oxygen dissolving tank 51.
It is controlled by a pump 34 which is regulated by a regulator 47. The treated water 50 is neutral, preferably at a pH of 6.5.
Adjusting to ~ 7.5 is desirable.

FIG. 8 shows the effect of the present invention using the apparatus of this embodiment. As the water to be treated, the nitrogen concentration is 20 ppm
Dechlorinated tap water to which potassium nitrate was added was used. The water to be treated was supplied at a constant flow rate of 300 ml / h, and the electrolytic current was increased stepwise to gradually increase the denitrification rate. The amount of denitrified water returned to the carbon dioxide gas dissolving tower is 3
It was adjusted to 00 to 400 ml / h. As a result, operation start 3
From around the 0th day, a treatment liquid of stable water quality was obtained. The nitrite nitrogen concentration in the treated water was 2 mg / L or less, and the nitrate nitrogen concentration was 5 mg / L or less. According to this example, it was possible to obtain good treated water having a low nitrous acid concentration in the treated water.

[0065]

According to the present invention, it has become possible to obtain an oxidized nitrogen removing solution having a low nitrous acid concentration. In addition, it is possible to reduce the amount of carbon dioxide used as a neutralizing agent and to maintain an environment suitable for the denitrification reaction of the hydrogen oxidizing and denitrifying bacterium, so that nitrate nitrogen and / or nitrite nitrogen can be more efficiently used. Can be removed.

[Brief description of the drawings]

FIG. 1 is a diagram showing one embodiment of a nitrate nitrogen removal apparatus according to the present invention.

FIG. 2 is a diagram showing one embodiment of a nitrate nitrogen removal apparatus according to the present invention.

FIG. 3 is a perspective view showing an outline of a cathode and projections of an embodiment of a nitrate nitrogen removal apparatus according to the present invention.

FIG. 4 is a perspective view showing an outline of a cathode and a projection of one embodiment of the nitrate nitrogen removal apparatus according to the present invention.

FIG. 5 is a perspective view showing an outline of a cathode and projections of one embodiment of the nitrate nitrogen removal apparatus according to the present invention.

FIG. 6 is a perspective view showing an outline of a cathode and a projection of one embodiment of the nitrate nitrogen removal apparatus according to the present invention.

FIG. 7 is a perspective view showing an outline of a cathode and projections of one embodiment of the nitrate nitrogen removal apparatus according to the present invention.

FIG. 8 is a characteristic diagram showing a result of a denitrification experiment of one embodiment of the nitrate nitrogen removal apparatus according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Water to be treated, 2 ... Carbon dioxide gas dissolution tank, 3 ... Carbon dioxide gas containing bubble, 4, 53 ... Aeration tube, 6 ... Electrolytic denitrification tank, 7, 8 ... Protrusion, 10 ... Anode, 11 ... Cathode, 12 ... Hydrogen ion permeable membrane, 13, 26 ... fixed layer of denitrifying bacteria, 14 ... anode chamber, 15 ...
Cathode chamber, 15a: Cathode reaction chamber, 15b: Denitrification chamber, 17 ...
Cathode water, 22 anode water, 25 denitrification tank, 51 oxygen dissolution tank, 52 air bubbles, 40 DC power supply, 31-34 pump, 36, 37 flow control valve, 41, 42, 43 p
H electrode, 45, 46, 47 ... pH controller.

Claims (12)

[Claims] 1. An electrolysis tank containing a cathode plate, an anode plate, and a hydrogen oxidizing and denitrifying bacterium is used to electrolyze water to be treated to generate hydrogen on the cathode surface, and the hydrogen is used for the treatment. An apparatus for removing nitrogen oxides dissolved in water by biological reduction to nitrogen, wherein mixing means for water to be treated is provided on the surface of the cathode plate. 2. The apparatus according to claim 1, wherein said mixing means is a projection provided on an electrode plate surface. 3. An apparatus according to claim 2, wherein said projection is made of a non-conductive material. 4. The oxidizing device according to claim 2, wherein the projections provided on the surface of the cathode plate and the projections provided on the surface facing the cathode plate are alternately arranged. Nitrogen removal equipment. 5. The cathode according to claim 4, wherein said electrolysis tank contains a cathode plate and a hydrogen oxidizing denitrifying bacterium by a diaphragm disposed between a cathode plate and an anode plate and not permeable to oxygen but permeable to hydrogen ions. Chamber and an anode chamber for accommodating the anode plate, wherein a surface facing the cathode plate is a diaphragm surface, and water to be treated is supplied to the cathode room. Removal device. 6. The apparatus according to claim 1, wherein the electrolytic cell is provided between the cathode plate and the anode plate by a diaphragm which does not transmit oxygen but transmits hydrogen ions. A cathode chamber containing oxidative denitrifying bacteria and an anode chamber containing an anode plate, and the cathode chamber accommodates a cathode reaction chamber and hydrogen oxidizing denitrifying bacteria by a cathode plate provided with a space through which water and ions can pass. The cathode plate is provided with the plate-shaped projections alternately arranged on the surface of the cathode plate facing the diaphragm and the surface of the cathode plate facing the denitrification chamber, respectively. The water to be treated is supplied to the cathode reaction chamber, and then the water to be treated is passed through the cathode reaction chamber and the denitrification chamber alternately through the cathode plate, whereby the water to be treated is dissolved in the water. Removal device. 7. The apparatus according to claim 6, wherein said projections are respectively brought into contact with opposing surfaces to substantially divide the cathode reaction chamber and the denitrification chamber into at least two or more, respectively. Oxidized nitrogen removal equipment. 8. The apparatus according to claim 6, further comprising a carbon dioxide dissolving tank for dissolving carbon dioxide in the water to be treated, wherein said hydrogen oxidizing and denitrifying bacterium is provided on a surface near a cathode plate of said denitrification chamber. Is provided with a denitrifying bacteria fixed layer having a gap through which water can pass, and the treated water in which carbon dioxide gas is dissolved is introduced into the cathode reaction chamber, and the treated water is separated from the cathode reaction chamber by the cathode reaction chamber. A device for removing oxidized nitrogen, wherein the device is alternately circulated between a nitrogen chamber through a cathode plate and a fixed layer of denitrifying bacteria. 9. The apparatus according to claim 6, wherein a flow path for returning the cathode water flowing out of the denitrification chamber to the carbon dioxide gas dissolving tank is provided, and at least a part of the cathode water is provided. An apparatus for removing oxidized nitrogen, which is returned to a carbon dioxide gas dissolving tank. 10. The apparatus according to claim 6, wherein the pH of the water to be treated for dissolving carbon dioxide gas flowing out of the carbon dioxide dissolving step is controlled by controlling the supply amount of carbon dioxide gas and / or the amount of cathode water returned. An apparatus for removing oxidized nitrogen, wherein the apparatus is adjusted. 11. The apparatus according to claim 6, further comprising a denitrification tank in which hydrogen oxidizing and denitrifying bacteria are fixed, wherein gas and cathode water flowing out of said denitrification chamber are added to said denitrification tank. Characterized by reducing unused nitrogen in the cathode water to nitrogen by utilizing unused hydrogen through at least a part of the apparatus. 12. The apparatus according to claim 6, wherein the denitrification-treated water and a part of the anodic water flowing out of the anode chamber are mixed under aeration, and the treated water is adjusted to pH 5.8 to 5.8.
An apparatus for removing oxidized nitrogen, comprising a treated water adjusting tank for adjusting the pH to 8.5, preferably 6.5 to 7.5.