JP2003071461A - Method and apparatus for removing nitrogen compound in wastewater - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an apparatus for removing a nitrogen compound in wastewater, compact, easy to control and enabling the simultaneous treatment of nitrate nitrogen and ammoniacal nitrogen. SOLUTION: The apparatus for removing the nitrogen compound in wastewater has an electrolytic cell 2 applying a current across the cathode (made of copper) and anode arranged in wastewater 1 containing nitrate nitrogen and ammoniacal nitrogen to decompose nitrate nitrogen into NOx and NH3 and transferring NOx to a gas phase in a gaseous state, a preheater 9 for preheating wastewater containing NH3 and ammoniacal nitrogen after electrolysis, a stripping device 3 for bringing preheated wastewater into contact with carrier gas 8 to transfer NH3 gas to the gas phase and a catalyst tank 4 for bringing the NOx gas and NH3 gas in the gas phase into contact with a catalyst prepared by mixing a composition comprising an oxide of at least one element among Ti, W and V with silica carrying at least one noble metal among Pt, Ir and Rh supported thereon toe generate nitrogen.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to wastewater containing nitrogen compounds, such as domestic wastewater, wastewater from thermal power plants, wastewater from chemical plants, wastewater discharged from semiconductor factories, sewage, etc.
The present invention relates to a method and apparatus for removing nitrate nitrogen and ammonia nitrogen contained in human waste treatment wastewater and the like.

[0002]

2. Description of the Related Art With the establishment of environmental and wastewater standards for nitrogen (N) and phosphorus (P) since 1993,
There is an increasing need to efficiently treat wastewater containing nitrogen compounds discharged from various plants such as coke ovens and semiconductor plants, and thermal power plants. Of these, the biological treatment method has been mainly used as the treatment method for N component.
However, in the biological treatment method, since fungi are difficult to grow in factory wastewater because it uses fungi, it is difficult to control fluctuations in concentration, temperature, treatment amount, etc., and it is difficult to keep the nitrogen compound concentration at the treatment outlet below a certain level. However, since there is a problem that a vast installation area is required, development of a processing method to replace this is desired.

[0003] In contrast, as a method for removing the ammonia nitrogen in waste water, 1) after the transition of NH ₃ in the gas by means such as stripping, a method of detoxifying the nitrogen on NH ₃ decomposing catalyst (JP There is a method of concentrating using a reverse osmosis membrane, a method of decomposing with hypochlorous acid, and the like.

As a method of removing nitrate / nitrite nitrogen in waste water, there is one described in JP-A-8-155461. This method involves electrolytic reduction of wastewater containing nitrate / nitrite nitrogen at the cathode to produce N ₂ such as NO ₂ and N ₂ O.
Ox and nitrogen are produced, part of these NOx and nitrogen are removed as a gas, and the rest is further reduced to produce NH ₄ ⁺ , and when the catholyte reaches the alkaline region with the progress of electrolysis, N
It is generated by the reaction of H ₄ ⁺ → NH ₃ , and this NH ₃ is removed to the gas phase by stripping with hydrogen gas. In this method, a porous electrode having gas-liquid permeability is used as the cathode. This porous electrode is formed by supporting a catalyst such as copper and silver, or palladium or platinum on a porous base material such as carbon or stainless steel.

[0005]

However, the N in the drainage is
Not many cases contain only NH ₃ as a component,
In most cases, it contains nitrate nitrogen. However, nitrate nitrogen cannot be removed by any of the methods for removing NH ₃ described above 1) to 3), so nitrate nitrogen remains in the waste water. The regulation of N content in wastewater is to prevent eutrophication of rivers and lakes, and not only ammonia nitrogen but also nitrate nitrogen is a target substance. It does not reduce the minutes. In addition, nitric acid in the above drainage /
In the method for removing nitrite nitrogen, the cathode for electrolysis is expensive because it is composed of a porous substrate of carbon or stainless steel carrying a catalyst using a noble metal such as silver or platinum.

The object of the present invention is to provide nitrate nitrogen, or wastewater containing nitrate nitrogen and ammonia nitrogen,
It is an object of the present invention to provide a compact and inexpensive nitrogen compound removal method and apparatus with high controllability, which can efficiently treat both.

[0007]

In order to achieve the above object, the first method for removing nitrogen compounds from waste water according to the present invention is
In the wastewater containing nitrate nitrogen and ammonia nitrogen,
By installing a cathode and an anode and energizing between these electrodes, nitrate nitrogen is converted to nitrogen oxide (NOx) and ammonia.
(NH ₃ ) or ammonium ions (NH ₄ ⁺ ) and decomposes nitrogen oxides (NOx) into the gas phase in a gaseous state, and the ammonia (N
H ₃ ) or ammonium ion (NH ₄ ⁺ ), and a second step in which the ammonia gas (NH ₃ ) generated by stripping the wastewater containing ammonia nitrogen is transferred into the gas phase, and the first step Generated nitrogen oxide gas (NO
x) and ammonia gas (NH ₃ ) generated in the second step
Is selected from the group consisting of oxides of one or more elements selected from titanium (Ti), tungsten (W) and vanadium (V), and platinum (Pt), iridium (Ir) and rhodium (Rh). And a third step of converting nitrogen oxides and ammonia in the gas into nitrogen by contacting with a catalyst prepared by mixing silica having one or more noble metals supported thereon. To do.

A second method for removing nitrogen compounds in wastewater according to the present invention is a method for treating wastewater containing nitrate nitrogen, which is the same as the first method for removing nitrogen compounds in wastewater. Except for nitrogen and its treatment, the others are the same. In the first and second methods, the cathode is preferably made of copper.

The third method for removing nitrogen compounds from wastewater according to the present invention is to oxidize nitrate nitrogen by arranging a cathode and an anode in wastewater containing nitrate nitrogen and energizing between the electrodes. Substance (NOx), ammonia (NH ₃ ) or ammonium ion (NH ₄ ⁺ ) is decomposed, and nitrogen oxide (NOx) is released in a gaseous state from the wastewater into the gas phase, and ammonia (NH ₃ ) or ammonium ion ( N
In the method of removing ammonia gas by moving residual ammonia (H ₄ ⁺ ) by stripping the residual waste gas, the cathode is made of copper.

The apparatus for removing nitrogen compounds from waste water according to the present invention comprises a cathode and an anode installed in waste water containing nitrate nitrogen and ammonia nitrogen, and nitric acid is supplied by energizing the electrodes. Nitrogen oxide (NOx) and ammonia (NH ₃ ) or ammonium ion (N
H ₄ ⁺ ) and nitrogen oxides (NOx) in the form of gas are transferred into the gas phase. Electrolysis means and ammonia (NH ₃ ) or ammonium ions (NH ₃ ) produced by electrolysis.
₄ ⁺ ), and stripping means for transferring ammonia gas from the waste water containing ammonia nitrogen into the gas phase, and T
A catalyst prepared by mixing a composition comprising an oxide of at least one element selected from i, W and V and silica carrying at least one noble metal selected from Pt, Ir and Rh. ,
It is characterized by having a gas treatment means for generating nitrogen by bringing the nitrogen oxide gas and the ammonia gas that have moved into the gas phase into contact with the catalyst. In this device, the cathode is preferably made of copper.

The inventors of the present invention have mainly considered the following steps as a method for removing nitrate nitrogen and ammonia nitrogen contained as nitrate ions in waste water.

In the first step, the nitrate ions in the wastewater are converted into ammonia nitrogen and NOx by electrolysis. In this step, when a pair of electrodes is placed in water containing nitrate ions and electricity is applied, various electrolytic reactions as shown below occur.

[0013] _{^{NO 3 - + 4H + + 3e}} → NO + 2H 2 O ... (1) NO 3 - + 3H + + 2e → HNO 2 + H 2 O ... (2) NO 3 - + 10H + + 8e → NH _{^{4 + + 3H 2 O ... (}} 3) NO 3 - + 4H + + 2e → 2NO + 2H 2 O ... (4) i.e., the subject of the electrolyte thus products generated by the nitrate ions is NO and NH _3. N of these decomposition products
Ox (NO) moves to the gas phase in the gas state after decomposition, but N
H ₃ remains in the liquid. As will be described later in Table 1 etc. in the section of Examples, it is preferable to use copper for the cathode when performing this electrolysis.

In the second step, the decomposition product NH _{3 is used.}
And NH ₃ contained in the waste water in advance are transferred to the gas phase by the NH ₃ stripping device installed in the downstream.
In this step, the waste water after electrolysis is heated to, for example, 100 ° C., then flows from the upper part of the tower of the stripping device, comes into contact with, for example, water vapor as a carrier gas introduced from the lower part of the tower, and contains a high concentration of NH _3. Gas containing gas is discharged to the outside of the stripping device through the flue above the tower.

In the third step, the gas generated by electrolysis and the gas discharged from the outlet of the stripping device are treated with the catalyst installed downstream. That is, NOx and ammonia that has moved into the gas phase are brought into contact with the catalyst and converted into nitrogen. NH ₃ and NOx decompose on the catalyst according to the following reaction.

[0016]

[Chemical 1] Here, the catalyst that decomposes NOx and NH ₃ is preferably a catalyst that can reduce NOx and oxidize NH ₃ . For example, a composition comprising an oxide of at least one element selected from titanium (Ti), tungsten (W), and vanadium (V) described in JP-A-5-14663, platinum (Pt), iridium ( A catalyst prepared by mixing silica containing one or more noble metals selected from Ir) and rhodium (Rh) is suitable. Through the above steps, it becomes possible to treat both the decomposition product generated by the electrolysis of nitrate ions and NH ₃ contained in the wastewater in advance.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a device for removing a nitride compound in waste water, which is an embodiment of the present invention, will be described. Figure 1
Shows an example of an apparatus for carrying out the present invention. In this device, the waste water 1 containing nitrate compounds such as nitrate nitrogen and ammonia nitrogen is used as a water pipe 6
Is supplied to the electrolytic cell 2 as an electrolyzing means at a predetermined rate. A pair of electrodes is installed in the electrolytic cell 2, and a current is supplied from a power supply device 11 between both electrodes. Power supply 11
When a current is supplied to the electrolytic cell 2 from the above, the above-mentioned decomposition reactions (1) to (4) of nitrate nitrogen proceed in the electrolytic cell 2 and nitrate ions become NOx and NH ₃ .

Here, a method of using a pair of electrodes using copper for the cathode and passing an electric current between the electrodes can efficiently convert into NH ₃ or NOx. An electrode made of a precious metal such as platinum or a titanium electrode coated with a precious metal, which is a good conductor of electricity, is suitable for the anode. The shape of the electrode may be any shape such as a plate shape, a coil shape, or a bar shape as long as it is a structure used as a normal electrode.

The electrolytic cell 2 has an anode 1 as shown in FIG.
3 and the cathode 12 may be installed, but when the anode 13 and the cathode 12 are separated by an electrodialysis membrane or an ion exchange membrane 14 as shown in FIG. 2 (b), nitrate ions are attracted to the anode 13. This is preferable because it prevents the decomposition rate from decreasing.

The waste water discharged from the electrolytic cell 2 is treated by stripping means after adding sodium hydroxide to adjust the pH of the solution to 10 or more, that is, the waste water is introduced into the preheater 9. After being heated to, for example, 100 ° C. in the preheater 9, the stripper tower 3 flows into the stripping tower 3 from its upper portion and comes into contact with, for example, air as the carrier gas 8 introduced from the lower portion, and contains a high concentration NH ₃ -containing gas The gas is released via the flue 7 at the top of the tower. The NH ₃ -containing gas released from the stripping tower 3 is diluted to a predetermined concentration with the dilution air 10 if necessary, and then flows into the catalyst layer 4 as a downstream gas treatment means.

The waste water discharged from the stripping tower 3 is discharged to the outside of the system through the water distribution pipe 6 after adjusting the pH. On the other hand, the NOx-containing gas generated from the electrolytic cell 2 is
It is led from the upper part of the electrolytic cell 2 to the upstream of the catalyst layer 4 from the flue 7.

A catalyst is installed in the catalyst layer 4, and NH ₃ in the gas is oxidized on the catalyst and NOx is reduced to be harmless to nitrogen and discharged from the chimney 5. The shape of the catalyst may be any shape, such as a plate shape, a honeycomb shape, or a granular shape, as long as it is a shape normally used as a denitration catalyst.

What is important here is that NOx is used as a decomposition catalyst.
A catalyst having both oxidation and reduction functions of reducing hydrogen and oxidizing NH ₃ , for example, T described in JP-A-514663.
a composition comprising an oxide of one or more elements selected from i, W and V, and 1 selected from Pt, Ir and Rh
It is to use a catalyst prepared by mixing with silica carrying at least one kind of noble metal. If a noble metal-supporting oxidation catalyst that has been generally used in the past is used as the NH ₃ decomposition catalyst, a large amount of NOx and N ₂ will be generated due to the oxidation of NH _3.
O is a byproduct. In order to remove NOx, it is necessary not only to install a denitration catalyst in the latter stage,
_{Since 2} O cannot be removed, it is directly discharged from the system.

Since N ₂ O is a warming substance like CO ₂ , if it is discharged as it is, it becomes a secondary pollutant. On the other hand, the above-mentioned mixed catalyst can convert NH ₃ into nitrogen without producing NOx or N ₂ O as a by-product, so that it is possible to perform the treatment in one stage without the need for a denitration device, so that the device has a compact structure. Becomes Furthermore, N ₂ O, which is a secondary pollutant, does not flow out.

Further, in the conventional biological treatment methods, it is difficult to control fluctuations in concentration, temperature, throughput, etc., and it is difficult to keep the concentration of nitrate nitrogen at the outlet below a certain level. In the method of the present invention, fluctuations in the concentration of nitrate nitrogen and the amount of treatment can be easily controlled only by changing the amount of current required for electrolysis, and the concentration of nitrate nitrogen in the outlet wastewater can be easily controlled.

As described above, according to the apparatus (method) for removing nitrogen oxides in wastewater according to the present invention, both wastewater containing only nitrate nitrogen contains both nitrate nitrogen and ammonia nitrogen. Either wastewater can be treated.

[0027]

Examples (Examples of electrolysis and comparative examples) Example 1 50 ml of water containing 2,000 ppm of NO ₃ ⁻ was placed in a beaker, and a copper plate (purity: 1.5 × 4 cm, thickness: 3 mm) was placed in the beaker. 99%) was used as a cathode, and a platinum wire having a diameter of 0.5 mm was coiled (wire length 40 cm) was used as an anode. A constant current power supply was connected and a current of 1.8 A was applied for 15 minutes. The pH of the solution after electrolysis was measured and found to be 11.

NO in water before and after electrolysis_Three ^-And NH_Threeconcentration
With an electrophoresis device (made by Yokogawa Analytical Systems)
Measure and NO from the increase and decrease_Three ^-Decomposition rate and NH_ThreeGeneration rate
Was measured. Also, in the upper part of the beaker that is energized, acid gas
A smell was confirmed. Comparative Example 1 The same platinum wire as the anode was used as the cathode in Example 1, and the same
Electrolyze, NO_Three ^-Decomposition rate and NH_ThreeThe generation rate is measured
It was Comparative example 2 Same as Example 1 except that the cathode is 3 mmφ × 4 cm carbon rod.
NO electrolysis_Three ^-Decomposition rate and NH_ThreeMeasure generation rate
did.

Table 1 shows NO of Example 1 and Comparative Examples 1 and 2.
_{3 ^-} shows the measurement results of the decomposition rate and NH ₃ production rate. It can be seen that when platinum and carbon are used for the cathode, nitrate ions are not decomposed at all, but when copper is used for the cathode, the nitrate ions are decomposed into NH ₃ . Further, in Example 1, an acidic gas odor was confirmed during energization, and it was estimated that part of nitrate ions was NOx.

[0030]

[Table 1] From the above, when electrolysis is performed with a pair of electrodes using copper for the cathode, nitrate ions can be efficiently cathodically reduced to NOx and NH ₃ . This cathode is much cheaper than an electrode using a catalyst in which a noble metal such as platinum is supported on carbon and copper or silver is supported. (Example of Transferring Ammonia Nitrogen into Gas Phase) The liquid after the electrolysis test of Example 1 was placed in a beaker, heated on an electric heater and boiled for 3 minutes. After replenishing the water content reduced by boiling, the NH ₃ concentration in the liquid was analyzed, and it was found that the NH ₃ concentration in the liquid was zero and NH ₃ was expelled from the system by boiling.

From these results, it can be seen that NH ₃ produced by electrolysis of nitrate ions can be easily expelled (stripping) to the gas phase by heating. (Example in which NOx and NH ₃ are converted to nitrogen by a catalyst) Titanium dioxide (MCH, SO ₄ content by Ishihara Sangyo: 2
wt%), ammonium paratungstate ((N
_{H 4) 10H 10 · W 12} O 46 · 6H 2 O), were kneaded using a kneader adding ammonium metavanadate and water, after granulating the resulting paste dried, calcined for 2 hours at 550 ° C.. The obtained granules are pulverized to have a composition of Ti / W /
A first component catalyst powder having V = 91/5/4 (atomic ratio) was obtained.

On the other hand, chloroplatinic acid (H ₂ [PtCl ₆ ] .6
In H ₂ O), fine silica powder (Tomita Seiyaku, adding microcomputer F) and evaporated to dryness on a sand bath, in air, a 0.01 wt% Pt · SiO ₂ was calcined for 2 hours at 500 ° C. It prepared and obtained the catalyst powder of the 2nd component.

Silica / alumina type inorganic fibers and water were added to the first component and the second component and kneaded with a kneader to obtain a catalyst paste. The catalyst paste is placed between catalyst base materials obtained by impregnating a net-like material made of E glassy fiber with a slurry of titania, silica sol and polyvinyl alcohol and drying at 150 ° C., and passing through a rolling roller into a plate shape. A catalyst was obtained. This was air-dried for 12 hours in the air and then baked at 500 ° C. for 2 hours. The second component / first component ratio of the first component and the second component in this catalyst is 1/99, and the Pt content corresponds to 2 ppm excluding the catalyst base material and the inorganic fibers.

A gas simulating the exhaust gas after stripping the obtained catalyst was passed through a flow reactor to measure the NH ₃ decomposition rate and NO and N ₂ O generation rate of the catalyst.

[0035]

[Equation 1] As a result, NH at the reactor outlet_ThreeConcentration is 5.0ppm, N
O is 0.5 ppm, N _TwoO is 23ppm, NH_ThreeDisassembly
The rate was 99.8%.

The above examples of electrolysis, examples of migrating ammonia nitrogen such as nitrogen oxide (NOx), ammonia (NH ₃ ) or ammonium ion (NH ₄ ⁺ ) into the gas phase, and nitrogen by a catalyst By performing through the example of converting oxides and ammonia into nitrogen, the nitrate ions contained in the wastewater can be rendered harmless to nitrogen with almost no generation of by-products.

[0037]

According to the method of the present invention, simultaneous treatment of nitrate nitrogen and ammonia nitrogen, which has been difficult in the past, can be carried out by a compact and highly controllable method and apparatus.

[Brief description of drawings]

FIG. 1 is a diagram showing a device for removing nitrogen oxides from waste water according to an embodiment of the present invention.

FIG. 2 is a diagram showing an electrolytic cell that is a component of a device for removing nitrogen oxides in waste water.

[Explanation of symbols]

Wastewater containing 1 nitrogen compound 2 electrolysis tank 3 stripping equipment 4 Catalyst layer 5 chimney 6 water pipe 7 flue 8 carrier gas 9 Preheater 10 dilution air 11 power supply 12 cathode 13 Anode 14 Cation exchange membrane

Continued front page    F-term (reference) 4D037 AA11 AB12 BA23 CA04                 4D048 AA06 AA08 AB02 BA03X                       BA06X BA07X BA23X BA27X                       BA30X BA41X BA42X BB03                       CD00 EA03                 4D061 DA08 DB19 DC14 DC15 EA02                       EB01 EB04 EB12 EB19 EB30                       EB31 FA03                 4G069 AA03 AA08 BA02B BA03B                       BA04B BA14B BC54A BC54B                       BC60A BC60B BC71A BC74A                       BC75A BC75B CA02 CA04                       CA05 CA08 CA11 CA13 DA06                       EA11 FB70

Claims (6)

[Claims] 1. Containing nitrate nitrogen and ammonia nitrogen
Install a cathode and an anode in the drainage and energize between the electrodes.
By converting the nitrate nitrogen into nitrogen oxides (NO
x), and ammonia (NH_Three) Or ammonium
Ion (NH_Four ⁺), The nitrogen oxide (NO
x) in the form of a gas in the gas phase,
Ammonia generated in the first step (NH_Three) Or Ammo
Ni-ion (NH_Four ⁺) And the ammoniacal nitrogen
Anne generated by stripping wastewater containing
Monia gas (NH_ThreeSecond step of transferring) into the gas phase
And the nitrogen oxide gas generated in the first step (NO
x) and the ammonia gas generated in the second step
(NH_Three) To titanium (Ti), tungsten (W) and
Oxide of one or more elements selected from vanadium (V)
And a composition comprising platinum (Pt) and iridium (Ir)
And one or more precious metals selected from rhodium (Rh)
By contacting with a catalyst prepared by mixing supported silica,
To convert nitrogen oxides and ammonia in gas to nitrogen
3. The nitrogen compound in the wastewater, characterized in that
How to remove things. 2. A cathode and an anode are installed in a wastewater containing nitrate nitrogen, and the nitrate nitrogen is converted into nitrogen oxide (NOx) and ammonia (N) by passing an electric current between the electrodes.
H ₃ ) or ammonium ions (NH ₄ ⁺ ) and decomposes the nitrogen oxides (NOx) into the gas phase in a gaseous state, and the ammonia (NH ₃₎ generated in the first step. ) Or a second step of transferring ammonia gas (NH ₃ ) generated by stripping wastewater containing ammonium ions (NH ₄ ⁺ ) into a gas phase, and nitrogen oxide generated in the first step. (NOx) gas and ammonia (NH ₃ ) gas generated in the second step are composed of oxides of one or more elements selected from titanium (Ti), tungsten (W) and vanadium (V). Of the nitrogen oxide in the gas by contacting the catalyst with a catalyst prepared by mixing a substance and silica carrying one or more noble metals selected from platinum (Pt), iridium (Ir) and rhodium (Rh). And method for removing nitrogen compounds in the waste water, characterized in that it comprises a third step of converting the nitrogen the ammonia. 3. The method for removing nitrogen compounds in waste water according to claim 1, wherein the cathode is made of copper. 4. A cathode and a positive electrode in wastewater containing nitrate nitrogen.
The electrodes are installed and electricity is applied between the electrodes.
Acid nitrogen is used as nitrogen oxide (NOx) and ammonia (N
H_Three) Or ammonium ion (NH_Four ⁺)
The nitrogen oxide (NOx) in gaseous form from the wastewater
The gas phase is transferred to the ammonia (NH _Three) Or Anne
Monium ion (NH_Four ⁺) Strip the remaining wastewater
Ammonia generated by pinging (NH_Three)gas
Removal of nitrogen compounds from wastewater
In the removal method, the cathode is composed of copper.
A method for removing nitrogen compounds from wastewater. 5. Containing nitrate nitrogen and ammonia nitrogen
Between the electrodes provided with a cathode and an anode installed in the drainage
Nitrate nitrogen is converted to nitrogen oxides (NO
x), and ammonia (NH_Three) Or ammonium
ON (NH_Four ⁺) Is decomposed into the nitrogen oxide (NOx)
Electrolysis means for transferring gas in the gaseous phase into the gas phase, and electrolysis
The ammonia (NH_Three) Or Ammoni
Um-ion (NH_Four ⁺) And the ammoniacal nitrogen
A stream that moves ammonia gas from wastewater into the gas phase
Tipping means and titanium (Ti), tungsten (W)
And an acid of one or more elements selected from vanadium (V)
And a composition containing platinum (Pt) and iridium (I
r), one or more precious metals selected from rhodium (Rh)
A catalyst prepared by mixing silica loaded with
To the nitrogen oxide gas and the gas
Gas processing hand to generate nitrogen by contacting ammonia gas
Removal of nitrogen compounds in wastewater characterized by having steps
apparatus. 6. The device for removing nitrogen compounds in wastewater according to claim 5, wherein the cathode is made of copper.