JPH07265877A - Treatment of ammonia-containing waste water - Google Patents

Abstract

PURPOSE:To enhance the efficiency in the catalytic decomposition of waste water by adding a nitrite to the ammonia-contg. waste water, bringing the waste water into contact with a metal-carrying catalyst under heating, separating and removing the generated gaseous nitrogen and then bringing the waste water into contact with a metal-carrying catalyst. CONSTITUTION:Plural fixed-bed catalystic reactors 5, 7 and 9 to treat ammonia- contg. waste water are arranged in series, nitrous acid or a nitrite is added to the waste water introduced into a raw water tank 1, and then sulfuric acid or sodium hydroxide is admixed and agitated to regulate the waste water to about pH6-7. The regulated waste water is delivered by a pump 2, preheated by a heat exchanger 3, then heated to about 130-180 deg.C by a heater 4 and introduced into a first reactor 5. The gaseous nitrogen generated by the catalytic decomposition of ammonia with a metal-carrying catalyst is extracted from a valve 6, and then treated water is sent to a second reactor 7. The generated gaseous nitrogen is extracted from a valve 8, the treated water is sent to a third reactor 9 and treated in the same way to obtain the desired treated water quality.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improved method for decomposing ammonia in wastewater containing ammonia. More specifically, the present invention is a method for decomposing and removing ammonia in wastewater using a catalyst, in which the contact state of the catalyst and the water to be treated is improved by extracting the gas generated with the progress of the reaction. The present invention relates to a good ammonia decomposition method.

[0002]

2. Description of the Related Art A considerable amount of ammonia is contained in reclaimed wastewater of a condensate desalination unit of a boiler for thermal power and nuclear power generation, wastewater of a fertilizer factory, and the like. The known nitrification denitrification method, ammonia stripping method, chlorine oxidation method, catalytic cracking method and the like are known. Among the above-mentioned methods for removing ammonia, the biological nitrifying and denitrifying method is a method in which nitrifying bacteria oxidize ammonia to nitrite or nitrate nitrogen and then denitrifying bacteria reduce it to nitrogen gas. However, this method
Since it is a microbial reaction, its decomposition activity is unstable with respect to various fluctuation factors, and it requires a large installation area and requires post-treatment of sludge. Further, the ammonia stripping method is a method of contacting with a large amount of air under alkaline conditions to diffuse ammonia into the atmosphere. However, this method is not economical because it requires a high alkali cost and needs to adsorb and concentrate the diffused ammonia again. On the other hand, the chlorine oxidation method is a method of oxidizing ammonium ions into nitrogen gas via chloramine by adding chlorine. This method requires approximately 10 times the amount of chlorine added to ammonia, and is not suitable for treating wastewater having a high ammonia concentration, and requires post-treatment of residual chlorine. In contrast to these methods, the catalytic cracking method is a treatment method having excellent characteristics such as a small installation area of the device, easy operation management, and generation of substances requiring post-treatment such as sludge and residual chlorine. Thus, for example, JP-A-4-293553 discloses a method of oxidatively decomposing ammonia by adding nitrous acid or nitrite as an oxidant to waste water containing ammonia and then heating the catalyst in the presence of a catalyst. Proposed. In this case, ammoniacal nitrogen in the water to be treated is decomposed according to the following reaction formula to generate nitrogen gas. _{^{NH 4 + + NO 2 - →}} NH 4 NO 2 → N 2 + 2H 2 O usually catalytic cracking reaction of ammonia is carried out in a fixed bed catalytic reactor, the water to be treated and a catalyst for the nitrogen gas generated here However, there is a problem that the above reaction does not proceed smoothly, and there has been a demand for a treatment method capable of always maintaining good contact between the water to be treated and the catalyst.

[0003]

DISCLOSURE OF THE INVENTION The present invention solves such a problem of the conventional method for removing ammonia by catalytically decomposing ammonia by using nitrous acid or nitrite as an oxidizing agent, and by decomposing ammonia, nitrogen is decomposed. Even if gas is generated, a method for treating wastewater containing ammonia that can maintain good treatment efficiency without disturbing the contact state of the granular solid catalyst and the water to be treated in the fixed bed catalytic reactor is provided. It was made for the purpose of providing.

[0004]

Means for Solving the Problems As a result of earnest studies on a method for reducing the influence of generated gas, the inventors of the present invention separated nitrogen gas generated during processing from the system to separate the catalyst and the water to be treated. It was found that the contact state can be kept good and the treatment effect can be improved, and the present invention has been completed based on this finding. That is, according to the present invention, (1) nitrite is added to ammonia-containing wastewater, the nitrite is brought into contact with a metal-supported catalyst under heating conditions, the generated nitrogen gas is separated and removed, and then the metal-supported catalyst is further heated under heating conditions. Disclosed is a method for treating ammonia-containing wastewater, which comprises contacting and decomposing ammonia.

The present invention will be described in detail below. In the method of the present invention, 0.5 to 2 mol, preferably 0.9 to 1.1 mol of nitrous acid or nitrite is added per 1 mol of ammonia nitrogen contained in the waste water. As the nitrite, an alkali metal salt of nitrite such as sodium nitrite and potassium nitrite can be preferably used. The amount of nitrous acid or nitrite used is 0.1 per mol of ammoniacal nitrogen.
If it is less than 5 mol, ammonia in the waste water cannot be sufficiently removed. When the amount of nitrite or nitrite used exceeds 2 mol per mol of ammoniacal nitrogen,
Nitrite ions are surely left in the waste water after removing the ammonia, which is not preferable. In the method of the present invention, the pH of the wastewater to which nitrous acid or nitrite has been added is adjusted by adding acid or alkali. As the acid used for pH adjustment, for example, sulfuric acid or hydrochloric acid can be preferably used. As the alkali used for pH adjustment, for example, alkali metal hydroxides such as sodium hydroxide and potassium hydroxide, and alkali metal carbonates such as sodium carbonate and potassium carbonate can be preferably used. In order to carry out catalytic decomposition of ammonia,
The pH is preferably 6-7.

In the method of the present invention, the pH-adjusted wastewater to which nitrous acid or nitrite is added is then sent to a fixed bed type catalytic reactor filled with a metal-supported catalyst. In this reactor, ammonia reacts with nitrite to form harmless nitrogen gas. As the metal catalyst, platinum, palladium, ruthenium, or the like can be used. As the carrier, alumina such as α-alumina and γ-alumina, silica, silica-alumina-based, titania-based metal oxide and the like can be used. The amount of metal supported is preferably 0.1 to 10% by weight. When the amount of metal supported is less than 0.1% by weight, the decomposition of ammonia tends to be insufficient. Even if the amount of metal supported exceeds 10% by weight, the decomposition of ammonia does not become easier in proportion to the increase in the amount supported. The temperature of the fixed bed type catalytic reactor is maintained at 100 to 300 ° C, more preferably 130 to 180 ° C, to carry out the reaction. If the temperature of the reactor is less than 100 ° C, decomposition of ammonia tends to be insufficient even if the contact time is lengthened, and if the temperature of the reactor exceeds 300 ° C, the reactor requires high pressure resistance. In addition, it is not preferable because it merely wastes energy unnecessarily and does not improve the decomposition efficiency of ammonia. When decomposing ammonia with a fixed bed catalytic reactor using a granular solid catalyst, the nitrogen gas generated in the treatment process reduces the contact efficiency between the catalyst and the water to be treated.
For example, when a theoretical equivalent amount of nitrite is added to water containing 2500 ppm of ammonia as nitrogen to remove ammonia, about 4 liters of nitrogen gas is generated from 1 liter of treated water in a standard state, and In the vicinity of the outlet, the volume occupied by nitrogen gas is larger than the volume occupied by the water to be treated even under pressurized conditions. In such a state, the inhibition of the contact reaction due to the generated gas cannot be ignored.

Next, one embodiment of the method of the present invention will be described with reference to the drawings. FIG. 1 is a flow sheet of an example of an apparatus for carrying out the method of the present invention. A plurality of fixed bed catalytic reactors for treating wastewater containing ammonia are arranged in series, and a valve for venting gas is provided between the reactors. After adding nitrite such as nitrous acid or sodium nitrite to the wastewater introduced into the raw water tank 1, add sulfuric acid or sodium hydroxide, etc. and stir the pH.
Adjust to 6-7. The effluent after pH adjustment by adding nitrous acid or nitrite is sent out by the pump 2 and the heat exchanger 3
After being preheated by (3), it is heated to 130 to 180 ° C. by the heater 4 and introduced to the first fixed bed type catalytic reactor 5. Nitrogen gas generated by the catalytic decomposition of ammonia is extracted from the valve 6 provided at the outlet of the first reactor, and the water to be treated is sent to the second fixed bed catalytic reactor 7. It is not preferable to separate the nitrogen gas and then return it to the same fixed bed type catalytic reactor, since this will deteriorate the quality of treated water. The nitrogen gas generated in the second fixed-bed catalytic reaction device 7 is extracted from the valve 8 provided at the outlet of the second reaction device, and the water to be treated is sent to the next fixed-bed catalytic reaction device. Repeat the same operation as necessary until the desired treated water quality is obtained,
In general, two fixed bed catalytic reactors often provide satisfactory results. The water to be treated sent out from the final fixed-bed catalytic reaction device 9 utilizes residual heat in the heat exchanger 3, and then is sent to the storage tank via the pressure regulating valve 10 to be naturally cooled. In the method of the present invention, the pH of the wastewater from which ammonia has been removed is adjusted by a known method, if necessary, and then discharged. In the method of the present invention, the total contact time required for the decomposition of ammonia is usually 3 to 100 minutes, and in most cases about 12 to 30 minutes. Liquid passing rate is usually 0.5-
20 hr ^-1, preferably advantageous range 2~8hr ^-1.
In the method of the present invention, it is appropriate that the ammonia-containing water to be treated contains ammonia as nitrogen in the range of 10 to 5000 mg / liter. Examples include recycled wastewater from salt equipment and wastewater from fertilizer plants.

[0008]

EXAMPLES Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these. Example 1 FIG. 2 is a flow sheet of the apparatus used in this example.
Waste water containing 1000 mg of ammonia nitrogen in 1 liter was put in the raw water tank 11 and sodium hydroxide was added to adjust the pH to 5.8, and then sodium nitrite equivalent to ammonia, that is, 4.93 g per liter of waste water. Of sodium nitrite was added and mixed uniformly. PH at this time
Was 6.5. This drainage is pumped to 0.5 by the pump 12.
40 ml of wt% platinum supported / TiO ₂ (particle size 3.0 mm) catalyst
Was charged and was sent to the first catalytic reaction tank 13 immersed in an oil bath at 140 ° C. at a flow rate of 240 ml / hr. After the water to be treated is cooled in the cooling tank 14, it is separated into gas and liquid in the storage tank 16, and further, to the second catalytic reaction tank 18 which is set under exactly the same conditions as the first catalytic reaction tank 13, the flow rate of 240 ml / hr is set by the pump 17. It was sent at. The water to be treated which had undergone the catalytic decomposition in the second catalytic reaction tank 18 was cooled in the cooling tank 19 and taken out from the valve 20. The concentration of ammonia nitrogen in the treated water was 43 mg / liter, and the concentration of nitrite nitrogen was 37 mg / liter. The removal rate with respect to the concentration of ammonia nitrogen in the raw water is 92% in terms of total nitrogen. Comparative Example 1 The procedure of Example 1 was repeated except that the cooling tank after the first catalytic reaction tank and the storage tank for gas-liquid separation were omitted, and the first catalytic reaction tank and the second catalytic reaction tank were directly connected. Under the same conditions as in Example 1, the drainage containing ammoniacal nitrogen was passed through. The concentration of ammonia nitrogen in the treated water was 115 mg / liter, and the concentration of nitrite nitrogen was 103 mg / liter. The removal rate with respect to the concentration of ammonia nitrogen in the raw water was 78% in terms of total nitrogen, and it can be seen that the removal rate of nitrogen is significantly reduced unless the nitrogen gas generated after the first catalytic reaction tank is separated and removed.

[0009]

Industrial Applicability By separating and removing the nitrogen gas generated in the fixed bed type catalytic reactor, the contact inhibition of the water to be treated and the catalyst is prevented, and the treatment efficiency of the catalytic decomposition of ammonia in the waste water is improved.

[Brief description of drawings]

FIG. 1 is a flow sheet of an example of an apparatus for carrying out the method of the present invention.

FIG. 2 is a flow sheet of the apparatus used in the examples.

[Explanation of symbols]

 1 Raw Water Tank 2 Pump 3 Heat Exchanger 4 Heater 5 First Fixed Bed Catalytic Reactor 6 Valve 7 Second Fixed Bed Catalytic Reactor 8 Valve 9 Last Fixed Bed Catalytic Reactor 10 Pressure Control Valve 11 Raw Water Tank 12 Pump 13 First catalytic reaction tank 14 Cooling tank 15 Valve 16 Storage tank 17 Pump 18 Second catalytic reaction tank 19 Cooling tank 20 Valve

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shuichi Sakamoto 3-4-7 Nishishinjuku, Shinjuku-ku, Tokyo Kurita Industry Co., Ltd. (72) Inventor Yasuhiko Takabayashi 3-4-7 Nishishinjuku, Shinjuku-ku, Tokyo Kurita Industry Co., Ltd. (72) Inventor Toshiji Nakahara 3-4-7 Nishishinjuku, Shinjuku-ku, Tokyo Kurita Industry Co., Ltd.

Claims (1)

[Claims] 1. A nitrite is added to wastewater containing ammonia,
A method for treating wastewater containing ammonia, which comprises contacting with a metal-supported catalyst under heating conditions, separating and removing generated nitrogen gas, and then contacting with the metal-supported catalyst under heating conditions to decompose ammonia.