JP3414513B2 - Treatment method of reclaimed wastewater from condensate desalination equipment - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for decomposing ammonia in reclaimed waste water of a condensate and desalination unit. More specifically, the present invention has a low total nitrogen concentration in treated water,
The present invention relates to a novel method for treating ammonia-containing wastewater that can obtain treated water having stable water quality. [0002] Boiler condensate desalination equipment, for example, thermal power,
A large amount of ammonia is contained in the reclaimed wastewater from the condensate desalination unit of a nuclear power boiler. Methods for removing this ammonia include, for example, biological nitrification denitrification, ammonia stripping, and chlorine oxidation. Methods and catalytic cracking methods are known. Among these methods for removing ammonia, the biological nitrification denitrification method is a method in which ammonia is oxidized to nitrite or nitrate nitrogen by nitrifying bacteria and then reduced to nitrogen gas by denitrifying bacteria. However, since this method is a microbial reaction, it has disadvantages such as unstable decomposition activity against various fluctuation factors, a large installation area, and post-treatment of sludge. are doing. Also, the ammonia stripping method
This is a method in which ammonia is released into the atmosphere by contact with a large amount of air under alkaline conditions. However, this method is not economical because the alkali cost is high and it is necessary to adsorb and remove the released ammonia again.
On the other hand, the chlorine oxidation method oxidizes ammonium ions into nitrogen gas via chloramine by adding chlorine. This method requires about 10 times the amount of chlorine added to ammonia, is not suitable for wastewater treatment with a high concentration of ammonia, and requires post treatment of residual chlorine. In contrast to these methods, the catalytic cracking method is a treatment method having excellent features such as a small installation area of the apparatus, easy operation management, and generation of substances requiring post-treatment such as sludge and residual chlorine. And is attracting attention. The present inventors have focused on this catalytic cracking method, and, first, after adding nitrite as an oxidizing agent to the water to be treated containing ammonia, and then adding heat in the presence of a catalyst, ammonia is efficiently produced. Oxidative decomposition (JP-A-4-293553).
No.). However, in this case, if the nitrite is added excessively, the ammonia is removed but the nitrite ion remains in the treated water, and there is a disadvantage that the purpose cannot be sufficiently achieved in terms of denitrification. In particular, in recent years, regulations on nitrogen in wastewater have been strengthened as measures to prevent eutrophication of lakes and inland seas, and some municipalities have established low standards of 10 mg / liter or less as total nitrogen, achieving such strict standards. Therefore, in the conventional method using only nitrite as an oxidizing agent, the ammonia concentration of the water to be treated is accurately measured, and the addition amount is strictly controlled so that the nitrite corresponding to the ammonia concentration is added. Needed. For example, in the case of treating water containing 1000 mg / liter of ammonia as nitrogen, an accuracy such that the magnitude of the error is 10 mg / liter or less as nitrogen, that is, a high measurement accuracy of a relative error of 1% or less is required. I was In addition, in order to determine the amount of nitrite to be added based on the measured value of the ammonia concentration of the water to be treated, control of the amount of nitrite added to the fluctuation of the ammonia concentration of the water to be treated is at least required for analysis. There was also a problem that it was delayed by the time, and as a result, stable treated water quality could not be obtained. As a means to solve such a problem, the present inventors add nitrite and oxygen or an oxygen source material as an oxidizing agent to an ammonia-containing wastewater, and then contact the metal catalyst with heating to decompose ammonia. The method was found to be effective. However, even by this method, A part of nitrous acid is nitric acid and nitric oxide by the following _{reaction, 3HNO 2 → 2NO + HNO 3} + H 2 O The resulting nitric acid, the oxidation degradation by nitrite and oxygen or oxygen source material Therefore, there is a problem that the total nitrogen concentration in the treated water cannot be sufficiently reduced because it remains in the treated water because it cannot be removed. [0003] The present invention solves the problem of the method for removing ammonia by catalytic decomposition of ammonia using nitrite and oxygen or an oxygen source material as such an oxidizing agent. It is not necessary to measure the ammonia concentration in the treated water with high accuracy, and a method for treating ammonia-containing wastewater that can achieve treated water with a low total nitrogen concentration and stable water quality and that can reduce equipment costs. It was made for the purpose of providing. In particular, ammonia is contained in a relatively high concentration in the reclaimed wastewater of the condensate desalination apparatus, and it is desired to stably reduce the total nitrogen concentration in the treated water. Means for Solving the Problems The inventors of the present invention have conducted intensive studies to achieve the above-mentioned object, and as a result, in a method for catalytically decomposing ammonia in reclaimed wastewater of a condensate desalination apparatus, After performing catalytic cracking by adding nitrite, and further performing catalytic cracking by adding hydrogen peroxide, it was found that the object could be achieved, and based on this finding, the present invention was completed. Was. That is, the present invention provides: (1) nitrite is added to ammonia-containing wastewater discharged during regeneration of a condensate desalination apparatus, and the pH is adjusted with an acid or alkali.
After adjusting to 5.6 to 7.2 and contacting with a noble metal catalyst under heating to decompose most of the ammonia, further adding hydrogen peroxide and contacting with the noble metal catalyst to remove residual ammonia It is intended to provide a method for treating reclaimed waste water of a condensate desalination apparatus characterized by decomposition. Further, as a preferred embodiment of the present invention, (2) the nitrite is sodium nitrite, and sodium nitrite is added so that the ratio of nitrite nitrogen / ammonium nitrogen becomes 0.8 to 1.0. (3) The treatment method according to (1) or (2), wherein the noble metal-based catalyst is a catalyst in which platinum is supported on a granular carrier such as titania, γ-alumina, and the like. ) The amount of hydrogen peroxide to be added is such that the weight ratio of hydrogen peroxide / ammoniacal nitrogen to the remaining ammonia is 3.6 to
(1) to (3) in which hydrogen peroxide is added so as to obtain 10.9
The processing method described in the item can be exemplified. Hereinafter, the present invention will be described in detail. In the method of the present invention, nitrite is added to the reclaimed wastewater of the condensate desalination unit, the pH is adjusted to 5 to 8 with an acid or alkali, and then the ammonia is decomposed by contact with a noble metal catalyst under heating. Let it. The order of the addition of nitrite and the adjustment of pH are optional. The pH may be adjusted to 5 to 8 after adding nitrite, or the nitrite may be added after adjusting the pH to 5 to 8 Can also. The reclaimed wastewater from the condensate desalination device is wastewater generated when regenerating the ion exchange resin in a device for desalinating condensate using an ion exchange resin, and contains ammonia at a high concentration. As the noble metal-based catalyst used in the method of the present invention, for example, α-alumina, γ
-A carrier such as alumina, titania, activated carbon, zirconia, zeolite, glass, silica, silica alumina, ion exchange resin, or the like, which supports a noble metal such as platinum, palladium, ruthenium, rhodium, silver, or copper can be used. Particularly, a catalyst in which platinum is supported on a granular carrier such as titania or γ-alumina is preferable. The amount of the metal carried is usually about 0.1 to 10% by weight based on the carrier.
It is preferable that these catalysts are packed in a column, and the reaction is carried out by passing an ammonia-containing wastewater to which nitrite has been added under heating, and in this case, an upward flowing liquid is desirable. The nitrite used in the method of the present invention can be used without particular limitation as long as it is soluble in water and the cation does not adversely affect the environment. Examples of such a nitrite include, for example, , Lithium nitrite, sodium nitrite, potassium nitrite, magnesium nitrite, calcium nitrite, and the like, with sodium nitrite being particularly preferred. Ammonia and sodium nitrite react according to the following equation and are removed as harmless nitrogen gas. NH ₃ + NaNO ₂ → N ₂ + NaOH + H ₂ O It is preferable to add nitrite so that the ratio of nitrite nitrogen / ammonia nitrogen becomes 0.8 to 1.0. If the ratio of nitrite nitrogen / ammonium nitrogen is less than 0.8, the amount of ammonia remaining without being decomposed by nitrite increases. If the ratio of nitrite nitrogen / ammonium nitrogen exceeds 1.0, excess nitrite remains in the waste water and cannot be removed in the subsequent decomposition step with hydrogen peroxide, which is not preferable. In the method of the present invention,
Decomposition of ammonia in the ammonia-containing wastewater by reaction with nitrite is performed by adjusting the pH of the wastewater to 5 to 5 with an acid or alkali.
8, preferably by adjusting the pH to 6-7. Drainage
The pH can be adjusted using an acid such as hydrochloric acid or sulfuric acid, or an alkali such as sodium hydroxide, potassium hydroxide or sodium carbonate. If the pH of the wastewater is less than 5, a part of the nitrite ion changes to a non-removable nitrate ion, and the nitrogen concentration in the treated water is undesirably increased. If the pH of the wastewater exceeds 8, the reaction rate between ammonia and nitrite decreases, and the concentration of nitrogen in the treated water may undesirably increase. In the method of the present invention, a treatment obtained by adding a nitrite, adjusting the pH to 5 to 8 with an acid or an alkali, and removing most of the ammonia by contact with a noble metal-based catalyst under heating. Hydrogen peroxide is further added to water to make contact with the noble metal catalyst to decompose residual ammonia. Ammonia and hydrogen peroxide react according to the following equation and are removed as harmless nitrogen gas. 2NH ₃ + 3H ₂ O ₂ → N ₂ + 6H ₂ O According to this formula, the theoretical weight ratio of hydrogen peroxide / ammoniacal nitrogen is 3.6, but the amount of added hydrogen peroxide depends on the remaining ammonia. On the other hand, it is preferable to add hydrogen peroxide / ammoniacal nitrogen in a weight ratio of 3.6 to 9.9. The amount of added hydrogen peroxide was 3.% by weight of hydrogen peroxide / ammoniacal nitrogen to the remaining ammonia.
If it is less than 6, ammonia is not completely decomposed and removed, but remains, which is not preferable. Even if the added amount of hydrogen peroxide exceeds 10.9 in terms of the weight ratio of hydrogen peroxide / ammoniacal nitrogen to the remaining ammonia, the efficiency of removing the remaining ammonia is commensurate with the increase in the added amount of hydrogen peroxide. Does not improve. Next, an apparatus for carrying out the method of the present invention will be described. FIG. 1 is a schematic view of an example of an apparatus for performing the method of the present invention. First, the ammonia-containing wastewater is guided to the treated water storage tank 1, and the pH is adjusted to 5 to 8 by adding an acid or an alkali. The aqueous nitrite solution stored in the aqueous nitrite storage tank 2 is added via the pump 3a to the water to be processed sent from the water storage tank by the pump 3b. At this time, the nitrite aqueous solution was added so that the amount of nitrite nitrogen was 0.8 to 1.0 with respect to the amount of ammonia nitrogen in the water to be treated. Is preferred. The water to be treated to which the aqueous nitrite solution has been added is sent to the first catalyst tower 6 through the heat exchanger 4 and the heater 5, and most of the ammonia in the water to be treated is decomposed by the decomposition reaction in the first catalyst tower. Decomposed. The aqueous hydrogen peroxide solution stored in the aqueous hydrogen peroxide storage tank 7 is added to the treated water that has exited the first catalyst tower via the pump 3c. At this time, the aqueous hydrogen peroxide solution is preferably added so that the weight ratio of hydrogen peroxide / ammonium nitrogen is 3.6 to 9.9. The treated water to which the aqueous hydrogen peroxide solution has been added is sent to the second catalyst tower 8, and the remaining ammonia is decomposed by a decomposition reaction in the second catalyst tower. The treated water leaving the second catalyst tower passes through a heat exchanger and is discharged out of the system via a pressure regulating valve 9. In the method of the present invention, ammonia is decomposed in two stages using nitrite and hydrogen peroxide as an oxidizing agent for decomposing ammonia, and an equivalent or excess amount of the entire oxidizing agent is added to the amount of ammonia. Therefore, a high ammonia removal rate can be achieved. Moreover, when focusing only on nitrite, since it is less than the equivalent to ammonia, unreacted nitrite ions do not remain in the treated water, and it is possible to obtain treated water of good quality. . Further, when hydrogen peroxide is excessively added, the excess hydrogen peroxide is decomposed into oxygen and water as shown in the following formula, so that the quality of the treated water is not affected at all. 2H ₂ O ₂ → O ₂ + 2H ₂ O When only hydrogen peroxide is used without using nitrite as an oxidizing agent, the decomposition efficiency is poor and an equivalent of 5
If not more than twice, ammonia cannot be sufficiently decomposed. In the method of the present invention, the conditions for decomposing ammonia in the catalyst tower are as follows.
Preferably, it is selected within the range of 80 to 250 ° C., and the reaction time is usually about 3 to 120 minutes, preferably about 12 to 30 minutes. SV is 0.5 to 20 hr ^-1 , preferably 2 to 5 hr ^-1.
Is advantageous. Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples. Example 1 A test for decomposing and removing ammonia was carried out by using an apparatus shown in FIG. The quality of the waste water is ammonia nitrogen 98
It was 0 mg / liter, nitrite nitrogen 1 mg / liter or less, nitrate nitrogen 1 mg / liter or less, and pH 2.5. In the treated water storage tank, the pH was adjusted to 6.5 by adding an aqueous solution of sodium hydroxide to the wastewater and stirring the solution. The water to be treated was sent out using a pump 3b, and an aqueous solution of sodium nitrite was added from an aqueous nitrite storage tank by the pump 3a so that the ratio of nitrite nitrogen / ammonia nitrogen was 0.9. The water to be treated to which the aqueous solution of sodium nitrite was added was heated by a heat exchanger and a heater, and charged to a first catalyst column filled with 1 liter of 0.5% by weight platinum / titania catalyst (average particle size: 1.5 mm). The solution was passed at a flow rate of liter / hr, that is, SV = 3 hr ⁻¹ , and the decomposition treatment was performed at 158 to 163 ° C. The quality of the treated water flowing out of the first catalyst tower is 96 mg / l for ammonia nitrogen, 1 mg / l or less for nitrite nitrogen, 1 mg / l or less for nitrate nitrogen,
pH was 11.9. Subsequently, the hydrogen peroxide solution was added to the treated water flowing out of the first catalyst tower by the pump 3c so that the concentration of the hydrogen peroxide became 670 mg / liter, and 0.5 was added.
0.3% by weight platinum / titania catalyst (average particle size: 1.5 mm)
The solution was passed through the second catalyst column filled with 1 liter at a flow rate of 3 liter / hr, that is, SV = 10 hr ^-1 , and 158 to 163
Decomposition treatment was performed at ℃. The quality of the treated water flowing out of the second catalyst tower is 1 mg / l or less for ammonia nitrogen, 1 mg / l or less for nitrite nitrogen, 1 mg / l or less for nitrate nitrogen, and pH 11.2. It was confirmed that the total nitrogen concentration in the treated water was 3 mg / liter or less. Comparative Example 1 The same operation as in Example 1 was carried out except that the regenerated wastewater of the cation exchange resin was used in the condensate treatment unit of the thermal power plant as in Example 1 and the pH was adjusted to 2.5 without adjusting the pH of the wastewater. The exact same operation was repeated. The quality of the treated water flowing out of the first catalyst tower was 173 mg / liter of ammonia nitrogen, 1 mg / liter or less of nitrite nitrogen, 63 mg / liter or less of nitrate nitrogen, and pH 2.6. The quality of the treated water flowing out from the second catalyst tower is 6 mg / l for ammonia nitrogen, 1 mg / l or less for nitrite nitrogen, and 6 mg / l for nitrate nitrogen.
If the pH of the waste water is not adjusted to 5 to 8 since nitrate nitrogen is generated in the first catalyst column and remains in the final treated water, the total nitrogen concentration is high. It became clear that it became. Example 2 and Comparative Example 2 Content of ammonia nitrogen: 1500 mg / liter, pH
The pH value of the water to be treated is 3.6 to
After adjusting to a range of 9.0, a test for decomposing and removing ammonia was conducted to examine the relationship between the pH of the water to be treated and the total nitrogen concentration in the treated water flowing out of the second catalyst tower. FIG. 2 is a graph showing the relationship between the pH of the water to be treated and the total nitrogen concentration in the treated water. An aqueous sodium hydroxide solution was added to the water to be treated and the mixture was stirred to adjust the pH to 3.6, and then an aqueous sodium nitrite solution was added so that the concentration of nitrite nitrogen became 1350 mg / liter. The water to be treated is 0.5% by weight platinum /
The solution was passed through a first catalyst tower filled with a titania catalyst (average particle size: 1.5 mm) at SV = 2 hr ^-1 and subjected to catalytic oxidative decomposition at 160 ° C. Further, an aqueous solution of hydrogen peroxide was added to the treated water flowing out of the first catalyst tower so that the concentration of hydrogen peroxide became 730 mg / liter, and a 0.5% by weight platinum / titania catalyst (average particle size: 1.5 mm ) Is packed in the second catalyst column.
= 10 hr ^-1 , and decomposition treatment was performed at 160 ° C. Total nitrogen concentration in treated water flowing out of the second catalyst tower is 100mg
/ Liter. This value corresponds to the leftmost point in the graph of FIG. Subsequently, using the same water to be treated, adjusting the pH of the water to be treated to 3.8, and repeating the same operation as above, the total nitrogen concentration in the treated water flowing out of the second catalyst tower was 80 mg / liter. Met. Hereinafter, similarly, various
Using the water to be treated adjusted to a pH value, perform catalytic oxidative decomposition under the same conditions as above, measure the total nitrogen concentration in the treated water flowing out of the second catalyst tower, and determine the pH of the treated water and the total nitrogen in the treated water. Points indicating the relationship between the concentrations were plotted on a graph, and the points on the graph were connected by a smooth curve to obtain FIG. From FIG. 2, it is necessary to control the pH of the water to be treated in the range of 5 to 8 in order to reduce the total nitrogen concentration in the treated water to 10 mg / liter or less. It can be seen that it is necessary to control the pH of the water to be treated in the range of 5.6 to 7.2 in order to make it less than liter. PH of water to be treated
Is less than 5, the total nitrogen concentration in the treated water sharply increases as the pH decreases. This is probably because nitric acid and nitric oxide are generated from nitrous acid on the acidic side, and the nitric acid generated at this time cannot be removed by catalytic oxidative decomposition. When the pH of the water to be treated exceeds 8, the total nitrogen concentration in the treated water gradually increases as the pH increases. This is considered to be due to the fact that the reaction rate between ammonia and nitrous acid decreases under alkaline conditions. According to the method of the present invention, it is not necessary to measure the ammonia concentration in the water to be treated with high accuracy, and the control of the injection amount of the oxidizing agent can be simplified, and as a result, the cost of the apparatus can be reduced. And the oxidizing agent can be added in excess, so that the effect of fluctuations in the ammonia concentration of the water to be treated is reduced, and the total nitrogen concentration is low and treated water of stable water quality can be obtained. .

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of an example of an apparatus for performing the method of the present invention. FIG. 2 is a graph showing the relationship between the pH of the water to be treated and the total nitrogen concentration of the treated water. [Description of Signs] 1 Treated water storage tank 2 Nitrite aqueous solution storage tank 3a Pump 3b Pump 3c Pump 4 Heat exchanger 5 Heater 6 First catalyst tower 7 Hydrogen peroxide aqueous solution storage tank 8 Second catalyst tower 9 Pressure regulating valve

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Toshiji Nakahara 3-4-7 Nishi-Shinjuku, Shinjuku-ku, Tokyo Inside Kurita Water Industries Co., Ltd. (72) Inventor Yasuhiko Takabayashi 3-4-1, Nishishinjuku, Shinjuku-ku, Tokyo No. 7 Kurita Kogyo Co., Ltd. (72) Yoshihiro Endo 3-7-4 Nishi-Shinjuku, Shinjuku-ku, Tokyo, Japan No. 7 Innovator Kurita Kogyo Co., Ltd. No. 4, No. 7, Kurita Water Industries Co., Ltd. (56) References JP-A-7-328653 (JP, A) JP-A-6-99180 (JP, A) JP-A-4-367784 (JP, A) JP-A-4-293553 (JP, A) JP-A-4-190882 (JP, A) JP-A-7-8974 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C02F 1 / 72 ZAB C02F 1/58 ZAB B01J 23/42

Claims (1)

(57) [Claims] [Claim 1] Nitrite is added to ammonia-containing wastewater discharged during regeneration of a condensate desalination unit, and the pH is adjusted to 5.6 to 7.2 with an acid or alkali. And condensing most of the ammonia by contacting it with a precious metal catalyst under heating, and then adding hydrogen peroxide to contact the precious metal catalyst to decompose the remaining ammonia. A method for treating reclaimed wastewater from a desalination unit.