A method for treating high-concentration ammonia-nitrogen wastewater

Abstract

The present application belongs to the technical field of wastewater treatment, and particularly relates to a treatment method of high-concentration ammonia-nitrogen wastewater. The treatment method comprises the following treatment steps: (1) pretreatment of high-ammonia-nitrogen wastewater: adding an acid solution to the high-ammonia-nitrogen wastewater to be treated, adjusting the pH value of the system to below 3, and then adding a regulating agent NaNO2 and uniformly mixing; (2) high-temperature hydrothermal treatment: placing the mixed wastewater pretreated in step (1) in a sealed hydrothermal reactor, heating to 120-180 DEG C for high-temperature hydrothermal reaction, cooling after the reaction is completed, obtaining treated water, and collecting the generated nitrogen. The treatment method is based on acid and sodium nitrite to regulate the hydrothermal reaction, and the method is carried out under high temperature and high pressure conditions, which can accelerate the reaction rate, shorten the treatment time, and improve the treatment efficiency. The problems of low ammonia-nitrogen treatment concentration, complex process technology, secondary pollution, etc. in the existing high-ammonia-nitrogen wastewater treatment technology are solved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of wastewater treatment, and particularly relates to a treatment method of high-concentration ammonia-nitrogen wastewater. BACKGROUND

[0002] The high-ammonia-nitrogen wastewater generated by the pharmaceutical industry has the characteristics of high ammonia-nitrogen content, high salinity, strong alkalinity, and high organic matter content. Ammonia-nitrogen mainly exists in the form of ion NH 4+ 4- and molecular NH3 in the wastewater. If not properly treated, the ammonia-nitrogen in the wastewater will be discharged into the natural water environment, accelerate the growth of algae, consume the dissolved oxygen in the water, cause serious water eutrophication, and threaten the ecological environment and human health.

[0003] Currently, the treatment methods of high-ammonia-nitrogen wastewater mainly include ion exchange method, stripping method, chemical precipitation method, and biological method. The ion exchange method is to exchange the ion NH 4+ 4- in the wastewater with the replacement ion on the ion exchange resin, so as to remove the ammonia-nitrogen. The ion exchange method is only suitable for treating medium and low concentration ammonia-nitrogen wastewater. With the increasing concentration of ammonia-nitrogen, the treatment capacity of the ion exchange resin is continuously weakened, and the exchange agent will be saturated in a short time. Therefore, the ion exchange resin has the problem of frequent regeneration, which greatly increases the economic cost and time cost of treatment. The stripping method is to adjust the pH value of the wastewater to strong alkalinity, so that the NH 4+ 4- in the wastewater is converted into free-state NH3, and then the NH3 in the water is stripped out by using hot air to achieve the purpose of removing ammonia-nitrogen. This method is easily affected by the environmental temperature, and a large amount of alkali needs to be added when treating high-ammonia-nitrogen wastewater, which has the hidden danger of secondary pollution. The chemical precipitation method is to add easily soluble chemical reagents, such as magnesium ions and phosphate ions, to the wastewater, so that they react with NH 4+ 4- in the wastewater to form a kind of difficultly soluble complex salt or precipitate, so as to achieve the purpose of removing ammonia-nitrogen. However, the addition of chemical reagents produces a large amount of sludge, which has high cost and low economic efficiency. The biological denitrification method generally refers to the process of oxidizing NH 4+ 4- in the wastewater to N2 through a series of processes such as microbial metabolism, ammonification, nitrification, and denitrification under the action of specific microorganisms in the activated sludge, so as to achieve wastewater denitrification. The process of removing ammonia-nitrogen by biological method is easily affected by environmental factors (toxic substances, temperature, etc.). When the biological activity is reduced, the nitrification and denitrification will be weakened. Most biological methods have low tolerance to high-concentration ammonia-nitrogen, and it is difficult to be directly applied to high-ammonia-nitrogen wastewater treatment. Usually, the pretreatment or combination with other technologies is needed.

[0004] In recent years, researchers have developed various methods for treating high-ammonia-nitrogen wastewater. Patent CN117865335A discloses a method for treating high-ammonia-nitrogen wastewater using a mixed algal film formed by Synechococcus S1 and green algae. Although this technology can tolerate wastewater with an ammonia-nitrogen concentration of up to 1000 mg / L, it has the problem of long algal cultivation time and hydraulic retention time. Patent CN114835265B discloses a composite microbial strain that can tolerate up to 6000 mg / L of ammonia-nitrogen, significantly improving the tolerance of biological treatment to ammonia-nitrogen concentration. However, it requires continuous supplementation of cultured bacteria solution to the biological pool during operation. In addition, when the salt content in wastewater is high, biochemical methods are not suitable for treatment, so this method has not been widely applied to the treatment of high-salt and high-ammonia-nitrogen wastewater. Patent CN108033592A discloses a method for treating high-ammonia-nitrogen wastewater using adsorption and filtration. The specific steps are as follows: (1) adding modified fly ash to high-ammonia-nitrogen wastewater and stirring to mix uniformly; (2) then heating and adjusting the pH value of the wastewater, adding crystal ferrous chloride for stirring, followed by adding chitosan acetic acid solution and ferrate for stirring and reaction; (3) filtering the high-ammonia-nitrogen wastewater treated in step (2) through a microfiltration membrane to obtain the product water. This method increases the specific surface area of modified fly ash by nearly 90 times, and the removal rate of wastewater with an ammonia-nitrogen concentration of 5000 mg / L reaches 99%. However, the process is relatively complex, multiple types of reagents are added, and there is a problem of saturated modified fly ash regeneration. Patent CN109851157B discloses a method for oxidizing high-concentration ammonia-nitrogen wastewater by combining high-concentration NaClO industrial wastewater with FeSO4 wastewater. The three types of wastewater are mixed and oxidized, then NaOH is added to the oxidized mixed wastewater for further reaction, and the wastewater is then subjected to sand filtration, dosing treatment, photocatalytic reaction, and MBR treatment to obtain the product water. This method can effectively remove high-ammonia-nitrogen wastewater and various by-products, with an ammonia-nitrogen removal rate of >98.8%, achieving the effect of simultaneous treatment of multiple types of wastewater. Although the Fe(OH)3 precipitate produced during the reaction process has a certain flocculation and sedimentation effect on wastewater, it still needs to be disposed of after accumulation and sedimentation. Therefore, there is an urgent need to develop a method that can treat high-concentration ammonia-nitrogen wastewater (NH 4+ -N concentration >10000 mg / L) with high efficiency, short cycle time, and no secondary pollutants.

[0005] Treatment of ammonia nitrogen wastewater by nitrite is a simple way, such as patent CN 1778707A and CN101648732A disclose treatment of ammonia nitrogen wastewater by nitrite under acidic and heating conditions, however, the heating conditions of the prior art are 50-60 DEG C or 70-80 DEG C, which cannot control parameters for different concentrations of high ammonia nitrogen wastewater, the reaction efficiency is low and excessive sodium nitrite is needed for treatment, although a high ammonia nitrogen removal rate is achieved, but nitrogen gas cannot be effectively produced and recovered, resulting in significant increase of nitrate nitrogen and total nitrogen content in the treated wastewater. In addition, sulfuric acid is used to adjust the acidity, which introduces sulfate impurity ions, the treatment cost is high and the burden of subsequent treatment is increased. SUMMARY

[0006] In view of the shortcomings and deficiencies of the prior art, the primary object of the present application is to provide a treatment method for high-concentration ammonia nitrogen wastewater.

[0007] The object of the present application is achieved by the following technical solutions:

[0008] A treatment method for high-concentration ammonia nitrogen wastewater, comprising the following treatment steps:

[0009] (1) Pretreatment of high ammonia nitrogen wastewater: adding an acidic solution to the high ammonia nitrogen wastewater to be treated, adjusting the pH value of the system to below 3, and then adding a regulating agent NaNO2 and mixing uniformly;

[0010] (2) High-temperature hydrothermal treatment: placing the mixed wastewater after pretreatment in step (1) in a sealed hydrothermal reactor and heating to 120-180 DEG C for high-temperature hydrothermal reaction, cooling after reaction, obtaining treated water and collecting generated nitrogen gas.

[0011] Further, the ammonia nitrogen content in the high ammonia nitrogen wastewater in step (1) is 1000-15000 mg / L; preferably, the ammonia nitrogen content is 10000-15000 mg / L, and the ammonia nitrogen composition is mainly high-salt high ammonia nitrogen wastewater of NH4Cl.

[0012] Further, the acidic solution in step (1) is nitric acid solution. The present application uses nitric acid solution to adjust the pH value of the system to be acidic, which can promote the reaction, and at the same time, does not introduce additional impurity ions, and has good treatment effect.

[0013] Further, the pH value of the system adjusted in step (1) is 0.5-3.

[0014] Further, the addition amount of the regulating agent NaNO2 in step (1) is 5-20 g / L of high ammonia nitrogen wastewater.

[0015] Further, the high-temperature hydrothermal reaction in step (2) is performed for 120-360 min; more preferably, the high-temperature hydrothermal reaction is performed at 150 DEG C for 180 min.

[0016] Further, the heating in step (2) is performed by using any one of an oil bath heating system, a steam heating system and an electric heating system.

[0017] Further, the cooling in step (2) is performed by using any one of a water cooling system, an air cooling system and an oil cooling system.

[0018] Further, the nitrogen gas is collected by using any one of a drainage method and a refrigerated air method.

[0019] Further, the ammonia nitrogen removal rate of the produced water is 92.2%-98.0%, the total nitrogen removal rate is 14.6%-90.5%, and the COD removal rate is 30.7%-63.6%.

[0020] The principle of the present application is that the ammonia nitrogen in the high-salt high-ammonia nitrogen wastewater generated in the pharmaceutical industry is mainly composed of NH4Cl, but the reaction of NH4Cl and NaNO2 is very slow at normal temperature, and the present application uses a hydrothermal technology to generate a high-temperature high-pressure reaction condition. Under the high-temperature high-pressure environment, NaNO2 and NH4Cl react violently, release a large amount of heat to maintain the reaction, and generate N2, NaCl and H2O, and the reaction products are harmless to the ecological environment. On this basis, the pH of the high-ammonia nitrogen wastewater is adjusted by adding an acidic solution to improve the reaction efficiency, thereby promoting the removal of ammonia nitrogen and the generation of nitrogen gas.

[0021] Compared with the prior art, the present application has the following beneficial effects:

[0022] (1) The present application provides a method for treating high-ammonia nitrogen wastewater by hydrothermal treatment based on acid and sodium nitrite, which is performed under high-temperature high-pressure conditions, can accelerate the reaction rate, shorten the treatment time and improve the treatment efficiency. The problems of low ammonia nitrogen treatment concentration, complex process technology and secondary pollution in the existing high-ammonia nitrogen wastewater treatment technology are solved.

[0023] (2) The present application has a significant effect on high-concentration ammonia nitrogen wastewater treatment, and can effectively cope with the challenge of wastewater treatment with high ammonia nitrogen concentration. By adjusting the reaction parameters, high-efficiency removal of high-concentration ammonia nitrogen wastewater (NH4 + -N concentration > 10000 mg / L) can be realized, and the highest removal efficiency reaches 98.0%, and TN and COD are also removed to a certain extent.

[0024] (3) The present application can be widely applied to the treatment process of high-concentration ammonia-nitrogen wastewater in the pharmaceutical, food, and chemical fertilizer industries, and is designed as a continuous reaction system, which has strong adaptability and can treat a large amount of wastewater. Meanwhile, the reaction generated in the present application is an exothermic reaction, and when the temperature in the hydrothermal reaction kettle reaches a certain reaction temperature, heating can be stopped, and the heat generated from the reaction process can maintain the progress of the reaction (with appropriate heating adjustment), which greatly reduces the operation cost and has a broad application prospect.

[0025] (4) The reaction product of the present application is mainly N2, NaCl, and H2O, without the generation of chemical precipitation, which reduces the risk of secondary pollution, and the generated N2 is recycled as a resource, which has good environmental benefits. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 The present application is a process flow chart of a high-concentration ammonia-nitrogen wastewater treatment method.

[0027] The reference signs are explained as follows: 1, high-ammonia-nitrogen wastewater storage tank; 2, stirring device; 3, heating device; 4, pressure gauge; 5, safety valve; 6, exhaust valve; 7, temperature gauge; 8, hydrothermal reactor; 9, cooling device; 10, gas collection device; 11, water production storage tank. DETAILED DESCRIPTION

[0028] The present application will be further described in detail below in combination with examples and drawings, but the embodiments of the present application are not limited thereto.

[0029] Example 1

[0030] The present application is a process flow chart of a high-concentration ammonia-nitrogen wastewater treatment method. Figure 1 as shown, comprising the following steps:

[0031] (1) Pretreatment of high-ammonia-nitrogen wastewater: 67% concentrated nitric acid solution is added to the high-ammonia-nitrogen wastewater to be treated (detailed data are shown in Table 1), the pH value of the system is adjusted to 1, and then the regulating agent NaNO2 is added and stirred to mix uniformly; the addition amount of NaNO2 is 20 g / L.

[0032] Table 1

[0033] Item Ammonia nitrogen TN COD TP pH Chloride ion Data 13800 mg / L 15600 mg / L 13240 mg / L 15 mg / L 9.15 38933 mg / L

[0034] (2) High-temperature hydrothermal treatment: the mixed wastewater after pretreatment in step (1) is placed in a sealed hydrothermal reactor and heated to 150℃ by oil bath, and hydrothermal reaction is carried out for 180 min. After the reaction is completed, the reactor is cooled by a water cooling system, and when the internal temperature of the reactor decreases to about 80℃, N2 is collected by the drainage method, and treated water is obtained.

[0035] The ammonia nitrogen concentration of the obtained water is 280 mg / L, the removal rate is 98.0%; the total nitrogen (TN) concentration is 1480 mg / L, the removal rate is 90.5%; and the COD concentration is 9180 mg / L, the removal rate is 30.7%. It is shown that the treatment method of the embodiment can efficiently remove ammonia nitrogen, and also has a certain removal effect on total nitrogen and COD.

[0036] Embodiment 2

[0037] The treatment method of the high-concentration ammonia nitrogen wastewater of the embodiment has a process flow chart as shown in Figure 1 The treatment method of the high-concentration ammonia nitrogen wastewater of the embodiment has a process flow chart as shown in

[0038] (1) Pretreatment of high ammonia nitrogen wastewater: 67% concentrated nitric acid solution is added to the high ammonia nitrogen wastewater to be treated (the detailed data is shown in Table 1), the pH value of the system is adjusted to 1.5, then the regulating agent NaNO2 is added and stirred to mix uniformly; the addition amount of NaNO2 is 17.5 g / L.

[0039] (2) High-temperature hydrothermal treatment: the mixed wastewater after pretreatment in step (1) is placed in a sealed hydrothermal reactor, heated to 160℃ by oil bath, and hydrothermal reaction is carried out for 180 min. After the reaction is completed, the reactor is cooled by using a water cooling system. When the internal temperature of the reactor is reduced to about 80℃, N2 is collected by using a drainage method, and the treated water is obtained.

[0040] The ammonia nitrogen concentration of the obtained water is 300 mg / L, the removal rate is 97.8%; the total nitrogen (TN) concentration is 3362 mg / L, the removal rate is 78.4%; and the COD concentration is 6020 mg / L, the removal rate is 54.5%. It is shown that the treatment method of the embodiment can efficiently remove ammonia nitrogen, and also has a certain removal effect on total nitrogen and COD.

[0041] Embodiment 3

[0042] The treatment method of the high-concentration ammonia nitrogen wastewater of the embodiment has a process flow chart as shown in Figure 1 The treatment method of the high-concentration ammonia nitrogen wastewater of the embodiment has a process flow chart as shown in

[0043] (1) Pretreatment of high ammonia nitrogen wastewater: 67% concentrated nitric acid solution is added to the high ammonia nitrogen wastewater to be treated (the detailed data is shown in Table 1), the pH value of the system is adjusted to 1.5, then the regulating agent NaNO2 is added and stirred to mix uniformly; the addition amount of NaNO2 is 17.5 g / L.

[0044] (2) High-temperature hydrothermal treatment: the pretreated mixed wastewater of step (1) was placed in a sealed hydrothermal reactor and heated to 150°C in an oil bath, and hydrothermal reaction was carried out for 240 min. After the reaction, the reactor was cooled by a water cooling system. When the internal temperature of the reactor dropped to about 80°C, N2 produced was collected by a drainage method, and treated water was obtained.

[0045] The ammonia nitrogen concentration of the treated water obtained in this example was 860 mg / L, and the removal rate was 93.8%. The total nitrogen concentration was 6770 mg / L, and the removal rate was 56.6%. The COD concentration was 7578 mg / L, and the removal rate was 42.8%. This shows that the treatment method of this example can efficiently remove ammonia nitrogen, and also has a certain removal effect on total nitrogen and COD.

[0046] Example 4

[0047] A high-concentration ammonia nitrogen wastewater treatment method of this example is shown in the process flow diagram as shown in Figure 1 , and includes the following steps:

[0048] (1) Pretreatment of high-ammonia nitrogen wastewater: 67% concentrated nitric acid solution was added to the high-ammonia nitrogen wastewater to be treated (detailed data are shown in Table 1), the pH value of the system was adjusted to 2.5, and then a regulating agent NaNO2 was added and stirred to mix uniformly. The addition amount of NaNO2 was 12.5 g / L.

[0049] (2) High-temperature hydrothermal treatment: the pretreated mixed wastewater of step (1) was placed in a sealed hydrothermal reactor and heated to 150°C in an oil bath, and hydrothermal reaction was carried out for 240 min. After the reaction, the reactor was cooled by a water cooling system. When the internal temperature of the reactor dropped to about 80°C, N2 produced was collected by a drainage method, and treated water was obtained.

[0050] The ammonia nitrogen concentration of the treated water obtained in this example was 730 mg / L, and the removal rate was 94.7%. The total nitrogen concentration was 11260 mg / L, and the removal rate was 27.8%. The COD concentration was 8578 mg / L, and the removal rate was 35.2%. This shows that the treatment method of this example can efficiently remove ammonia nitrogen, and also has a certain removal effect on total nitrogen and COD.

[0051] Example 5

[0052] A high-concentration ammonia nitrogen wastewater treatment method of this example is shown in the process flow diagram as shown in Figure 1 , and includes the following steps:

[0053] (1) Pretreatment of high ammonia-nitrogen wastewater: 67% nitric acid solution was added to the high ammonia-nitrogen wastewater to be treated (detailed data are shown in Table 1), the pH value of the system was adjusted to 3, and then the regulating agent NaNO2 was added and stirred to mix uniformly. The addition amount of NaNO2 was 10 g / L.

[0054] (2) High-temperature hydrothermal treatment: the mixed wastewater pretreated in step (1) was placed in a sealed hydrothermal reactor and heated to 140°C in an oil bath, and hydrothermal reaction was carried out for 150 min. After the reaction, the reactor was cooled by a water cooling system. When the internal temperature of the reactor dropped to about 80°C, N2 produced was collected by drainage method, and the treated water was obtained.

[0055] The ammonia-nitrogen concentration of the treated water obtained in this example was 10800 mg / L, and the removal rate was 21.7%. The total nitrogen concentration was 10940 mg / L, and the removal rate was 29.9%. The COD concentration was 6600 mg / L, and the removal rate was 50.2%.

[0056] Example 6

[0057] In this example, sulfuric acid solution was used instead of nitric acid solution to adjust the pH value of the system to 1, and the rest was the same as in Example 1.

[0058] The ammonia-nitrogen concentration of the treated water obtained in this example was 10800 mg / L, and the removal rate was 21.7%. The total nitrogen concentration was 10940 mg / L, and the removal rate was 29.9%. The COD concentration was 6600 mg / L, and the removal rate was 50.2%.

[0059] From the comparison results of Example 6 and Example 1, it can be seen that when sulfuric acid solution is used to adjust the pH value, the removal effect of the system on ammonia-nitrogen and total nitrogen is greatly reduced, ammonia-nitrogen cannot be effectively converted into nitrogen gas, and sulfate impurity ions are introduced, increasing the difficulty and cost of subsequent treatment.

[0060] Comparative Example 1

[0061] In this comparative example, hydrothermal treatment was carried out without adding the regulating agent NaNO2, compared with Example 1.

[0062] The ammonia-nitrogen concentration of the treated water obtained in this example was 7565 mg / L, and the removal rate was 45.2%. The total nitrogen concentration was 7620 mg / L, and the removal rate was 51.1%. The COD concentration was 5016 mg / L, and the removal rate was 62.1%.

[0063] From the comparison results of Comparative Example 1 and Example 1, it can be seen that, under the condition of not adding NaNO2, only by adjusting pH with an acidic solution and then performing hydrothermal treatment, although there is a certain treatment effect on ammonia nitrogen, total nitrogen and COD, the removal of ammonia nitrogen cannot meet the target requirements.

[0064] Comparative Example 2

[0065] This comparative example is compared with Example 1, and hydrothermal treatment is performed without adding an acidic solution.

[0066] The ammonia nitrogen concentration of the produced water in this comparative example is 11628 mg / L, and the removal rate is 15.7%; the total nitrogen concentration is 12900 mg / L, and the removal rate is 17.3%; and the COD concentration is 6020 mg / L, and the removal rate is 54.5%.

[0067] From the comparison results of Comparative Example 2 and Example 1, it can be seen that, under the condition of not adding an acidic solution, the removal effects of ammonia nitrogen and total nitrogen are extremely poor, and the removal rate is only about 15%.

[0068] Comparative Example 3

[0069] This comparative example is compared with Example 1, and high-temperature and high-pressure hydrothermal reaction is not performed in a sealed hydrothermal reactor, but the reaction is directly performed in a distillation flask at 100℃ for 180 min.

[0070] The ammonia nitrogen concentration of the produced water in this comparative example is 16531 mg / L; the total nitrogen concentration is 23560 mg / L; and the COD concentration is 10830 mg / L.

[0071] From the comparison results of Comparative Example 3 and Example 1, it can be seen that, under the condition of not using high-temperature hydrothermal conditions, the concentrations of ammonia nitrogen and total nitrogen in the produced water do not decrease but increase, which indicates that the high-temperature and high-pressure reaction environment provided by hydrothermal treatment is extremely important for treating high-ammonia-nitrogen wastewater.

[0072] The above examples are preferred embodiments of the present application, but the embodiments of the present application are not limited by the above examples, and any changes, modifications, substitutions, combinations and simplifications made without departing from the spirit and principles of the present application shall be equivalent replacement modes, and all shall be included in the protection scope of the present application.

Claims

1. A method for treating high-concentration ammonia-nitrogen wastewater, characterized by comprising the steps of, The method comprises the following processing steps: (1) Pretreatment of high ammonia-nitrogen wastewater: adding an acidic solution to the high ammonia-nitrogen wastewater to be treated to adjust the pH value of the system to below 3, and then adding a regulating agent NaNO2 and mixing uniformly; (2) High-temperature hydrothermal treatment: placing the mixed wastewater after the pretreatment in step (1) in a sealed hydrothermal reactor, heating to 120-180℃ for high-temperature hydrothermal reaction, cooling after the reaction is completed, obtaining treated water, and collecting the generated nitrogen gas; The high ammonia-nitrogen wastewater in step (1) is high-salt high ammonia-nitrogen wastewater with an ammonia-nitrogen content of 10000-15000 mg / L, and the ammonia-nitrogen composition is mainly NH4Cl; The acidic solution is nitric acid solution; adjusting the pH value of the system to below 3 means adjusting the pH value to 0.5-3; The adding amount of the regulating agent NaNO2 is 5-20 g / L of high ammonia-nitrogen wastewater.

2. The method of claim 1, wherein the high-concentration ammonia-nitrogen wastewater is treated by the method of claim 1. The time of the high-temperature hydrothermal reaction in step (2) is 120-360 min.

3. The method of claim 1, wherein the high concentration ammonia-nitrogen wastewater is treated by the method of claim 1. The heating in step (2) adopts any one of an oil bath heating system, a steam heating system and an electric heating system.

4. The method of claim 1, wherein the high concentration ammonia-nitrogen wastewater is treated by the method of claim 1. The cooling in step (2) adopts any one of a water cooling system, an air cooling system and an oil cooling system.

5. The method of claim 1, wherein the high concentration ammonia-nitrogen wastewater is treated by the method of claim 1. The collection mode of the nitrogen gas in step (2) is any one of a drainage method and a frozen air method.

6. The method of claim 1, wherein the high concentration ammonia-nitrogen wastewater is treated by the method of claim 1. The ammonia-nitrogen removal rate of the treated water in step (2) is 92.2%-98.0%, the total nitrogen removal rate is 14.6%-90.5%, and the COD removal rate is 30.7%-63.6%.

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