System and method for treating ammonia-nitrogen wastewater through combination of air stripping and electrochemical oxidation

Abstract

The invention discloses a system and a method for treating ammonia-nitrogen wastewater by combining air stripping and electrochemical oxidation. The system comprises a pretreatment device, an air stripping device and an electrochemical oxidation device. The pretreatment device is used for adjusting the pH value of wastewater, so that ammonium ions are converted into free ammonia nitrogen, the stripping device is used for heating the wastewater and introducing gas to convert the ammonia nitrogen into ammonia gas to be discharged, and the electrochemical oxidation device comprises a countercurrent mixing chamber and an electrochemical reaction chamber; an electrolyte in the electrochemical reaction chamber generates absorption liquid containing hypochlorous acid and chlorine free radicals under the action of the electrode group, ammonia gas in the countercurrent mixing chamber is in reverse contact with the absorption liquid and is absorbed and oxidized into nitrogen gas, and the reacted absorption liquid flows back to the electrochemical reaction chamber for circulation. Through combination of gas-liquid separation and electrochemical oxidation, pollution caused by direct contact of an electrode and complex wastewater is avoided, efficient harmless treatment of ammonia nitrogen is realized, the problems of low treatment efficiency, easy generation of secondary pollution and the like in the prior art are solved, and the method is suitable for ammonia nitrogen wastewater treatment in industries of petrochemical engineering, food processing and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of ammonia-nitrogen wastewater treatment, and particularly relates to a system and method for treating ammonia-nitrogen wastewater by air stripping combined with electrochemical oxidation. BACKGROUND

[0002] In the past few decades, China's rapid industrialization and agricultural intensification have led to a sharp increase in the production and discharge of nitrogen-containing compounds. In the industrial fields of petroleum chemical industry, fertilizer manufacturing, food processing, pharmaceuticals, and aquaculture, raw materials used in the production process (such as synthetic ammonia, urea) and untreated wastewater carry a large amount of ammonia-nitrogen components directly or indirectly into the environment. At the same time, the excessive use of fertilizers and pesticides in agricultural activities through surface runoff and soil infiltration has increased the concentration of ammonia-nitrogen in natural water bodies, and the decomposition of nitrogen-containing organic matter in domestic wastewater during urbanization has also exacerbated this pollution trend.

[0003] Ammonia-nitrogen pollution usually occurs in the form of wastewater or exhaust gas. In wastewater, ammonia-nitrogen mainly exists in the form of free ammonia (NH3) and ammonium ion (NH4 + ), with concentrations varying greatly depending on the industry, for example, coking wastewater can reach 5-7000 mg / L, and landfill leachate can exceed 3000 mg / L. More importantly, such wastewater has a complex composition: it is often turbid and black, accompanied by a pungent ammonia odor, has poor biodegradability, contains high concentrations of salts (TDS), nitrate nitrogen, nitrite nitrogen, heavy metal ions (such as Ca 2+ ,Mg 2+ ,Fe 2+ / Fe 3+ ,Mn 2 + ,Cu 2+ ,Ni 2 +, and a variety of organic pollutants (COD). This complex matrix poses a serious challenge to subsequent treatment technologies, especially those that rely on direct contact with wastewater for reaction (such as electrochemical oxidation).

[0004] Currently, various technical combinations have been formed in the field of ammonia-nitrogen pollution control, but the mainstream processes still face systematic technical bottlenecks, making it difficult to balance high-efficiency removal, harmless decomposition, operational stability, and wide adaptability to different water qualities:

[0005] (1) Biological treatment system: Although it has cost advantages, its technical ceiling has already been reached: ① Nitrifying bacteria are sensitive to temperature, with a 70% activity reduction below 15°C; ② High salt (TDS > 15 g / L) environments cause osmotic pressure imbalance in microorganisms, leading to the collapse of nitrification; ③ Imbalance of carbon-nitrogen ratio requires external carbon source, with methanol dosage in denitrification stage reaching 3 times the COD equivalent. Monitoring data shows that the ammonia-nitrogen removal rate of wastewater treatment plants using A / O process decreases by 30%-50% in winter.

[0006] (2) Ammonia stripping method: By adjusting the pH to alkaline and assisted by high-temperature steam stripping of free ammonia, although it can quickly reduce the total nitrogen concentration in wastewater and has strong tolerance to complex water quality (high salt, heavy metals, organic matter) (the removal effect is less affected by coexisting substances), it has significant defects: the most fundamental limitation is its physical separation mechanism - only transferring ammonia nitrogen from the liquid phase to the gas phase, without realizing harmless decomposition. If the ammonia-containing exhaust gas generated during the stripping process is not equipped with a high-efficiency absorption device, it will form a cross-media pollution chain of "water → atmosphere → water". While configuring an ammonia gas absorption device (such as acid absorption), although it can capture ammonia gas, it ultimately produces high-concentration ammonia nitrogen waste liquid (such as ammonium sulfate, ammonium chloride solution), which still needs to be disposed of as hazardous waste or resourcefully utilized, essentially failing to completely solve the pollution problem and significantly increasing the operating cost and complexity.

[0007] (3) Electrochemical ammonia oxidation technology: As a new method, it has recently received widespread attention, and its principle is to directly decompose pollutants into nitrogen gas (N2) through electrode reactions, theoretically having the advantages of zero reagent addition and harmless decomposition. However, when this technology is applied in the traditional way (e.g., electrodes are directly immersed in wastewater to be treated), its actual efficiency is severely restricted by the complex matrix of wastewater, revealing two core defects: ① Coexisting ion interference and electrode pollution / failure: Ca 2+ ,Mg 2+ ions in wastewater are reduced and deposited on the cathode surface, causing electrode passivation; heavy metal ions can also poison the active coating of the electrode. These factors collectively cause rapid performance degradation and shortening of the service life of the electrode, especially the cathode. For example, the annual corrosion rate of titanium-based ruthenium-iridium electrodes in wastewater containing chlorine reaches 15%, and the replacement cycle of the equipment is shortened to 12-18 months; ② Organic matter competition and low current efficiency: coexisting organic pollutants (COD) will compete with ammonia nitrogen for oxidizing substances (such as ·OH, Cl2 / HClO) generated at the anode, significantly reducing the current efficiency of ammonia nitrogen oxidation. When COD exceeds 500 mg / L, the ammonia nitrogen removal efficiency decreases by more than 40%; ③ Conductivity limitation and energy consumption problem: many industrial wastewater (such as some aquaculture wastewater, leachate, and low-salt organic wastewater) has insufficient conductivity, resulting in excessive internal resistance in the electrochemical treatment unit, which requires high voltage to be applied. Not only does this significantly increase energy consumption (a case in a certain coking plant shows that the power consumption per ton of water reaches 25 kW·h), but high voltage also accelerates anode corrosion, further shortening the service life of the electrode; ④ Byproduct risk: in wastewater containing chlorine ions, toxic byproducts such as chlorinated organic compounds may be produced.

[0008] The core contradiction of the prior art is that "harmless decomposition ability" and "complex water quality universality" are difficult to be compatible. For example, the technology of electrochemical oxidation which can harmlessly decompose ammonia nitrogen seriously depends on wastewater quality, and coexisting substances interference leads to poor stability; and the technology of stripping which has high universality only realizes phase transfer of pollutants, and finally causes secondary pollution. This contradiction makes a single technology or simple process combination unable to simultaneously meet the needs of "high-efficiency removal, harmless decomposition and resistance to water quality fluctuation". Therefore, how to develop a new coupling process which can separate ammonia nitrogen by physical means to avoid water quality interference and completely oxidize it into nitrogen by chemical means to fundamentally solve the pollution problem is an urgent problem to be solved at present. SUMMARY

[0009] In view of the technical problems existing in the prior art, a first object of the present application is to provide a system for treating ammonia nitrogen wastewater by combining stripping with electrochemical oxidation, which breaks through the contradiction between "harmlessness" and "universality" in the prior art by the process paths of stripping and non-contact electrochemical oxidation, realizes high-efficiency removal and complete harmless decomposition of ammonia nitrogen, and provides an innovative solution for the treatment of complex water quality ammonia nitrogen wastewater.

[0010] A second object of the present application is to provide a method for treating ammonia nitrogen wastewater by combining stripping with electrochemical oxidation.

[0011] In order to achieve the above object, the present application adopts the following technical solutions:

[0012] A system for treating ammonia nitrogen wastewater by combining stripping with electrochemical oxidation comprises a pretreatment device for receiving and adjusting the pH value of ammonia nitrogen wastewater; a stripping device located downstream of the pretreatment device, wherein the ammonia nitrogen wastewater from the pretreatment device is heated and introduced into a gas to convert free ammonia nitrogen in the ammonia nitrogen wastewater into ammonia gas and separate and discharge it; and an electrochemical oxidation device located downstream of the stripping device, wherein the electrochemical device comprises a shell having a partition plate to separate the space in the shell into an up-down arranged countercurrent mixing chamber and an electrochemical reaction chamber, a plurality of communication ports are formed on the partition plate; an electrolyte and an electrode group immersed below the electrolyte level are contained in the interior of the electrochemical reaction chamber, the electrolyte undergoes electrochemical reaction under the action of constant current direct current applied by the electrode group to generate an absorption liquid containing hypochlorous acid and chlorine free radicals; the lower part of the countercurrent mixing chamber receives the ammonia gas from the upstream, the upper part thereof communicates with the electrochemical reaction chamber through a circulation pipeline to receive the absorption liquid, the ammonia gas is reversely contacted with the absorption liquid and reacts with the hypochlorous acid and chlorine free radicals in the absorption liquid to be converted into nitrogen gas, and the nitrogen gas is discharged from the upper part of the countercurrent mixing chamber; and the reacted absorption liquid flows back to the electrochemical reaction chamber through the communication ports on the partition plate.

[0013] According to an example, the electrode group comprises a plurality of anode plates and a plurality of cathode plates, which are arranged in an interleaved manner in the electrochemical reaction chamber.

[0014] According to an example, the anode plates are ruthenium iridium titanium-based coated electrode plates, and the cathode plates are pure titanium plates.

[0015] According to an example, the electrolyte is a sodium chloride electrolyte, with a concentration of 1-3M, preferably 2M.

[0016] According to an example, the counter-flow mixing chamber comprises a first air injection pipe arranged at the lower part thereof and a first spray pipe arranged at the upper part thereof, the first air injection pipe receiving the ammonia gas from the upstream, and the first spray pipe being connected to the circulation pipeline.

[0017] According to an example, a first packing layer is arranged between the first air injection pipe and the first spray pipe, and a second packing layer is arranged above the first spray pipe.

[0018] According to an example, the upstream of the pretreatment device is further provided with an ammonia-nitrogen wastewater storage device and an alkaline agent storage device connected thereto.

[0019] According to an example, the stripping device comprises a stripping zone and a heating zone arranged in a top-bottom manner, the lower part of the stripping zone being provided with a second air injection pipe and the upper part thereof being provided with a second spray pipe, the heating zone receiving the ammonia-nitrogen wastewater from the pretreatment device and being provided with a heater therein, the heater being located below the liquid level, wherein the second spray pipe receives the ammonia-nitrogen wastewater heated in the heating zone, and contacts the gas in the second air injection pipe, so as to convert the free ammonia-nitrogen in the ammonia-nitrogen wastewater into ammonia gas and discharge the ammonia gas from the upper part of the stripping zone.

[0020] According to an example, a third packing layer is arranged between the second air injection pipe and the second spray pipe, and a fourth packing layer is arranged above the second spray pipe.

[0021] The method for treating ammonia-nitrogen wastewater by using the stripping combined with electrochemical oxidation includes the following steps: delivering the ammonia-nitrogen wastewater to the pretreatment device, adjusting the pH value of the ammonia-nitrogen wastewater to be alkaline, converting the ammonium ion in the ammonia-nitrogen wastewater into free ammonia-nitrogen; delivering the pretreated ammonia-nitrogen wastewater to the heating area of the stripping device, delivering the heated ammonia-nitrogen wastewater to the stripping area, contacting the ammonia-nitrogen wastewater with the gas from the lower part of the stripping area, converting the free ammonia-nitrogen into ammonia gas and discharging the ammonia gas from the top of the stripping area; applying constant current direct current to the electrolyte through the electrode group of the electrochemical reaction chamber, making the electrolyte generate electrochemical reaction to generate the absorption liquid containing hypochlorous acid and chlorine free radicals; reversely contacting the ammonia gas with the absorption liquid through the reverse flow mixing chamber, making the ammonia gas be absorbed by the absorption liquid and converted into nitrogen gas by reacting with the hypochlorous acid and chlorine free radicals in the absorption liquid and then being discharged.

[0022] The present application has the following advantages:

[0023] The "non-contact ammonia oxidation" coupling process proposed by the present application fundamentally solves the technical bottleneck of the traditional ammonia-nitrogen wastewater treatment process by decoupling the space and reaction medium of the two key steps of "ammonia-nitrogen separation" and "harmless decomposition". In the ammonia-nitrogen separation stage, the stripping technology is used to convert the ammonia-nitrogen in the wastewater into gaseous ammonia. This step uses the strong tolerance of stripping to complex water quality to effectively deal with the interference of coexisting substances such as high salt, heavy metals and organic matter, ensuring that various high ammonia-nitrogen wastewater can realize stable and efficient ammonia-nitrogen removal. In the harmless decomposition stage, pure ammonia gas is innovatively introduced into a specially designed electrochemical reactor for oxidation, completely avoiding the direct contact of the electrochemical reactor with complex wastewater matrix. By accurately controlling the reaction conditions such as electrolyte composition, concentration, pH, temperature, etc., not only the best electrochemical environment for ammonia oxidation is created, but also the problems of pollution, poisoning and scaling of the electrode by coexisting ions and organic matter in the wastewater and the problem of competitive consumption of oxidizing substances are completely eliminated. Therefore, this process can realize efficient and high-selectivity conversion of ammonia gas to nitrogen gas, the current efficiency is close to the theoretical maximum, the energy consumption is significantly reduced, the electrode works in a clean environment, the service life is greatly extended, and the system operation stability is extremely high.

[0024] Compared with the prior art, the present application solves the contradiction between "harmless decomposition" and "universality". The stripping step overcomes the complexity of water quality and ensures the universality of the process; the non-contact electrochemical oxidation realizes efficient harmless decomposition, and the two complement each other. The process also eliminates the problem of secondary pollution, and the ammonia gas stripped is directly converted into harmless nitrogen gas for emission, avoiding the disadvantages of traditional stripping processes that produce ammonia-containing waste gas or high-concentration ammonia-nitrogen waste liquid. At the same time, since the non-contact electrochemistry operates under ideal conditions, its current efficiency is high, electrode loss is small, and energy consumption is much lower than that of traditional electrochemical systems for direct wastewater treatment and traditional stripping + absorption processes. In actual application, by optimizing the process conditions, the present application can realize continuous operation of wastewater treatment, with an ammonia-nitrogen removal rate of more than 99%, an ammonia-nitrogen concentration reduced to below 100 mg / L, a nitrogen gas conversion rate of more than 90%, and a nitrogen gas conversion rate of 100% under some working conditions, laying a solid foundation for subsequent wastewater disposal and recycling. In addition, the absorption liquid "absorption-oxidation-absorption" circulation mechanism further improves the economic efficiency and sustainability of the process, making the present application exhibit significant advancement and practicality in the field of ammonia-nitrogen wastewater treatment. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 is a process flow diagram of the stripping combined with electrochemical oxidation system for treating ammonia-nitrogen wastewater of the present application.

[0026] Figure 2 is a perspective structural schematic diagram of the electrochemical oxidation device of the present application.

[0027] Figure 3 is a perspective structural schematic diagram of the electrochemical oxidation device of the present application from another angle.

[0028] Figure 4 is a perspective structural schematic diagram of the electrochemical oxidation device of the present application with the electrode group opened.

[0029] Figure 5 is a perspective sectional view of the electrochemical oxidation device of the present application.

[0030] Figure 6 is a perspective structural schematic diagram of the electrochemical reaction chamber of the present application.

[0031] Figure 7 is a curve change diagram of the ammonia-nitrogen removal efficiency of the present application in the stripping device under different pH values of ammonia-nitrogen wastewater.

[0032] Figure 8 is a curve change diagram of the ammonia-nitrogen removal efficiency of the present application in the stripping device under different temperatures of ammonia-nitrogen wastewater.

[0033] Figure 9 is a curve change diagram of the ammonia-nitrogen removal efficiency of the present application under different current densities of the electrode group of the electrochemical oxidation device.

[0034] Wherein, 1 is a pretreatment device, 101 is a stirrer, 102 is a pH monitor, 2 is a stripping device, 2a is a stripping zone, 2b is a heating zone, 201 is a tower body, 201a is an ammonia-nitrogen wastewater inlet, 201b is an ammonia gas outlet, 202 is a second air injection pipe, 203 is a second spray pipe, 204 is a third filler layer, 205 is a fourth filler layer, 206 is a heater, 207 is a wastewater temporary storage device, 208 is a second pump body, 3 is an electrochemical oxidation device, 3a is a countercurrent mixing chamber, 3b is an electrochemical reaction chamber, 301 is a shell, 301a is an ammonia gas inlet, 301b is a nitrogen gas outlet, 301b1 is a grille, 301c is an absorption liquid outlet, 301d is an absorption liquid inlet, 301e is a liquid discharge port, 301f is a mounting port, 301g is a limiting block, 301h is a branch pipe, 302 is a partition plate, 302a is a communication port, 303 is an electrolyte, 304 is an electrode group, 304a is an anode plate, 304b is a cathode plate, 304b1 is a groove, 304c is a turning plate, 304c1 is a handle, 305 is a first air injection pipe, 306 is a first spray pipe, 307 is a first filler layer, 307a is a first perforated plate, 307b is a first Pall ring, 308 is a second filler layer, 308a is a second perforated plate, 308b is a second Pall ring, 309 is a first pump body, 309a is a circulation pipeline, 310 is a fan, 4 is an ammonia-nitrogen wastewater storage device, and 5 is an alkaline agent storage device. DETAILED DESCRIPTION

[0035] The application will be described in further detail below with reference to the drawings and specific embodiments. Unless otherwise specified, the reagents, methods, and equipment used in the application are conventional reagents, methods, and equipment in the technical field. The test methods in the following examples, unless otherwise specified, are generally in accordance with conventional experimental conditions. Unless otherwise specified, the reagents and raw materials used in the application are commercially available.

[0036] REFERENCE Figure 1 It shows one specific embodiment of a system for treating ammonia-nitrogen wastewater by stripping combined with electrochemical oxidation, which mainly comprises a pretreatment device 1, a stripping device 2, and an electrochemical oxidation device 3. By combining gas-liquid separation with electrochemical oxidation, direct contact between the electrode and the complex wastewater is avoided, which leads to pollution. The application realizes efficient and harmless treatment of ammonia-nitrogen, and solves the problems of low treatment efficiency and secondary pollution in the prior art.

[0037] The pre-treatment device 1 is used for receiving and adjusting the pH value of ammonia-nitrogen wastewater, and comprises a tank body, a first liquid inlet for receiving ammonia-nitrogen wastewater and a second liquid inlet for receiving an alkaline agent, and a liquid outlet for discharging alkaline ammonia-nitrogen wastewater, which are formed on the side wall of the tank body, and a stirrer 101 and a pH monitor 102 are arranged on the top of the tank body, the stirrer 101 is used for mixing the ammonia-nitrogen wastewater and the alkaline agent to make the wastewater reach a predetermined pH value, and the pH monitor 102 is used for monitoring the pH of the wastewater in real time. The upstream of the pre-treatment device 1 is further provided with an ammonia-nitrogen wastewater storage device 4 and an alkaline agent storage device 5 connected thereto, the outlet of the ammonia-nitrogen wastewater storage device 4 is connected to the first liquid inlet through a pipeline, the outlet of the alkaline agent storage device 5 is connected to the second liquid inlet through a pipeline, and a metering pump is arranged on the pipeline of each of them. When in operation, the pH value of the ammonia-nitrogen wastewater is adjusted to 11-13 by adding liquid sodium hydroxide or lime water solution to the tank body, so as to create favorable conditions for subsequent ammonia-nitrogen stripping.

[0038] With reference to the above Figure 1 , the stripping device 2 is located downstream of the pre-treatment device 1, and in which the ammonia-nitrogen wastewater from the pre-treatment device 1 is heated and introduced into a gas to convert free ammonia-nitrogen in the ammonia-nitrogen wastewater into ammonia gas and separate and discharge the ammonia gas. The stripping device 2 comprises a tower body 201, the side of which is provided with an ammonia-nitrogen wastewater inlet 201a, and the top of which is provided with an ammonia gas outlet 201b, and the inside of the tower body 201 has a stripping zone 2a and a heating zone 2b arranged in sequence. In the stripping zone 2a, a second air injection pipe 202 is arranged in the lower part thereof and the air injection direction is upward, and a second spray pipe 203 is arranged in the upper part thereof and the liquid injection direction is downward, so as to be arranged in countercurrent. A third filler layer 204 is arranged between the second air injection pipe 202 and the second spray pipe 203, and a fourth filler layer 205 is arranged above the second spray pipe 203, and the two filler layers are both composed of a Pall ring filler made of PP material, which is used for increasing the gas-liquid contact area and making the gas-liquid mixing more sufficient.

[0039] The heating zone 2b receives the alkaline wastewater of the pre-treatment device 1 through the ammonia-nitrogen wastewater inlet 201a, and a heater 206 is arranged in the heating zone 2b below the liquid level, specifically a constant-temperature heater 206, which is used for heating the wastewater to a constant temperature, for example, 20-40℃. The heated wastewater is lifted to the second spray pipe 203 in the upper part of the stripping zone 2a through an external second pump body 208, and is uniformly sprayed to the third filler layer 204 through a plurality of nozzles arranged on the second spray pipe 203. At this time, the second air injection pipe 202 introduces air or steam as a gas source into the third filler layer 204 to contact the falling wastewater in countercurrent, so as to promote the free ammonia-nitrogen to volatilize into ammonia gas, the ammonia gas carries a small amount of water vapor to rise through the fourth filler layer 205 to intercept the liquid droplets, so that the water vapor condenses on the surface of the filler, and the remaining dry gas is discharged from the ammonia gas outlet 201b at the top of the tower body 201.

[0040] The wastewater after stripping ammonia nitrogen falls to the heating zone 2b and is circulated by the second pump body 208 to the second spray pipe 203, forming a tower internal circulation, and the circulation time is 4-8 hours, preferably 6 hours. The side of the heating zone 2b is externally connected with a wastewater temporary storage device 207, and the wastewater treated to reach the standard is temporarily stored in the device and enters the subsequent treatment process. The stripping device 2 is based on the principle that ammonia nitrogen is easy to volatilize under alkaline conditions, and through countercurrent gas-liquid exchange and circulating heating, efficient removal of free ammonia nitrogen is realized, low ammonia nitrogen load wastewater is provided for subsequent electrochemical oxidation, and the problem of electrode pollution caused by direct electrolysis of high-concentration ammonia nitrogen wastewater is avoided.

[0041] With reference to Figures 1-6 , the electrochemical oxidation device 3 is located downstream of the stripping device 2, and its core function is to convert the ammonia gas generated by stripping into nitrogen gas through electrochemical means, realizing harmless treatment of ammonia nitrogen, and. The electrochemical device includes a shell 301, one side wall of the shell 301 forms an ammonia gas inlet 301a, the other side forms an absorption liquid inlet 301d, an absorption liquid outlet 301c and a liquid discharge port 301e, and the top forms a nitrogen gas outlet 301b. The shell 301 is divided into countercurrent mixing chambers 3a and electrochemical reaction chambers 3b arranged in the upper and lower parts, and they realize the circulation flow of the absorption liquid through the plurality of communication ports 302a on the partition plate 302. In this embodiment, the shell 301 is a vertical cuboid structure, and of course it can also be a cylindrical, polygonal or other structure.

[0042] The inside of the electrochemical reaction chamber 3b contains an electrolyte and an electrode group 304 immersed below the liquid level. The electrolyte is a sodium chloride electrolyte with a concentration of 1-3M, and the preferred concentration is 2M. The electrode group 304 is composed of a plurality of staggered anode plates 304a and cathode plates 304b, wherein the anode plate 304a is a ruthenium iridium titanium-based coated electrode plate, a plurality of anode plates 304a are electrically connected, the cathode plate 304b is a pure titanium plate, a plurality of cathode plates 304b are electrically connected, and the anode plate and the cathode plate are respectively electrically connected with the positive electrode and the negative electrode of the external power supply, forming an electric field for electrolyzing the electrolyte. By applying a constant current direct current of 10mA / cm 2- 30mA / cm 2 to the electrode group 304, the electrolyte undergoes an electrochemical reaction to generate an absorption liquid containing hypochlorous acid and chlorine free radicals.

[0043] The countercurrent mixing chamber 3a is mainly used for ammonia gas absorption and oxidation reaction. A first air injection pipe 305 is arranged at the lower part of the countercurrent mixing chamber 3a. One end of the first air injection pipe 305 is connected to the ammonia gas inlet 301a of the side wall of the shell 301 for receiving ammonia gas from the upstream. The other end of the first air injection pipe 305 extends horizontally in the shell 301 and the air injection direction is upward. A first spray pipe 306 is arranged at the upper part of the countercurrent mixing chamber 3a. One end of the first spray pipe 306 is connected to the absorption liquid inlet 301d and connected to the absorption liquid outlet 301c of the electrochemical reaction chamber 3b through a circulation pipe 309a. The other end of the first spray pipe 306 extends horizontally and the liquid injection direction is downward. The first spray pipe 306 and the first air injection pipe 305 are arranged in countercurrent. In order to increase the gas-liquid contact area, a first packing layer 307 is arranged between the first air injection pipe 305 and the first spray pipe 306. The first packing layer 307 includes two first hole plates 307a arranged in an upper-lower manner. The four side edges of the first hole plates 307a are fixed to the inner wall of the shell 301. A plurality of first Pall rings 307b are arranged between the two first hole plates 307a. A second packing layer 308 is arranged above the first spray pipe 306 for blocking water vapor from being discharged from the nitrogen gas outlet 301b. The second packing layer 308 includes a second hole plate 308a. A plurality of second Pall rings 308b are arranged between the second hole plate 308a and the top wall of the shell 301. A grid 301b1 is arranged at the nitrogen gas outlet 301b for blocking the second Pall rings 308b. The shapes of the first Pall rings 307b and the second Pall rings 308b can be spherical, polygonal, cylindrical, etc.

[0044] With reference to Figure 4 and Figure 6 The side wall of the shell 301 is provided with a mounting opening 301f. A flap 304c is rotatably arranged at the mounting opening 301f. A plurality of anode plates 304a are arranged at the inner side of the flap 304c at intervals. When the flap 304c is sealed and coincides with the mounting opening 301f, the anode plates 304a are embedded in the electrochemical reaction chamber 3b along with the rotation of the flap 304c and located between two cathode plates 304b, so that the anode plates 304a and the cathode plates 304b are arranged in a staggered manner. The flap structure facilitates the disassembly and maintenance of the electrode plates. The outer side of the flap 304c is provided with a handle 304c1 for facilitating the opening and closing of the flap 304c by the operator. A plurality of cathode plates 304b are arranged at intervals along the length direction of the electrochemical reaction chamber 3b. The lower part of the cathode plates 304b is clamped in a plurality of limiting blocks 301g arranged at the bottom of the electrochemical reaction chamber 3b. In addition, the absorption liquid outlet 301c and the liquid discharge opening 301e correspond to the side part of the electrochemical reaction chamber 3b. The absorption liquid outlet 301c is provided with a branch pipe 301h extending horizontally to the middle part of the electrochemical reaction chamber 3b for receiving absorption liquid. The bottom of the cathode plate 304b is formed with a groove 304b1 for the branch pipe 301h to pass through.

[0045] The ammonia gas discharged from the stripping device 2 enters the countercurrent mixing chamber 3a through the ammonia gas inlet 301a and is uniformly diffused upward to the first filler layer 307 through the first air injection pipe 305. At the same time, the absorption liquid containing hypochlorous acid and chlorine radicals generated in the electrochemical reaction chamber 3b is lifted to the first spray pipe 306 through the external first pump body 309 and is uniformly distributed to the first filler layer 307. In the first filler layer 307, the ammonia gas is in countercurrent mixing contact with the absorption liquid, and after the ammonia gas is absorbed, it rapidly reacts with the hypochlorous acid and chlorine radicals in the absorption liquid to convert into nitrogen gas. The generated nitrogen gas passes through the second filler layer 308 and is discharged from the nitrogen gas outlet 301b at the upper part of the countercurrent mixing chamber 3a, and finally is pumped to the external environment by the fan 310. The reacted absorption liquid flows back to the electrochemical reaction chamber 3b through the communication port 302a on the partition plate 302 to continue to participate in the electrochemical reaction, forming a circulating treatment process, realizing efficient and harmless decomposition of ammonia gas, and the whole process ammonia gas does not directly contact the electrode group 304, truly solving the fundamental interference problem of complex water quality on the electrochemical process.

[0046] A method for treating ammonia-nitrogen wastewater by using the above stripping combined with electrochemical oxidation, comprising the following steps:

[0047] The ammonia-nitrogen wastewater is transported from the ammonia-nitrogen wastewater storage device 4 to the pretreatment device 1 through the pipeline, and the pH monitor 102 is used to monitor the pH value of the wastewater in real time. According to the monitoring data, the metering pump of the outlet pipeline of the alkaline agent storage device 5 is controlled to add liquid alkali or lime water into the tank, and the stirrer 101 is started to mix the wastewater and the alkaline agent sufficiently, so that the pH value of the ammonia-nitrogen wastewater is adjusted to 11-13. Under this alkaline condition, the ammonium ion (NH4 + ) in the wastewater is converted into free ammonia-nitrogen (NH3), which creates favorable conditions for the removal of ammonia-nitrogen in the subsequent process. When the pH value reaches the predetermined range, the wastewater with adjusted pH value is discharged from the pretreatment device 1 through the liquid outlet.

[0048] The pretreated ammonia-nitrogen wastewater is fed into the heating zone 2b of the stripping device 2 through a pipeline, and the constant-temperature heater 206 submerged below the liquid level heats the wastewater to 20-40℃. After reaching the set temperature, the heated wastewater is lifted by the second pump body 208 to the second spray pipe 203 at the top of the stripping zone 2a, and is uniformly distributed to the third filler layer 204 below through the multiple nozzles on the spray pipe. At the same time, the second air injection pipe 202 introduces air or steam into the stripping zone 2a, and the gas flows from bottom to top, and is mixed and contacted with the wastewater sprayed from top to bottom in the third filler layer 204 in a countercurrent manner. Under the synergistic effect of alkalinity and temperature, the free ammonia-nitrogen volatilizes from the liquid phase to the gas phase and is converted into ammonia gas. The gas stream carrying the ammonia gas continues to rise, passes through the fourth filler layer 205, and is finally discharged from the ammonia gas outlet 201b at the top of the stripping zone 2a. The wastewater after the ammonia-nitrogen is stripped falls downward to the heating zone 2b, is pumped away by the second pump body 208 and lifted to the second spray pipe 203 again, forming a tower internal circulation. After multiple cycles, when the ammonia-nitrogen concentration in the wastewater is reduced to a predetermined value, the wastewater is transported to the wastewater temporary storage device 207 connected to the side of the heating zone 2b.

[0049] A 2M sodium chloride electrolyte is injected into the electrochemical reaction chamber 3b, and the electrode group 304 with the anode plate 304a being a ruthenium iridium titanium-based coated electrode plate and the cathode plate 304b being a pure titanium plate is submerged below the liquid level of the electrolyte. A constant current direct current of 10-30 mA / cm 2 is applied to the electrode group 304 by a power supply, and under the action of the electric field, the electrolyte undergoes an electrochemical reaction. An oxidation reaction occurs on the surface of the anode, and hypochlorous acid (HClO) and chlorine radicals (Cl·) are catalytically generated, thereby generating an absorption liquid with strong oxidizing property, which provides reaction materials for the subsequent oxidation of ammonia gas.

[0050] The ammonia gas discharged from the stripping device 2 enters the countercurrent mixing chamber 3a through the ammonia gas inlet 301a in the side wall of the shell 301, and is uniformly diffused upward to the first filler layer 307 through the first air injection pipe 305. At the same time, the first pump body 309 outside the electrochemical reaction chamber 3b lifts the absorption liquid containing hypochlorous acid and chlorine radicals generated in the electrochemical reaction chamber 3b to the first spray pipe 306 at the upper part of the countercurrent mixing chamber 3a through the circulation pipeline 309a, and uniformly distributes to the first filler layer 307. In the first filler layer 307, the ammonia gas is in countercurrent contact with the absorption liquid, and the ammonia gas is quickly absorbed by the absorption liquid, and the nitrogen element in the ammonia gas undergoes an oxidation-reduction reaction with the hypochlorous acid and chlorine radicals in the absorption liquid to be converted into nitrogen gas (N2). The generated nitrogen gas continues to rise through the second filler layer 308, is discharged from the nitrogen gas outlet 301b at the upper part of the countercurrent mixing chamber 3a, and is finally pumped away by the fan 310 and discharged to the external environment. The reacted absorption liquid flows back to the electrochemical reaction chamber 3b through the communication port 302a on the partition plate 302, continues to participate in the electrochemical reaction, forms a recycling use of the electrolyte, and continuously realizes the harmless decomposition treatment of the ammonia gas.

[0051] Effect verification

[0052] Reference Figure 7 , which shows the curve of ammonia nitrogen removal efficiency of ammonia nitrogen wastewater at different pH values in the stripping device 2. The specific experimental conditions are as follows: the initial concentration of ammonia nitrogen wastewater is 3000 mg / L, the residence time of the stripping device 2 is 6 hours, the temperature of the heating zone 2b is controlled at 40℃, and the gas-liquid ratio of stripping is set to 7000:1. During the experiment, first, the raw ammonia nitrogen wastewater is adjusted in pH by the pretreatment device 1. By adding 50% NaOH solution, the pH value of the wastewater is adjusted to 11, 12 and 13 respectively. After pH adjustment, the wastewater enters the heating zone 2b of the stripping device 2 for temperature rise, and then the countercurrent contact stripping treatment is carried out in the tower by the second spray pipe 203 and the gas introduced from the second spray pipe 202. The ammonia gas produced by stripping is introduced into the electrochemical oxidation device 3. Figure 7 The experimental data shows that the ammonia nitrogen in the wastewater under different pH conditions is effectively removed within 6 hours of stripping time, and the experimental results show obvious regularity. With the increase of pH value from 11 to 13, the ammonia nitrogen removal efficiency increases significantly. When the pH value reaches 13, the ammonia nitrogen in the wastewater is almost completely removed, which verifies the positive promoting effect of alkaline conditions on the stripping efficiency of ammonia nitrogen.

[0053] Reference Figure 8 , which shows the curve of ammonia nitrogen removal efficiency of ammonia nitrogen wastewater at different temperatures in the stripping device 2. The specific experimental conditions are as follows: the initial concentration of ammonia nitrogen wastewater is 3000 mg / L, the pretreatment pH is adjusted to 13, the stripping residence time is 6 hours, the gas-liquid ratio of stripping is set to 7000:1, and the wastewater temperature is set to 20℃, 30℃ and 40℃ respectively by adjusting the heater 206. After the pretreatment is adjusted to pH 13, the wastewater enters the heating zone 2b of the stripping device 2, and the temperature is raised to the corresponding temperature by the heater 206, and then the countercurrent stripping is carried out in the tower by the second spray pipe 203 and the gas introduced from the second spray pipe 202. The experimental data shows that the ammonia nitrogen can be effectively removed under each temperature condition within 6 hours of stripping time, and the removal efficiency increases significantly with the increase of temperature. When the temperature reaches 40℃, the ammonia nitrogen removal rate tends to 100%, which verifies the promoting effect of temperature on the stripping process of ammonia nitrogen, and higher water temperature accelerates the volatilization rate of free ammonia nitrogen.

[0054] Reference Figure 9 , which shows the curve of ammonia nitrogen removal efficiency of ammonia nitrogen wastewater at different current densities of the electrode group 304 of the electrochemical oxidation device 3. The specific experimental conditions are as follows: the initial concentration of ammonia nitrogen wastewater is 3000 mg / L, the residence time of the stripping device 2 is 6 hours, the pH is 13, the gas-liquid ratio is 7000:1, and the wastewater temperature is 40℃. The ammonia gas produced by stripping is introduced into the electrochemical oxidation device 3. In the experiment, the current output of the electrode group 304 is adjusted, and the current density is set to 10 mA / cm 2, 20 mA / cm 2 , 30 mA / cm 2 Three current density conditions were used to oxidize ammonia nitrogen in the circulating absorption solution. The experimental data showed that under the three current density conditions, ammonia nitrogen in the absorption solution could be quickly removed within 3 hours of electro-oxidation reaction, and the removal efficiency significantly increased with the increase of current density. When the current density reached 30 mA / cm 2 , ammonia nitrogen was completely oxidized. The results showed that by adjusting the current density, the ammonia nitrogen oxidation efficiency could be controlled. Considering the energy consumption and treatment effect, 30 mA / cm 2 was the preferred working parameter. It is worth noting that the oxidized electrolytic absorption solution was detected, and no accumulation of NO3 - and NO2 - was detected, which confirmed that under this electrochemical system, ammonia nitrogen could be completely converted into nitrogen gas, without forming nitrogen oxides, and the conversion rate could reach 100%.

[0055] The detailed description and the accompanying drawings or diagrams are support and description of the present teachings, but the scope of the present teachings is only limited by the claims. Although some of the best modes and other embodiments for implementing the present teachings have been described in detail, there are various alternative designs and embodiments for practicing the present teachings defined in the appended claims. Furthermore, the present disclosure expressly includes combinations and sub-combinations of the elements and features set forth above and below.

Claims

1. A system for treating ammonia nitrogen wastewater by stripping combined with electrochemical oxidation, characterized in that, include: A pretreatment device for receiving and adjusting the pH value of ammonia nitrogen wastewater; A stripping device is located downstream of the pretreatment device, in which the ammonia nitrogen wastewater from the pretreatment device is heated and gas is introduced to convert the free ammonia nitrogen in the ammonia nitrogen wastewater into ammonia gas and separate and discharge it. An electrochemical oxidation device, located downstream of the stripping device, comprises: The housing has a partition plate to divide the space inside the housing into a countercurrent mixing chamber and an electrochemical reaction chamber arranged vertically, and the partition plate has a plurality of communication ports. The electrochemical reaction chamber contains an electrolyte and an electrode assembly submerged below the electrolyte level. The electrolyte undergoes an electrochemical reaction under the constant current direct current applied by the electrode assembly to generate an absorbent containing hypochlorous acid and chlorine free radicals. The lower part of the countercurrent mixing chamber receives the ammonia gas from the upstream, and its upper part is connected to the electrochemical reaction chamber through a circulation pipeline to receive the absorbent. The ammonia gas comes into countercurrent contact with the absorbent and reacts with hypochlorous acid and chlorine free radicals in the absorbent to convert into nitrogen gas. The nitrogen gas is discharged from the upper part of the countercurrent mixing chamber. The absorbed liquid after the reaction flows back to the electrochemical reaction chamber through the communication port on the partition plate.

2. The system according to claim 1, characterized in that, The electrode assembly includes multiple anode plates and multiple cathode plates, which are arranged alternately in the electrochemical reaction chamber.

3. The system according to claim 2, characterized in that, The anode plate is a ruthenium-iridium titanium-based coated electrode plate, and the cathode plate is a pure titanium plate.

4. The system according to claim 1, characterized in that, The electrolyte is a sodium chloride electrolyte with a concentration of 1-3M.

5. The system according to claim 1, characterized in that, The countercurrent mixing chamber includes a first jet pipe disposed at its lower part and a first spray pipe disposed at its upper part. The first jet pipe receives the ammonia gas from upstream, and the first spray pipe is connected to the circulation pipeline.

6. The system according to claim 5, characterized in that, A first packing layer is provided between the first jet pipe and the first spray pipe, and a second packing layer is provided above the first spray pipe.

7. The system according to claim 1, characterized in that, The upstream of the pretreatment device is also equipped with an ammonia nitrogen wastewater storage device and an alkaline reagent storage device connected thereto.

8. The system according to claim 1, characterized in that, The stripping device includes a stripping zone and a heating zone arranged vertically. A second jet pipe is provided at the lower part of the stripping zone, and a second spray pipe is provided at the upper part. The heating zone receives the ammonia nitrogen wastewater from the pretreatment device and is equipped with a heater located below the liquid level. The second spray pipe receives the heated ammonia nitrogen wastewater from the heating zone and contacts the gas from the second jet pipe to convert the free ammonia nitrogen in the ammonia nitrogen wastewater into ammonia gas, which is then discharged from the upper part of the stripping zone.

9. The system according to claim 8, characterized in that, A third packing layer is provided between the second jet pipe and the second spray pipe, and a fourth packing layer is provided above the second spray pipe.

10. A method for treating ammonia nitrogen wastewater using a stripping combined with electrochemical oxidation system as described in any one of claims 1-9, characterized in that, Includes the following steps: The ammonia nitrogen wastewater is transported to the pretreatment device, and the pH value of the ammonia nitrogen wastewater is adjusted to alkaline, so that the ammonium ions in the ammonia nitrogen wastewater are converted into free ammonia nitrogen. The pretreated ammonia nitrogen wastewater is transported to the heating zone of the stripping device, heated, and then transported to the stripping zone, where it comes into contact with gas introduced from the bottom of the stripping zone, causing the free ammonia nitrogen to be converted into ammonia gas and discharged from the top of the stripping zone. A constant current direct current is applied to the electrolyte through the electrode group in the electrochemical reaction chamber, causing the electrolyte to undergo an electrochemical reaction and generate an absorbent containing hypochlorous acid and chlorine free radicals. The ammonia gas is brought into countercurrent contact with the absorbent liquid through the countercurrent mixing chamber, so that the ammonia gas is absorbed by the absorbent liquid and reacts with hypochlorous acid and chlorine free radicals in the absorbent liquid to be converted into nitrogen gas and discharged.

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