Magnesium anode pore plate hydrodynamic cavitation generator and method thereof

Abstract

The invention provides a hydraulic cavitation generator for a magnesium anode pore plate. The hydraulic cavitation generator for the pore plate comprises a first pore plate, a second pore plate and three sections of detachable cylindrical reaction containers, the first pore plate is a magnesium anode plate, the second pore plate is an inert cathode plate, and a flow channel is arranged in the cylindrical reaction container; the pore plate is clamped between the cylindrical reaction containers, and the pore plate is connected with the cylindrical reaction containers in a sealing manner. The problems that a common hydrodynamic cavitation generator is limited in capacity of treating organic matters in wastewater and cannot treat nitrogen and phosphorus in the wastewater are solved.

Description

Technical Field

[0001] The present invention relates to the technical field of environmental engineering water treatment, and particularly to a device and method for a magnesium anode orifice plate hydrodynamic cavitation generator. Background Art

[0002] With the rapid development of industrialization and urbanization, organic matter and nitrogen and phosphorus pollution in wastewater have become one of the important environmental problems faced globally. Agricultural wastewater, aquaculture wastewater, food processing wastewater, etc. all contain a large amount of organic matter and relatively high nitrogen and phosphorus concentrations. Excessive nitrogen and phosphorus content can lead to eutrophication of water bodies and the occurrence of water blooms, causing serious impacts on the water ecosystem. Currently, the treatment method for such organic matter and nitrogen and phosphorus mixed wastewater usually uses biological methods to first degrade organic matter, and then specifically remove ammonia nitrogen and phosphorus step by step. This method has disadvantages such as complex technology, difficult control, and large floor area. To meet the market's demand for cost reduction and floor area reduction, the research and development of technologies for degrading organic matter in wastewater while removing and recovering nitrogen and phosphorus from wastewater has become a frontier topic in the field of water treatment technology.

[0003] The sacrificial magnesium anode electrolysis method refers to a method that uses a magnesium metal plate as the anode and an inert electrode plate as the cathode to electrolytically recover nitrogen and phosphorus in water in the form of magnesium ammonium phosphate precipitation, which has received much attention in recent years. When a positive current is applied to the magnesium anode, Mg 2+ is released, and it reacts with NH 4 + and PO 4 3- ions in the water body to generate MgNH 4 PO 4 ·6H 2 O, that is, magnesium ammonium phosphate, commonly known as struvite, which can be used as an effective slow-release fertilizer. The sacrificial magnesium anode electrolysis method has been proven to be an excellent way to simultaneously recover nitrogen and phosphorus in wastewater that meets the requirements of sustainable resource utilization. However, this process has a key problem of low mass transfer efficiency in the device. Ordinary solution stirring methods cannot disperse Mg 2+ on the surface of magnesium in time, and the uneven distribution of Mg 2+ causes the magnesium plate precipitate to cover too quickly, affecting the efficiency of the magnesium plate.

[0004] Hydrodynamic cavitation refers to the phenomenon that when a high-speed flowing liquid passes through a specific hydraulic component, and the local pressure of the fluid suddenly drops below the vaporization pressure corresponding to the liquid temperature in this area for some reason, part of the liquid vaporizes to generate a large number of bubbles. When these bubbles burst, a high temperature of up to 4927 °C and a high pressure of 1000 atm will be generated in the surrounding space. By continuously acting at a high frequency, a high-speed jet with a speed of 400 km / h is formed, and a strong shock wave is generated, which can degrade organic matter, improve the mass transfer efficiency of the fluid and break the cell walls of microorganisms, thus achieving a bactericidal effect. At present, hydrodynamic cavitation technology is often applied to the removal of suspended solids and organic matter in wastewater, wastewater disinfection, chemical process intensification, medical device cleaning, etc., and has received extensive attention in the fields of environmental protection, chemical industry, medicine, etc. However, there are few reports on the synchronous removal of organic matter, nitrogen, and phosphorus by combining it with magnesium anode electrolysis method in the same cavitation device.

[0005] There are many problems in simply using hydrodynamic cavitation technology to treat wastewater: Traditional hydrodynamic cavitation generators such as orifice plate type and Venturi type generators have simple structures and flexible operations, but their efficiency in removing organic matter is relatively low. Rotary and vortex hydrodynamic cavitation generators have higher efficiencies, but they have high device costs, high energy consumption, and much lower control flexibility than orifice plates and Venturi tubes.

[0006] The removal effect of hydrodynamic cavitation technology on COD is relatively low (i.e., less than 40%), so it is not considered as a separate wastewater treatment method. However, due to its easy implementation advantages, it is often used as an intensification means or a pretreatment method in combination with other processes.

[0007] The present invention proposes a device and method for a magnesium anode orifice plate hydrodynamic cavitation generator, aiming to simplify the process of removing nitrogen, phosphorus, and organic matter in wastewater and improve the removal rates of organic matter and microorganisms in wastewater. Summary of the Invention

[0008] The purpose of the present invention is to overcome the problem that the ordinary hydrodynamic cavitation generator has limited ability to treat organic matter in wastewater and cannot treat nitrogen and phosphorus in wastewater. A device and method for synchronously removing nitrogen, phosphorus, organic matter, and microorganisms in wastewater by magnesium anode orifice plate hydrodynamic cavitation method are provided to solve the problem that nitrogen and phosphorus need further treatment when the ordinary hydrodynamic cavitation generator treats wastewater containing nitrogen, phosphorus, and organic matter. Combining the magnesium anode and the orifice plate hydrodynamic cavitation generator in the same device saves space and eliminates the problem of passivation of the anode plate caused by uneven distribution of magnesium ions in the magnesium anode electrolysis process.

[0009] The technical solution adopted by the present invention is as follows: A magnesium anode orifice plate hydrodynamic cavitation generator, wherein the orifice plate hydrodynamic cavitation generator includes: A first orifice plate, and the first orifice plate is a magnesium anode plate; A second orifice plate, and the second orifice plate is an inert cathode plate; A three-section detachable cylindrical reaction vessel, with a flow channel inside the cylindrical reaction vessel; The orifice plate is clamped between the cylindrical reaction vessels, and the orifice plate is hermetically connected to the cylindrical reaction vessels.

[0010] Furthermore, the magnesium anode plate is pure magnesium or magnesium alloy; the mass ratio of magnesium in the magnesium alloy is more than 90%; the thickness of the magnesium anode plate is 2 - 5 mm.

[0011] Furthermore, the inert cathode plate is an inert electrode plate such as a titanium oxide plate, a titanium plate or a stainless steel plate; the thickness of the inert electrode plate is the same as that of the magnesium anode plate.

[0012] Furthermore, the orifice plate structure is a single-hole orifice plate structure or a multi-hole orifice plate structure; the diameter of the holes is 1 - 5 mm, the holes are evenly distributed, and are radially distributed from the central hole to the outside.

[0013] Furthermore, the holes on the anode plate and the cathode plate are not completely correspondingly arranged.

[0014] Furthermore, the distance between the anode plate and the cathode plate is 20 - 70 mm.

[0015] Furthermore, the outer diameter of the orifice plate is larger than the outer diameter of the cylindrical reaction vessel, and is composed of a fastener and a sealing ring connection; the part of the orifice plate exposed outside the outer diameter of the cylindrical reaction vessel is connected to a power supply.

[0016] The present invention also provides a nitrogen and phosphorus-containing wastewater treatment system, including the above-mentioned magnesium anode orifice plate hydrodynamic cavitation generator. The treatment system further includes a filter, a circulation tank, a pressurizing device and a pressure gauge; the upstream of the magnesium anode orifice plate hydrodynamic cavitation generator is connected to the circulation tank, a pressurizing device is provided between the magnesium anode orifice plate hydrodynamic cavitation generator and the circulation tank, pressure gauges are connected to both the upstream and downstream of the magnesium anode orifice plate hydrodynamic cavitation generator; the downstream of the magnesium anode orifice plate hydrodynamic cavitation generator is connected to the filter, and the filter is connected to the circulation tank.

[0017] Furthermore, the filter is a filter bag filter, a filter membrane filter or a paper tape filter; the filtration accuracy of the filter is 1 - 50 μm.

[0018] Furthermore, the circulation tank is provided with an organic matter, nitrogen and phosphorus on-line monitoring device for measuring the organic matter, nitrogen and phosphorus concentrations of the wastewater in the circulation tank.

[0019] The present invention also provides a method for synchronously removing organic matter, nitrogen and phosphorus, and microorganisms in wastewater by using the magnesium anode orifice plate hydrodynamic cavitation generator according to any one of claims 1 - 10. The method includes the following steps: The first step is to adjust the pH value of the sewage to 7 - 9 and introduce it into the circulation tank; In the second step, turn on the water pump to input the sewage in the circulation pool into the magnesium anode cavitation generator, and adjust the water pump to make the upstream pressure 3 - 8 bar; in the third step, after the magnesium anode cavitation generator is completely filled with water, turn on the DC power supply to start electrolysis, set the constant current mode, and the current range is between 0.1 - 0.5 A. Operate for a period of time until the indicators of the wastewater reach the standard, then turn off the DC power supply, and then turn off the water pump for drainage; In the fourth step, collect the magnesium ammonium phosphate crystals in the filter.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows: During the reaction process, the magnesium anode orifice plate hydrodynamic cavitation generator can simultaneously remove nitrogen, phosphorus, organic matter and microorganisms in the wastewater. All reactions are carried out in the same device, saving space and also solving the problem that general hydrodynamic cavitation generators cannot remove nitrogen and phosphorus in the wastewater. The hydrogen gas generated by the sacrificial magnesium anode electrolysis can also increase the number of cavitation nuclei in the wastewater and improve the cavitation intensity. The cathode can decompose water to generate hydroxyl radicals, enhancing the degradation effect on refractory organic matter. The recovered product can be used as a slow-release fertilizer.

[0021] Due to the turbulent flow effect generated by the cavitation microjet, the magnesium anode orifice plate hydrodynamic cavitation process provided by the present invention can solve the problem of low mass transfer efficiency in the magnesium anode electrolysis process, reduce the coverage of precipitates on the surface of the magnesium anode, thereby promoting the release of magnesium from the magnesium anode, and has a synergistic effect, and the synergistic effect is higher than that of using the magnesium anode electrolysis process or the orifice plate hydrodynamic cavitation process alone.

[0022] During the magnesium ammonium phosphate crystallization process, hydrogen ions will be generated, resulting in a decrease in the alkalinity of the solution and affecting the precipitation crystallization efficiency. The water decomposition brought about by the magnesium anode oxidation and cavitation will release hydroxide ions, slowing down the decrease in the solution pH, thereby improving the precipitation efficiency and slowing down the passivation of the anode plate. Description of the Drawings

[0023] Figure 1 It is a schematic structural diagram of a magnesium anode orifice plate hydrodynamic cavitation generator.

[0024] Figure 2 It is Figure 1 a partial explosion diagram of

[0025] Figure 3 It is a schematic orifice plate structure diagram of another embodiment.

[0026] Figure 4 It is a system structure diagram of a magnesium anode orifice plate hydrodynamic cavitation generator.

[0027] Figure 5 It is an enlarged view of part A. Embodiment

[0028] Typical embodiments embodying the features and advantages of the present invention will be described in detail in the following description. It should be understood that the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. The present invention can have different variations in different embodiments, all without departing from the scope of the present invention.

[0029] As Figures 1 - 2 shown, the connection mode of the magnesium anode orifice plate hydrodynamic cavitation generator is: cylindrical reaction vessel 12 - first orifice plate 10 - cylindrical reaction vessel 14 - second orifice plate 11 - cylindrical reaction vessel 13; the first orifice plate 10 is a magnesium anode orifice plate, and the second orifice plate 11 is a corrosion-resistant inert conductive orifice plate. As Figure 4 , the cylindrical reaction vessel 12 is provided with a liquid inlet hole 120, and the cylindrical reaction vessel 14 is provided with a liquid outlet 130. As Figure 5 shown, flanges are provided on both sides where the cylindrical reaction vessel is connected to the orifice plate. Sealing grooves are provided on the flanges, and sealing members 15 are provided in the sealing grooves. The flanges are connected by threaded fasteners 16. The outer diameters of the first orifice plate 10 and the second orifice plate 11 are larger than the outer diameter of the cylinder of the cylindrical reaction vessel. As Figure 3 , the first orifice plate 10 and the second orifice plate 11 are provided with connection terminals 100 and 110 that respectively expose the cylindrical reaction vessel. The connection terminal 100 is connected to the positive pole of the power supply, and the connection terminal 110 is connected to the negative pole of the power supply. The magnesium anode orifice plate hydrodynamic cavitation generator is an improvement based on the orifice plate generator and is composed of two orifice plates connected in series. The orifice plate is a circular flat plate with multiple holes for fluid passage.

[0030] As Figure 4 shown, a nitrogen and phosphorus-containing wastewater treatment system. The liquid inlet 120 of the magnesium anode orifice plate hydrodynamic cavitation generator 1 is connected to the circulation pool 3. A liquid inlet pump 2 and a liquid inlet valve are provided between the magnesium anode orifice plate hydrodynamic cavitation generator 1 and the circulation pool 3. The liquid outlet 130 of the magnesium anode orifice plate hydrodynamic cavitation generator 1 is connected to the bag filter 4. A liquid inlet pump 2 and a liquid outlet valve are provided between the magnesium anode orifice plate hydrodynamic cavitation generator 1 and the bag filter 4. The liquid outlet end of the bag filter 4 is connected to the circulation pool 3. Pressure gauges 5 are provided upstream and downstream of the magnesium anode orifice plate hydrodynamic cavitation generator. An on-line monitoring device for monitoring organic matter, nitrogen, and phosphorus can be provided in the circulation pool 3. The bag filter 4 is provided with a filter bag 41 and a slag discharge valve 40 is provided below.

[0031] Working principle of the nitrogen and phosphorus-containing wastewater treatment system: After the wastewater to be treated is introduced into the circulation pool, the flow rate of the water pump 2 is adjusted to control the pressure difference between the upstream and downstream of the magnesium anode orifice plate hydrodynamic cavitation generator. Under the condition of power on, sacrificial magnesium anode electrolysis and cavitation treatment are carried out on the wastewater, and the treated wastewater returns to the circulation pool through the filter.

[0032] The wastewater enters the magnesium anode orifice plate hydrodynamic cavitation generator from the circulation pool through a water pump. When it reaches the first orifice plate, due to the sudden reduction in the flow-through area, the flow velocity surges, the pressure drops sharply, and cavitation bubbles are generated. Subsequently, when leaving the orifice, the pressure instantaneously rebounds, the cavitation bubbles collapse and a transmission chain effect occurs, generating smaller cavitation bubbles that continuously collapse. Then, the wastewater passes through the second orifice plate and cavitation occurs again, strengthening the degradation of organic substances. Pressure gauges are installed on both the upstream pipeline and the downstream pipeline of the magnesium anode orifice plate hydrodynamic cavitation generator to control the cavitation intensity and detect the pressure difference. In addition, after the wastewater flows out of the second orifice plate, it first passes through a filter and then returns to the circulation pool. The wastewater treatment process goes through multiple cycles until the water quality reaches the expected target and then is discharged.

[0033] Furthermore, the first orifice plate is a perforated magnesium metal plate, which can be pure magnesium, magnesium alloy, etc. The mass proportion of magnesium in the magnesium alloy is more than 90%, and the thickness of the electrode plate can be selected from 2 - 5 mm. First, the perforated magnesium metal plate is connected to the positive pole of an external power supply and can act as a magnesium source to generate magnesium ions and hydrogen when energized. Second, the perforated magnesium anode can also be used as a hydrodynamic component for hydrodynamic cavitation. By changing the flow-through area, it affects the fluid flow velocity, thereby generating a cavitation effect to degrade organic substances and kill microorganisms.

[0034] Furthermore, the second orifice plate is a perforated inert electrode plate, which can be a titanium oxide plate, a titanium plate, a stainless steel plate, etc. The thickness of the inert electrode plate is the same as that of the anode plate. This perforated inert electrode plate is connected to the negative electricity of an external power supply. Under the condition of being energized, water and oxygen are reduced at the cathode to generate hydrogen and hydroxide ions or hydroxyl radicals. The generated hydrogen can supplement the number of cavitation nuclei in the wastewater and enhance the cavitation effect.

[0035] The above sacrificial magnesium anode electrolysis method can be represented by the following reactions: Anode (oxidation): Mg - e - →Mg + Cathode (reduction): Precipitation reaction: Furthermore, the orifice plate structure includes orifice plate structures with different configurations such as single-hole orifice plates and multi-hole orifice plates. The size of the holes can be selected from 1 to 5 mm in diameter, and the holes are evenly distributed, radiating from the center point to the outside. When the wastewater passes through the orifice plate, due to the reduction of the flow-through area, the flow velocity increases, and the pressure drops below the saturated vapor pressure, generating cavitation bubbles. Subsequently, due to the recovery of the pressure, the cavitation bubbles burst, producing a cavitation effect. When the magnesium anode orifice plate hydro-cavitation generator treats wastewater, it is jointly affected by cavitation reaction and electrolysis reaction. First, the thermal effect, physical effect, and hydroxyl radicals generated by cavitation can break down macromolecular organic matter and degrade it into smaller molecules that are easier to treat, thereby reducing the COD of the wastewater. Secondly, the magnesium ions electrolyzed from the magnesium anode can react with nitrogen and phosphorus in the wastewater to form magnesium ammonium phosphate precipitate, which is an effective slow-release fertilizer, thereby recovering nitrogen and phosphorus in the water. At the same time, the turbulent flow effect brought by cavitation can accelerate the solute transfer efficiency, promote ion exchange in the wastewater, thereby improving the magnesium ammonium phosphate precipitation efficiency and reducing the adsorption of this precipitate on the surface of the magnesium anode, reducing its inhibition of the magnesium anode dissolution. In addition, the hydrolysis reaction occurring on the cathode surface generates hydrogen and hydroxyl groups. The generated hydrogen can be used as cavitation nuclei to enhance the cavitation intensity, and the generated hydroxide ions can adjust the pH value of the wastewater. The alkaline environment is more suitable for the formation of magnesium ammonium phosphate. Finally, cavitation and electrolysis can damage the cell walls of microorganisms, thereby playing a bactericidal role. Therefore, the magnesium anode orifice plate hydro-cavitation generator has the ability to simultaneously remove nitrogen, phosphorus, organic matter, and microorganisms from wastewater.

[0036] The cavitation method described above can be represented by the following reactions: H 2 O → H · + · OH O 2 → 2O H · + O → OH The holes on the anode plate and the cathode plate are not completely corresponding, forming a curved flow channel to strengthen the collision of the wastewater with the cathode plate after flowing through the anode plate, and a shearing effect occurs to enhance the degradation effect of organic matter.

[0037] The closer the distance between the anode and cathode plates, the stronger the electrolysis effect. The optional plate spacing is 20 - 70 cm.

[0038] The filter is a precision filtering device such as a filter bag filter, a filter membrane filter, or a paper belt filter. The filtering accuracy can be selected from 1 to 50 μm to filter the magnesium ammonium phosphate precipitate for subsequent recycling.

[0039] The system is also equipped with on-line monitoring devices for organic matter, nitrogen and phosphorus in the circulation pool, which are used to measure the concentrations of organic matter, nitrogen and phosphorus in the wastewater in the circulation pool to judge the number of circulation times. If the system is used to treat wastewater with stable properties, it is only necessary to measure in advance the number of circulation times required for sample treatment, and the on-line monitoring devices for organic matter, nitrogen and phosphorus can be omitted.

[0040] A method for removing organic matter, nitrogen, phosphorus and microorganisms from wastewater based on the above-mentioned magnesium anode orifice plate hydrodynamic cavitation generator comprises the following steps: In the first step, adjust the pH value of the sewage to 7-9 (since hydroxide ions will be generated by the reduction of water at the cathode during the electrolysis process, increasing the alkalinity of the wastewater, and the hydroxide ions consumed due to magnesium ammonium phosphate in the device can be supplemented), and introduce it into the circulation pool.

[0041] In the second step, turn on the water pump to input the sewage in the circulation pool into the magnesium anode cavitation generator, and adjust the water pump to make the upstream pressure 3-8 bar; In the third step, after the magnesium anode cavitation generator is completely filled with water, turn on the DC power supply to start electrolysis, set the constant current mode, and the current range is between 0.1-0.5 A. Operate for a period of time until the indicators of the wastewater reach the standard, turn off the DC power supply, and then turn off the water pump for drainage.

[0042] In the fourth step, collect the magnesium ammonium phosphate crystals in the filter for resource recovery and utilization.

[0043] Although the specific implementation manners of the present invention have been described in detail in conjunction with the accompanying drawings, it should not be construed as a limitation on the protection scope of this patent. Within the scope described in the claims, various modifications and deformations that can be made by those skilled in the art without creative labor still fall within the protection scope of this patent.

Claims

1. A magnesium anode orifice hydrodynamic cavitation generator, characterized in that, the magnesium anode orifice hydrodynamic cavitation generator comprises: a first orifice plate, which is a magnesium anode plate; a second orifice plate, which is an inert cathode plate; a three-section detachable cylindrical reaction vessel with a flow channel therein; the orifice plates are clamped between the cylindrical reaction vessels, and the orifice plates are hermetically connected to the cylindrical reaction vessels.

2. The magnesium anode orifice hydrodynamic cavitation generator according to claim 1, characterized in that, the magnesium anode plate is pure magnesium or a magnesium alloy; the mass proportion of magnesium in the magnesium alloy is more than 90%; the thickness of the magnesium anode plate is 2-5 mm.

3. The magnesium anode orifice hydrodynamic cavitation generator according to claim 1, characterized in that, the inert cathode plate is an inert electrode plate such as a titanium oxide plate, a titanium plate, or a stainless steel plate; the thickness of the inert electrode plate is the same as that of the magnesium anode plate.

4. The magnesium anode orifice hydrodynamic cavitation generator according to claim 1, characterized in that, the orifice plate structure is a single-orifice plate structure or a multi-orifice plate structure; the diameter of the orifices is 1-5 mm, the orifices are evenly distributed, and are radially distributed from the central orifice to the outside.

5. The magnesium anode orifice hydrodynamic cavitation generator according to claim 1, characterized in that, the orifices on the anode plate and the cathode plate are not completely corresponding.

6. The magnesium anode orifice hydrodynamic cavitation generator according to claim 1, characterized in that, the distance between the anode plate and the cathode plate is 20-70 mm.

7. The magnesium anode orifice hydrodynamic cavitation generator according to claim 1, characterized in that, the outer diameter of the orifice plate is larger than the outer diameter of the cylindrical reaction vessel, a sealing ring is provided between the orifice plate and the cylindrical reaction vessel, and they are connected by fasteners; the part of the orifice plate exposed outside the outer diameter of the cylindrical reaction vessel is connected to a power supply.

8. A nitrogen and phosphorus-containing wastewater treatment system, characterized in that, it includes the magnesium anode orifice hydrodynamic cavitation generator according to any one of claims 1-7, and the treatment system further includes a filter, a circulation pool, a pressurizing device, and a pressure gauge; the upstream of the magnesium anode orifice hydrodynamic cavitation generator is connected to the circulation pool, a pressurizing device is provided between the magnesium anode orifice hydrodynamic cavitation generator and the circulation pool, pressure gauges are connected to both the upstream and downstream of the magnesium anode orifice hydrodynamic cavitation generator; the downstream of the magnesium anode orifice hydrodynamic cavitation generator is connected to the filter, and the filter is connected to the circulation pool.

9. The nitrogen and phosphorus-containing wastewater treatment system according to claim 8, characterized in that, the filter is a filter bag filter, a filter membrane filter, or a paper tape filter; the filtration accuracy of the filter is 1-50 μm; the circulation pool is provided with on-line monitoring devices for organic matter, nitrogen, and phosphorus to measure the concentrations of organic matter, nitrogen, and phosphorus in the wastewater in the circulation pool.

10. A method for synchronously removing organic matter, nitrogen, phosphorus, and microorganisms from wastewater by using the magnesium anode orifice hydrodynamic cavitation generator according to any one of claims 1-10, characterized in that, the method comprises the following steps: The first step is to adjust the pH value of the sewage to 7-9 and introduce it into the circulation pool; Step 2: Turn on the water pump to input the sewage in the circulation pool into the magnesium anode cavitation generator, and adjust the water pump to make the upstream pressure 3 - 8 bar; Step 3: After the magnesium anode cavitation generator is completely filled with water, turn on the DC power supply to start electrolysis, set the constant current mode, and the current range is between 0.1 - 0.5 A. Operate for a period of time until the indicators of the wastewater reach the predetermined target, then turn off the DC power supply and then turn off the water pump for drainage; Step 4: Collect the magnesium ammonium phosphate crystals in the filter.

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