A method for separating low-conductivity phosphorus-containing wastewater by electrochemically generating Fe-Ca green rust in situ

Abstract

The present invention discloses a method for separating low-conductivity phosphorus-containing wastewater by electrochemically generating Fe-Ca green rust in situ, which comprises the following three steps: electrochemically generating Fe-Ca green rust flocs in situ, capturing phosphorus in situ by the flocs, and controlling the release of phosphorus from the flocs for separation and recovery. 2+ Phosphorus in low-conductivity wastewater is captured ex situ by generating Fe-Ca green rust flocs. The Fe-Ca green rust flocs are produced in large quantities, have high stability, and a capture rate of over 93.4%. The captured phosphorus is released and recycled through the transformation of the Fe-Ca green rust floc structure, with a recovery rate of over 76.78%. During the floc generation stage, the electrode plates do not come into contact with the wastewater, avoiding electrode passivation and facilitating the generation of flocs. Compared with the electrochemical method, the capture rate is high and the energy consumption is low for treating the same wastewater, which greatly reduces the cost of treating low-conductivity wastewater.

Description

Technical Field

[0001] The present invention relates to a sewage treatment method, in particular to a method for separating phosphorus-containing wastewater by electrochemically generating Fe-Ca green rust in situ. Background Art

[0002] With the rapid development of modern industry and agriculture, a large amount of phosphorus enters the surface water environment through urban sewage and industrial and agricultural wastewater, leading to eutrophication of water bodies and disrupting the ecological balance. At present, the commonly used phosphorus removal methods mainly include chemical precipitation, biological method, adsorption method and electrochemical method. Chemical precipitation is the existing mainstream deep phosphorus removal technology. It can achieve ultra-low total phosphorus concentration in effluent, but the required chemical agents will cause secondary pollution, the treatment process is complicated and the cost is high. The biological method has high removal efficiency, but the cycle is long and the biological bacteria are selective for sewage components. The adsorption method produces less sludge, has a wide range of applications and is easy to operate, but lacks adsorbents for efficient phosphorus removal. Electrocoagulation method for phosphorus removal uses metal anode electrolysis to produce metal ion coagulants and cathode electrolysis to produce hydroxide ions, which spontaneously hydrolyze to form various monomer and polymer species, and finally form insoluble hydroxides. The newly formed amorphous flocculant has a large surface area, which is conducive to the rapid adsorption or capture of phosphorus in the water. The hydroxyl radicals it contains undergo coordination exchange with phosphate ions to achieve rapid phosphorus removal. At the same time, the metal cations produced by electrolysis can also co-precipitate with phosphate ions to remove them. The electroflocculation method has a high phosphorus removal efficiency, but long-term electrolysis causes the electrodes to be easily passivated, and the flocs or precipitates produced will be adsorbed on the cathode surface, resulting in an increase in overpotential, affecting the phosphorus removal efficiency. Especially for low-concentration phosphorus-containing wastewater, the low conductivity of the solution leads to an increase in ohmic drop, an increase in cell voltage, and an increase in energy consumption. Chinese invention patent CN114790018A effectively removes pollutants in wastewater through flocculation and ectopic capture, preventing other ions in the sewage from passivating the electrodes during the electroflocculation process, but the flocculent stability is poor; at the same time, this method only achieves the removal of pollutants and the treatment process is complicated, and the pollutants cannot be separated and recovered. Summary of the Invention

[0003] Purpose of the invention: The purpose of the present invention is to efficiently capture phosphorus in low-conductivity wastewater and release, separate and recover the captured phosphorus, and provide a method for ex situ separation of low-conductivity phosphorus-containing wastewater by electrochemically generating Fe-Ca green rust.

[0004] Technical solution: The method of the present invention for the ex situ separation of low-conductivity phosphorus-containing wastewater by electrochemically generating Fe-Ca green rust comprises the following three steps:

[0005] (a) adding an electrolyte solution composed of Na2SO4 and Na2CO3 to an electrolytic cell, controlling the amount of Na2SO4 and Na2CO3 added to adjust the conductivity of the electrolyte, adding CaCl2 to control the iron-calcium ratio, and controlling the pH value, dissolved oxygen content, current density, and electrolysis time to produce Fe-Ca green rust flocs;

[0006] (b) adjusting the pH and DO values ​​of the wastewater and controlling the residence time to allow the low-conductivity phosphorus-containing wastewater to flow from the water inlet of the electrolytic cell through the flocculent body to capture phosphorus;

[0007] (c) regulating the redox potential of the flocculent solution, controlling the pH and temperature of the flocculent to release phosphorus; introducing a phosphorus-rich solution into the flocculent solution to collect the phosphorus released from the solution and separating and collecting the phosphorus-containing precipitate.

[0008] Furthermore, in step (a), the electrode spacing of the electrolytic cell is 5-50 mm, and barrier membranes are provided on both sides of the electrodes near the water inlet and outlet of the electrolytic cell, which confines the floccules in the middle of the electrolytic cell, thereby facilitating the flow of phosphorus-containing wastewater through the floccules while preventing the floccules from entering the water outlet for secondary treatment; the electrode cell structure is a flat frame type or cylindrical.

[0009] Furthermore, the barrier membrane is a microfiltration membrane, which is made of nylon, polyamide, PET polyester fiber or PP polypropylene, with a mesh size of 300-800 mesh, preferably 400-600 mesh.

[0010] Furthermore, the electrode is a flat electrode, a particle electrode or a porous electrode, and its material is iron or carbon steel, wherein the particle electrode needs to be fixed with a titanium mesh.

[0011] Furthermore, step (a) includes the following steps:

[0012] (a1) Add 250-5000 mg / L of Na2SO4 and 25-500 mg / L of Na2CO3 to an electrolyte solution and adjust the conductivity to 550-11000 μs / cm, SO4 2- It is conducive to the rapid formation of flocs, CO3 2- It can buffer the pH change of the solution and thus reduce the destruction of the floc structure caused by pH change;

[0013] (a2) Add CaCl2 to control the iron-calcium ratio to 1 / 10-1 / 5, Ca 2+ Fe supplementation 2+ The formation of green rust further avoids the 2+ Converted to Fe 3+ Effect on the yield of green rust; at the same time, Ca 2+ It is not easy to be oxidized, and the generated flocs have a stable structure and high yield, thereby improving the capture rate of phosphorus elements; the iron ion content of the electrolysis in the electrolytic cell is calculated by the formula Where F is the Faraday constant F = 9.65 × 10000 C / mol, and the amount of calcium ions added is calculated;

[0014] (a3) Adjust the solution pH to 5-7. A neutral environment is conducive to the formation of green rust.

[0015] (a4) Control the dissolved oxygen concentration to 1-10 mg / L by nitrogen aeration, and the aeration rate is 0.1-3 L / (L·min) to avoid excessive oxygen content that may cause Fe in the solution 2+ Converted to Fe 3+ Reduce the production of green rust;

[0016] (a5) Adjust the current density to 1-10 mA / cm 2 , the reaction takes 10-60 minutes to generate Fe-Ca green rust flocs.

[0017] Furthermore, step (b) comprises the following steps:

[0018] (b1) adjusting the dissolved oxygen content of the wastewater to 1-15 mg / L by adding a combination of Na2SO3 and acetaldehyde oxime or carbohydrazide, with the amount of Na2SO3 being 8-20 mg / L, the amount of carbohydrazide being 5-20 mg / L, and the amount of acetaldehyde oxime being 7-50 mg / L, to prevent green rust oxidation from reducing the capture efficiency of phosphorus;

[0019] (b2) adjusting the pH value of the wastewater to 6-9;

[0020] (b3) Control the wastewater to flow through the flocs in a single pass or cyclic manner to capture phosphorus, and control the retention time of the flow to 10-30 minutes before discharge; a small amount of phosphate and free Ca 2+ 、Fe 2+ or Fe 3+ The generated precipitate is blocked between the flocculent layers.

[0021] Furthermore, step (c) comprises the following steps:

[0022] (c1) regulating the redox potential of the floc solution by combining H2O2 with oxygen or air, wherein the amount of H2O2 is 8-13 mg / L, the oxygen aeration rate is 0.1-0.2 L / (L·min), and the air aeration rate is 0.6-1 L / (L·min);

[0023] (c2) adjusting the solution pH to 6-10 and the temperature to 25° C.-35° C. to promote incomplete oxidation of green rust to magnetite and release phosphate anions;

[0024] (c3) controlling the phosphorus-rich liquid to pass through the floc layer and staying there for 5-10 minutes to collect the phosphate anions released into the solution;

[0025] (c4) separating the magnetite and the phosphorus-containing precipitate by magnetic separation, and collecting the phosphorus-containing precipitate.

[0026] Reaction mechanism: Under the action of the electric field, the anode electrolyzes the metal cation Fe 2+ or Fe 3+ , H2 and OH are electrolyzed at the cathode - , control the oxygen content of the solution to make the metal cation Fe 2+ or Fe 3+ With OH - Combined in the electrolyte solution, it forms green rust flocs with strong adsorption properties. 2+ Easily oxidized to Fe 3+ , introducing Ca 2+ Supplement and bind to the octahedron of green rust to form green rust flocs with a more stable structure. Green rust is a double metal hydroxide with a special layered structure and an LDHs structure. The intercalated layers are rich in exchangeable anions with strong binding ability to divalent or trivalent phosphate anions. The layered structure has a high specific surface area and abundant surface hydroxyl groups. It coordinates with phosphate anions in a monodentate mononuclear (bridging), bidentate mononuclear and bidentate binuclear manner to form complexes, and captures phosphate anions in wastewater through physical adsorption, surface complexation reaction, and coordination adsorption. After complete capture, oxides are introduced into the flocs, and the pH value and temperature are controlled to promote the incomplete oxidation of green rust into magnetite. Magnetite has a weak binding force with phosphate anions, and phosphorus is collected through phosphorus-rich liquid. A small amount of phosphate in the solution reacts with free Ca 2+ 、Fe 2+ or Fe 3+ The generated precipitate is blocked between the floc layers, and the phosphorus-containing precipitate is separated and recovered by magnetic separation.

[0027] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: 1. Fe-Ca green rust flocs ectopically capture phosphorus in low-conductivity wastewater, and the Fe-Ca green rust flocs are produced in large quantities, have high stability, and a capture rate of over 93.4%; 2. The captured phosphorus is released and recovered through the transformation of the Fe-Ca green rust floc structure, and the recovery rate reaches over 76.78%; 3. The electrode plates do not contact the wastewater during the floc production stage, avoiding electrode passivation and facilitating the production of flocs; 4. Compared with the electrochemical method, the capture rate is high and the energy consumption is low for treating the same wastewater, which greatly reduces the cost of treating low-conductivity wastewater. DETAILED DESCRIPTION

[0028] Example 1

[0029] Simulating a city's domestic sewage, the phosphorus content is 5mg / L and the conductivity is 50μs / cm. The electrolytic cell is cylindrical, with carbon steel as the flat electrode, the electrode spacing is 50mm, and the barrier membrane is made of nylon with a mesh size of 800. The phosphorus-containing wastewater is recycled and treated:

[0030] (1) adding an electrolyte solution consisting of 250 mg / L Na2SO4 and 25 mg / L Na2CO3 to adjust the conductivity of the wastewater to 550 μs / cm;

[0031] (2) Add CaCl2 to control the calcium-iron ratio in the solution to 1 / 10;

[0032] (3) The pH value was adjusted to 7, and the dissolved oxygen content of the solution was controlled to 10 mg / L by nitrogen aeration at 3 L / (L·min). The current density was 10 mA / cm 2 , the reaction lasted for 10 min to produce Fe-Ca green rust flocs;

[0033] (4) Add 8 mg / L of Na2SO3 and 5 mg / L of carbohydrazide to the phosphorus-containing wastewater to control the dissolved oxygen content of the phosphorus-containing wastewater to 15 mg / L;

[0034] (5) Adjust the pH value to 6, pass the phosphorus-containing wastewater through the flocculent body in a single-pass flow, control the flow retention time to 10 min, and then remove the phosphorus content at the outlet;

[0035] (6) 8 mg / L H2O2 was added to the flocs while exposing them to air at 0.6 L / (L·min) to adjust the redox potential of the floc solution. The pH of the solution was adjusted to 6, and the temperature was controlled at 25°C. The phosphorus-rich liquid was controlled to flow through the flocs in a single pass and was discharged after staying for 5 minutes.

[0036] (7) Recover the phosphorus element from the phosphorus-rich solution, and at the same time separate the phosphorus-containing precipitate in the solution by magnetic separation, and calculate the recovery rate of the phosphorus element.

[0037] The phosphorus content at the outlet of the test step (5) is 0 mg / L, the capture rate of phosphorus in the flocs is 100%, and the total energy consumption is 1.216 KWh / m 3 ; The recovery rate of phosphorus in step (7) is calculated to be 97.2%.

[0038] Comparative Example 1

[0039] The electrolytic cell is cylindrical, with carbon steel as the flat electrode, the electrode spacing is 50mm, the pH value is adjusted to 7, and the oxygen content of the solution is controlled to 10mg / L by nitrogen aeration at 3L / (L·min). An equal amount of the above-mentioned simulated domestic sewage from a certain city is passed into the electrolytic cell at a current density of 10mA / cm 2, After 10 minutes of reaction, remove the electrolytic cell and detect the phosphorus content at the outlet.

[0040] After treating phosphorus-containing wastewater in the above manner, the phosphorus content at the outlet was 0.05 mg / L, the removal rate of phosphorus was 99.9%, and the total energy consumption was 4.157 KWh / m 3 .

[0041] Example 2

[0042] The runoff water from a certain farmland was simulated, with a phosphorus content of 60 mg / L and a conductivity of 800 μs / cm. The electrolytic cell used a flat frame type, with carbon steel as the porous electrodes, an electrode spacing of 40 mm, and a barrier membrane made of nylon with a mesh size of 700. The phosphorus-containing wastewater was recycled and treated:

[0043] (1) Adding an electrolyte solution composed of 1000 mg / L Na2SO4 and 100 mg / L Na2CO3 to adjust the conductivity of the wastewater to 2200 μs / cm;

[0044] (2) Add CaCl2 to control the calcium-iron ratio in the solution to 1 / 9;

[0045] (3) The pH value was adjusted to 7, and the dissolved oxygen content of the solution was controlled to 8 mg / L by nitrogen aeration at 2.5 L / (L·min). The current density was 8 mA / cm 2 , Fe-Ca green rust flocs were produced after 30 min of reaction;

[0046] (4) adding 8 mg / L of Na2SO3 and 7 mg / L of acetaldehyde oxime to the phosphorus-containing wastewater to control the dissolved oxygen content of the phosphorus-containing wastewater to 12 mg / L;

[0047] (5) Adjust the pH value to 6, pass the phosphorus-containing wastewater through the flocculent body in a single-pass flow, control the flow retention time to 15 minutes, and then remove the phosphorus content at the outlet;

[0048] (6) 10 mg / L H2O2 was added to the flocs while exposing them to air at 0.8 L / (L·min) to adjust the redox potential of the floc solution. The pH of the solution was adjusted to 7, and the temperature was controlled at 25°C. The phosphorus-rich liquid was controlled to flow through the flocs in a single pass and was discharged after staying for 6 minutes.

[0049] (7) Recover the phosphorus element from the phosphorus-rich solution, and at the same time separate the phosphorus-containing precipitate in the solution by magnetic separation, and calculate the recovery rate of the phosphorus element.

[0050] The phosphorus content at the outlet of the test step (5) is 0.02 mg / L, the capture rate of phosphorus in the flocs is 99.9%, and the total energy consumption is 0.763 KWh / m 3; The recovery rate of phosphorus in step (7) is calculated to be 89.15%.

[0051] Comparative Example 2

[0052] The electrolytic cell adopts a flat frame type, with carbon steel as porous electrodes, an electrode spacing of 40 mm, and a pH value of 7. The oxygen content of the solution is controlled to 8 mg / L by nitrogen aeration at 2.5 L / (L·min). An equal amount of the above simulated farmland runoff water is passed into the electrolytic cell at a current density of 8 mA / cm 2 , remove the electrolytic cell after 30 minutes of reaction and detect the phosphorus content at the outlet.

[0053] After treating phosphorus-containing wastewater in the above manner, the phosphorus content at the outlet was 5.9 mg / L, the removal rate of phosphorus was 90.2%, and the total energy consumption was 2.431 KWh / m 3 .

[0054] Example 3

[0055] We simulated wastewater from a farm with a phosphorus content of 300 mg / L and a conductivity of 2000 μs / cm. The electrolytic cell was cylindrical, with iron electrodes, a 30 mm electrode spacing, and a 600 mesh PET fiber barrier film. The following was done to recycle the phosphorus-containing wastewater:

[0056] (1) adding an electrolyte solution consisting of 2600 mg / L of Na2SO4 and 260 mg / L of Na2CO3 to adjust the conductivity of the wastewater to 5600 μs / cm;

[0057] (2) adding CaCl2 to control the calcium-iron ratio in the solution to 1 / 8;

[0058] (3) The pH value was adjusted to 6, and the dissolved oxygen content of the solution was controlled to 7 mg / L by nitrogen aeration at 2 L / (L·min). The current density was 5 mA / cm 2 , Fe-Ca green rust flocs were produced after 40 min of reaction;

[0059] (4) adding 10 mg / L of Na2SO3 and 12 mg / L of carbohydrazide to the phosphorus-containing wastewater to control the dissolved oxygen content of the phosphorus-containing wastewater to 10 mg / L;

[0060] (5) Adjust the pH value to 7, pass the phosphorus-containing wastewater through the flocculent body in a single-pass flow, control the flow retention time to 20 min, and then remove the phosphorus content at the outlet;

[0061] (6) 10 mg / L H2O2 was added to the flocs and air was exposed at 1 L / (L·min) to adjust the redox potential of the floc solution. The pH of the solution was adjusted to 8 and the temperature was controlled at 30°C. The phosphorus-rich solution was controlled to flow through the flocs in a single pass and was discharged after staying for 7 minutes.

[0062] (7) Recover the phosphorus element from the phosphorus-rich solution, and at the same time separate the phosphorus-containing precipitate in the solution by magnetic separation, and calculate the recovery rate of the phosphorus element.

[0063] The phosphorus content at the outlet of the test step (5) is 5.1 mg / L, the capture rate of phosphorus in the flocs is 98.3%, and the total energy consumption is 0.632 KWh / m 3 ; The recovery rate of phosphorus in step (7) is calculated to be 85.43%.

[0064] Comparative Example 3

[0065] The electrolytic cell is cylindrical, with iron as the flat electrode and an electrode spacing of 30 mm. The pH value is adjusted to 6, and the oxygen content of the solution is controlled to 7 mg / L by nitrogen aeration at 2 L / (L·min). An equal amount of the above-mentioned simulated farm wastewater is passed into the electrolytic cell at a current density of 5 mA / cm 2 , After 40 minutes of reaction, remove the electrolytic cell and detect the phosphorus content at the outlet.

[0066] After treating phosphorus-containing wastewater in the above manner, the phosphorus content at the outlet was 84.6 mg / L, the removal rate of phosphorus was 71.8%, and the total energy consumption was 1.743 KWh / m 3 .

[0067] Example 4

[0068] We simulated severely polluted groundwater with a phosphorus content of 1000 mg / L and a conductivity of 3000 μs / cm. The electrolytic cell used a flat-plate frame, with iron as the porous electrode, a 20 mm inter-electrode spacing, and a 400 mesh PP barrier film. The following was done to recycle the phosphorus-containing wastewater:

[0069] (1) Adding an electrolyte solution consisting of 4000 mg / L Na2SO4 and 400 mg / L Na2CO3 to adjust the conductivity of the wastewater to 8800 μs / cm;

[0070] (2) adding CaCl2 to control the calcium-iron ratio in the solution to 1 / 7;

[0071] (3) The pH value was adjusted to 5, and the dissolved oxygen content of the solution was controlled to 3 mg / L by nitrogen aeration at 1.5 L / (L·min), and the current density was 3 mA / cm 2 , Fe-Ca green rust flocs were produced after 55 min of reaction;

[0072] (4) adding 20 mg / L of Na2SO3 and 20 mg / L of carbohydrazide to the phosphorus-containing wastewater to control the dissolved oxygen content of the phosphorus-containing wastewater to 15 mg / L;

[0073] (5) Adjust the pH value to 8, pass the phosphorus-containing wastewater through the flocculent body in a single-pass flow, control the flow retention time to 25 min, and then remove the phosphorus content at the outlet;

[0074] (6) 12 mg / L H2O2 was added to the flocs while exposing them to oxygen at 0.1 L / (L·min) to adjust the redox potential of the floc solution. The pH of the solution was adjusted to 9, and the temperature was controlled at 35°C. The phosphorus-rich liquid was controlled to flow through the flocs in a single pass and was discharged after staying for 8 minutes.

[0075] (7) Recover the phosphorus element from the phosphorus-rich solution, and at the same time separate the phosphorus-containing precipitate in the solution by magnetic separation, and calculate the recovery rate of the phosphorus element.

[0076] The phosphorus content at the outlet of the test step (5) is 54 mg / L, the capture rate of phosphorus in the flocs is 94.6%, and the total energy consumption is 0.524 KWh / m 3 ; The recovery rate of phosphorus in step (7) is calculated to be 78.45%.

[0077] Comparative Example 4

[0078] The electrolytic cell adopts a flat frame type, with iron as the porous electrode, the electrode spacing is 20mm, the pH value is adjusted to 5, and the oxygen content of the solution is controlled to 3mg / L by nitrogen aeration at 1.5L / (L·min). The same amount of the above-mentioned simulated farm wastewater is passed into the electrolytic cell with a current density of 3mA / cm 2 , remove the electrolytic cell after 55 minutes of reaction and detect the phosphorus content at the outlet.

[0079] After treating phosphorus-containing wastewater in the above manner, the phosphorus content at the outlet was 397 mg / L, the removal rate of phosphorus was 60.3%, and the total energy consumption was 1.421 KWh / m 3 .

[0080] Example 5

[0081] The phosphorus-containing wastewater from a certain factory was simulated, with a phosphorus content of 3000 mg / L and a conductivity of 5000 μs / cm. The electrolytic cell used a flat-plate frame, with iron particles as electrodes secured by a titanium mesh, a 5 mm inter-electrode spacing, and a nylon barrier membrane with a mesh size of 300. The phosphorus-containing wastewater was recycled and treated as follows:

[0082] (1) Adding an electrolyte solution consisting of 5000 mg / L of Na2SO4 and 500 mg / L of Na2CO3 to adjust the conductivity of the wastewater to 11000 μs / cm;

[0083] (2) adding CaCl2 to control the calcium-iron ratio in the solution to 1 / 5;

[0084] (3) The pH value was adjusted to 5, and the dissolved oxygen content of the solution was controlled to 1 mg / L by nitrogen aeration at 0.1 L / (L·min). The current density was 1 mA / cm 2 , Fe-Ca green rust flocs were produced after 60 min of reaction;

[0085] (4) Add 20 mg / L of Na2SO3 and 50 mg / L of acetaldehyde oxime to the phosphorus-containing wastewater to control the dissolved oxygen content of the phosphorus-containing wastewater to 1 mg / L;

[0086] (5) Adjust the pH value to 9, and pass the phosphorus-containing wastewater through the flocculent body in a circulating flow. After the flow retention time is controlled to 30 min, the phosphorus content at the outlet is detected;

[0087] (6) 13 mg / L H2O2 was added to the flocs while exposing them to oxygen at 0.2 L / (L·min) to adjust the redox potential of the floc solution. The pH of the solution was adjusted to 10, and the temperature was controlled at 35°C. The phosphorus-rich liquid was controlled to flow through the flocs in a single pass and was discharged after staying for 10 minutes.

[0088] (7) Recover the phosphorus element from the phosphorus-rich solution, and at the same time separate the phosphorus-containing precipitate in the solution by magnetic separation, and calculate the recovery rate of the phosphorus element.

[0089] The phosphorus content at the outlet of the test step (5) is 198 mg / L, the capture rate of phosphorus in the flocs is 93.4%, and the total energy consumption is 0.412 KWh / m 3 ; The recovery rate of phosphorus in step (7) is calculated to be 76.78%.

[0090] Comparative Example 5

[0091] The electrolytic cell adopts a flat frame type, with iron as the porous electrode, the electrode spacing is 20mm, the pH value is adjusted to 5, and the oxygen content of the solution is controlled to 1mg / L by nitrogen aeration at 0.1L / (L·min). The same amount of the above-mentioned simulated phosphorus-containing wastewater from a certain factory is passed into the electrolytic cell at a current density of 1mA / cm 2 After 60 minutes of reaction, the electrolytic cell was removed and the phosphorus content at the outlet was detected. After treating the phosphorus-containing wastewater in the above manner, the phosphorus content at the outlet was 1632 mg / L, the removal rate of phosphorus was 45.6%, and the total energy consumption was 1.164 KWh / m 3 .

[0092] Comparative Example 6

[0093] Compared with Example 3, without introducing Ca 2+ Supplementary binding to the octahedron of green rust changes the generation amount and stability of green rust flocs, and treats an equal amount of simulated wastewater of Example 3. The phosphorus content at the outlet is detected to be 61.5 mg / L, the capture rate of phosphorus in the flocs is 79.5%, and the total energy consumption is 0.632 KWh / m 3 ; The calculated recovery rate of phosphorus is 67.15%.

[0094] Comparative Example 7

[0095] Compared with Example 4, without introducing Ca 2+ Supplementary binding to the octahedron of green rust changes the generation amount and stability of green rust flocs, and treats an equal amount of simulated wastewater of Example 4. The phosphorus content at the outlet is detected to be 293 mg / L, the capture rate of phosphorus in the flocs is 70.7%, and the total energy consumption is 0.524 KWh / m 3 ; The calculated recovery rate of phosphorus is 60.65%.

[0096] Comparative Example 8

[0097] Compared with Example 5, without introducing Ca 2+ Supplementary binding to the octahedron of green rust changes the generation and stability of green rust flocs, and treats an equal amount of simulated wastewater from Example 5. The phosphorus content at the outlet is detected to be 666 mg / L, the capture rate of phosphorus in the flocs is 77.8%, and the total energy consumption is 0.412 KWh / m 3 ; The calculated recovery rate of phosphorus was 64.17%.

[0098] Table 1 Recovery rates of phosphorus-containing wastewater under different recovery conditions

[0099]

[0100] By comparing Examples 1-5 with Comparative Examples 1-5, it is found that the addition of electrolytes Na2SO4 and Na2CO3 to adjust the conductivity of the electrolyte and introduce Ca 2+ Controlling the calcium-iron ratio to generate flocs to capture phosphorus has a higher capture rate than the simple iron electrochemical reaction to generate flocs to capture phosphorus. This is because SO4 2- It is conducive to the rapid formation of flocs, CO3 2-The pH value of the buffer solution changes to prevent the floc structure from being destroyed. In Examples 1-5, as the phosphorus content in the wastewater continues to increase, the addition of electrolytes Na2SO4 and Na2CO3 to adjust the conductivity of the electrolyte to generate flocs maintains a phosphorus capture rate of over 93.4%, with no significant change. However, in Comparative Examples 1-5, which generate flocs using a simple iron electrochemical reaction, the phosphorus capture rate drops from 94.4% to 45.6%, and the energy consumption is also higher than that of Examples 1-5. This is because the phosphorus-containing wastewater has a low conductivity, and the cell voltage required for the same current increases, leading to increased energy consumption. Furthermore, other pollutants in the phosphorus-containing wastewater precipitate and adsorb on the electrode surface upon contact with the charged electrode, resulting in electrode passivation and affecting the amount of floc produced. Consequently, insufficient flocs can be generated to capture the large amount of phosphorus in the wastewater. Therefore, the iron electrochemical reaction treatment of wastewater with high phosphorus content is less effective.

[0101] By comparing Examples 3-5 and Comparative Examples 6-8, it is found that adding electrolytes Na2SO4 and Na2CO3 to adjust the conductivity of the electrolyte while introducing Ca 2+ Controlling the calcium-iron ratio to generate flocs to capture phosphorus has a higher capture rate than simply adding electrolytes Na2SO4 and Na2CO3 to adjust the conductivity of the electrolyte to generate flocs to capture phosphorus. This is because the Fe 2+ Easily oxidized to Fe 3+ Affect the formation of green rust, introduce Ca 2+ Can replace Fe 2+ Combined with the octahedron of green rust to continue to form new green rust, increasing the amount of green rust generated, while Ca 2+ The fact that it is not easily oxidized increases the stability of green rust, thereby improving the capture rate of phosphorus and thus improving the recovery rate of phosphorus.

Claims

1. A method for ex situ separation of low conductivity phosphorus-containing wastewater by electrochemically generating Fe-Ca green rust, characterized in that: It includes the following three steps: (a) adding an electrolyte solution composed of Na2SO4 and Na2CO3 to an electrolytic cell, controlling the amount of Na2SO4 and Na2CO3 added to adjust the conductivity of the solution, adding CaCl2 to control the iron-calcium ratio, and controlling the pH value, dissolved oxygen content, current density, and electrolysis time to produce Fe-Ca green rust flocs; (b) Adjusting the pH and DO values ​​of the wastewater and controlling the residence time to allow the low-conductivity phosphorus-containing wastewater to flow from the inlet of the electrolytic cell through the flocculent bodies to capture phosphorus; (c) regulating the redox potential of the floc solution, controlling the pH and temperature of the flocs to release phosphorus; introducing a phosphorus-rich solution into the floc solution to collect the phosphorus released from the solution and separating and collecting the phosphorus-containing precipitate; Step (a) comprises the following steps: (a1) adding an electrolyte solution consisting of 250-5000 mg / L of Na2SO4 and 25-500 mg / L of Na2CO3 to adjust the conductivity to 550-11000 μs / cm; (a2) Add CaCl2 to control the iron-calcium ratio to 1 / 10-1 / 5; (a3) adjusting the pH of the solution to 5-7; (a4) Adjusting the dissolved oxygen concentration to 1-10 mg / L by nitrogen aeration; (a5) Adjust the current density to 1-10 mA / cm 2 , react for 10-60 minutes to generate Fe-Ca green rust flocs; Step (b) comprises the following steps: (b1) adjusting the dissolved oxygen content of the wastewater to 1-15 mg / L by combining Na2SO3 with acetaldehyde oxime or carbohydrazide; (b2) Adjust the pH value of the wastewater to 6-9; (b3) Controlling the flow of wastewater through the flocs to capture phosphorus in a single-pass or cyclic manner, and controlling the residence time of the flow to be 10-30 minutes before discharge; Step (c) comprises the following steps: (c1) regulating the redox potential of the floc solution by combining H2O2 with oxygen or air; (c2) adjusting the solution pH to 6-10 and the temperature to 25°C-35°C to release phosphate anions; (c3) Control the phosphorus-rich liquid to pass through the floc layer and stay for 5-10 minutes to collect the phosphate anions released into the solution; (c4) separating the magnetite and the phosphorus-containing precipitate, and collecting the phosphorus-containing precipitate.

2. The method for ex situ separation of low-conductivity phosphorus-containing wastewater by electrochemically generating Fe-Ca green rust according to claim 1, characterized in that: In step (a), the distance between the electrodes of the electrolytic cell is 5-50 mm, and barrier membranes are provided on both sides of the electrodes near the water inlet and outlet of the electrolytic cell; the electrode cell is a flat frame type or cylindrical.

3. The method for ex situ separation of low-conductivity phosphorus-containing wastewater by electrochemically generating Fe-Ca green rust according to claim 2, characterized in that: The barrier membrane is a microfiltration membrane, which is made of nylon, polyamide, PET polyester fiber or PP polypropylene, and has a mesh size of 300-800 meshes.

4. The method for ex situ separation of low-conductivity phosphorus-containing wastewater by electrochemically generating Fe-Ca green rust according to claim 2, characterized in that: The electrode is a flat plate electrode, a particle electrode or a porous electrode, and its material is iron or carbon steel. The particle electrode needs to be fixed with a titanium mesh.

5. The method for ex situ separation of low-conductivity phosphorus-containing wastewater by electrochemically generating Fe-Ca green rust according to claim 1, characterized in that: The nitrogen usage in step (a4) is 0.1-3 L / (L·min).

6. The method for ex situ separation of low-conductivity phosphorus-containing wastewater by electrochemically generating Fe-Ca green rust according to claim 1, characterized in that: In step (b1), the amount of Na2SO3 used is 8-20 mg / L, the amount of carbohydrazide used is 5-20 mg / L, and the amount of acetaldehyde oxime used is 7-50 mg / L.

7. The method for ex situ separation of low-conductivity phosphorus-containing wastewater by electrochemically generating Fe-Ca green rust according to claim 1, characterized in that: In step (c1), the amount of H2O2 used is 8-13 mg / L, the oxygen aeration amount is 0.1-0.2 L / (L·min), and the air aeration amount is 0.6-1 L / (L·min).

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