Multi-stage sewage treatment device and method combining biological treatment with electro-Fenton

Abstract

The invention discloses a multistage sewage treatment device and method combining biological treatment and electro-Fenton, and relates to the field of water pollution control, the device is composed of a three-chamber microbial fuel cell reactor, and the three-chamber microbial fuel cell reactor is sequentially composed of a microbial anode chamber, an electro-Fenton cathode chamber and a photosynthesis chamber from left to right. The biological anode chamber consists of filled anaerobic granular sludge and a carbon brush electrode serving as an electron collector; and the core structure of the electro-Fenton cathode chamber is a gas diffusion cathode loaded with nano FeCo2O4. The microbial anode chamber and the electro-Fenton cathode chamber are isolated by a proton exchange membrane, and the gas diffusion electrode separates the electro-Fenton cathode chamber and the photosynthesis chamber at the same time. According to the invention, the combination of two clean energy sources, namely microbial energy and light energy, is realized, the efficient removal capability of aerobic and anaerobic biological treatment technologies and the synergistic mechanism of microorganisms and algae for purifying sewage are combined, the efficiency of the microbial fuel cell is further optimized, and the removal of ammonia nitrogen and refractory organic pollutants in the sewage is realized.

Description

Technical Field

[0001] The invention relates to the field of water pollution control, and in particular to a multi-stage sewage treatment device and method combining biological treatment with electro-Fenton. Background Art

[0002] As the global water shortage and water pollution problems become increasingly serious, it is imperative to find efficient, energy-saving and environmentally friendly sewage treatment technologies. Traditional sewage treatment methods include physical, chemical and biological methods. Traditional sewage treatment methods consume a lot of electricity, which increases energy costs, and produce greenhouse gases during the treatment process, which has a negative impact on the environment. In addition, biological treatment has low efficiency, is difficult to remove non-degradable organic pollutants, and may also cause secondary pollution.

[0003] Microbial fuel cells (MFCs) are an emerging sewage treatment technology that purifies sewage and recycles electricity at the same time, achieving an organic combination. Microorganisms convert organic matter into electricity, removing pollution while basically requiring no additional energy input, with low operating costs and no secondary pollution. Sewage can be treated in real time with higher efficiency. However, the industrial application of MFCs still faces problems such as limited degradation capacity, inability to completely remove difficult-to-degrade organic pollutants, low power generation, weak buffering capacity, and easy electrode contamination.

[0004] The electro-Fenton process removes difficult-to-degrade organic pollutants by electrocatalytically activating molecular oxygen to produce highly oxidizing active species such as hydroxyl radicals. It has the advantages of high efficiency, saving on reagent costs, simple and easy operation, small footprint, and low sludge output. However, the electro-Fenton process has strict requirements on electrode materials, high energy consumption, and a single application scenario.

[0005] The bio-electro-Fenton system combines the anode biodegradation of microbial fuel cells with the cathode electro-Fenton cathode degradation, and can achieve deep removal of organic pollutants in sewage under self-powered conditions. However, the overall efficiency of the current bio-electro-Fenton technology is still low when it is self-powered due to the large overpotential of the bio-anode chamber, the low dissolved oxygen in the cathode chamber, the activity and long-term stability of the cathode catalyst. Higher electrochemical activity of the bio-anode is a precursor to the efficient operation of the system under self-powered conditions, but the traditional bio-anode has a long enrichment time, and the preparation of three-dimensional porous electrodes with high specific surface area is complex and costly, and faces many problems when the reactor is scaled up. For the problem of dissolved oxygen in the cathode chamber, aeration can usually be used to increase the dissolved oxygen concentration, but this will greatly increase the energy cost, and due to the low solubility of O2 in water, the traditional aeration method is inefficient and is not suitable for electrode surface reactions such as electro-Fenton. The selection of suitable cathode catalysts and configurations is also difficult, especially in maintaining cathode performance for a long time while meeting basic performance. Summary of the invention

[0006] The purpose of the present invention is to provide a multi-stage sewage treatment device and method combining biological treatment with electro-Fenton, which can efficiently and stably remove COD, ammonia nitrogen and difficult-to-degrade organic pollutants in sewage.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is:

[0008] A multi-stage sewage treatment device combining biological treatment with electro-Fenton comprises a microbial anode chamber, an electro-Fenton cathode chamber and a photosynthesis chamber. The sewage enters the electro-Fenton cathode chamber after being treated in the microbial anode chamber. The microbial anode chamber is composed of filled anaerobic granular sludge and a microbial anode electrode as an electron collector. The microbial anode chamber and the electro-Fenton cathode chamber are separated by a proton exchange membrane. A gas diffusion cathode is loaded between the electro-Fenton cathode chamber and the photosynthesis chamber. The microbial anode electrode and the gas diffusion cathode form a loop and transmit electrons to the electro-Fenton cathode chamber. The CO2 gas generated in the microbial anode chamber is transported to the photosynthesis chamber and converted into O2 by the photosynthesis chamber. The gas diffusion cathode reduces and activates the O2 to produce highly oxidizing active species and enters the electro-Fenton cathode chamber. The electro-Fenton reaction occurs in the electro-Fenton cathode chamber to remove organic pollutants in the sewage.

[0009] The method for treating sewage based on the multi-stage sewage treatment device comprises the following steps:

[0010] The first step is to construct a microbial fuel cell and electrically acclimate the anaerobic granular sludge in the microbial anode chamber; the sewage enters the microbial anode chamber, and the microorganisms in the anaerobic granular sludge perform primary decomposition of organic matter in the sewage to achieve COD removal and generate electrons and CO2; the electrons reach the gas diffusion cathode through the external circuit, and the CO2 enters the photosynthesis chamber through the gas loop, providing CO2 for the photosynthesis of green algae in the photosynthesis chamber, and the generated O2 enters the cathode chamber through the gas diffusion cathode, and is reduced and activated by the nano-FeCo2O4 in the gas diffusion cathode to produce highly oxidizing active species;

[0011] In the second step, the water outlet of the anode chamber enters the cathode chamber, where the refractory organic pollutants that are not removed by microorganisms are secondary degraded and removed by the strong oxidizing active species produced in the electro-Fenton cathode chamber; the protons produced by the microorganisms decomposing the sewage on the microbial anode electrode enter the cathode chamber through the proton exchange membrane and participate in the electro-Fenton reaction of the FeCo2O4 gas diffusion cathode;

[0012] In the third step, the treated wastewater is discharged from the electro-Fenton cathode chamber.

[0013] In view of the problems faced by the bio-electro-Fenton system in potential applications, the present invention redesigns the overall configuration of the bio-electro-Fenton reactor and optimizes various parts of the reactor. First, compared with the mainstream dual-chamber configuration, the present invention adds an additional photosynthesis chamber. The photosynthesis of hydrogel-immobilized algae can reduce the CO2 emission of the bio-anode chamber and provide O2 for the electro-Fenton cathode chamber to increase the dissolved oxygen concentration in the cathode chamber, promote the electro-Fenton reaction and pollutant degradation.

[0014] In order to solve the problem of large overpotential in the biological anode chamber, the present invention uses anaerobic granular sludge as the inoculation source and a carbon brush electrode with high porosity and comparative area as an electron collector, which can effectively provide the biological capacity of the anode chamber. After running for a period of time, the proportion of the electroactive microbial population in the anaerobic granular sludge will further increase, thereby improving the performance.

[0015] For the electro-Fenton cathode chamber, the present invention uses a gas diffusion cathode modified with nano-FeCo2O4. Co and Fe sites can respectively reduce O2 to H2O2 and activate H2O2 to produce active species such as hydroxyl radicals under the continuous supply of cathode electrons, while the metal oxide can maintain long-term stability when operating at neutral pH.

[0016] The reactor configuration of the present invention effectively integrates microbial treatment, electro-Fenton and photosynthesis, and realizes synergistic sewage purification through water and gas exchange between different reactor chambers, thus solving the problems of low efficiency and difficulty in effective maintenance of the bio-electro-Fenton system in self-powered removal of organic pollutants.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0018] 1. The present invention realizes the combination of two major clean energy sources, microbial energy and light energy, and combines the efficient removal capabilities of aerobic and anaerobic biological treatment technologies and the synergistic mechanism of microorganisms and algae in purifying sewage, further optimizing the efficiency of microbial fuel cells and achieving the removal of ammonia nitrogen and difficult-to-degrade organic pollutants in sewage.

[0019] 2. The present invention realizes the fixation and conversion of O2 and CO2 under the energy self-sufficiency of the system. The sewage can be decomposed into CO2 by anaerobic granular sludge digestion reaction, and the in-situ photosynthetic oxygen production of green algae replaces mechanical aeration, and no gas or energy supply is required throughout the process.

[0020] 3. The present invention can adjust a series of operating parameters of the microbial fuel cell, such as organic load, sewage concentration, hydraulic retention time, dissolved oxygen, water inlet mode, etc., and ultimately stabilize parameters such as voltage and dissolved oxygen.

[0021] 4. The present invention has the characteristics of high decontamination efficiency, simple treatment process and low operating cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the structure of the device of the present invention.

[0023] Figure 2 It is a physical schematic diagram of the device of the present invention.

[0024] The meanings of the reference numerals in the figure are:

[0025] 1-microorganism anode chamber, 2-electro-Fenton cathode chamber, 3-photosynthesis chamber, 4-microorganism anode electrode, 5-proton exchange membrane, 6-gas diffusion cathode, 7-green algae hydrogel, 8-anode chamber water inlet pipe, 9-anode chamber water outlet pipe, 10-cathode chamber water inlet pipe, 11-cathode chamber water outlet pipe, 12-anode chamber exhaust port, 13-photosynthesis chamber air inlet, 14-resistance. DETAILED DESCRIPTION

[0026] The present invention is further described in detail below in conjunction with embodiments and drawings.

[0027] Example 1

[0028] like Figure 1 and 2 As shown, the multi-stage sewage treatment device combining biological treatment and electro-Fenton provided by the present invention adopts a three-chamber microbial fuel cell configuration, including an anaerobic microbial anode chamber 1, an aerobic electro-Fenton cathode chamber 2 and a photosynthesis chamber 3, a proton exchange membrane 5 is arranged between the microbial anode chamber 1 and the electro-Fenton cathode chamber 2, and a gas diffusion cathode 6 is arranged between the electro-Fenton cathode chamber 2 and the photosynthesis chamber 3. The specific structure of each component is as follows:

[0029] 1. Microbial anode chamber

[0030] The microbial anode chamber 1 is a closed structure with a cavity inside, and a water inlet is opened at the bottom. The water inlet is connected to the anode chamber water inlet pipe 8 and is connected to the external water supply device. The anode chamber water inlet pipe 8 is also provided with a water pump and a water inlet valve. The water pump is used to provide power for the sewage inlet of the microbial anode chamber 1, and the water inlet valve is used to control the water inlet flow of sewage into the microbial anode chamber 1.

[0031] An anode chamber exhaust port 12 is provided at the top of the microbial anode chamber 1, which is connected to the inner chamber of the photosynthesis chamber 3 through the anode chamber exhaust port 12. A switch valve is provided on the anode chamber exhaust port 12 to control the opening and closing of the exhaust port.

[0032] A microbial anode electrode 4 is also fixed in the internal chamber of the microbial anode chamber 1. The surface of the microbial anode electrode 4 is used to attach anaerobic microbial membranes, i.e., enriched electroactive microorganisms. In order to make the microbial anode electrode 4 better provide attachment sites for microorganisms, a carbon brush material is used. The carbon brush is acid-treated and heat-treated before use, which can increase the output power.

[0033] When preparing the microbial anode electrode 4, the carbon brush was first soaked in acetone for 12 hours, taken out and soaked in a 2 mol / L hydrochloric acid solution for 4 hours, and finally, it was repeatedly ultrasonically cleaned with deionized water until the pH value reached 7.0, and dried in an oven at 60°C for use.

[0034] The microbial anode chamber 1 is connected to the electro-Fenton cathode chamber 2 through a proton exchange membrane 5, and only protons are allowed to pass through. In addition, a connecting pipeline is provided between the microbial anode chamber 1 and the electro-Fenton cathode chamber 2, namely, an anode chamber outlet pipe 9 and a cathode chamber inlet pipe 10 connected to each other, and a valve is provided on the connecting pipeline to control the flow rate of sewage and adjust the hydraulic retention time of sewage in the electro-Fenton cathode chamber 2.

[0035] 2. Electro-Fenton cathode chamber

[0036] The electro-Fenton cathode chamber 2 is a transparent structure with a hollow chamber, and is made of organic glass to ensure the effects of light transmission, corrosion resistance and insulation.

[0037] The bottom of the electro-Fenton cathode chamber 2 is connected to the outside through a drainage pipe 11 for discharging sewage treated by the device.

[0038] The electro-Fenton cathode chamber 2 and the photosynthesis chamber 3 are separated by a gas diffusion cathode 6, which is composed of a titanium mesh substrate, a catalyst layer FeCo2O4 and a diffusion layer PTFE in sequence. The diffusion layer PTFE is closely attached to the photosynthesis chamber 3, while the catalyst layer FeCo2O4 faces the side of the electro-Fenton cathode chamber 2.

[0039] The preparation steps of the gas diffusion cathode are as follows:

[0040] Step 1: Preparation of carbon paste and titanium mesh:

[0041] The preparation process of carbon paste is as follows: weigh 3g of carbon black in a 250mL beaker, add ethanol to the 100mL mark for dispersion, use a pipette to measure 1mL of PTFE emulsion in the dispersed carbon black, stir slowly with a magnetic stirrer for 30min, then place it in an ultrasonic cleaning pot for 10min, take it out and absorb the ethanol to make a carbon paste. The preparation of titanium mesh includes alkaline washing and acid washing of titanium mesh, heating and boiling for 30min under the conditions of adding 10% sodium hydroxide and oxalic acid respectively, then rinse it with distilled water, dry it, and then soak it in 30% PTFE emulsion, repeatedly soak and dry it until the titanium mesh can be well hydrophobic, that is, a diffusion layer of PTFE is formed.

[0042] Step 2: Hot pressing and high temperature calcination:

[0043] The formed carbon paste is evenly applied on the prepared titanium mesh, and the carbon paste is repeatedly rolled with a glass rod to evenly cover one side of the titanium mesh, maintaining a thickness of 2-3 mm, and then hot-pressed at 95°C for 15 minutes. The hot-pressed electrode is placed in a muffle furnace, calcined at 300°C for 5 minutes, and then taken out.

[0044] Step 3: Preparation of catalyst layer coating:

[0045] First, weigh 100 mg of ferrous chloride FeCl2, 250 mg of cobalt nitrate Co(NO3)2·6H2O, 30 mg of urea, and 150 mg of ammonium fluoride NH4F, add 100 mL of deionized water, and stir magnetically for 2 hours; transfer the obtained mixed solution into a 100 mL hydrothermal autoclave and keep it at 100-200°C for 8 hours; after the reaction is completed, wash it with deionized water and anhydrous ethanol respectively, vacuum dry it, and keep it at 60°C for 10-15 hours to obtain FeCo2O4 catalyst. The catalytic layer of the gas diffusion cathode was prepared by a coating method. 20 mg of FeCo2O4 material, 2 mg of conductive acetylene black, and 4 mg of polyvinylidene fluoride (PVDF) were weighed in turn and ground in an agate mortar for 1 to 3 h. 300 μL of NMP was drawn into the agate mortar with a pipette and ground again to obtain a uniform slurry. The slurry was evenly coated on the other side of the titanium mesh and dried at 60°C in a vacuum drying oven for 10 to 15 h. Finally, the prepared electrode sheet was kept at a pressure of 8 to 15 MPa for 2 min under a tablet press to obtain a FeCo2O4 catalytic layer.

[0046] At this time, the prepared electrode is a final formed gas diffusion cathode, one side of which is a diffusion layer of PTFE and the other side is a catalyst layer of FeCo2O4.

[0047] A resistor 14 is connected in series with the microbial anode electrode 4 and the gas diffusion cathode 6 via a wire to form a circuit, and the electrons generated by the microbial anode electrode 4 reach the gas diffusion cathode 6 through the circuit.

[0048] 3. Photosynthesis Chamber

[0049] The photosynthesis chamber 3 is a transparent structure with a hollow chamber, and is made of organic glass to ensure light transmission, corrosion resistance and insulation effects.

[0050] The photosynthesis chamber 3 is loaded with green algae hydrogel 7, which can absorb CO2 produced by the microbial anode chamber 1. The produced O2 passes through the diffusion layer PTFE of the gas diffusion cathode 6, is reduced and activated by the catalytic layer FeCo2O4 to produce highly oxidizing active species, and enters the electro-Fenton cathode chamber 2.

[0051] The green algae include one or more of Chlorella vulgaris, Phaeodactylumtricornutum, Nannochloropsis sp., Scenedesmus obliquus or Dunaliella salina.

[0052] The preparation steps of green algae hydrogel are as follows: take 40mL Chlorella algae solution (the concentration of Chlorella algae solution is OD 600 =1.0) in a beaker, add sodium alginate to a final (mass) concentration of 0.3%, stir thoroughly and let stand to obtain a chlorella solution; prepare 10g / L calcium chloride solution in another beaker, drop the chlorella solution into the constantly stirred calcium chloride solution at a speed of 5r / min using a peristaltic pump, filter out excess calcium chloride solution, and then form algae gel balls.

[0053] Green algae concentration detection method: Use an ultraviolet spectrophotometer to measure the absorbance of the green algae suspension at a specific wavelength to determine the concentration of the green algae suspension.

[0054] BG11 medium basic formula: N salt: NaNO3 1.5g / L; P salt: K2HPO4 0.04g / L; S salt: MgSO4·7H2O0.075g / L; CaCl2·2H2O 0.036g / L; Na2CO3 0.02g / L; A4: C6H8O7 0.60mg / L, (NH4)3Fe(C6H5O7)20.60mg / L, Na2EDTA 0.10mg / L; A5: H3BO3 2.86×10 -3 g / L, MnCl2·4H2O1.81×10 -3 g / L, ZnSO4·7H2O 0.22×10 -3 g / L、Na2MoO4·2H2O 0.39×10 -3g / L, CuSO4·5H2O0.80×10 -4 g / L、Co(NO3)2·6H2O 0.49×10 -4 g / L.

[0055] The pH of the medium was adjusted to 6.5 with 1 mol / L NaOH or HCl to obtain BG11 medium.

[0056] Example 2

[0057] The method for treating COD, ammonia nitrogen and the difficult-to-degrade organic pollutant BPA in sewage by using the above-mentioned sewage treatment device is as follows:

[0058] The first step is to assemble the sewage treatment device. The reaction tank cavity is made of acrylic (PMMA) and is sealed with silicone pads and screws. The top of the shell is removable. A carbon brush electrode is fixed in the anode chamber as a microbial anode electrode, and then the anaerobic granular sludge is transferred and filled.

[0059] During the operation of the microbial fuel cell, the anaerobic granular sludge in the anode liquid is enriched with electricity-producing microorganisms through electrical acclimation. The anode solution is composed of M9 culture medium, and sodium acetate is added as a carbon source. A 50mM potassium ferrocyanide solution is used as an electron acceptor in the cathode chamber. The anaerobic granular sludge is enriched using anode potential control. Under anode potential control (+20mV vs.Ag / AgCl), after 7 days of operation, the proportion of electroactive microbial populations in the anaerobic granular sludge will further increase, the electricity production performance will be improved, the output current of the MFC will be significantly increased, the microorganisms will gradually adapt and grow and reproduce on the carbon brush electrode, and microorganisms with electrochemical activity will preferentially be enriched on the electrode surface. At this point, the electrical acclimation of anaerobic granular sludge is completed.

[0060] In the second step, simulated domestic sewage is pumped into the microbial anode chamber through the water inlet pipe. The anaerobic granular sludge oxidizes the organic matter in the sewage through anaerobic digestion by using gravity to fully mix the sewage and fully contact the sewage with the anaerobic granular sludge. The anaerobic digestion process includes hydrolysis and fermentation, production of small molecular acids, methane, and CO2. The generated CO2 gas is discharged through the exhaust port, collected and transported to the photosynthesis chamber; while anaerobic granular sludge oxidizes organic matter, it produces electrons and protons. The electrons flow through the wire to the cathode electrode to generate current, and the protons reach the cathode chamber through the proton exchange membrane to participate in the electro-Fenton reaction in the cathode chamber.

[0061] Among them, simulated domestic sewage: peptone 80-150 mg / L, as a source of nitrogen-containing organic matter, simulates substances such as protein in domestic sewage. Potassium nitrate 30-60 mg / L, disodium hydrogen phosphate 20-30 mg / L, provide nitrogen and phosphorus elements for sewage. Trace elements: ferrous sulfate 5-10 mg / L, manganese sulfate 2-5 mg / L, meet the needs of microbial growth for trace elements such as iron and manganese. Add a small amount of sodium chloride to control the salt concentration at about 0.1% to 0.3% to simulate a low-salt environment.

[0062] The proton exchange membrane is soaked in 1 mol / L HCl for 12 h before use and rinsed with deionized water several times until it reaches neutrality in order to assemble the dual-chamber MFC. The thickness of the proton exchange membrane is preferably 0.22 μm.

[0063] The third step is that the sewage treated by the microbial anode chamber enters the electro-Fenton cathode chamber. The photosynthesis chamber receives the CO2 gas transported by the microbial anode chamber. Under light conditions, while the green algae synthesize their own life forms, the O2 produced by photosynthesis is diffused and transported to the gas diffusion cathode. The catalyst layer FeCo2O4 of the gas diffusion cathode directly contacts the cathode sewage, and the electro-Fenton reaction occurs in the cathode chamber. Under the continuous supply of cathode electrons, the Co and Fe sites can respectively reduce O2 to H2O2 and activate H2O2 to produce active species such as hydroxyl free radicals, thereby efficiently removing biodegradable organic pollutants in sewage.

[0064] The anode chamber of the microbial fuel cell cannot degrade hard-to-degrade organic pollutants, but only has an adsorption effect with an adsorption rate of 10%. The hydroxyl free radicals produced by electro-Fenton in the cathode chamber have strong oxidizing properties and are sufficient to remove most hard-to-degrade organic pollutants.

[0065] Table 1: Test results under light conditions (unit: mg / L)

[0066]

[0067] Step 4: The treated sewage is discharged through the cathode chamber outlet pipe, and the sewage is centrifuged by a centrifugal agitator and then left to stand to separate and settle the impurities contained in the sewage. The impurities are retained on the filter; the treated sewage is discharged from the drain pipe, realizing the removal process of COD, ammonia nitrogen and difficult-to-degrade organic pollutants in the sewage.

[0068] Example 3

[0069] The only difference from Example 2 is that there is no lighting.

[0070] In the absence of light, the anode chamber works normally, but the photosynthesis chamber cannot work, resulting in insufficient oxygen supply. Electro-Fenton at the cathode can hardly occur, and the difficult-to-degrade organic pollutant BPA remaining in the sewage cannot be degraded.

[0071] In actual application, the cathode chamber can be connected in parallel with multiple anaerobic anode chambers to adapt to a variety of different water qualities. Different anode chambers are set with different hydraulic retention times to acclimate the anaerobic biofilm that adapts to the corresponding water quality. In order to reduce the load on the cathode chamber with multiple anode chambers in parallel, the size of the cathode chamber and the anode chamber can be changed according to actual conditions.

[0072] In summary, the multi-stage sewage treatment device and method combining biological treatment with electro-Fenton proposed in the present invention utilizes the coupling of microorganisms and light energy to perform multi-stage sewage treatment, realizes COD degradation and CO2 fixation conversion under energy self-sufficiency conditions, and simultaneously removes ammonia nitrogen and difficult-to-degrade organic pollutants in sewage. The process does not require additional electrical input, aeration, simple process, and easy operation, overcoming the time-consuming and energy-consuming defects of traditional sewage treatment technology and the low power generation efficiency and poor treatment efficiency of ordinary microbial fuel cells.

[0073] The above contents are merely examples and explanations of the concept of the present invention. The technicians in this technical field may make various modifications or additions to the specific embodiments described or replace them in a similar manner. As long as they do not deviate from the concept of the invention or exceed the scope defined by the claims, they should all fall within the protection scope of the present invention.

Claims

1. A multi-stage sewage treatment device combining biological treatment with electro-Fenton, characterized in that: It includes a microbial anode chamber, an electro-Fenton cathode chamber and a photosynthesis chamber. The sewage enters the electro-Fenton cathode chamber after being treated in the microbial anode chamber. The microbial anode chamber is composed of filled anaerobic granular sludge and a microbial anode electrode as an electron collector. The microbial anode chamber and the electro-Fenton cathode chamber are separated by a proton exchange membrane, and a gas diffusion cathode is loaded between the electro-Fenton cathode chamber and the photosynthesis chamber. The microbial anode electrode and the gas diffusion cathode form a loop and transmit electrons to the electro-Fenton cathode chamber. The CO2 gas generated in the microbial anode chamber is transported to the photosynthesis chamber and converted into O2 through the photosynthesis chamber. The gas diffusion cathode reduces and activates O2 to produce highly oxidizing active species and enters the electro-Fenton cathode chamber. The electro-Fenton reaction occurs in the electro-Fenton cathode chamber to remove organic pollutants in the sewage.

2. The multi-stage sewage treatment device according to claim 1, characterized in that: When preparing the microbial anode electrode, the carbon brush was first soaked in acetone for 12 hours, taken out and soaked in a 2 mol / L hydrochloric acid solution for 4 hours, and finally, it was repeatedly ultrasonically cleaned with deionized water until the pH value reached 7.0, and dried in an oven at 60°C.

3. The multi-stage sewage treatment device according to claim 1, characterized in that: The gas diffusion cathode is composed of a titanium mesh substrate, a catalytic layer FeCo2O4 and a diffusion layer PTFE in sequence. The diffusion layer PTFE is closely attached to the photosynthesis chamber, while the catalytic layer FeCo2O4 faces one side of the electro-Fenton cathode chamber.

4. The multi-stage sewage treatment device according to claim 3, characterized in that: The preparation steps of the gas diffusion cathode are as follows: Step 1: Preparation of carbon paste and titanium mesh: The preparation process of carbon paste is as follows: weigh 3g of carbon black in a 250mL beaker, add ethanol to the 100mL mark for dispersion, use a pipette to measure 1mL of PTFE emulsion in the dispersed carbon black, stir slowly for 30min with a magnetic stirrer, then place it in an ultrasonic cleaning pot for 10min, take it out and absorb the ethanol to make a carbon paste; the preparation of titanium mesh includes alkaline washing and acid washing of titanium mesh, respectively, heating and boiling for 30min under the condition of adding 10% sodium hydroxide and oxalic acid, then rinsing with distilled water, drying, and then soaking in 30% PTFE emulsion, repeatedly soaking and drying until the titanium mesh can be well hydrophobic, that is, forming a diffusion layer PTFE; Step 2: Hot pressing and high temperature calcination: The formed carbon paste is evenly applied on the prepared titanium mesh, and the carbon paste is repeatedly rolled with a glass rod to evenly cover one side of the titanium mesh, maintaining a thickness of 2 to 3 mm, and then hot-pressed at 95°C for 15 minutes; the hot-pressed electrode is placed in a muffle furnace, calcined at 300°C for 5 minutes, and then taken out; Step 3: Preparation of catalyst layer coating: First, weigh 100 mg of ferrous chloride FeCl2, 250 mg of cobalt nitrate Co(NO3)2·6H2O, 30 mg of urea, and 150 mg of ammonium fluoride NH4F, add 100 mL of deionized water, and stir magnetically for 2 h; transfer the obtained mixed solution into a 100 mL hydrothermal kettle and keep it at 100-200 ° C for 8 h; after the reaction, wash it with deionized water and anhydrous ethanol respectively, vacuum dry it, and keep it at 60 ° C for 10-15 h to obtain FeCo2O4 catalyst; use the coating method to prepare the catalytic layer of the gas diffusion cathode, weigh 20 mg FeCo2O4 material, 2 mg of conductive acetylene black, and 4 mg of polyvinylidene fluoride (PVDF) were ground in an agate mortar for 1 to 3 hours; 300 μL of NMP was pipetted into the agate mortar and ground again to obtain a uniform slurry; the slurry was evenly coated on the other side of the titanium mesh and dried at 60°C in a vacuum drying oven for 10 to 15 hours; finally, the prepared electrode sheet was kept at 8 to 15 MPa for 2 minutes under a tablet press to obtain a FeCo2O4 catalyst layer; At this time, the prepared electrode is a final formed gas diffusion cathode, one side of which is a diffusion layer of PTFE and the other side is a catalyst layer of FeCo2O4.

5. The multi-stage sewage treatment device according to claim 1, characterized in that: A resistor is connected in series between the microbial anode electrode and the gas diffusion cathode through a wire to form a circuit, and the electrons generated by the microbial anode electrode reach the gas diffusion cathode through the circuit.

6. The multi-stage sewage treatment device according to claim 1, characterized in that: The green algae comprises one or more of Chlorella vulgaris, Phaeodactylum tricornutum, Nannochloropsis sp., Scenedesmus obliquus or Dunaliella salina; The preparation steps of green algae hydrogel are as follows: take 40mL Chlorella algae solution (the concentration of Chlorella algae solution is OD 600 =1.0) in a beaker, add sodium alginate to a final (mass) concentration of 0.3%, stir thoroughly and let stand to obtain a chlorella solution; prepare 10g / L calcium chloride solution in another beaker, drop the chlorella solution into the constantly stirred calcium chloride solution at a speed of 5r / min using a peristaltic pump, filter out excess calcium chloride solution, and then form algae gel balls.

7. A method for treating sewage using a multi-stage sewage treatment device according to any one of claims 1 to 6, characterized in that: Here are the steps: The first step is to construct a microbial fuel cell and electrically acclimate the anaerobic granular sludge in the microbial anode chamber; the sewage enters the microbial anode chamber, and the microorganisms in the anaerobic granular sludge perform primary decomposition of organic matter in the sewage to achieve COD removal and generate electrons and CO2; the electrons reach the gas diffusion cathode through the external circuit, and the CO2 enters the photosynthesis chamber through the gas loop, providing CO2 for the photosynthesis of green algae in the photosynthesis chamber, and the generated O2 enters the cathode chamber through the gas diffusion cathode, and is reduced and activated by the nano-FeCo2O4 in the gas diffusion cathode to produce highly oxidizing active species; In the second step, the water outlet of the anode chamber enters the cathode chamber, where the refractory organic pollutants that are not removed by microorganisms are secondary degraded and removed by the strong oxidizing active species produced in the electro-Fenton cathode chamber; the protons produced by the microorganisms decomposing the sewage on the microbial anode electrode enter the cathode chamber through the proton exchange membrane and participate in the electro-Fenton reaction of the FeCo2O4 gas diffusion cathode; In the third step, the treated wastewater is discharged from the electro-Fenton cathode chamber.

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