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

Abstract

A multi-stage sewage treatment device and method combining biological treatment with electro-Fenton, relating to the field of water pollution control, comprises a three-chamber microbial fuel cell reactor, consisting of, from left to right, a microbial anode chamber, an electro-Fenton cathode chamber, and a photosynthesis chamber. The biological anode chamber is composed of anaerobic granular sludge filled with the chamber and a carbon brush electrode serving as an electron collector; 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 also separates the electro-Fenton cathode chamber from the photosynthesis chamber. The present invention combines two major clean energy sources, microbial energy and light energy, combining the efficient removal capabilities of aerobic and anaerobic biological treatment technologies with 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.

Description

Technical Field

[0001] The present 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] With global water shortages and water pollution becoming increasingly serious, finding efficient, energy-saving, and environmentally friendly wastewater treatment technologies has become a pressing priority. Traditional wastewater treatment methods include physical, chemical, and biological methods. These methods consume significant amounts of electricity, increasing energy costs and generating greenhouse gases during the treatment process, negatively impacting the environment. Furthermore, biological treatment is inefficient, struggles to remove persistent organic pollutants, and can cause secondary pollution.

[0003] Microbial fuel cells (MFCs), an emerging wastewater treatment technology, purify wastewater while simultaneously recovering electrical energy, achieving an organic integration. Microorganisms convert organic matter into electricity, removing pollutants with minimal external energy input, resulting in low operating costs and zero secondary pollution. They can treat wastewater in real time, achieving higher efficiency. However, the industrial application of MFCs faces challenges such as limited degradation capacity, an inability to completely remove recalcitrant organic pollutants, low power generation, weak buffering capacity, and susceptible 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 production. However, the electro-Fenton process has strict requirements on electrode materials, high energy consumption, and a single application scenario.

[0005] The bioelectro-Fenton system combines anode biodegradation with cathode electro-Fenton degradation, enabling deep removal of organic pollutants from wastewater under self-powered conditions. However, current bioelectro-Fenton technology remains inefficient under self-powered operation due to issues such as high overpotential in the bioanode chamber, low dissolved oxygen in the cathode chamber, cathode catalyst activity, and long-term stability. High electrochemical activity in the bioanode is a prerequisite for efficient operation under self-powered conditions, but traditional bioanodes have long enrichment times, and the preparation of three-dimensional porous electrodes with high surface areas is complex and costly, presenting numerous challenges during reactor scale-up. Aeration is often used to increase dissolved oxygen concentration in the cathode chamber, but this significantly increases energy costs. Furthermore, due to the low solubility of O2 in water, traditional aeration methods are inefficient and unsuitable for electrode surface reactions like electro-Fenton. Selecting an appropriate cathode catalyst and configuration is also challenging, particularly in ensuring that cathode performance is maintained over a long period while meeting basic performance requirements. Summary of the Invention

[0006] The purpose of the present invention is to propose a multi-stage sewage treatment device and method combining biological treatment with electro-Fenton, which can efficiently and stably remove COD, ammonia nitrogen and refractory 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. After being treated in the microbial anode chamber, the sewage enters the electro-Fenton cathode chamber. The microbial anode chamber is composed of filled anaerobic granular sludge and a microbial anode electrode serving 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 installed 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. 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, which enter the electro-Fenton cathode chamber. An 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. Sewage enters the microbial anode chamber, where microorganisms in the anaerobic granular sludge perform a primary decomposition of organic matter in the sewage, removing COD and generating electrons and CO2. Electrons travel through an external circuit to the gas diffusion cathode, and CO2 enters the photosynthesis chamber through a gas loop, providing CO2 for photosynthesis of green algae in the photosynthesis chamber. The generated O2 enters the cathode chamber through the gas diffusion cathode and is reduced and activated by 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 have not been removed by microorganisms are secondary degraded and removed by the highly oxidizing active species produced in the electro-Fenton cathode chamber. The protons produced by the microorganisms on the microbial anode electrode decomposing the wastewater 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] To address the challenges faced by potential applications of bio-electro-Fenton systems, this paper redesigns the overall configuration of the bio-electro-Fenton reactor and optimizes various components. First, compared to the mainstream dual-chamber configuration, this paper adds an additional photosynthesis chamber. The photosynthesis of hydrogel-immobilized algae reduces CO₂ emissions from the bio-anode chamber and provides O₂ to the electro-Fenton cathode chamber, increasing the dissolved oxygen concentration in the cathode chamber and promoting the electro-Fenton reaction and pollutant degradation.

[0014] To address the problem of high 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 a period of operation, the proportion of electroactive microbial populations in the anaerobic granular sludge will further increase, thereby improving performance.

[0015] For the electro-Fenton cathode chamber, the present invention utilizes a gas diffusion cathode modified with nano-FeCo2O4. With a 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 radicals. The metal oxide maintains long-term stability when operating at a neutral pH.

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

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] 1. The present invention combines two major clean energy sources, microbial energy and light energy, and combines the efficient removal capabilities of aerobic and anaerobic biological treatment technologies with 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. This invention achieves the fixation and conversion of O2 and CO2 while maintaining energy self-sufficiency. The anaerobic granular sludge digestion reaction decomposes wastewater into CO2, while the in-situ photosynthetic oxygen production by green algae replaces mechanical aeration, eliminating the need for air or energy supply.

[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 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 figures are:

[0025] 1-microbial anode chamber, 2-electro-Fenton cathode chamber, 3-photosynthesis chamber, 4-microbial 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 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 with 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 provided between the microbial anode chamber 1 and the electro-Fenton cathode chamber 2, and a gas diffusion cathode 6 is provided 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. A water inlet is provided 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. A water pump and a water inlet valve are also provided on the anode chamber water inlet pipe 8. 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 flow rate of sewage entering the microbial anode chamber 1.

[0031] An anode chamber exhaust port 12 is provided on 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 within the internal chamber of the microbial anode chamber 1. Its surface is used to attach anaerobic microbial membranes, i.e., to enrich electroactive microorganisms. To provide a better attachment site for microorganisms, the microbial anode electrode 4 is made of a carbon brush. The carbon brush has been acid- and heat-treated to increase 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 then dried in an oven at 60°C for use.

[0034] The microbial anode chamber 1 is connected to the electro-Fenton cathode chamber 2 via a proton exchange membrane 5, allowing only protons to pass through. A connecting pipe is also provided between the microbial anode chamber 1 and the electro-Fenton cathode chamber 2, namely, a connected anode chamber outlet pipe 9 and cathode chamber inlet pipe 10. Valves are installed on the connecting pipe to control the flow rate of the wastewater and adjust the hydraulic retention time of the wastewater 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 cavity, and is made of organic glass to ensure 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 base, a catalytic layer FeCo2O4 and a diffusion layer PTFE in sequence. The diffusion layer PTFE is closely attached to the photosynthesis chamber 3, while the catalytic 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 carbon paste is prepared as follows: 3g of carbon black is weighed into a 250mL beaker and dispersed by adding ethanol to the 100mL mark. 1mL of PTFE emulsion is pipetted into the dispersed carbon black and slowly stirred with a magnetic stirrer for 30 minutes. The mixture is then ultrasonically cleaned in an ultrasonic cleaning pot for 10 minutes. The ethanol is then removed and the carbon paste is formed. The titanium mesh is prepared by alkaline and acid washing, heating and boiling the mesh in 10% sodium hydroxide and oxalic acid for 30 minutes, respectively. The mesh is then rinsed with distilled water, dried, and then immersed in 30% PTFE emulsion. This immersion and drying cycle is repeated until the mesh is effectively hydrophobic, forming a diffusion layer of PTFE.

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

[0043] The formed carbon paste is evenly applied to the prepared titanium mesh. Repeatedly roll with a glass rod to evenly coat one side of the titanium mesh, maintaining a thickness of 2-3 mm. Then, hot press at 95°C for 15 minutes. The hot-pressed electrode is placed in a muffle furnace and calcined at 300°C for 5 minutes before removal.

[0044] Step 3: Coating preparation of catalyst layer:

[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; the resulting mixed solution is transferred to a 100 mL hydrothermal autoclave and maintained at 100-200°C for 8 hours; after the reaction is complete, wash with deionized water and anhydrous ethanol respectively, vacuum dry, and keep at 60°C for 10-15 hours to obtain the FeCo2O4 catalyst. The catalytic layer of the gas diffusion cathode was prepared using a coating method. 20 mg of FeCo2O4 material, 2 mg of conductive acetylene black, and 4 mg of polyvinylidene fluoride (PVDF) were weighed in sequence and 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 pressed at 8 to 15 MPa for 2 minutes under a tablet press to obtain the 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 cavity, and is made of organic glass to ensure light transmission, corrosion resistance and insulation.

[0050] The photosynthesis chamber 3 is loaded with green algae hydrogel 7, which can absorb the 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, Phaeodactylum tricornutum, Nannochloropsis sp., Scenedesmus obliquus or Dunaliella salina.

[0052] The preparation steps of green algae hydrogel are as follows: take 40mL of 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. Then, prepare 10 g / L calcium chloride solution in another beaker, and add the chlorella solution dropwise into the constantly stirred calcium chloride solution at a speed of 5 r / min using a peristaltic pump. Filter out the excess calcium chloride solution to 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 culture 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 using the above-mentioned sewage treatment device is as follows:

[0058] The first step is to assemble the sewage treatment unit. The reaction chamber is made of acrylic (PMMA) and sealed with silicone gaskets and screws. The top of the shell is removable. A carbon brush electrode is fixed in the anode chamber to serve as the microbial anode electrode, and then 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 consists 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 enrich on the electrode surface. At this point, the electrical acclimation of the anaerobic granular sludge is completed.

[0060] In the second step, simulated domestic sewage is pumped through the inlet pipe into the microbial anode chamber. Using gravity and ensuring thorough mixing of the sewage and sufficient contact between the sewage and the anaerobic granular sludge, the anaerobic granular sludge oxidizes the organic matter in the sewage through anaerobic digestion. The anaerobic digestion process includes hydrolysis and fermentation, production of small molecule acids, methane, and CO2. The generated CO2 gas is discharged through the exhaust port, collected, and transported to the photosynthesis chamber. The anaerobic granular sludge also produces electrons and protons as it oxidizes organic matter. Electrons flow through a wire to the cathode electrode, generating current, while protons pass through the proton exchange membrane to the cathode chamber, participating in the electro-Fenton reaction there.

[0061] For simulated domestic sewage, the following ingredients are added: 80-150 mg / L of peptone, serving as a source of nitrogenous organic matter and simulating substances such as protein found in domestic sewage. 30-60 mg / L of potassium nitrate and 20-30 mg / L of disodium hydrogen phosphate provide nitrogen and phosphorus to the sewage. Trace elements: 5-10 mg / L of ferrous sulfate and 2-5 mg / L of manganese sulfate meet the microbial growth requirements for trace elements such as iron and manganese. A small amount of sodium chloride is added to keep the salt concentration around 0.1% to 0.3% to simulate a low-salinity environment.

[0062] Before use, the proton exchange membrane was soaked in 1 mol / L HCl for 12 hours and rinsed with deionized water several times until it reached neutrality for assembly of the dual-chamber MFC. The thickness of the proton exchange membrane was preferably 0.22 μm.

[0063] In the third step, the wastewater treated by the microbial anode chamber enters the electro-Fenton cathode chamber. The photosynthesis chamber receives CO2 gas from the microbial anode chamber. Under light conditions, while the green algae synthesize their own life forms, they photosynthesize and produce O2, which is then diffused and transported to the gas diffusion cathode. The FeCo2O4 catalytic layer of the gas diffusion cathode directly contacts the cathode wastewater, and an electro-Fenton reaction occurs in the cathode chamber. With a 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 radicals, thereby effectively removing refractory organic pollutants in the wastewater.

[0064] The anode chamber of the microbial fuel cell cannot degrade refractory organic pollutants, only adsorbing them at a rate of 10%. However, the hydroxyl radicals produced by electro-Fenton in the cathode chamber are highly oxidizing and sufficient to remove most refractory organic pollutants.

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

[0066]

[0067] In the fourth step, the treated sewage is discharged through the cathode chamber outlet pipe, and the sewage is centrifuged by a centrifugal agitator, and then allowed to stand to separate and settle the impurities contained in the sewage, and 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 adapted 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, achieving COD degradation and CO2 fixation conversion under energy self-sufficiency conditions, and simultaneously removing ammonia nitrogen and difficult-to-degrade organic pollutants in sewage. This process does not require additional electrical energy input or aeration, has a simple process, and is easy to operate. It overcomes the time-consuming and energy-consuming shortcomings of traditional sewage treatment technology and the problems of low power generation efficiency and poor treatment efficiency of ordinary microbial fuel cells.

[0073] The above content is merely an example and explanation of the concept of the present invention. Those skilled in the art may make various modifications or additions to the described specific embodiments 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 scope of protection 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. After being treated in the microbial anode chamber, the sewage enters the electro-Fenton cathode chamber. The microbial anode chamber is composed of filled anaerobic granular sludge and a microbial anode electrode serving 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 installed 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 produced in the microbial anode chamber is transported to the photosynthesis chamber and converted into O2 by the photosynthesis chamber. The photosynthesis chamber is loaded with green algae hydrogel. The gas diffusion cathode reduces and activates O2 to produce highly oxidizing active species, which enter the electro-Fenton cathode chamber. The electro-Fenton reaction occurs in the electro-Fenton cathode chamber to remove organic pollutants in the sewage. The gas diffusion cathode 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, while the catalyst layer FeCo2O4 faces the side of the electro-Fenton cathode chamber. 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 3 g of carbon black into a 250 mL beaker, add ethanol to the 100 mL mark for dispersion, use a pipette to measure 1 mL of PTFE emulsion into the dispersed carbon black, stir slowly with a magnetic stirrer for 30 minutes, then place it in an ultrasonic cleaning pot for 10 minutes, remove it and absorb the ethanol to make a carbon paste; the preparation of titanium mesh includes alkaline washing and acid washing of the titanium mesh, heating and boiling for 30 minutes under the conditions of adding 10% sodium hydroxide and oxalic acid respectively, then rinsing with distilled water, drying, and then immersing it in 30% PTFE emulsion, repeatedly immersing and drying until the titanium mesh is well hydrophobic, that is, forming a diffusion layer of PTFE; Step 2: Hot pressing and high temperature calcination: The formed carbon paste was evenly applied to the prepared titanium mesh and repeatedly rolled with a glass rod to evenly cover the carbon paste on one side of the titanium mesh, maintaining a thickness of 2-3 mm. The electrode was then hot-pressed at 95°C for 15 minutes. The hot-pressed electrode was placed in a muffle furnace and calcined at 300°C for 5 minutes before being removed. Step 3: Coating preparation of catalyst layer: First, 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) were weighed, added to 100 mL of deionized water, and magnetically stirred for 2 h. The resulting mixed solution was transferred to a 100 mL hydrothermal autoclave and maintained at 100-200 °C for 8 h. After the reaction, it was washed with deionized water and anhydrous ethanol, dried under vacuum, and kept at 60 °C for 10-15 h to obtain FeCo2O4 catalyst. The catalytic layer of the gas diffusion cathode was prepared by the coating method. 20 mg of FeCo2O4 material, 2 mg of conductive acetylene black, and 4 mg of polyvinylidene fluoride were weighed in sequence and ground in an agate mortar for 1-3 h. 300 mg of FeCo2O4 was pipetted with a pipette. Add 100 μL of NMP to an agate mortar and grind again to obtain a uniform slurry. The slurry is evenly coated on the other side of the titanium mesh and dried in a vacuum drying oven at 60°C for 10-15 hours. Finally, the prepared electrode sheet is pressed under a pressure of 8-15 MPa for 2 minutes to obtain the 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.

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 h, taken out and soaked in a 2 mol / L hydrochloric acid solution for 4 h, and finally, repeatedly ultrasonically cleaned with deionized water until the pH value reached 7.0, and then dried in an oven at 60 °C.

3. 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 via a wire to form a circuit, and electrons generated by the microbial anode electrode reach the gas diffusion cathode through the circuit.

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

5. A method for treating sewage based on the multi-stage sewage treatment device according to any one of claims 1 to 4, 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. Sewage enters the microbial anode chamber, where microorganisms in the anaerobic granular sludge perform a primary decomposition of organic matter in the sewage, removing COD and generating electrons and CO2. Electrons travel through an external circuit to the gas diffusion cathode, and CO2 enters the photosynthesis chamber through a gas loop, providing CO2 for photosynthesis of green algae in the photosynthesis chamber. The generated O2 enters the cathode chamber through the gas diffusion cathode and is reduced and activated by 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 have not been removed by microorganisms are secondary degraded and removed by the highly oxidizing active species produced in the electro-Fenton cathode chamber. The protons produced by the microorganisms on the microbial anode electrode decomposing the wastewater 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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