Method, system and application of iron cycle enhanced biological electrochemical cathode denitrification

Abstract

This invention relates to the field of environmental microbial electrochemistry technology, and discloses a method, system, and application of iron-cycle-enhanced bioelectrochemical cathode denitrification. The method comprises: The method is carried out in a bioelectrochemical system, including: (1) introducing synthetic influent into the bioelectrochemical system, then inoculating the cathode with sludge, controlling the cathode potential at -0.6 V (vs. SHE) in a sequencing batch process for 9-30 days, allowing *Geobacterium* microorganisms to be directionally enriched in the biofilm of the cathode; (2) introducing the influent to be treated, and controlling the cathode potential at -0.6 V (vs. SHE) for denitrification. This invention utilizes the construction of Fe... 2+ / Fe 3+ The cycle significantly enhances long-range electron transfer within the biofilm, effectively driving efficient nitrogen removal by the outer denitrifying bacteria community and greatly improving the cathode nitrogen removal efficiency of the bioelectrochemical system.

Description

Technical Field

[0001] This invention relates to the field of environmental microbial electrochemistry, specifically to a method, system, and application of iron-cycle-enhanced bioelectrochemical cathode denitrification. Background Technology

[0002] Bioelectrochemical systems (BES) are a novel water treatment technology that uses electrodes as electron donors or acceptors to drive microbial metabolism. In the cathode denitrification process, microorganisms use the cathode as an inorganic electron donor to convert nitrate nitrogen (NO3) into nitrogen. - -N) and nitrite (NO2) - The process gradually reduces nitrogen (-N) to nitrogen gas, which has advantages such as no need for external carbon sources, low carbon emissions, and flexible process control. It has good application potential in the field of deep denitrification treatment of wastewater with low carbon-to-nitrogen ratio (such as aquaculture wastewater and secondary effluent from municipal sewage).

[0003] However, in actual operation, significant electron transport resistance exists within the cathode biofilm. Electroactive microorganisms with direct electron transport capabilities (such as *Geobacterium*) can typically only effectively acquire electrons in the inner region immediately adjacent to the electrode surface. Their electron transport distance, relying on outer membrane redox proteins or conductive nanowires, is extremely limited (usually on the micrometer scale), while the outer denitrifying bacteria suffer metabolic limitations due to insufficient electron supply, resulting in low overall denitrification efficiency of the system. Furthermore, when *Geobacterium* microorganisms metabolize independently, they readily drive the dissimilatory nitrate reduction to ammonium (DNRA) pathway, converting nitrogen to NH4+. + The nitrogen remains in the water, further limiting the system's total nitrogen removal rate.

[0004] In the prior art, CN116253425A discloses a wastewater treatment device and method based on bioelectrochemical principles. This method involves periodically adding ferric iron (Fe₂O₃) to a reaction tank, utilizing iron-reducing bacteria to achieve ferric ammonia oxidation (Feammox) for nitrogen removal, while simultaneously using the ferrous salt generated in the reaction for phosphorus removal. Although this technology involves the utilization of iron cycling, its core mechanism lies in using ferric iron as an electron acceptor to drive the ammonia oxidation reaction. It primarily addresses the removal of ammonia nitrogen under low carbon-to-nitrogen ratio conditions, but does not offer solutions to problems such as the limitation of electron transfer distance within the cathode biofilm or insufficient electron supply to the external denitrifying bacteria. Furthermore, the ferric iron is added periodically in this technology, requiring continuous replenishment, and it does not synergize with cathode potential control, making it difficult to achieve continuous and stable operation of the iron cycle.

[0005] Therefore, how to overcome the limitations of electron transfer distance within the biofilm and achieve efficient electron supply to the external denitrifying bacteria during the cathode denitrification process of a bioelectrochemical system, thereby improving the total nitrogen removal rate of the system, remains a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] The present invention aims to solve the problems of limited electron transfer distance within the biofilm and difficulty in achieving efficient electron supply to the outer denitrifying bacteria community during the cathode denitrification process of existing bioelectrochemical systems, which result in the need to further improve the total nitrogen removal rate.

[0007] To achieve the above objectives, a first aspect of the present invention provides a method for enhanced bioelectrochemical cathode denitrification based on iron cycling, the method being carried out in a bioelectrochemical system, comprising: Step (1) Introduce artificially synthesized influent into the bioelectrochemical system, then inoculate sludge into the cathode, control the cathode potential at -0.6 V (vs. SHE) and acclimate it in a sequencing batch process for 9-30 days to allow Geobacter spp. microorganisms to be directionally enriched in the biofilm of the cathode; Among them, the artificially synthesized influent contains Fe 3+ Concentration of 30-50 mg / L, NO3 - -N concentration is 45-55 mg / L; Step (2) introduce the influent to be treated and control the cathode potential at -0.6 V (vs. SHE) to carry out denitrification.

[0008] A second aspect of the present invention provides a bioelectrochemical system for implementing the method described in the first aspect, the bioelectrochemical system comprising an autotrophic denitrification reactor, wherein the autotrophic denitrification reactor is provided with: The cathode is a carbon felt electrode used to load a cathode biofilm, which is enriched with Geobacterium spp. and denitrifying bacteria. The anode is a stainless steel mesh electrode; An isolation layer, made of nylon mesh, is disposed between the cathode and the anode; The bioelectrochemical system also includes: Potential control unit, used to keep the cathode potential constant at -0.6 V (vs. SHE).

[0009] The third aspect of this invention provides the application of the system described in the second aspect in the deep denitrification treatment of nitrate-containing wastewater.

[0010] Compared with the prior art, the present invention has at least the following beneficial effects: (1) The method of the present invention can significantly improve the cathode denitrification efficiency and system stability. This is achieved by precisely controlling the cathode potential at -0.6 V (vs. SHE) during the acclimation stage and controlling the Fe content in the influent. 3+ Concentration, constructing Fe 2+ / Fe 3+The circulation significantly enhances the long-range electron transport capacity within the biofilm. Compared to the unacclimated control group, the nitrate removal rate constant and total nitrogen removal rate were significantly improved, and the time for the system to reach steady-state operation was shortened from 60 hours to 48 hours, achieving a dual improvement in denitrification efficiency and operational stability.

[0011] (2) The method of this invention can achieve targeted enrichment and metabolic cooperation between electroactive microorganisms and denitrifying bacteria. At the cathode potential and Fe... 3+ Under synergistic domestication, the relative abundance of *Geobacterium* microorganisms in the cathode biofilm significantly increased from 3.2% to 18.3%, while denitrifying functional bacteria (such as *UBA6170* and *Sulfurella*) also maintained a high abundance. Functional gene analysis showed that outer membrane polyhemoglobin cytochrome genes (omcS, omcF) and iron reduction / oxidation related genes (ndh2, dmkB, cyc2, mtoA) were significantly enriched, while DNRA pathway genes were not upregulated, indicating that electrons are preferentially used for iron cycle-mediated electron transfer rather than ammonia nitrogen accumulation, effectively avoiding the side reaction of dissimilatory nitrate reduction to ammonium.

[0012] (3) The method of the present invention is simple to operate and does not require an external carbon source. It is suitable for deep denitrification of wastewater with low carbon-to-nitrogen ratio. It only requires controlling the Fe content of the influent during the acclimatization stage in a conventional bioelectrochemical system. 3+ The system achieves high concentration, requires no external organic carbon source, and has a small environmental footprint. Experimental results show that even with an influent carbon-to-nitrogen ratio below 3 (such as aquaculture wastewater and secondary effluent from municipal sewage), the total nitrogen removal rate can still reach over 90%, providing an efficient and economical technical solution to address the industry challenge of insufficient carbon source for nitrogen removal in low-carbon-to-nitrogen wastewater. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the structure of the autotrophic denitrification reactor of the bioelectrochemical system in a preferred embodiment of the present invention; Figure 2 This is a graph showing the changes in influent nitrate nitrogen and effluent total nitrogen concentration, nitrate nitrogen, nitrite nitrogen, and ammonia nitrogen concentration over time during continuous flow operation of the bioelectrochemical system in a preferred embodiment of the present invention. Figure 3 This is a graph showing the changes in influent nitrate nitrogen and effluent total nitrogen concentration, nitrate nitrogen, nitrite nitrogen, and ammonia nitrogen concentration over time during continuous flow operation of the bioelectrochemical system in the comparative example of this invention. Figure 4 In the preferred embodiments and comparative examples of this invention, the cathode nitrate removal rate constant and the intracellular iron concentration of the cathode biomembrane in the bioelectrochemical system vary with the amount of Fe synthesized influent. 3+ Concentration change graph; Figure 5These are laser confocal scanning microscope images of the biofilm on the surface of the cathode carbon felt electrode in the preferred embodiment and comparative example of the present invention. Figure 6 This is a comparison diagram of the relative abundance composition of the main functional microbial communities in the pretreated sludge, the preferred embodiment, and the comparative example of the cathode biofilm in this invention. Figure 7 The heatmap shows the differences in the expression levels of genes related to iron metabolism, nitrogen metabolism, and electron transport chain in the cathode biofilm of pretreated sludge compared to preferred embodiments and comparative examples of the present invention.

[0014] Explanation of reference numerals in the attached figures 1-Cathode electrode interface; 2-Anode electrode interface; 3-Inlet; 4-Outlet; 5-Cathode biofilm; 6-Carbon felt electrode; 7-Nylon mesh; 8-Stainless steel mesh electrode. Detailed Implementation

[0015] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0016] The technical principle of this invention lies in the fact that a cathode potential of -0.6 V (vs. SHE) provides a continuous low-potential electron source for *Geobacterium* microorganisms, driving them to emit Fe... 3+ The electron acceptor undergoes dissimilar iron reduction to produce Fe. 2+ Fe 2+ On the one hand, it can act as a soluble electron carrier to diffuse to the outside of the biofilm, transferring electrons to denitrifying bacteria; on the other hand, it can be oxidized and regenerated into Fe under the action of microorganisms or anodic action. 3+ This forms a closed loop. Simultaneously, Fe... 3+ / Fe 2+ During the cycle, secondary minerals such as magnetite generated in situ form a solid-phase conductive network within the biofilm, further extending the electron transport distance. This dual mechanism synergistically overcomes the traditional limitations on electron transport distance within biofilms, ensuring an ample electron supply to the outer denitrifying bacteria community and achieving efficient nitrogen removal.

[0017] It should be noted that: In this invention, the *Geobacterium* microorganisms are a type of electroactive microorganism with extracellular electron transfer capabilities, capable of reducing dissimilar iron using Fe(III) as the terminal electron acceptor; The dissimilar iron reduction refers to the process by which microorganisms in an anaerobic environment use extracellular insoluble iron oxides as terminal electron acceptors to obtain energy through the reduction of Fe(III) coupled with oxidative electron donors. Long-range electron transfer refers to the process by which electrons are transferred from the electrode surface through the conductive network (such as conductive proteins, nanowires, and minerals) within the biofilm to the outer microorganisms, a distance that can reach tens of micrometers or more.

[0018] As previously stated, the first aspect of this invention provides a method for enhanced bioelectrochemical cathode denitrification based on iron cycling, the method being carried out in a bioelectrochemical system, comprising: Step (1) Introduce artificially synthesized influent into the bioelectrochemical system, then inoculate sludge into the cathode, control the cathode potential at -0.6 V (vs. SHE) and acclimate it in a sequencing batch process for 9-30 days to allow Geobacter spp. microorganisms to be directionally enriched in the biofilm of the cathode; Among them, the artificially synthesized influent contains Fe 3+ Concentration of 30-50 mg / L, NO3 - -N concentration is 45-55 mg / L; Step (2) introduce the influent to be treated and control the cathode potential at -0.6 V (vs. SHE) to carry out denitrification.

[0019] More preferably, in step (1), the acclimatization time for the sequential batch pattern is 9-15 days.

[0020] Preferably, in step (1), the sequential batch mode domestication is carried out in an intermittent manner, with a fixed time sequence and a hydraulic residence time of 3-5 days for each cycle.

[0021] More preferably, each cycle includes water intake, reaction, sedimentation, and drainage in sequence.

[0022] More preferably, in step (1), the Fe content in the artificially synthesized feed water is adjusted by adding FeCl3 solution. 3+ The concentration is 30-50 mg / L.

[0023] According to a preferred embodiment, in step (1), the sludge inoculated at the cathode is activated sludge from the secondary sedimentation tank of a wastewater treatment plant, and the concentration of the inoculation is 2000-3000 mg VSS / L. Preferably, the method of the present invention further includes: in step (1), before applying the sludge to the inoculation, the sludge is pretreated for 15-20 days, including: replacing the wastewater with tap water every 5 days followed by nitrogen purging for 15 minutes to consume the existing iron, dissolved oxygen, and organic matter.

[0024] In a preferred embodiment, in the bioelectrochemical system, the cathode uses carbon felt as the electrode material, the anode uses stainless steel mesh as the electrode material, and the cathode and the anode are isolated by nylon mesh.

[0025] In a preferred embodiment, the method of the present invention further includes: in step (1) and / or step (2), controlling the temperature of the bioelectrochemical system to be 25-35°C, the pH to be 6.5-8.0, and the dissolved oxygen to be no more than 0.2 mg / L.

[0026] In a preferred embodiment, in step (2), the NO3 in the influent to be treated - -N concentration not higher than 100 mg / L.

[0027] Preferably, the Fe in the influent to be treated 3+ The concentration should not exceed 50 mg / L.

[0028] Preferably, in step (2), the denitrification process is operated in a continuous flow mode with a hydraulic retention time of 10-12 h.

[0029] As previously stated, a second aspect of the present invention provides a bioelectrochemical system for implementing the method described in the first aspect, the bioelectrochemical system comprising an autotrophic denitrification reactor, wherein the autotrophic denitrification reactor is provided with: The cathode is a carbon felt electrode used to load a cathode biofilm, which is enriched with Geobacterium spp. and denitrifying bacteria. The anode is a stainless steel mesh electrode; An isolation layer, made of nylon mesh, is disposed between the cathode and the anode; The bioelectrochemical system also includes: Potential control unit, used to keep the cathode potential constant at -0.6 V (vs. SHE).

[0030] Preferably, the cathode is inoculated with sludge, which is activated sludge from the secondary sedimentation tank of a wastewater treatment plant, and the inoculation concentration is 2000-3000 mg VSS / L.

[0031] In a preferred embodiment, the system of the present invention further includes a temperature control unit, a pH adjustment unit, and a dissolved oxygen control unit, for maintaining the system temperature at 25-35°C, the pH at 6.5-8.0, and the dissolved oxygen at no more than 0.2 mg / L.

[0032] The following combination Figure 1 A preferred embodiment of the bioelectrochemical system described in this invention will be described, the bioelectrochemical system comprising: Figure 1 The autotrophic denitrification reactor shown is equipped with: Water inlet 3 and water outlet 4 are connected. Water enters the reactor through water inlet 3 and water is discharged through water outlet 4. The cathode is a carbon felt electrode 6, which is used to load the cathode biofilm 5, which is enriched with Geobacterium spp. microorganisms and denitrifying bacteria. The anode is made of stainless steel mesh electrode 8; An isolation layer, disposed between the cathode and the anode, is made of nylon mesh 7; The cathode is connected to a cathode electrode interface 1, and the anode is connected to an anode electrode interface 2, for connection to an external potential control unit; The system also includes: A potential control unit, comprising a reference electrode and a potentiostat, wherein the reference electrode is disposed near the cathode, and the potentiostat is connected to the cathode via a cathode electrode interface 1 and to the anode via an anode electrode interface 2, for maintaining the cathode potential constant at -0.6 V (vs. SHE).

[0033] As previously stated, the third aspect of the present invention provides the application of the system described in the second aspect in the deep denitrification treatment of nitrate-containing wastewater.

[0034] According to a preferred embodiment, the nitrate-containing wastewater is aquaculture wastewater or secondary effluent from municipal sewage, with a carbon-to-nitrogen ratio not exceeding 3 and a total nitrogen removal rate of not less than 90%.

[0035] It should be noted that, in this invention, the carbon-nitrogen ratio (C / N) is the ratio of the mass concentration of chemical oxygen demand (COD) (mg / L) to the mass concentration of total nitrogen (TN) (mg / L).

[0036] Preferably, the NO3 in the nitrate-containing wastewater - -N concentration not exceeding 100 mg / L, Fe 3+ The concentration should not exceed 50 mg / L.

[0037] The present invention will be described in detail below through examples. Unless otherwise specified, the raw materials used are all commercially available products.

[0038] Sludge: Taken from the activated sludge of the secondary sedimentation tank of the sewage treatment plant, with a sludge volume index (SVI) of 80-100 mg / L and a mixed liquor suspended solids concentration (MLSS) of 3500-4000 mg / L.

[0039] Artificially synthesized influent: NO3 -The concentrations of Fe2+, NaHCO3, NaCl, MgSO4·7H2O, and CaCl2 were 50 mg / L, 2.0 g / L, 0.5 g / L, 0.41 g / L, 0.02 g / L, pH 7.2 ± 0.2, dissolved oxygen < 0.1 mg / L, and 50 mM phosphate buffer. No organic carbon source was added. 3+ The concentration was adjusted by adding FeCl3 solution, depending on the specific example.

[0040] The influent to be treated was artificially synthesized, with the following concentrations: NaHCO3 2.0 g / L, NaCl 0.5 g / L, MgSO4·7H2O 0.41 g / L, CaCl2 0.02 g / L, phosphate buffer 50 mM, and NO3-. - -N concentration was 50 mg / L, pH was 7.2 ± 0.2, dissolved oxygen was < 0.1 mg / L, no organic carbon source was added, and Fe... 3+ Concentration < 50 mg / L.

[0041] Pretreatment of sludge: To eliminate interference from residual iron in the inoculation sludge, the sludge was pretreated for 15 days before inoculation. This included replacing the wastewater with tap water every 5 days and then purging with nitrogen for 15 minutes to consume the iron, organic matter and dissolved oxygen originally present in the sludge. No iron was added to the influent during this period, resulting in pretreated sludge ready for inoculation.

[0042] Example 1

[0043] This embodiment illustrates that the iron-cycle-enhanced bioelectrochemical cathode denitrification method provided by the present invention is carried out according to the following steps: This embodiment employs a bioelectrochemical system, which includes an autotrophic denitrification reactor (such as...). Figure 1 As shown, the main body is made of plexiglass (with an effective volume of 1000 mL), and the autotrophic denitrification reactor is equipped with: Inlet 3 and outlet 4; water enters the autotrophic denitrification reactor through inlet 3 and water is discharged through outlet 4. Cathode: Carbon felt electrode 6 (5 cm radius, 5 mm thickness) is used. Anode: Stainless steel mesh electrode 8 (5 cm radius) is used, and nylon mesh 7 is used for physical isolation between the anode and cathode; The cathode is connected to a cathode electrode interface 1, and the anode is connected to an anode electrode interface 2, for connection to an external potential control unit; The potential control unit includes a reference electrode and a potentiostat. The reference electrode is located near the cathode, and the potentiostat is connected to the cathode through a cathode electrode interface 1 and to the anode through an anode electrode interface 2.

[0044] Step (1) Four identical autotrophic denitrification reactors were set up, and synthetic influent was introduced into each reactor through inlet 3. Pretreated sludge was inoculated at the cathode of each reactor at a concentration of 2000 mg VSS / L. The cathode potential of each bioelectrochemical system was precisely controlled at -0.6 V (vs. SHE) using a potentiostat (CHI1000C, Shanghai Chenhua). The Fe content of the synthetic influent in each reactor was controlled. 3+ The concentrations were 30, 40, and 50 mg / L, respectively, and the mixture was acclimated for 9 days using a sequential batch process. The operation was intermittent and cyclical, with a hydraulic retention time of 3 days per cycle. The temperature was controlled at 30±1℃ using a constant-temperature water bath to allow the *Geobacterium* microorganisms to be directionally enriched in the cathode biofilm 5. The *Geobacterium* microorganisms enriched in the cathode biofilm were then used to... 3+ Reduced to Fe 2+ At the same time, Fe 2+ Oxidized to Fe 3+ This forms an iron cycle to enhance electron transport; Step (2) Introduce the influent to be treated, continue to control the cathode potential of each bioelectrochemical system at -0.6 V (vs. SHE), run each reactor in continuous flow mode for 600 h, with a hydraulic retention time of 12 h, control the temperature at 30±1℃ through a constant temperature water bath, carry out denitrification, and finally obtain the effluent at the outlet 4 of each reactor.

[0045] Record the Fe in the artificially synthesized influent 3+ The influent nitrate nitrogen and effluent total nitrogen concentrations of the reactor in step (2) are 30 mg / L. Figure 2 (a) shows the results, indicating that the total nitrogen in the effluent stabilized at 48 hours, with the highest calculated total nitrogen removal rate (NRE) of 93.3%. Analyze and record the Fe in the artificially synthesized influent. 3+ The inorganic nitrogen concentration in the effluent from the reactor with a concentration of 30 mg / L in step (2) Figure 2 (b) shows the results, indicating that NO3 in the effluent... - -N was 2.0 ± 0.6 mg / L, NO2 - -N was 1.0 ± 0.3 mg / L, NH4+ + The average concentration of -N was 0.3 ± 0.2 mg / L.

[0046] Comparative Example 1 This comparative example uses the same system as Example 1 and includes the following steps: The influent to be treated was introduced into the autotrophic denitrification reactor. The cathode potential of each bioelectrochemical system was controlled at -0.6 V (vs. SHE). The reactor was operated in continuous flow mode for 600 h with a hydraulic retention time of 12 h. The temperature was controlled at 30±1℃ using a constant temperature water bath for denitrification. Record the concentrations of nitrate nitrogen in the influent and total nitrogen in the effluent of the reactor. Figure 3 (a) shows the results, indicating that the total nitrogen in the effluent stabilized at 60 hours, with the highest calculated total nitrogen removal rate (NRE) of 84.7%. Analyze and record the concentration of inorganic nitrogen in the water. Figure 3 (b) shows the results, indicating that NO3 in the effluent... - -N was 5.2 ± 0.8 mg / L, NO2 - -N was 2.1 ± 0.1 mg / L, NH4+ + The average concentration of -N was 0.4 ± 0.2 mg / L.

[0047] Comparative Example 2 This comparative example was conducted using a method similar to that of Example 1, except that in step (1), three identical reactors were set up, and the Fe content in the three reactors was... 3+ The concentrations were controlled at 0, 10 and 20 mg / L respectively; the final effluent obtained in step (2) was named DP1, DP2 and DP3 respectively.

[0048] Test Example 1 The nitrate removal rate constant in the effluent obtained in step (2) of each reactor group in Example 1 and Comparative Example 2 and the intracellular iron content in the biofilm of the acclimated cathode in step (1) were detected and analyzed. The results are as follows: Figure 4 As shown in the figure: With DP1 (Fe 3+ Compared to DP1 (0 mg / L), the nitrate removal rate constants of the other groups increased to 1.3 times that of DP1 (Fe 3+ 10 mg / L), 1.7 times (Fe 3+ 20 mg / L), 1.8 times (Fe 3+ 30 mg / L), 1.9 times (Fe 3+ 40 mg / L) and 1.7 times (Fe 3+ 50 mg / L).

[0049] The intracellular iron content of the cathode biofilm varies with the amount of Fe synthesized in the influent. 3+ The concentration increases with increasing concentration, respectively compared to DP1 (Fe 3+0 mg / L) increased by 103% (Fe 3+ 10 mg / L), 194% (Fe 3+ 20 mg / L), 314% (Fe 3+ 30 mg / L), 314% (Fe 3+ 40 mg / L) and 337% (Fe 3+ 50 mg / L).

[0050] The above results indicate that at a cathode potential of -0.6 V and Fe 3+ Under the synergistic effect of domestication, the system's nitrogen removal efficiency is significantly improved.

[0051] Test Example 2 The microbial adhesion on the surface of the acclimated cathode electrode in step (1) of Final Example 1 and Comparative Example 1 was detected, and the results are as follows: Figure 5 As shown in the figure, the amount of microorganisms attached to the surface of the cathode electrode in Comparative Example 1 is sparse, the fluorescence signal points are sparsely and discretely distributed, and a small number of isolated microbial aggregates can be seen. The cathode electrode surface has limited adsorption and enrichment capacity for microorganisms.

[0052] In Example 1, the number of fluorescent signal points on the cathode electrode surface increased significantly, and the density of microbial aggregates was significantly higher than that in Comparative Example 1. Furthermore, the microorganisms were distributed continuously in a network pattern along the carbon fiber skeleton of the electrode. This indicates that ferric iron significantly promoted the attachment and accumulation of microorganisms on the cathode electrode surface, which is conducive to the formation of a dense and uniformly covered functional biofilm. This provides a structural basis for the construction of long-range electron transport channels and the formation of functional hierarchical structures within the biofilm.

[0053] Test Example 3 The relative abundance composition of major functional bacteria in the biofilm on the surface of the pretreated sludge, the acclimatized cathode electrode in step (1) of Example 1, and Comparative Example 1 was analyzed, and the results are as follows: Figure 6 As shown.

[0054] As shown in the figure, the functional microbial community in the pretreated sludge is dominated by heterotrophic or sulfur autotrophic denitrifying bacteria such as Thiobacillus sp., Rhodocyclus sp., and Desulfobacillus sp., while Geobacter anodireducens has a relatively low abundance.

[0055] Compared with the pretreated sludge, the total relative abundance of the main functional bacteria in the cathode biofilm of Comparative Example 1 decreased (from about 49.2% to 40.6%), and the proportion of each bacterial group did not change significantly, indicating that the targeted enrichment effect of the functional bacteria was limited.

[0056] In Example 1, the total relative abundance of major functional bacteria in the biofilm of the cathode increased to 60.2%, with the relative abundance of Geobacterium (18.3%) significantly higher than that of the pretreated sludge and Comparative Example 1. Meanwhile, denitrification bacteria such as UBA6170 (9.2%) and Sulfurivermis fontis (8.3%) also maintained high abundance. This indicates that ferric iron effectively drove the enrichment of Geobacterium, and that ferric iron effectively drove the directional enrichment of electroactive microorganisms and promoted the co-enrichment of electroactive microorganisms and denitrifying bacteria.

[0057] Test Example 4 The differences in the expression levels of genes related to iron metabolism, nitrogen metabolism, and electron transport chain in the biofilm on the surface of the domesticated cathode electrode in step (1) of Final Example 1 and Comparative Example 1 were analyzed, and the results are as follows: Figure 7 As shown.

[0058] As can be seen from the figure, in terms of iron metabolism functional genes, the abundance of outer membrane polyhemoglobin cytochrome genes (such as omcS, omcF), iron reduction-related genes (such as ndh2, dmkB), and iron oxidation-related genes (such as cyc2, mtoA) of *Geotrichum* spp. in the biofilm of the cathode in Example 1 was enriched to varying degrees compared with that in Comparative Example 1, indicating that Fe... 3+ Domestication enhanced the extracellular electron transport protein synthesis and iron metabolism of G. anodireducens, which is beneficial for long-range electron transport mediated by the Fe(III) / Fe(II) cycle in the biomembrane.

[0059] In Example 1, the abundance of most genes related to the electron transport chain (such as nouC, sdhC) and the DNRA pathway (nrfH) was reduced, indicating that Geobacter mainly uses electrons for iron metabolism, which promotes the division of labor and cooperation between electroactive microorganisms and denitrifying bacteria inside the biofilm.

[0060] The above data show that the method provided by the present invention can enhance the long-range electron transfer within the biofilm by enriching electroactive microorganisms of the genus Geobacterium and enhancing their iron metabolism capacity, and form a metabolic cooperation mode between electroactive microorganisms and denitrifying bacteria, thereby significantly improving the cathode denitrification efficiency of the system.

[0061] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for enhanced bioelectrochemical cathode denitrification based on iron cycling, characterized in that, This method is performed in a bioelectrochemical system and includes: Step (1) Introduce artificially synthesized influent into the bioelectrochemical system, then inoculate sludge into the cathode, control the cathode potential at -0.6 V and acclimate it in a sequential batch mode for 9-30 days, so that the genus Geobacterium can be directionally enriched in the biofilm of the cathode. Among them, the artificially synthesized influent contains Fe 3+ Concentration of 30-50 mg / L, NO3 - -N concentration is 45-55 mg / L; Step (2) introduce the influent to be treated and control the cathode potential at -0.6 V to carry out denitrification; In steps (1) and (2), -0.6 V is the value of vs SHE, which is relative to the standard hydrogen electrode.

2. The method according to claim 1, characterized in that, In step (1), the batch domestication is carried out in an intermittent manner with a fixed time sequence, and the hydraulic residence time of each cycle is 3-5 days.

3. The method according to claim 1 or 2, characterized in that, In step (1), the sludge is activated sludge from the secondary sedimentation tank of a wastewater treatment plant, and the inoculation concentration is 2000-3000 mg VSS / L.

4. The method according to claim 1, characterized in that, In the bioelectrochemical system, the cathode uses carbon felt as the electrode material, the anode uses stainless steel mesh as the electrode material, and the cathode and the anode are isolated by nylon mesh.

5. The method according to claim 1, characterized in that, The method further includes: in step (1) and / or step (2), controlling the temperature of the bioelectrochemical system to be 25-35℃, the pH to be 6.5-8.0, and the dissolved oxygen to be no more than 0.2 mg / L.

6. The method according to claim 1, characterized in that, In step (2), the NO3 in the influent to be treated - -N concentration is 45-55 mg / L.

7. A bioelectrochemical system for implementing the method according to any one of claims 1-6, characterized in that, The bioelectrochemical system includes an autotrophic denitrification reactor, which is equipped with: The cathode is a carbon felt electrode used to load a cathode biofilm, which is enriched with Geobacterium spp. and denitrifying bacteria. The anode is a stainless steel mesh electrode; An isolation layer, made of nylon mesh, is disposed between the cathode and the anode; The bioelectrochemical system also includes: The potential control unit is used to keep the cathode potential constant at -0.6 V, where -0.6 V is the value relative to the standard hydrogen electrode (vs SHE).

8. The bioelectrochemical system according to claim 7, characterized in that, The system also includes a temperature control unit, a pH adjustment unit, and a dissolved oxygen control unit, which are used to maintain the system temperature at 25-35℃, the pH at 6.5-8.0, and the dissolved oxygen at no more than 0.2 mg / L.

9. The application of the system according to claim 7 or 8 in the deep denitrification treatment of nitrate-containing wastewater.

10. The application according to claim 9, characterized in that, The nitrate-containing wastewater is either aquaculture wastewater or secondary effluent from municipal sewage, with a carbon-to-nitrogen ratio not exceeding 3 and a total nitrogen removal rate of not less than 90%.

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