Iron-carbon micro-electrolysis coupled with soil infiltration system, application and sewage treatment method

Abstract

The present application belongs to the technical field of sewage treatment, and particularly relates to an iron-carbon micro-electrolysis coupled soil infiltration system, application and a sewage treatment method. The technical scheme of the present application is an iron-carbon micro-electrolysis coupled soil infiltration system, which comprises, from top to bottom, a soil covering layer, a gravel water distribution layer, a substrate filler layer and a gravel supporting layer; the substrate filler layer is filled with at least iron-carbon filler. The present application further provides application of the iron-carbon micro-electrolysis coupled soil infiltration system in sewage treatment and a sewage treatment method. The present application provides a new option for removal of pollutants in domestic sewage and greenhouse gas emission reduction.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of sewage treatment, and particularly relates to an iron-carbon micro-electrolysis coupled soil infiltration system, application and sewage treatment method. BACKGROUND

[0002] The high-cold region of China includes Qinghai, Tibet and parts of Sichuan, Yunnan, Gansu and Xinjiang, and has the characteristics of low temperature, low oxygen and strong ultraviolet. With the improvement of rural living standards, the amount of sewage discharge increases, but the lack of sewage discharge system and treatment facilities leads to low sewage treatment rate. Direct discharge of untreated domestic wastewater threatens ecology and public health, and the greenhouse gases generated in the operation of the existing treatment system exacerbate environmental pressure, so it is urgent to improve the treatment efficiency and solve the technical problems of low-temperature operation and low C / N ratio.

[0003] High-cold rural sewage is divided into gray water (accounting for 50-80%, containing washing pollutants) and black water (accounting for 30%, containing breeding toilet wastewater). The treatment mainly includes biological method (activated sludge, biofilm) and ecological method (artificial wetland, etc.). The biological method is difficult to popularize due to high cost, and the ecological method is still restricted by water quality fluctuation and climate conditions, and has defects such as large occupation area and temperature sensitivity.

[0004] The high-cold region adopts the double mode of standard discharge and resource utilization, and the decentralized agricultural area is suitable for resource utilization, and the population-dense area needs to meet the standard for pollution prevention. Direct pollution discharge leads to excessive nitrate in groundwater, eutrophication of surface water and soil degradation. Although the state promotes environmental remediation, facilities generally face problems such as unsuitable technology, expensive operation and maintenance, and lack of management, and it is urgent to develop low-cost, low-energy and easy-to-maintain treatment technology. SUMMARY

[0005] In view of the problems in the prior art, the application provides an iron-carbon micro-electrolysis coupled soil infiltration system, application and sewage treatment method, so as to solve the problems of poor pollutant removal and greenhouse gas emission reduction effect of the prior art on high-cold region domestic sewage, and also solve the problem of insufficient adaptability of microorganisms to the environment.

[0006] The technical scheme of the present application is as follows: The present application provides an iron-carbon micro-electrolysis coupled soil infiltration system, which comprises, from top to bottom, a soil cover layer, a gravel water distribution layer, a substrate filler layer and a gravel support layer; the substrate filler layer is filled with at least iron-carbon filler. Through careful design of the iron-carbon micro-electrolysis coupled soil infiltration system, the soil cover layer at the top can preliminarily intercept part of the pollutants, the gravel water distribution layer plays a filtering and supporting role, and the iron-carbon filler added in the key substrate filler layer, on the one hand, utilizes the principle of iron-carbon micro-electrolysis, iron and carbon form countless micro-batteries in the sewage, generate strong oxidation-reduction effect, break the chain of complex organic matter in the sewage, decompose the complex organic matter, reduce the toxicity and improve the biodegradability; on the other hand, the iron-carbon filler provides a special environment for microorganisms, screens out microbial populations that are suitable for high oxidation-reduction potential and specific substrates, promotes microbial metabolic activity, synergistically degrades residual organic pollutants, simultaneously inhibits the activity of greenhouse gas producing bacteria such as methanogens, reduces the emission of greenhouse gases such as methane, and finally realizes the remarkable effect of efficient removal of pollutants in the sewage and greenhouse gas emission reduction. In addition, the addition of the iron-carbon filler has a screening effect on microorganisms, so that the microorganisms develop towards the direction of favorable populations that are suitable for the environment of the system.

[0007] Preferably, the substrate filler layer further comprises zeolite filler. The substrate filler layer is composed of filler and soil, and the volume ratio of the filler to the soil is 1:2. The soil cover layer is composed of soil. The gravel water distribution layer is composed of gravel filler, and the gravel support layer is composed of gravel filler, and the particle size of the gravel filler is 4-6 mm. By carefully setting the filler of each layer, the stability of the iron-carbon micro-electrolysis coupled soil infiltration system can be increased, and the maintenance cost and workload can be reduced. Preferably, the substrate filler layer is composed of iron-carbon filler, zeolite filler and soil, the zeolite filler layer is located in the upper layer, the iron-carbon filler layer is located in the lower layer, and soil is doped in the zeolite filler layer and the iron-carbon filler layer. Among them, the soil is promoted to naturally settle and compact in the iron-carbon filler and the zeolite filler by means of water injection. The layered design fully utilizes the high adsorption performance of zeolite to preferentially intercept pollutants in water, and then further degrades and converts pollutants through the reducing effect of the iron-carbon filler, while combining the biological purification function of soil to realize multi-stage treatment of pollutants, significantly improving the water quality purification efficiency and the overall ecological function of the system. Preferably, the volume ratio of the zeolite filler layer to the iron-carbon filler layer is 1-3:1-3. The volume ratio of the zeolite filler layer to the iron-carbon filler layer is 3:1, 1:1 or 1:3. Preferably, the volume ratio of the zeolite filler layer to the iron-carbon filler layer is 1:1. Preferably, the mass ratio of Fe / C in the iron-carbon filler is 3-5:1. Preferably, the mass ratio of Fe / C in the iron-carbon filler is 3:1. Preferably, the soil is brown soil, and the organic matter content of the soil is greater than 8%, the nitrogen content is greater than 0.4%, the phosphorus content is greater than 0.2%, the potassium content is greater than 17%, and the pH value is greater than 5%.

[0008] Preferably, the soil has an organic matter content of 8.46%, a nitrogen content of 0.469%, a phosphorus content of 0.25%, a potassium content of 17.4%, and a pH value of 5.94%. By reasonably matching the volume ratio of the zeolite filler layer and the iron-carbon filler layer and using brown soil rich in organic matter as the soil, the adsorption performance of the zeolite and the reduction performance of the iron-carbon can be fully utilized, and at the same time, the brown soil rich in organic matter provides a good growth environment for microorganisms, thereby significantly improving the water purification effect and the stability of the ecological system, and realizing efficient and stable ecological restoration and water purification functions. Preferably, the volume ratio of the soil cover layer, the gravel water distribution layer, the substrate filler layer, and the gravel support layer is 1:1:16:2. Preferably, the thicknesses of the soil cover layer, the gravel water distribution layer, the substrate filler layer, and the gravel support layer are 5cm, 5cm, 80cm, and 10cm, respectively. By reasonably allocating the volume ratio or thickness of each layer, the ecological function of the soil, the drainage and support effect of the gravel, and the purification capacity of the substrate filler layer can be effectively utilized, the optimization balance of the ecological function and the engineering performance is realized, and the stability and operating efficiency of the system are improved. Preferably, the iron-carbon micro-electrolysis coupled soil infiltration system has at least one of the following characteristics: the particle size of the iron-carbon filler is 8-12mm; and the particle size of the zeolite filler is 4-8mm. By reasonably selecting the particle sizes of the iron-carbon filler and the zeolite filler, the porosity and water flow passability of the filler layer can be effectively improved, and at the same time, the sufficient contact between the filler and the pollutants is ensured, thereby enhancing the pollutant removal efficiency and operating stability of the system. Preferably, the iron-carbon micro-electrolysis coupled soil infiltration system further comprises a water inlet and a water outlet; the water inlet is located in the gravel water distribution layer and is 7cm away from the upper surface of the soil cover layer; and the water outlet is located in the gravel support layer and is 3cm away from the bottom of the gravel support layer. By skillfully setting the water inlet and the water outlet, the hydraulic load and the dry-wet ratio can be more effectively adjusted, the iron-carbon micro-electrolysis coupled soil infiltration system can achieve the best effect of removing sewage pollutants and reducing greenhouse gases, and domestic sewage can be more effectively treated. By reasonably setting the positions of the water inlet and the water outlet, uniform water distribution and sufficient filtration are ensured, and the overall purification efficiency and stability of the system are improved.

[0009] The application also provides an application of the iron-carbon micro-electrolysis coupled soil infiltration system in sewage treatment. The high-efficiency reduction and degradation effect of the iron-carbon micro-electrolysis and the biological purification function of the soil infiltration can be fully utilized, the collaborative removal of various pollutants in sewage can be realized, the sewage treatment efficiency and the effluent water quality can be significantly improved, and the application has the advantages of low operating cost, simple operation, and environmental friendliness.

[0010] The application also provides a sewage treatment method, comprising the following steps: S1, inoculating and culturing activated sludge in domestic sewage in an alpine region; S2, introducing the domestic sewage in the alpine region inoculated with the cultured activated sludge into the iron-carbon micro-electrolysis coupled soil infiltration system for treatment, and discharging the treated sewage from a water outlet, wherein the hydraulic load of the iron-carbon micro-electrolysis coupled soil infiltration system is 0.001-0.01 m 3 / m 2 ·h, and the dry-wet ratio is 1-5:1; the pH value of the domestic sewage in the alpine region is 6.5-8.0, the SS is 100-200 mg / L, the COD is 200-450 mg / L, the BOD5 is 200-300 mg / L, the NH4 + -N is 20-90 mg / L, and the TP is 2.0-6.5 mg / L. According to the characteristics of the domestic sewage in the alpine region, the problem of sewage treatment in the alpine region is effectively solved by inoculating and culturing activated sludge and using the iron-carbon micro-electrolysis coupled soil infiltration system for treatment, in combination with the appropriate hydraulic load, dry-wet ratio and temperature conditions, so that the sewage can be discharged up to the standard, and the method has the advantages of high efficiency, stability, strong adaptability and environmental friendliness.

[0011] The inoculated sludge mainly plays a role in culturing denitrifying bacteria, which act in the substrate filler layer. The role of denitrifying bacteria in sewage treatment includes: 1) ammonification, many organic nitrogen compounds such as proteins and amino acids cannot be directly absorbed and utilized by microorganisms, and denitrifying bacteria can decompose these complex organic nitrogen into ammonia nitrogen (NH4 + ). For example, in the process of protein decomposition, denitrifying bacteria hydrolyze proteins into peptides and amino acids by secreting various enzymes, and finally release ammonia. 2) nitrification, converting ammonia nitrogen into nitrate nitrogen (NO3 - ). This process usually consists of two stages: nitrosation, ammonia-oxidizing bacteria (such as nitrite bacteria) oxidize ammonia (NH3) or ammonium ion (NH4 + ) to nitrite (NO2 - ). For example, under suitable temperature, pH and dissolved oxygen conditions, nitrite bacteria use intracellular enzymes such as ammonia monooxygenase to oxidize ammonia to nitrite. Nitrification, nitrite bacteria further oxidize nitrite to nitrate (NO3 -). Nitrobacteria convert nitrite to nitrate through the action of enzymes such as nitrite oxidase. 3) Denitrification, which reduces nitrate nitrogen to gaseous nitrogen (such as N2, NO, N2O, etc.), thereby removing nitrogen from water. Denitrifying bacteria use nitrate as an electron acceptor under anaerobic conditions, reducing nitrate to nitrite and further to gaseous nitrogen. For example, during denitrification, denitrifying bacteria use organic matter as an electron donor to ultimately convert nitrate to nitrogen gas, which is released into the atmosphere. Through these actions, denitrifying bacteria can effectively reduce the nitrogen content in wastewater, reduce the pollution of water bodies by nitrogen, prevent water eutrophication, and thus improve water quality. At the same time, denitrifying bacteria work together with other microorganisms in the wastewater treatment system to form a complex microbial community, which collectively removes organic matter, nitrogen, phosphorus and other pollutants in wastewater, improving the efficiency of wastewater treatment. The activated sludge is mixed sludge from the anaerobic tank and the aerobic tank of the Danma Sewage Treatment Station in Damama Town, Huzhu County, Haidong City, Qinghai Province, in a mass ratio of 1:1. The activated sludge contains mixed bacteria from the anaerobic tank and the aerobic tank.

[0012] Further, the iron-carbon micro-electrolysis coupled soil infiltration system operates with a hydraulic load of 0.001 m 3 / m 2 ·h, 0.003 m 3 / m 2 ·h, 0.006 m 3 / m 2 ·h or 0.01 m 3 / m 2 ·h; and a dry-wet ratio of 5:1, 3:1 or 1:1. Preferably, the hydraulic load is 0.003 m 3 / m 2 ·h, and the dry-wet ratio is 5:1. By precisely controlling the hydraulic load and the dry-wet ratio during operation of the system, efficient pollutant removal and stable system operation can be achieved, while reducing energy consumption and operating costs.

[0013] The iron-carbon micro-electrolysis coupled soil infiltration system of the present application can effectively treat domestic sewage, efficiently remove pollutants in the sewage, and reduce the emission of greenhouse gases. The system also has good denitrification ability. The type of filler and the depth of soil significantly affect the structure of the microbial community. The addition of iron-carbon filler has a screening effect on microorganisms, allowing them to develop towards a population that is adapted to the environment. The system has application value in the field of wastewater treatment technology. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1Fig. 1 is a schematic diagram of the shapes of the experimental soil and fillers, (a) gravel, (b) soil, (c) iron-carbon, and (d) zeolite; Figure 2 Fig. 2 is a schematic diagram of the structure of the iron-carbon micro-electrolysis coupled soil infiltration system; Figure 3 Fig. 3 is a graph of the COD concentration change in different HL stages; Figure 4 Fig. 4 is a graph of the COD concentration change in different D / W stages; Figure 5 Fig. 5 is a graph of the COD concentration and removal rate change in different filler experimental columns, the column chart represents the concentration, and the scatter chart represents the removal rate; Figure 6 Fig. 6 is a graph of the NH4 + -N concentration change in different HL stages; Figure 7 Fig. 7 is a graph of the NH4 + -N concentration change in different D / W stages; Figure 8 Fig. 8 is a graph of the NH4 + -N concentration and removal rate change in different filler experimental columns, the column chart represents the concentration, and the scatter chart represents the removal rate; Figure 9 Fig. 9 is a graph of the NO3 - -N concentration change in different HL stages; Figure 10 Fig. 10 is a graph of the NO3 - -N concentration change in different D / W stages; Figure 11 Fig. 11 is a graph of the NO3 - -N concentration change in different filler experimental columns; Figure 12 Fig. 12 is a graph of the NO2 - -N concentration change in different HL stages; Figure 13 Fig. 13 is a graph of the NO2 - -N concentration change in different D / W stages; Figure 14 Fig. 14 is a graph of the NO2 - -N concentration change in different filler experimental columns; Figure 15 Fig. 15 is a graph of the TN concentration change in different HL stages; Figure 16 Fig. 16 is a graph of the TN concentration change in different D / W stages; Figure 17 Fig. 17 is a graph of the TN concentration and removal rate change in different filler experimental columns, the column chart represents the concentration, and the scatter chart represents the removal rate; Figure 18 Fig. 18 is a graph of the TP concentration change in different HL stages; Figure 19 Fig. 19 is a graph of the TP concentration change in different D / W stages; Figure 20 Fig. 20 is a graph of the TP concentration and removal rate change in different filler experimental columns, the column chart represents the concentration, and the scatter chart represents the removal rate; Figure 21 Fig. 21 is a graph of the CH4 emission flux change in different HL stages; Figure 22 Fig. 22 is a graph of the CH4 emission flux change in different D / W stages; Figure 23 Fig. 23 is a graph of the CH4 emission flux change in different filler experimental columns; Figure 24 Fig. 24 is a graph of the N2O emission flux change in different HL stages; Figure 25 Fig. 25 is a graph of the N2O emission flux change in different D / W stages; Figure 26A graph showing the variation of N2O emission flux in experimental columns with different packing materials; Figure 27 The images show SEM analysis of the zeolite packing. The left image is a SEM image of the original zeolite packing magnified 20,000 times, and the right image is a SEM image of the zeolite packing in column #5 magnified 5,000 times. Figure 28 The images show SEM analysis results of the iron-carbon packing material. The left image is a SEM image of the original iron-carbon packing material magnified 20,000 times, and the right image is a SEM image of the iron-carbon packing material in column #5 magnified 5,000 times. Figure 29 XPS spectrum of original iron-carbon filler; Intensity: strength; Binding energy: binding energy; Figure 30 XPS spectra of the lower layer of iron-carbon packing material in the experimental column; Intensity: strength; Binding energy: binding energy; Figure 31 This is a dilution curve plot; Sobs index: observed species index; Number of Reads Sampled: number of reads; Rarefaction curves: dilution curves; Figure 32 Venn diagrams for samples at different depths; Figure 33 Principal component analysis (PCA) plot of the sample at the genus level; Principal component analysis at the genus level. Figure 34 Hierarchical clustering tree at the genus level (>1%); Tree Plot; Group; Taxa. Figure 35 Relative abundance at the phylum level (>1%) for different samples; Figure 36 Relative abundance at the class level (>1%) for different samples; Relative abundance at the class level: relative abundance at the class level. Figure 37 Relative abundance at the Genus level (>1%): Relative abundance at the Genus level; Figure 38 Redundancy analysis diagram of microbial community structure and environmental factors; RDA on Genus level: Genus-level redundancy analysis; where COD Concentration: COD concentration; COD Removed efficiency: COD removal efficiency; Time (days): time (days); stage: stage; NH4 +- N Concentration: NH4 + - N Concentration: NH4 + - N Removed efficiency: NH4 + - N Removed efficiency: NH4 - N Removed efficiency: NH4 DETAILED DESCRIPTION

[0015] The present application is further illustrated by the following examples without limiting the present application in any way.

[0016] The following experimental materials and water quality are used: Experimental materials: soil: after natural air drying treatment, pass through 2mm sieve. Its profile color changes less, the surface layer presents grayish brown, the lower layer mainly transitions from brown to light brown, soil property: brown soil; organic matter is 8.46%; nitrogen content is 0.469%; phosphorus content is 0.25%; potassium content is 17.4%; pH value is 5.94. Soil shape is shown in Figure 1 (b). Select gravel filler specifications 4-6mm, zeolite filler specifications 4-8mm, iron-carbon filler specifications 8-12mm, shapes are shown in Figure 1 (a), 1(d) and 1(c). The mass ratio of Fe / C in the iron-carbon filler is 3~5:1. Rural domestic sewage quality in alpine region: pH: 6.5~8.0; SS: 100~200mg / L; COD: 200~450mg / L; BOD5: 200~300 mg / L; NH4 + - N: 20~90 mg / L; TP: 2.0~6.5 mg / L.

[0017] Example 1

[0018] As Figure 2As shown, an iron-carbon micro-electrolysis coupled soil infiltration system includes a soil column device, the soil column has a diameter of 20 cm and a height of 100 cm, and the material is organic glass. A 5 cm space is reserved at the upper part of the soil column to prevent soil leakage, the water inlet is located 7 cm away from the upper surface of the soil cover layer, and the water outlet is 3 cm away from the bottom. Five substrate sampling points are set, and the sampling points are 10 cm, 30 cm, 50 cm, 70 cm and 90 cm away from the soil surface, respectively. In this embodiment, a total of six soil columns are provided, which are named 1# column, 2# column, 3# column, 4# column, 5# column and 6# column. Among them, 1# column, 4# column, 5# column and 6# column are embodiments, and 2# column and 3# column are control embodiments. The soil column device from top to bottom is 5 cm of soil cover layer, 5 cm of gravel distribution layer, 80 cm of substrate filler layer and 10 cm of gravel supporting layer. Among them, the substrate filler layer of 3# column is filled with 80 cm of gravel filler, the substrate filler layer of 1# column is filled with 80 cm of iron-carbon filler, the substrate filler layer of 2# column is filled with 80 cm of zeolite filler, and the substrate filler layers of 4# column, 5# column and 6# column are filled with zeolite filler and iron-carbon filler according to the volume ratio of 3:1, 1:1 and 1:3, respectively. The zeolite filler and the iron-carbon filler in the substrate filler layer are mixed with soil, and the volume ratio of the zeolite filler and the iron-carbon filler to the soil is 1:2. In order to prevent the short flow effect from affecting the reliability of the experimental data, before filling the substrate, vaseline is uniformly smeared on the inner wall of the column to enhance the sealing performance. After the filling of the substrate is completed, the soil is naturally settled and compacted by water injection. In order to simulate the lightless environment, the soil column is wrapped with aluminum foil before the formal experiment.

[0019] Embodiment 2

[0020] A sewage treatment method, comprising the following steps; S1, inoculating and culturing activated sludge in high-cold region domestic sewage; S2, introducing the high-cold region domestic sewage inoculated and cultured with activated sludge into the water inlets of 1# column, 2# column, 3# column, 4# column, 5# column and 6# column in the iron-carbon micro-electrolysis coupled soil infiltration system in embodiment 1 respectively, so as to treat the high-cold region domestic sewage; wherein the cultured activated sludge is mixed sludge with a mass ratio of 1:1 taken from the anaerobic tank and the aerobic tank of the Danma sewage treatment station in Huzhu County, Haidong City, Qinghai Province; the inoculated sludge is mainly cultured denitrifying bacteria. The high-cold region domestic sewage has the following water quality: C / N = 5, COD is 250 mg / L, TN is 50 mg / L, NH4 + -N is 50 mg / L, BOD5 is 250 mg / L, TP is 4.5 mg / L, pH is 7.5, and SS is 150 mg / L; during the treatment of the high-cold region domestic sewage, NH4 + -N, TN, TP, COD, NO3 - -N, NO2 --N is used for testing. The system is considered stable after the effluent quality has been stable for one week. The operating parameters of the iron-carbon micro-electrolysis coupled soil infiltration system are shown in Table 1.

[0021] Table 1 Operating parameters of the iron-carbon micro-electrolysis coupled soil infiltration system

[0022]

[0023] Pollutant removal efficiency analysis: The water quality of the inlet and outlet of domestic sewage from the high-altitude cold region treated with different hydraulic loads and wet-dry ratios in Example 2 was measured. 0.5L water samples were collected from the inlet and outlet using 200mL polyethylene plastic bottles every two days. All samples were analyzed on the day of sampling. Monitoring indicators included chemical oxygen demand (COD) and ammonia nitrogen (NH4+). + -N), nitrate nitrogen (NO3) - -N), nitrite nitrogen (NO2) - The determination of total nitrogen (TN), total phosphorus (TP), and pH value was carried out according to the methods specified in "Methods for Monitoring and Analysis of Water and Wastewater (Fourth Edition)". pH was determined using the electrode method with a PHS-3C apparatus, COD was determined using the rapid digestion spectrophotometric method with a Hach digester, and NH4+ was determined... + -N, NO2 - -N, NO3 - -N and TN were determined by UV-Vis spectrophotometer using Nash reagent spectrophotometry (HJ665-2013), naphthylethylenediamine hydrochloride spectrophotometry (HJ668-2013), UV spectrophotometry (HJ / T346─2007), and alkaline potassium persulfate spectrophotometry (HJ636—2012), respectively. TP was determined by TOC-LCPH using ammonium molybdate spectrophotometry (HJ670-2013).

[0024] 1. COD Removal Efficiency Analysis: 1) Impact of Hydraulic Loading (HL) on COD Removal Efficiency. The changes in influent and effluent COD concentrations under different HL are shown below. Figure 3 As shown, the COD concentration in the effluent decreases significantly with decreasing HL. The decrease in HL significantly increases the contact time between COD and the packing matrix. This change not only promotes the adsorption of COD by the packing but also creates more favorable conditions for microbial degradation of COD. Consequently, the COD removal rate at different HL stages decreases as HL decreases from 0.01 m... 3 / m 2 h(Ia) decreased to 0.001m 3 / m 2• h(Id), COD removal rate overall upward trend. Shows that with the decrease of HL, COD removal effect gradually changed good, in the whole experimental stage to maintain a high removal rate. 2) the influence of dry-wet ratio on COD removal effect. Different D / W under the COD influent concentration and effluent concentration as Figure 4 shown, with D / W from 5:1 (IIa) to 1:1 (IIc), the COD effluent concentration of six groups of experimental column gradually increased with time in the steady state. It is shown that the decrease of D / W leads to the decrease of DO content in the system, which is not conducive to the oxidation and decomposition of COD; in addition, with the extension of influent time, the duration of hydraulic scouring effect on the substrate is also increased, which will reduce the adsorption efficiency of the substrate to COD. It can be further known that the COD removal rate of different D / W stages decreases with the decrease of dry-wet ratio in the steady state. It is shown that the decrease of D / W prolongs the influent time and leads to the decrease of COD removal rate, but the overall removal rate is not affected too much, and the overall removal rate is still high. 3) the influence of filler on COD removal effect. Figure 5 The COD effluent concentration and removal efficiency of different filler experimental columns at each HL stage and D / W stage are shown. It is shown that when Fe element, NO3 - and organic matter coexist in the reaction system, two chemical reactions can occur: 10Fe 2+ + 6H2O + 2NO3 - = 10Fe 3+ + 12OH - + N2 (I) 8Fe 3+ + organic matter + 4H2O = 8Fe 2+ + 2HCO3 - + 9H + (II) When the above reactions reach steady state, the system not only effectively controls the accumulation of specific Fe ions, but also significantly improves the removal efficiency of organic pollutants. Therefore, the effluent concentration of the control group is higher than that of the experimental column filled with iron-carbon filler, and the effluent concentration of 1# column is abnormally high, which may be due to the fewer microbial growth points provided by the iron-carbon filler, and the number of microorganisms is much lower than that of other experimental columns.

[0025] 2、NH4 + -N removal effect analysis: 1) the influence of HL on NH4 + -N removal effect. NH4 + -N in different HL stages under the influent concentration and effluent concentration as Figure 6 shown, with the decrease of HL, the NH4 + -N effluent concentration of each experimental column decreases. The contact time of wastewater with filler substrate is prolonged with the decrease of HL, which is beneficial to the removal of NH4 +-N is better adsorbed by the filler. At the same time, it is beneficial to the reoxygenation of the system and promotes the nitrification process of nitrifying bacteria, so as to realize the removal of NH4 + -N. Further, it can be known that NH4 + -N removal rate at different HL stages, with the decrease of HL, NH4 + -N removal rate gradually increases. 2) The influence of dry-wet ratio on NH4 + -N removal effect. NH4 + -N influent concentration and effluent concentration at different D / W stages are shown in Figure 7 , the NH4 + -N effluent concentration increases with the decrease of D / W. Because the influent time of the system is prolonged with the decrease of D / W, which is not conducive to the reoxygenation of the system, and further inhibits the metabolic activity of nitrifying bacteria, eventually leading to the inhibition of nitrification reaction and the increase of effluent pollutant concentration. Further, it can be known that NH4 + -N removal rate at different D / W stages, with the decrease of D / W, NH4 + -N removal rate also gradually decreases. Although the removal rate of NH4 + -N shows a certain downward trend, but even the removal rate of the lowest iron-carbon group (1#) is more than 80%, which shows that the removal of NH4 + -N will be affected by D / W, but the overall removal rate is within the acceptable range. 3) The influence of filler on NH4 + -N removal effect. The effluent concentration and removal rate of NH4 + -N at different stages of different filler experimental columns are shown in Figure 8 , the NH4 + -N average effluent concentration is: 1#>6#>5#>4#>2#>3#, it can be known that the effluent concentration of NH4 + -N of the experimental column with iron-carbon filler and zeolite filler is higher than that of 3# column. NH4 + -N is mainly removed by microbial nitrification, and iron-carbon filler and zeolite filler can provide a huge living space and a good living environment for nitrifying bacteria to meet the activities of nitrifying bacteria; in addition, Fe 3+ generated in the reaction can also act as an electron acceptor to promote the oxidation of NH4 + -N, and zeolite can better adsorb NH4 + -N through ion exchange between cations. Therefore, the combination of iron-carbon filler and zeolite filler is helpful to the removal of NH4 + -N, of which the lowest effluent concentration is 1.73 mg / L. Fe can reduce part of NO3 - -N and NO2 - -N to NH4 + -N, NO3- -N and NO2 - -N is reduced to NH4 + -N reaction equation: 4Fe + 7H2O + NO3 - =4Fe 2+ +10OH - +NH4 + (III) 3Fe + 8H + +NO2 - =3Fe 2+ +2H2+NH4 + (IV). Therefore, the experimental column with iron-carbon filler NH4 + -N effluent concentration is higher than that of 3# column.

[0026] 3, NO3 - -N removal effect analysis: 1) the influence of hydraulic load on NO3 - -N removal effect. The effluent concentration of NO3 - -N in different HL stages is shown in Figure 9 , the effluent concentration of NO3 - -N in six experimental columns decreases with the decrease of HL. Because the soil is negatively charged, it is difficult to adsorb the same negatively charged NO3 - -N, and the decrease of HL helps to strengthen the adsorption of soil to NO3 - -N. Therefore, when the HL decreases, the effluent concentration of NO3 - -N also decreases. 2) the influence of dry-wet ratio on NO3 - -N removal effect. The effluent concentration of NO3 - -N in different D / W stages is shown in Figure 10 , with the decrease of D / W, the effluent concentration of NO3 - -N in six experimental columns gradually increases. Because with the decrease of D / W value, the duration of influent is prolonged, the continuous flow of water produces scouring effect on the filler, which makes the NO3 - -N attached to the surface of the filler be stripped and discharged with effluent. 3) the influence of filler on NO3 - -N removal effect. The effluent concentration of NO3 - -N in different filler experimental columns is shown in Figure 11 , because the carbon-nitrogen ratio (C / N) of sewage is low, the denitrification process lacks sufficient carbon source supply, which causes the continuous accumulation of NO3 - -N in the system and discharge with effluent. The effluent concentration of NO3 - -N in different filler experimental columns is ranked as: 3#>2#>4#>5#>6#>1#. Among them, the NO3 -The NO3- concentration was consistently higher than that of other experimental groups and remained at a high level for an extended period. This indicates that the addition of iron-carbon packing material can significantly improve denitrification efficiency, thereby reducing NO3- concentration. - The residual amount of -N. Iron (Fe) in the iron-carbon packing acts as the anode, releasing electrons during the reduction reaction, providing a sufficient electron source for the denitrification process, and promoting NO3-. - As an electron acceptor, it is reduced and removed. Simultaneously, the microelectrolysis system generates a large amount of reduced hydrogen atoms [H] and Fe. 2+ / Fe 3+ The autotrophic microorganisms in the system can use these substances as electron donors for autotrophic denitrification, converting NO3 into nitrogen. - -N is ultimately reduced to N2. In addition, Fe is produced during the electrolysis process. 2+ / Fe 3+ It can enhance the metabolic activity of microorganisms, thereby further improving the denitrification efficiency of the system. The activated carbon component in the iron-carbon packing can also serve as a supplementary carbon source, providing necessary organic matter support for the metabolic activities of denitrifying bacteria. This characteristic allows activated carbon to play a dual role in enhancing biological denitrification: both as an electron transport medium and as a carbon source supplier available to microorganisms. Therefore, the addition of iron-carbon packing can improve the denitrification efficiency of the system. - Removal of -N is highly effective.

[0027] 4. NO2 - -N Removal Effect Analysis: 1) Effect of Hydraulic Load on NO2 - -N removal efficiency: NO2 at different HL stages - -N effluent concentration, such as Figure 12 As shown, NO2 decreases with decreasing HL. - -N concentration in the effluent showed a decreasing trend. This indicates that HL has a positive effect on NO2 concentration. - The concentration of NO2- in the effluent is significantly affected, possibly due to the decrease in HL, leading to a more complete denitrification process. - -N conversion is more thorough, NO2 - -N accumulation is relatively low. 2) Dry-wet ratio of NO2 - -N removal efficiency: NO2 under different D / W - -N effluent concentration, such as Figure 13 As shown, as the D / W ratio decreases from 5:1 (IIa) to 1:1 (IIc), NO2 - The concentration of -N in the effluent gradually increases. This indicates that the D / W ratio affects the NO2 concentration. - The NO- effluent concentration is significantly affected because the reduced wet-dry ratio increases the influent time, creating a better anaerobic environment that is conducive to denitrification. However, continuous influent leads to the accumulation of NO2. - -N was not fully reacted before flowing out with the water. Overall, NO2... --N effluent concentration remains at a low level. Figure 13 In the middle, NO2 - The effluent concentration of -N is higher than that of the influent because a small amount of NO3 is produced during the denitrification process. - -N is converted to NO2 - -N. 3) Packing material for NO2 - The Influence of NO2 Removal Efficiency on NO2 in Different Packing Columns at Various Stages - -N effluent concentration, such as Figure 14 As shown, NO2 is emitted from all reaction columns. - -N concentrations were maintained at a low level, below 0.08 mg / L. The effluent NO2 concentration from the iron-carbon composite packing reactor column was... - The relatively high NO2 concentration (p<0.05) is attributed to the effective enhancement of the denitrification process by the iron-carbon medium through micro-electrolysis. The NO2 concentration in the experimental column with added iron-carbon packing was [not specified]. - The -N concentration in the effluent is significantly higher than that in a pure iron-carbon reactor. The main reasons are: 1. The reduced proportion of iron and carbon components in the mixed system leads to insufficient electron donors, resulting in higher concentrations of the denitrification intermediate product NO2. - -N was not completely reduced; secondly, the non-uniform distribution of the packing material may form local mass transfer barriers, affecting the electron transfer efficiency of the reaction system.

[0028] 5. TN Removal Efficiency Analysis: 1) Influence of Hydraulic Load on TN Removal Efficiency. The changes in influent and effluent TN concentrations under different HL conditions are shown below. Figure 15 As shown, the TN concentration in the effluent of the six experimental columns decreased with decreasing HL. This is because the decrease in HL causes NO3... - -N and NH4 + As the concentration of nitrogen (N) in the effluent decreases, the concentration of nitrogen (TN) in the effluent also decreases. Therefore, it can be seen that the TN removal rate increases as HL decreases under different HL conditions. This indicates that the TN removal rate is related to the proportion of iron-carbon packing material added; the more iron-carbon packing material added, the higher the TN removal rate. This is mainly because the iron-carbon packing material, acting as an electron donor, provides electrons to compensate for the insufficient carbon source in wastewater with a low C / N ratio. Furthermore, changes in hydraulic loading (HL) have a significant impact on the denitrification efficiency of the soil infiltration system. The mechanism of this phenomenon may involve two factors: first, the relatively slow proliferation rate of autotrophic nitrite-oxidizing bacteria (NOB); second, a low influent C / N ratio inhibits the denitrification process. 2) The effect of wet-dry ratio on TN removal efficiency. The changes in TN influent and effluent concentrations under different D / W conditions are shown below. Figure 16 As shown, the TN effluent concentration in the six experimental columns increases with decreasing D / W. This is mainly due to the increased influent time, which leads to increased NO3 concentration. -- The effluent concentration of TN increases, which leads to the increase of the effluent concentration of TN. Therefore, the TN removal rate under different D / W conditions decreases with the decrease of D / W. It can be seen that D / W has a significant effect on the removal of TN in the soil infiltration system. 3) The effect of fillers on the removal of TN. The effluent concentration and removal rate of TN in different filler experimental columns are shown in Table 2. Figure 17 - - The effluent concentration and removal rate of TN are consistent with those of NO3 - - The effluent concentration and removal rate of TN are consistent with those of NO3 - - The effluent concentration and removal rate of TN are consistent with those of NO3 + - The effluent concentration and removal rate of TN are consistent with those of NO3 + - The effluent concentration and removal rate of TN are consistent with those of NO3 + - The effluent concentration and removal rate of TN are consistent with those of NO3

[0029] 6. Analysis of TP removal effect: 1) Effect of hydraulic load on TP removal effect. The effluent concentration of TP under different HL conditions is shown in Table 3. Figure 18 3 2 3 2 When the HL decreases from 0.01 m 3 2 3 2 The effluent concentration of TP in the six experimental columns shows a decreasing trend. Because the decrease of HL reduces the scouring intensity of water flow on the filler matrix, which is beneficial to the adsorption of TP by the filler matrix. It can be seen that the removal rate of TP under different HL conditions increases with the decrease of HL. The results show that the removal rate of TP in each group remains at a high level, which is consistent with the results of other experiments. 2) Effect of dry-wet ratio on TP removal effect. The influent and effluent concentration of TP under different D / W conditions is shown in Table 4. Figure 19As shown, the effluent TP concentration of the six experimental columns significantly increased when the D / W ratio decreased from 5:1 (IIa) to 1:1 (IIc). This indicates that the adsorption capacity of the filler matrix for TP tends to decrease with the accumulation of running time. Firstly, the hydraulic scouring effect causes the adsorbed TP to gradually desorb from the filler surface; secondly, part of the TP that is not completely fixed will be lost with the effluent, causing the effluent TP concentration to increase. Further, it can be seen that the TP removal rate under different D / W ratios significantly decreases when the D / W ratio decreases from 5:1 (IIa) to 1:1 (IIc). This indicates that the removal efficiency of the system for TP tends to decrease with the extension of running time. However, even under long-term running conditions, the TP removal rate can still be maintained at a relatively ideal level, indicating that the experimental columns have superior performance in removing TP. Figure 20 As shown, the addition of iron-carbon has a good effect on the removal of TP, and the ratio of iron-carbon added also has a significant impact on the removal of TP. Phosphorus in wastewater can form a precipitate with Fe 2+ / Fe 3+ complexes and thus be removed, so the TP removal rate increases with the increase of the ratio of iron-carbon filler. Based on the above analysis of the effect of adding iron-carbon micro-electrolysis filler and zeolite on the removal of pollutants in the soil infiltration system, the following conclusions can be drawn: 1) The HL results show that when the HL decreases from 0.010 m 3 / m 2 ·h to 0.001 m 3 / m 2 ·h, the effluent concentrations of COD, NH4 + -N, NO3 - -N, NO2 - -N, TN, TP and other pollutants tend to decrease, and the removal rates of various pollutants tend to increase. 2) The D / W results show that when the D / W decreases from 5:1 to 1:1, the effluent concentrations of COD, NH4 + -N, NO3 - -N, NO2 - -N, TN, TP and other pollutants tend to increase, and the removal rates of various pollutants tend to decrease. 3) The results of the filler show that the best operating parameters are HL = 0.003 m³ / ㎡·h, D / W = 5:1, and the volume ratio of iron-carbon to zeolite is 1:1, at which the best removal effect is obtained. The removal rates of COD, NH4 + -N, TN, TP are 94.94%, 94.79%, 81.06%, and 99.25%, respectively.

[0030] Greenhouse gas emission reduction performance analysis: By analyzing the effects of different hydraulic loads HL, dry-wet ratios (D / W) and filler ratios on CH4 and N2O emissions, the greenhouse gas emission reduction performance of the iron-carbon micro-electrolysis coupled soil infiltration system was explored. During the parameter experiment, greenhouse gas monitoring was carried out simultaneously. Greenhouse gases mainly collect and analyze CH4 and N2O, using the static dark box method. The gas sampling box is a 100 cm high and 20 cm diameter organic plastic column. To mix the gas evenly, a simple gas mixing fan is installed in the organic plastic column. Before the greenhouse gas collection begins, cover the organic plastic column over the soil column and seal it with water to avoid gas leakage. At each stable stage, greenhouse gases are measured every two days, and each stage is detected three times. To avoid external environmental interference during sampling, choose to sample the gas every hour between 11 am and 14 pm, a total of 4 times. The syringe will draw the gas sample into a 100 mL aluminum bag, and the gas bag will collect the greenhouse gas, which will be detected by a gas chromatograph. The greenhouse gas flux calculation formula is as formula (V): (V); In formula (V): F is the gas emission flux (CH4 unit is mg·m -2 h -1 and N2O unit is μg·m -2 h -1 ); H is the height of the organic plastic column (m); T is the temperature during sampling (K); P is the atmospheric pressure during sampling (Pa); P0 is the atmospheric pressure under standard conditions (Pa); ρ is the density of a certain measured gas (molar mass / gas molar volume under standard conditions, g·L -1 ); dc / dt is the concentration change rate of a certain measured gas in the sampling box during sampling (CH4 unit is cm 3 ·m -3 h -1 and N2O unit is mm 3 ·m -3 h -1 ). In this study, SPSS 20 statistical software was used to process and analyze the experimental data, and the single-factor variance analysis (One-way ANOVA) method was used to test the differences between groups. The statistical significance level was set as follows: when the P value is less than 0.05, it is considered to be significantly different, and when the P value is less than 0.01, it is considered to be extremely significantly different. At the same time, Origin 2018 drawing software was used to visualize the N2O and CH4 emission characteristics of each experimental stage.

[0031] 1. CH4 emission reduction performance analysis: 1) Effect of hydraulic load on CH4 emission reduction performance: Figure 21The CH4 emission of each experimental column under different HL is shown. It can be seen that the CH4 emission decreases as the HL decreases from Ia to Id. The decrease of HL leads to the increase of DO concentration, and more COD is oxidized to CO2 under aerobic conditions. 2) Effect of dry-wet ratio on CH4 emission reduction performance: Figure 22 The CH4 emission of each experimental column under different D / W is shown. It can be seen that when the dry-wet ratio (D / W) decreases, the water retention time of the system is correspondingly extended. This change in working conditions changes the redox conditions inside the reactor, prompting more organic matter (COD) to be converted through the methanation pathway, ultimately leading to an increase in methane (CH4) production. 3) Effect of filler on CH4 emission reduction performance: The CH4 emission flux of different filler experimental columns is shown in Figure 23 The CH4 emission of each experimental column is shown in order from large to small: 3# > 1# > 6# > 5# > 4# > 2#. It can be seen that the addition of iron-carbon filler can effectively reduce the production of CH4. Iron-carbon filler mainly inhibits the production of methane (CH4) through the following two mechanisms: ① its high-efficiency removal of organic matter significantly reduces the substrate concentration available to methanogens; ② the Fe³⁺ produced during the filler reaction promotes the growth and reproduction of iron-reducing bacteria, which form a substrate competition relationship with methanogens, effectively inhibiting the production of methane. In addition, it is also known that the experimental column with only zeolite filler has the lowest CH4 emission, because the unique crystal structure of zeolite helps to adsorb and store CH4; and the porous structure of zeolite may also improve the local dissolved oxygen environment to promote the oxidation of CH4. As can be seen from the above, iron-carbon filler and zeolite filler can effectively reduce the emission of CH4.

[0032] 2, N2O emission reduction performance analysis. 1) Effect of hydraulic load on N2O emission reduction performance: The N2O emission of the six groups of soil infiltration systems under different hydraulic loads is shown in Figure 24 It can be seen that as the HL decreases, the N2O emission of each experimental column also gradually decreases, which may be due to the decrease of HL leading to the decrease of NO3 - -N involved in the reaction, resulting in a decrease in the production of intermediate N2O. 2) Effect of dry-wet ratio on N2O emission reduction performance: The N2O emission of each experimental column under different dry-wet ratios is shown in Figure 25 It can be seen that as the D / W decreases, the N2O emission flux of each experimental column increases significantly, which is because the extension of the water inlet time forms a good anaerobic environment, which is conducive to the denitrification process of microorganisms, and more N2O is produced and emitted. 3) Effect of filler on N2O emission reduction performance: The N2O emission of different filler experimental columns is shown in Figure 26It can be seen that the N2O emissions from columns #1, #4, #5, and #6 are higher than those from columns #2 and #3. This is because the addition of iron-carbon packing promotes the denitrification process, producing more N2O, thus resulting in higher N2O emissions. Furthermore, it can be observed that N2O emissions gradually increase as the proportion of iron-carbon packing increases. Columns #2 and #3, due to insufficient carbon source, have more difficulty in carrying out the denitrification process, thus resulting in lower N2O emissions.

[0033] In summary, the analysis of the effects of adding iron-carbon micro-electrolysis filler and zeolite on the pollutant removal efficiency and greenhouse gas emission reduction of the soil infiltration system leads to the following conclusions: 1) The results of HL on greenhouse gas emission fluxes indicate that decreasing HL reduces the emission fluxes of N2O and CH4. 2) The results of D / W on pollutant removal indicate that decreasing D / W increases the emission fluxes of N2O and CH4. 3) The results of the filler on greenhouse gas emission fluxes indicate that the optimal operating parameter is HL = 0.003 m. 3 / m 2 With h, D / W = 5:1, and iron-carbon / zeolite volume ratio of 1:1, the optimal greenhouse gas emission reduction effect is achieved. The emission fluxes of N2O and CH4 are 0.172 mg / m³. 2 ·h and 0.007 mg / m 2 ·h.

[0034] Exploring the Mechanism of Wastewater Treatment:

[0035] 1. Characterization and Analysis of Packing Material. Scanning electron microscopy (SEM) was used to characterize and reveal the evolution of the mesoscopic structure of the packing material. During the experimental study, column #5 was selected, and two characteristic sampling points were set at points c (30 cm from the top of the system) and d (70 cm from the top of the system) along the longitudinal spatial gradient. The topological changes in the microstructure of the matrix surface before and after the operation cycle were compared, and the spatiotemporal evolution of the reactive sites at the packing material interface was quantitatively analyzed. X-ray photoelectron spectroscopy (XPS) was used for in-depth analysis of the chemical state of the packing material surface. After the system operation cycle was completed, representative matrix samples were collected from sampling points c and d in column #5. XPS and SEM were used to compare and analyze the original iron-carbon packing material and the packing material samples in the lower layer of column #5 to examine the changes in their physicochemical properties during the operation of the soil infiltration system, focusing on the evolution of the material surface morphology and elemental composition; and SEM analysis was performed on the physical changes of the zeolite packing material before and after operation to explore its apparent morphological changes. 1) Apparent Morphology Analysis (SEM) Figure 27 As shown, by Figure 27As can be seen from the left picture, the zeolite filler presents a typical porous crystal structure, with clear and evenly distributed surface channels of consistent pore size, indicating its high adsorption potential. Meanwhile, the crystal surface is smooth, with no obvious attachments or impurities, indicating that the original zeolite filler has an intact physical structure, making it suitable for adsorption and ion exchange of pollutants. Figure 27 As can be seen from the right picture, the zeolite surface is covered with a layer of obvious deposits or biofilms, with some of the original channels being blocked, resulting in a significant decrease in porosity. The surface roughness increases, possibly due to the adsorption of organic matter, nitrogen and phosphorus pollutants, or metal ions (such as NH4 + , PO4 3- , Fe 3+ , etc.) in the wastewater, forming chemical precipitates or complexes. Meanwhile, filamentous bacteria or granular colonies are observed in some areas, indicating that the zeolite can serve as a microbial carrier during wastewater treatment, providing space for microbial activity and participating in the biodegradation of NH4 + -N, COD, and other pollutants. As can be seen from the pictures of the iron-carbon filler before and after the reaction, Figure 28 the iron-carbon filler undergoes significant changes before and after the reaction. The surface of the pre-reaction filler is smooth and complete, while the post-reaction filler is rough with holes and cracks. This indicates that the iron-carbon filler undergoes chemical reactions during wastewater treatment, particularly the dissolution of iron and the corrosion of carbon-based materials, which is caused by micro-electrolysis. These changes help to improve the adsorption capacity and reactivity of the filler, enhancing the effectiveness of wastewater treatment. At the same time, it can be seen that there is some precipitation on the surface of the iron-carbon filler in the 5# experimental column, which is because Fe 3+ and OH - combine to form Fe(OH)2 and Fe(OH)3 precipitates during the reaction. These precipitates adsorbed on the surface of the iron-carbon filler inhibit the transfer of electrons and affect the denitrification process. Meanwhile, Fe 3+ and Fe 2+ form complex precipitates with PO4 3- , indicating that the iron-carbon filler has good phosphorus removal capacity in wastewater. 2) Apparent morphology analysis. Figure 29 and Figure 30 are the XPS characterization results before and after the experiment of the 5# column. It shows that the iron element in the iron-carbon filler undergoes chemical reactions during the experiment. When acting as an electron donor, Fe (II) is oxidized to Fe (III) by losing electrons; when acting as an electron acceptor, Fe (III) can be reduced to Fe (II) by accepting electrons. In addition, the increase of Fe (II) and Fe (III) indicates that Fe (0) is oxidized.

[0036] 2. Microbial Alpha and Beta Diversity Analysis. Collect the substrate samples at the sampling ports 30 cm (upper layer b point) and 70 cm (lower layer d point) from the top of the system, and immediately store them in a -80°C ultra-low temperature environment. For easy identification, the upper layer samples are uniformly marked as U (UP), and the lower layer as D (DOWN), such as A1U represents the upper layer sample of 1# experimental column, and A1D corresponds to its lower layer sample. DNA is extracted using the FastDNA® SPIN Kit for soil extraction kit, and then agarose gel electrophoresis is used to detect the purity and concentration to meet the subsequent determination and analysis requirements. The PCR amplification primer sequence is shown in the table. Then use the MiseqPE300 / NovaSeqPE250 platform of Illumina Company to sequence, and compare the sequence with the Genbank database. According to 97% similarity, the sequenced sequences are divided into different operational taxonomic units (OTUs), and the microbial diversity is analyzed by using the Meiji biological cloud platform. 1) OTU clustering analysis. Figure 31 It is described whether the current sequencing depth of 12 samples is sufficient to reflect the microbial population diversity contained in the samples. As Figure 31 , when the sequencing amount is less than 10000, the number of OTUs shows a rapid upward trend; when the sequencing amount increases to 20000, the growth rate of OTU slows down obviously; when the sequencing amount is 50000, the curve is basically stable. This change rule shows that although it has not reached the complete saturation state, the existing sequencing depth can effectively capture the main microbial sequence information in the 12 samples, and the data amount is sufficient to fully reflect the microbial community composition characteristics in each reaction sample. The OTU level population distribution Venn diagram between different depth samples is shown in Figure 32 , which shows the similarity and difference between the upper and lower layers in the system. The number of OTU level species in the upper and lower samples is 10427 and 9039 respectively, and the number of common species is 3426. Further analysis, with the increase of the running depth of the system, the number of microbial species shows a decreasing trend, from the initial 10427 to 9039. The number change shows that the change of the running depth of the system will significantly change the structure and composition of the microbial community. 2) Microbial Alpha diversity analysis. Six indicators of Sobs, Shannon, Simpson, Ace, Chao1 and Coverage are used for quantitative analysis of microbial community samples, as shown in Table 2.

[0037] Table 2 Microbial Alpha Diversity Analysis

[0038]

[0039] As shown in Table 2, the coverage values of the 12 samples were all greater than 95%, indicating that the sequencing structure could accurately reflect the diversity and richness of microorganisms. By comparing the detection data of each experimental column, it was found that the Sobs index, Shannon index, Ace index and Chao1 index of column 3 at multiple sampling time points were significantly better than those of other experimental columns. It revealed that column 3 had a more abundant number of microbial populations, higher species diversity and better community richness. It was known that the iron-carbon filler played an important role in microbial screening, promoting the evolution of microbial communities towards a more suitable environmental condition. The Sobs index, Shannon index, Ace index and Chao1 index of column 2 were higher than those of column 3, which might be due to the fact that the internal space of zeolite filler was more than that of gravel, making it more suitable for microbial survival and reproduction. By comparing and analyzing different depth soil layers, it was found that the Sobs, Shannon, Ace and Chao1 diversity indices of the surface layer of most experimental columns were significantly better than those of the deep layer samples. It showed that the surface soil had higher microbial richness, more complex community diversity and richer species composition. This distribution characteristic showed that with the increase of soil depth, the number and species of microbial communities showed a decreasing trend. Figure 33 It was known that the principal component analysis of the genus level of the samples showed that P1+P2 was 34.77%, indicating that there were significant differences between the samples. The overlapping area of the two ellipses representing the upper and lower samples was very small, indicating that the sample difference between the upper and lower samples was very large, proving that the depth of the experimental column had a significant impact on the diversity and richness of the microbial population. The P1 axis divided most of the lower layer samples of the experimental column on the left and a small part on the right, and they were far apart, indicating that the difference between the lower layer samples was large. The P2 axis divided most of the upper layer samples on the upper side and a small part on the lower side, indicating that the difference between the upper layer samples was also large. It was proved that the filler of the experimental column had a significant impact on the diversity and richness of the microbial population. Two samples in the lower layer were close to the upper layer samples, indicating that the composition of these two samples at the genus level was similar to that of the upper layer samples, which might be the 2# and 3# columns without adding iron-carbon. Figure 34 A system clustering tree diagram based on genus-level classification units was displayed, and the branch distance reflected the difference in microbial composition between samples. The shorter the branch distance, the higher the similarity between the communities. The results showed that samples from the same source (such as A1D and A4D, A2U and A6U) formed independent clustering units, indicating that their microbial community structure had significant homology. It was further found that when the upper and lower layers of the experimental device used the same filling material (such as A1U and A2D), their microbial composition had the highest degree of convergence, revealing the screening effect of the environmental matrix on microorganisms. This visual analysis directly presented the correlation between the spatial distribution characteristics of the microbial community and the selection pressure of the matrix.

[0040] 3. Microbial community composition analysis. (1) Microbial community composition analysis at the phylum level. Figure 35 The microbial community composition of different samples at the phylum classification level was displayed. It was shown that all samples presented convergent characteristics in the composition of dominant flora. By screening microbial groups with a relative abundance threshold of >1%, a total of 14 core bacterial phyla were identified. Actinobacteriota (5.7~24.54%), Proteobacteria (11.69~45.24%), Acidobacteriota (1.5~19.56%), Chloroflexi (3.2~14.86%), and Firmicutes (3.6~64.7%) constituted the main functional flora. The total abundance of these dominant phyla in the samples reached more than 80%, reflecting the structural stability of microbial groups. Myxococcota (1.2~4.9%) and Gemmatimonadota (0.1~10.74%) were also included. The functional analysis of the flora involved in this study showed that the Actinobacteriota contained strains with organic matter metabolism and biological phosphorus accumulation functions, which explained the biological mechanism of stable removal of COD and TP from a microbiological perspective. Proteobacteria was the core functional flora of the wastewater biological treatment system, which not only dominated the degradation of organic pollutants, but also was confirmed to participate in the processes of ammonification, nitrification, and denitrification, which significantly promoted the nitrogen removal efficiency of the system. The correspondence between the structure of the flora and the removal efficiency of pollutants revealed the coupling characteristics of microbial functional groups and water purification processes. Some microorganisms in the Acidobacteriota had the function of dissimilatory iron reduction, so the relative abundance of Acidobacteriota was higher in the experimental column with iron-carbon filler. Chloroflexi could oxidize NO2 - -N and CH4 and fix CO2, realizing the processes of nitrification and carbon sequestration. Firmicutes and Myxococcota were also related to denitrification. Nitrospirota, as a denitrifying functional bacteria, could directly oxidize NH4 + -N to NO3 2- -N in the system. Some bacteria in Bacteroidota (1.7~5.7%) also had denitrification ability. The rich denitrification-related flora at the phylum level indicated that the system with iron-carbon filler had good denitrification potential. (2) Microbial community composition analysis at the class level. Figure 36By setting the relative abundance threshold > 1% screening criteria, a total of 33 dominant bacteria classes were identified. The composition of the dominant bacterial community between samples showed significant homology, and the core community included Actinobacteria, Alphaproteobacteria (α-proteobacteria), Gammaproteobacteria (γ-proteobacteria), Vicinamibacteria, Symbiobacteriia, Clostridia (Clostridia), Chloroflexia (Chloroflexia), Bacilli (Bacilli), Anaerolineae (Anaerolineae), etc. α-proteobacteria has good organic matter removal capacity, γ-proteobacteria is involved in NO3 - -N, NO2 - -N reduction and iron oxidation. Chloroflexia, Bacilli, Anaerolineae, etc. There are denitrifying bacteria in the class. Nitrospiria (Nitrospirae, 0.25-11.26%) is related to nitrification. The above shows that the system has good denitrification capacity. (3) Analysis of microbial community composition characteristics at the genus level. Figure 37 By setting the relative abundance threshold > 1% screening criteria, a total of 57 dominant bacteria were identified, including unclassifiedfMicro coccaceae, Symbiobacterium, norank_f_Vicinamibacteraceae, norank0Vicinamibacterale, norank_f_ Gemmatimonadaceae, norank_f_JG30-KF-CM45, Bacillus (Bacillus), Sphingomonas (Sphingomonas), etc. The total abundance is greater than 20%. Among them, norank_f_JG30-KF-CM45 and Bacillus belong to NOB genus. Sphingomonas has excellent organic matter degradation capacity. There are Nitrosomonas (Nitrosomonas), Nitrospira (Nitrospira) and other nitrifying bacteria in the system.

[0041] 4. Correlation analysis of microbial community and environmental factors. Figure 38 It is revealed that the environmental factors (COD, NH4 +The correlation between the microbial community structure and environmental factors (pH, Eh, DO, NO3--N, NO2--N, TN, TP) was analyzed by RDA. RDA1 and RDA2 explained 16.23% and 7.19% of the total variation of the microbial community structure, respectively. The arrows represented the environmental factors, and their lengths indicated the degree of influence on the microbial population structure. The angle represented the positive or negative correlation. The sample points were clustered and labeled based on the spatial location of the experimental column (upper and lower layers) and the type of filler. The orthogonal projection could quantify the regulatory weight of environmental factors on microbial structure. The distance of the sample projection point from the coordinate origin was directly proportional to the biological selection of the factor. The results showed that each environmental parameter significantly reshaped the functional group distribution of microbial genera through differential selection pressure, confirming the interaction between pollutant metabolism and biogeochemical cycling. Therefore, different environmental factors had a significant impact on the microbial population structure at the genus level. In summary, through the analysis of filler characterization, microbial Alpha and Beta diversity, microbial community composition characteristics, and the correlation between microbial communities and environmental factors, the pollutant removal mechanism and greenhouse gas emission reduction mechanism of the iron-carbon micro-electrolysis coupled soil infiltration system (SIS) were revealed. The following conclusions were drawn: 1) SEM showed that the surface roughness of the iron-carbon filler increased after operation, with holes and cracks appearing, indicating the dissolution of iron elements and the corrosion of carbon-based materials during the micro-electrolysis reaction, which enhanced the adsorption capacity and reactivity of the filler. At the same time, the Fe(OH)2 and Fe(OH)3 precipitates generated on the surface of the iron-carbon may inhibit electron transfer but promote the complexation and precipitation removal of phosphorus. The zeolite filler was covered with a biofilm and sediment after operation, and the porosity decreased, but it could still adsorb NH4 + -N through cation exchange and provide habitat for microorganisms. XPS analysis showed that there was a valence state conversion between Fe 2+ and Fe 3+ during the reaction, indicating that the iron elements in the iron-carbon filler not only act as electron donors to participate in the denitrification process but also as electron acceptors to promote the redox reaction of pollutants. 2) The results of microbial diversity analysis showed that the system depth had a significant impact on the microbial population structure. With the increase of system depth, the number of OTUs in the sample decreased. The results of Alpha diversity analysis showed that the microbial diversity (Sobs index, Shannon index) of the upper layer samples was significantly higher than that of the lower layer, indicating that the soil depth had a screening effect on the distribution of microorganisms. The addition of iron-carbon filler had a screening effect on microorganisms, making them develop towards a population that was adapted to the environment. 3) The microbial composition characteristics analysis showed that at the phylum level, Actinobacteria (5.7%~24.54%) and Proteobacteria (11.69%~45.24%) were the dominant groups, which were closely related to organic matter degradation, phosphorus removal, and nitrogen cycling. At the class level, Alphaproteobacteria had good organic matter removal capacity, and γ-Proteobacteria was closely related to NO3 - -N, NO2 -The reduction of N and the oxidation of iron are related, and the bacteria capable of denitrification exist in the class of Chloroflexia, Bacilli, Anaerolineae, Nitrospiria (0.25-11.26%), and the like, which proves that the system has good denitrification capacity. At the genus level, the detection of Nitrosomonas and Nitrospira verifies the nitrification capacity of the system, and the enrichment of NOB microbial species such as norank_f_JG30-KF-CM45 and Bacillus strengthens the denitrification efficiency.

[0042] In summary, the advantages of the present application are as follows: 1) when HL decreases, the removal rates of COD, NH4 + -N, NO3 - -N, NO2 - -N, TN, TP and the like show an upward trend. The gas emission fluxes of N2O and CH4 show a downward trend. 2) when D / W decreases, the removal rates of COD, NH4 + -N, NO3 - -N, NO2 - -N, TN, TP and the like show a downward trend. 3) the optimal operating parameters are HL=0.003 m³ / m 2 ·h, D / W=5:1, and the volume ratio of iron-carbon to zeolite is 1:1, so that the best pollutant removal effect and greenhouse gas emission reduction effect are obtained. The removal rates of COD, NH4 + -N, TN, TP are 94.94%, 94.79%, 81.06%, 99.25% respectively. The gas emission fluxes of N2O and CH4 are 0.172 mg / m 2 ·h and 0.007 mg / m 2 ·h respectively. 4) SEM shows that the dissolution of iron and the corrosion of carbon-based materials during the micro-electrolysis reaction process enhance the adsorption capacity and reaction activity of the filler; and the Fe(OH)2 and Fe(OH)3 precipitates generated on the surface of the iron-carbon inhibit the electron transfer, but promote the complexation and precipitation removal of phosphorus. The zeolite filler is covered with a biological membrane and deposits on the surface after operation, and the porosity is reduced, but it can still adsorb NH4 + -N through cation exchange and provide habitat for microorganisms. XPS shows that there are Fe 2+ and Fe 3+Conversion, both as an electron donor involved in the denitrification process, and as an electron acceptor to promote the redox of pollutants. 5) Alpha diversity shows that the microbial diversity (Sobs index, Shannon index) of the upper sample is significantly higher than that of the lower sample, and soil depth has a screening effect on microbial distribution; the addition of iron-carbon filler has a screening effect on microorganisms, making microorganisms develop towards a population adapted to the environment. 6) Beta diversity shows that filler type and soil depth significantly affect microbial community structure. At the phylum level, Actinobacteria and Proteobacteria are the dominant bacterial groups, which are closely related to organic matter degradation, phosphorus removal, and nitrogen cycling; at the class level, Alphaproteobacteria has good organic matter removal capacity, and gamma-Proteobacteria is related to NO3 - -N, NO2 - reduction and iron oxidation, and there are also denitrifying bacterial genera in the classes and phyla of Chloroflexia, Bacilli, Anaerolineae, Nitrospiria (0.25~11.26%), etc., which all indicate that the system has good denitrification capacity. At the genus level, the detection of Nitrosomonas and Nitrospira verifies the nitrification capacity of the system, and the enrichment of NOB genera such as norank_f_JG30-KF-CM45 and Bacillus enhances the denitrification efficiency.

Claims

1. A soil infiltration system coupled with iron-carbon micro-electrolysis, characterized in that, From top to bottom, it consists of: a soil cover layer, a gravel water distribution layer, a matrix filler layer, and a gravel support layer; The matrix filler layer is at least filled with iron-carbon filler; The matrix filler layer also includes zeolite filler; The matrix filler layer is composed of filler and soil, and the volume ratio of the filler and soil is 1:2; The soil cover layer is composed of soil; The gravel water-bearing layer is composed of gravel filler, the gravel support layer is composed of gravel filler, and the particle size of the gravel filler is 4~6mm; The matrix filler layer is composed of iron-carbon filler, zeolite filler and soil, with the zeolite filler located in the upper layer and the iron-carbon filler located in the lower layer. The soil is mixed into both the zeolite filler layer and the iron-carbon filler layer. The volume ratio of the zeolite filler layer to the iron-carbon filler layer is 1~3:1~3; The Fe / C mass ratio in the iron-carbon filler is 3~5:1; The soil is brown soil, with an organic matter content greater than 8%, a nitrogen content greater than 0.4%, a phosphorus content greater than 0.2%, a potassium content greater than 17%, and a pH value of 5.

94. The volume ratio of the soil cover layer, the gravel water-bearing layer, the matrix filler layer and the gravel support layer is 1:1:16:

2. The thicknesses of the soil cover layer, the gravel water-bearing layer, the matrix filler layer, and the gravel support layer are 5cm, 5cm, 80cm, and 10cm, respectively. The iron-carbon micro-electrolysis coupled soil infiltration system has at least one of the following characteristics: The particle size of the iron-carbon filler is 8~12mm; The particle size of the zeolite filler is 4~8mm; The system also includes an inlet and an outlet; the inlet is located in the gravel water distribution layer, 7 cm from the upper surface of the soil cover layer; the outlet is located in the gravel support layer, 3 cm from the bottom of the gravel support layer.

2. The application of the iron-carbon micro-electrolysis coupled soil infiltration system as described in claim 1 in wastewater treatment.

3. A wastewater treatment method, characterized in that, Includes the following steps: S1. Inoculate and cultivate activated sludge in domestic sewage in high-altitude and cold regions; S2. Domestic sewage from high-altitude cold regions, inoculated with cultured activated sludge, is introduced into the iron-carbon micro-electrolysis coupled soil infiltration system described in claim 1 for treatment until it meets the standards, and then discharged from the outlet. The hydraulic load of the iron-carbon micro-electrolysis coupled soil infiltration system is 0.001~0.01 m³. 3 / m 2 •h, the wet-dry ratio is (1~5):1; The domestic sewage from the high-altitude cold region has a pH of 6.5–8.0, SS of 100–200 mg / L, COD of 200–450 mg / L, BOD5 of 200–300 mg / L, and NH4+. + -N is 20–90 mg / L, and TP is 2.0–6.5 mg / L.

4. The wastewater treatment method according to claim 3, characterized in that, The hydraulic load of the iron-carbon micro-electrolysis coupled soil infiltration system is controlled at 0.001 m during operation. 3 / m 2 ·h、0.003 m 3 / m 2 ·h、0.006 m 3 / m 2 ·h or 0.01 m 3 / m 2 •h; the wet-dry ratio is 5:1, 3:1 or 1:1.

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