Anaerobic denitrification reactor and method for accelerating degradation of nitrate nitrogen in micro-polluted water

Abstract

The invention belongs to the technical field of biological filters, and relates to an anaerobic denitrification reactor and a method for accelerating degradation of nitrate nitrogen in micro-polluted water, and the reactor mainly comprises a biological filter, a water inlet tank, a peristaltic pump and a water outlet tank. The biological filter is composed of a filler composed of a modified polyvinylidene fluoride hollow fiber membrane and a high-temperature treatment iron wire, and the wastewater treatment process of the biological filter comprises a biofilm culturing stage and a domestication stage. In the biofilm culturing stage, the activated sludge enters the biofilter along with inlet water, and is loaded on the surface of membrane filaments to form a layer of biofilm; in the domestication stage, the wastewater slowly enters a biological filter and stays in a reactor, iron and nitrate nitrogen in the wastewater are subjected to denitrification reaction under the action of denitrifying bacteria, and the purpose of rapidly removing the nitrate nitrogen is achieved. According to the method, nitrate nitrogen in the wastewater can be rapidly reduced under the condition of not additionally adding a carbon source, so that the treatment process is low-carbon, aeration is not needed, energy is saved, and the method is suitable for industrial popularization.

Description

Technical Field

[0001] The invention belongs to the technical field of biological filter tanks, and relates to an anaerobic denitrification reactor and a method for accelerating the degradation of nitrate nitrogen in micro-polluted water. Background Art

[0002] With the increasing application of membrane filtration technology in the field of water treatment, waste membrane fibers have become an urgent problem to be solved at this stage. Waste membrane fibers (PVDF) are often used in membrane aeration bioreactors (MABR), microbial filter technology development and application research on the purification of micro-polluted water in rural areas.

[0003] In recent years, autotrophic denitrification technology using inorganic electron donors such as hydrogen, sulfur, and iron as substrates has received increasing attention. Among them, the electron donor for sulfur autotrophic denitrification is mainly HS - , S 2- , S 0 , and S2O3 2- , which will cause SO4 2- The increase in concentration and acidity; the low solubility of hydrogen in hydrogen autotrophic denitrification may hinder the rate of H2 mass transfer from gas to liquid. In addition, the explosive properties of H2 during storage and use will limit its application. The advantage of iron autotrophic denitrification technology is that it avoids residual organic carbon in the effluent and reduces sludge production. Therefore, it is regarded as an effective alternative to heterotrophic denitrification. Iron autotrophic denitrification technology has the characteristics of safe and easy availability of raw materials, low price, and synergistic removal of pollutants such as phosphorus and arsenic, and has broad application prospects. Summary of the invention

[0004] The present invention combines the biofilm method and the iron autotrophic denitrification technology to build an anaerobic denitrification reactor for accelerating the degradation of nitrate nitrogen in micro-polluted water.

[0005] The first technical solution of the present invention is an anaerobic denitrification reactor, the main part of which is a biological filter tank, including a three-layer structure, wherein the upper and lower parts are both supporting layers, and the middle part is a filler layer;

[0006] The packing layer is a carrier of the denitrification reaction, and is composed of iron wire and membrane wire, wherein the iron wire is calcined at high temperature, and the membrane wire is made of PVDF hollow fiber membrane material, and the PVDF hollow fiber membrane is obtained by modification with L-DOPA;

[0007] The iron wires and membrane wires in the packing layer are composed of dispersed and wrapped types. The dispersed type means that the iron wires and membrane wires are evenly dispersed, and the wrapped type means that the iron wires penetrate into the membrane wires.

[0008] The upper and lower ports of the reactor are sealed, and the water inlet mode is bottom-in and top-out;

[0009] The water inlet is spaced apart from the lower port of the reactor, and the water outlet is spaced apart from the upper port.

[0010] Furthermore, the packing layer is made of discarded PVDF hollow fiber membrane material, so that the discarded PVDF is recycled as a resource.

[0011] Furthermore, it also includes a peristaltic pump connected to the biofilter, and the outlet of the peristaltic pump is connected to the inlet of the biofilter.

[0012] Furthermore, the height ratio of the upper supporting layer, the middle filling layer and the lower supporting layer is 1:1:1.

[0013] Furthermore, the distance between the water inlet and the lower port of the reactor is 30 mm, and the distance between the water outlet and the upper port of the reactor is 30 mm.

[0014] The second technical solution of the present invention is a method for accelerating the degradation of nitrate nitrogen in micro-polluted water by using the above-mentioned anaerobic denitrification reactor, combining the biofilm method and iron autotrophic denitrification to accelerate the degradation of nitrate nitrogen in micro-polluted water, including a biofilm stage and an acclimation stage;

[0015] 1) Nitrogen is introduced into the reactor to remove oxygen in the feed and make the dissolved oxygen less than 0.25 mg / L;

[0016] 2) Biofilm formation stage: After the slightly polluted water containing activated sludge enters the reactor, it passes through the lower support layer and enters the packing layer. The activated sludge is adsorbed on the surface of the membrane by the adsorption performance of the modified hollow fiber membrane to form a biofilm.

[0017] The hydraulic retention time of biofilm formation stage is 0.5-1h, pH is 7.2±1, and temperature is room temperature;

[0018] 3) After the biofilm formation stage ends at 15±2 days, the acclimatization stage begins to treat slightly polluted water. After the slightly polluted water enters the reactor, the nitrate nitrogen in the water undergoes a denitrification reaction with iron under the action of denitrifying bacteria, and the nitrate nitrogen is reduced to nitrogen gas, thereby achieving the removal of nitrate nitrogen;

[0019] During the acclimation phase, the hydraulic retention time was 8-10 h, the pH was 7.2 ± 1, and the temperature was room temperature;

[0020] The bacteria involved in iron autotrophic denitrification are mainly denitrifying Campylobacter, Dechlorocampylobacter, Dechloromonas, Brevundimonas, Chitinophora, and Pseudomonas;

[0021] This method does not require an external carbon source.

[0022] Furthermore, in an anaerobic environment, dissolved oxygen (DO) is less than 0.25 mg / L.

[0023] Furthermore, the modification method of PVDF is as follows: first prepare a 0.1 mol / L, pH=8.5 tris(hydroxymethyl)aminomethane-hydrochloric acid buffer solution, then dissolve levodopa in a Tris-HCl buffer solution, prepare a 2 g / L DOPA solution, immerse the PVDF membrane in the DOPA solution, shake in the dark for 24 hours, take it out, wash it with deionized water until the washing liquid is clear and colorless, use filter paper to absorb the moisture on the surface of the membrane filament, dry it naturally, and obtain a modified PVDF membrane.

[0024] The present invention utilizes the adsorption performance of modified waste membrane filaments (PVDF) on microorganisms, adsorbs microorganisms on the membrane surface to form a biofilm, and then utilizes the autotrophic denitrification process of iron to reduce nitrate nitrogen and nitrite nitrogen in water to nitrogen gas:

[0025] 4Fe(II)+2NO3 - +8H + →4Fe(III)+N2+4H2O

[0026] 10Fe(II)+2NO2 - +12H + →10Fe(III)+N2+6H2O

[0027] This invention modifies waste membrane fibers and uses the biofilm method and iron autotrophic denitrification technology to build a waste membrane fiber-iron wire autotrophic denitrification reactor, and develops a bioreactor for accelerating the degradation of nitrate nitrogen in wastewater, thereby realizing the resource recycling process, improving economic benefits, and being conducive to the improvement and development of this type of denitrification process, which will help promote the continuous improvement and innovation of water quality purification technology and processes for micro-polluted water in rural areas.

[0028] Compared with traditional denitrification technology, the present invention has the following advantages:

[0029] 1. The packing layer of the reactor is made of waste PVDF hollow fiber membrane material, which can recycle and reuse the waste PVDF resources; the waste PVDF hollow fiber membrane can increase the surface roughness after being modified with L-DOPA, which is conducive to the attachment of microorganisms.

[0030] 2. The activated sludge after acclimation under iron autotrophic conditions showed strong denitrification and ferrous iron utilization capabilities. This process combines the advantages of two technologies, namely, the denitrifying bacteria in the biofilm loaded by the hollow fiber membrane can accelerate the progress of iron autotrophic denitrification and accelerate the degradation rate of nitrate nitrogen;

[0031] 3. This technology does not require an external carbon source, and the raw materials are safe, easily available, and inexpensive, with low processing costs. It can also synergistically remove pollutants such as phosphorus and arsenic. It will help promote the continuous improvement and innovation of water purification technology and processes for micro-polluted water in rural areas, and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a schematic diagram of the device of the anaerobic denitrification reactor in the embodiment of the present invention; the reactor is a laboratory-scale biofilter reactor, which mainly includes a biofilter, a water inlet tank, a peristaltic pump, a water outlet tank, etc. The non-aerated filler reactor is composed of a cylindrical acrylic material with a height of 300mm and a diameter of 50mm, with an effective volume of 589mL. The interior of the biofilter includes a supporting layer (100mm high) and a filler layer, with an effective height of 100mm. Both the upper and lower ports of the reactor are sealed, the distance between the water inlet and the lower end is 30mm, and the distance between the water outlet and the upper end is also 30mm. The water inlet method is bottom-in and top-out.

[0033] Figure 2 The four-cycle reactor nitrate nitrogen (NO3 - -N), nitrite nitrogen (NO2 - -N), concentration and pH changes; 1-Fe_1-sludge(distributed) represents a dispersed group, and 1-Fe_1-sludge(wrapped) represents a wrapped group;

[0034] Figure 3 It is the first batch of nitrate nitrogen (NO3 - -N), nitrite nitrogen (NO2 - -N), concentration and pH changes; 1-Fe_1-sludge is the encapsulated group, which is the target group, and the other three groups are the control groups;

[0035] Figure 4 The second batch of nitrate nitrogen (NO3 - -N), nitrite nitrogen (NO2 - -N), concentration and pH changes; 1-Fe_1-sludge(distributed) represents the dispersed group, 1-Fe_1-sludge(wrapped) represents the wrapped group; the remaining two groups are control groups;

[0036] Figure 5 It is the dominant bacteria at the genus level of the reactor packing layer after four cycles of operation. The first four groups represent the dominant bacteria in the sludge, iron wire, inner surface of the membrane wire, and outer surface of the membrane wire in the group with dispersed packing, and the last four groups represent the dominant bacteria in the sludge, iron wire, inner surface of the membrane wire, and outer surface of the membrane wire in the group with packed packing. DETAILED DESCRIPTION

[0037] The present invention will be further described below through specific embodiments and accompanying drawings:

[0038] Example 1

[0039] Experimental steps:

[0040] A laboratory-scale denitrification reactor apparatus (e.g., Figure 1 ), mainly including a biofilter, an inlet tank, a peristaltic pump, an outlet tank, etc. The biofilter is made of a cylindrical acrylic material with a height of 300mm and a diameter of 50mm. The interior includes a supporting layer (pebbles, 100mm high) and a packing layer, with an effective height of 100mm. The packing is composed of iron wire calcined at high temperature and waste PVDF membrane wire modified with L-DOPA, wherein the diameter of the iron wire is 0.3mm, the inner diameter of the modified waste PVDF membrane wire is 1.1mm, and the outer diameter is 2mm. Both the upper and lower ports of the reactor are sealed, the distance between the water inlet and the lower end is 30mm, and the distance between the water outlet and the upper end is also 30mm, and the water inlet method is bottom-in and top-out.

[0041] The modification method of PVDF is as follows: first prepare a 0.1 mol / L, pH=8.5 tris(hydroxymethyl)aminomethane-hydrochloric acid (Tris-HCl) buffer solution, then dissolve levodopa in the Tris-HCl buffer solution, prepare a 2 g / L DOPA solution, immerse the PVDF membrane in the DOPA solution, shake it in the dark for 24 hours (80 rpm, 25°C), take it out, wash it with deionized water until the washing liquid is clear and colorless, use filter paper to absorb the moisture on the surface of the membrane, and dry it naturally to obtain a modified PVDF membrane.

[0042] The operation process of the device is divided into the biofilm formation stage and the acclimation stage. In the biofilm formation stage, the reactor is first inoculated with sludge collected from the Tianjin sewage treatment plant to allow the filler to form biofilm. After the biofilm formation, the biofilter is provided with slightly polluted wastewater from the inlet tank (1L) in the acclimation stage. The initial concentration of nitrate nitrogen in the inlet water is 50mg / L, which is added in the form of sodium nitrate. At the same time, the nutrient elements NaHCO3 (200mg / L), KH2PO4 (25mg / L), MgSO4 (25mg / L), CaCl2 (20mg / L) and 1mL trace element H3BO3 (14mg / L), NiCl3·6H2O (190mg / L), CuSO4·5H2O (250mg / L), MnCl2·4H2O (990mg / L), CoCl2·6H2O (240mg / L), NaMoO4·2H2O (220mg / L), ZnSO4·7H2O (430mg / L), NaWO4·2H2O (50mg / L), all purchased from Tianjin Xiens Co., Ltd. Nitrogen was introduced into the reactor for 20 minutes to remove oxygen in the feed and make the dissolved oxygen (DO) lower than 0.25mg / L. The hydraulic retention time was set to 8h, and the water inlet mode was bottom-in and top-out. During the operation, the concentrations of nitrate, nitrite, ammonium, Fe(II) and Fe(III) in the outlet tank were monitored every day to observe the denitrification effect, that is, the degradation rate of nitrate nitrogen.

[0043] A series of batch tests were conducted under Fe(II) oxidation and nitrate reduction conditions to verify whether the observed reactors were microbially mediated processes. First, for the evaluation of the autotrophic metabolism of Fe, a group of experiments with both sludge and Fe was set up, with Fe inserted into the PVDF membrane in the form of iron wire, without the need for an external carbon source. In addition, considering the surface adsorption of Fe, the surface adsorption of PVDF, and the carbon reduction in sludge, three groups of experiments were set up, namely, only Fe, only PVDF, and only sludge filler. This is the first batch of solutions;

[0044] For the biological test of activated sludge, that is, the contact area between Fe and activated sludge, a group of materials containing both sludge and Fe is required. Unlike the first batch, the iron wire does not need to be inserted into the PVDF membrane wire, and no external carbon source is required. In addition, considering the surface adsorption of Fe, the surface adsorption of PVDF and the carbon reduction in the sludge, it is necessary to set up fillers with only Fe, or only Fe and sludge. This is the second batch of solutions.

[0045] Ammonia nitrogen (NH4 + -N). NO3 was determined by ion chromatography (DX-500, Dionex Corporation). - -N and NO2 - -N. Fe, including Fe(II) and total Fe, was determined by the phenanthroline method.

[0046] 16SrDNA sequencing can be used to examine the dominant bacteria involved in iron autotrophic denitrification at the genus level from a microscopic perspective. In order to perform parallel sequencing of multiple samples at one time, 16SrDNA sequencing is currently used more frequently. This technology can analyze and compare the types of microorganisms, microbial abundance and population structure in the samples, reflecting the differences between microbial populations. The differences between microbial populations are closely related to the type of domesticated substrate. Therefore, 16SrDNA sequencing is used to identify microbial classification and population indicators.

[0047] Result analysis:

[0048] Figure 2This indicates that within the four cycles, the target reactor can reduce nitrate nitrogen to a very low value (below 2 mg / L), and the removal efficiency of nitrate nitrogen reaches 98.42%, which meets the nitrate standard of drinking water in my country (10 mg / L). In the short term, the degradation rate of nitrate nitrogen in the dispersed group is faster than that in the encapsulated group. This may be because the PVDF membrane encapsulation hinders the autotrophic denitrification of iron, which is obviously not the case in the dispersed group. However, its iron loss is faster than that in the encapsulated group, resulting in its inability to operate for a long time. On the contrary, the encapsulated group can operate for a long time. Nitrite accumulates in the reaction, which may be the product of autotrophic denitrification of iron. At the same time, Fe and NO3 - The chemical reaction between them may inhibit the autotrophic denitrification process, resulting in NO3 - The iron ions in the solution were not detected in the reactor because the pH after the reaction was high and most of the iron ions formed precipitates. The amount of precipitates is difficult to quantify. Therefore, an accurate mass balance calculation has not been performed for this chemical reaction. Only a qualitative description of Fe and NO3 - The pH range is always maintained in an alkaline environment. Under neutral and alkaline conditions, although the denitrifying bacteria are more active, Fe 2+ It is more prone to mineralization, and under acidic conditions, Fe 2+ It is relatively stable but the activity of denitrifying bacteria becomes weaker.

[0049] 4Fe(II)+2NO3 - +8H + →4Fe(III)+N2+4H2O(1-1)

[0050] Fe+2H2O→Fe(II)+H2+2OH - (1-2)

[0051] Theoretically, the reaction process in this study will produce H + accumulation, because Fe(II) autotrophic denitrification is an acid-producing process as shown in reaction (1-1), Fe 0 The alkali production process is as shown in reaction (1-2), and the final pH is maintained at 8-9, indicating that biological denitrification plays a major role in the system. The increase in pH may also be due to Fe 0 The presence of Fe 0 It also reduces nitrate nitrogen, but is accompanied by OH - However, the increase in pH will lead to the formation of nitrite nitrogen (NO2 - -N) accumulation, which can also be confirmed from the figure.

[0052] In addition, almost no ammonia nitrogen generation was detected in the reactor. The generation of ammonia nitrogen may be caused by: the death and rupture of microorganisms releasing ammonia nitrogen; the nitrate reduction-ferrous oxidation process in the reactor involves some non-iron autotrophic denitrifying bacteria, and the dissimilatory nitrate reduction to ammonium (DNRA) occurs. The DNRA process is mainly divided into two stages. In the first step, under the catalysis of NAR, NO3 - Reduction of NO2 - ; In the second step, Nir converts NO2 - Finally reduced to NH4 + Therefore, the ammonia nitrogen cannot be monitored because the activity of microorganisms in the DNRA process is reduced by the low temperature, which in turn affects the generation of NH4 in the DNRA process. + .

[0053] Figure 3 The nitrate nitrogen (NO3 - -N) degradation process mainly includes four pathways: Fe surface adsorption, membrane surface adsorption, carbon reduction in sludge, and Fe autotrophic denitrification, which contribute to nitrate nitrogen removal by 8.71%, 4.83%, 15.42% and 65.81%, respectively. It can be concluded that the method of passing iron wire into PVDF membrane can utilize the denitrifying bacteria in the microorganisms adsorbed on the PVDF surface to provide electrons with Fe(II) to degrade nitrate nitrogen, which can achieve better treatment effect, and at the same time play a physical, chemical and biological role, so that nitrate nitrogen can be degraded faster.

[0054] There is nitrite nitrogen (NO2 - -N) accumulation, while NH4 is almost undetectable + , indicating that nitrate is first reduced to nitrite, mainly by Fe 2+ Provide electrons, and a small amount of carbon in the sludge also provides electrons, while only a small amount of Fe 0 Participate in the reaction. The pH in the reactor gradually rises to around 7.5, which is an alkaline condition. Most of the Fe in the system exists in the form of precipitation. The concentration of Fe ions in the system is very low, so it is difficult to quantitatively detect the changes in Fe(II) and Fe(III).

[0055] Figure 4 This indicates that the surface adsorption of Fe, carbon reduction in sludge, and autotrophic denitrification of Fe in this group of reactors with wrapped fillers have a significant effect on NO3 - -N removal accounted for 6.43%, 0.76% and 31.3% respectively, while the surface adsorption of Fe, carbon reduction in sludge and autotrophic denitrification of Fe in the group of reactors with dispersed fillers had a significant impact on NO3- -N removal accounted for 6.43%, 0.76% and 34.79% respectively. It can be seen that the two groups of reactors have a significant impact on NO3 - The removal effects of -N are similar, and the removal rate is not high. This may be because the bacteria in the microorganisms need a domestication process to show a stronger removal effect.

[0056] During the operation of the reactor, nitrite nitrogen (NO2 - -N) accumulation, while NH4 is almost undetectable + The conclusions drawn are consistent with those of the first batch of reactors. Due to the initial operation of the reactor (acclimation stage), the NO3 - -N removal did not show significant differences, so NO2 - -N detection results also showed no significant differences; during operation, the pH gradually increased and became alkaline, so it was difficult to quantitatively detect changes in Fe(II) and Fe(III).

[0057] It can be seen that when the reactor runs for a long time, the denitrifying bacteria in the sludge are domesticated and can better participate in the denitrification reaction. The dispersed filler has a larger specific surface area, which can provide a larger contact space for Fe(II) and microorganisms, accelerating NO3 - -N reduction, while in the early stage of reactor operation, although the dispersed filler has a larger specific surface area, the difference is not obvious because the denitrifying bacteria have not yet been domesticated.

[0058] Figure 5 It reflects the dominant bacteria at the genus level in the iron wire of the packing layer, the surface of the membrane wire and the sludge. From the figure, it can be seen that the bacteria involved in denitrification are Denitratisoma, Dichloroiodate, Dechloromonas, Brevundimonas, Chitinophaga, Pseudomonas, Hyphomicrobium, Simplicispira, Limnobacter, Aquabacterium, Anaerolinea, Romboutsia, etc.

[0059] From this we can draw the following conclusions:

[0060] The method of inserting iron wire into PVDF membrane can utilize the denitrifying bacteria among the microorganisms adsorbed on the surface of PVDF, and Fe 2+Providing electrons to degrade nitrate nitrogen can achieve better treatment results, while playing a physical, chemical and biological role, making nitrate nitrogen degrade faster;

[0061] Different forms of fillers can affect NO3 - -N degradation rate, in the early stage of reactor operation, the NO3 - The degradation effect of -N was slightly better than that of the group of reactors with dispersed fillers, but there was no obvious difference. When the reactor was running for a long time, the denitrifying bacteria in the sludge were domesticated and could better participate in the denitrification reaction. The dispersed filler had a larger specific surface area, which could provide a larger contact space for Fe(II) and microorganisms, accelerating the degradation of NO3 - -N reduction, the filler is a dispersed NO3 - -N degradation effect is better than that of the group of reactors with wrapped fillers;

[0062] NO3 in the reactor system - -N degradation mainly includes four pathways: Fe surface adsorption, membrane surface adsorption, carbon reduction in sludge, and Fe autotrophic denitrification, and there will be competition during the operation of the reactor. In the early stage of the reaction, Fe autotrophic denitrification is dominant, and NO3 - -N degradation is faster, in the middle and late stages of the reaction, other pathways dominate, NO3 - -N degradation is slow.

[0063] The bacteria involved in iron autotrophic denitrification are mainly Denitratisoma, Dichloroiodate, Dechloromonas, Brevundimonas, Chitinophaga, Pseudomonas, etc.

[0064] The above description is only a preferred embodiment of the present invention, and therefore cannot be used to limit the scope of the present invention. That is, equivalent changes and modifications made according to the patent scope of the present invention and the contents of the specification should still fall within the scope of the present invention.

Claims

1. An anaerobic denitrification reactor, characterized in that: The main part is the biofilter, which includes a three-layer structure, in which the upper and lower parts are support layers, and the middle part is a filler layer; The packing layer is a carrier of the denitrification reaction, and is composed of iron wire and membrane wire, wherein the iron wire is calcined at high temperature, and the membrane wire is made of PVDF hollow fiber membrane material, and the PVDF hollow fiber membrane is obtained by modification with L-DOPA; The iron wires and membrane wires in the packing layer are composed of dispersed and wrapped types. The dispersed type means that the iron wires and membrane wires are evenly dispersed, and the wrapped type means that the iron wires penetrate into the membrane wires. The upper and lower ports of the reactor are sealed, and the water inlet mode is bottom-in and top-out; There is a distance between the water inlet and the lower port of the reactor, and there is a distance between the water outlet and the upper port.

2. An anaerobic denitrification reactor according to claim 1, characterized in that: The packing layer is made of discarded PVDF hollow fiber membrane material, and the discarded PVDF is recycled as a resource.

3. An anaerobic denitrification reactor according to claim 1, characterized in that: It also includes a peristaltic pump connected to the biofilter, and the outlet of the peristaltic pump is connected to the inlet of the biofilter.

4. An anaerobic denitrification reactor according to claim 1, characterized in that: The height ratio of the upper supporting layer, the middle filling layer and the lower supporting layer is 1:1:

1.

5. An anaerobic denitrification reactor according to claim 1, characterized in that: The distance between the water inlet and the lower port of the reactor is 30 mm, and the distance between the water outlet and the upper port of the reactor is 30 mm.

6. A method for accelerating the degradation of nitrate nitrogen in micro-polluted water using the anaerobic denitrification reactor described in claims 1 to 5, characterized in that: Combining biofilm method and iron autotrophic denitrification to accelerate the degradation of nitrate nitrogen in micro-polluted water, including biofilm formation stage and acclimatization stage; 1) Nitrogen is introduced into the reactor to remove oxygen in the feed and make the dissolved oxygen less than 0.25 mg / L; 2) Biofilm formation stage: After the slightly polluted water containing activated sludge enters the reactor, it passes through the lower support layer and enters the packing layer. The activated sludge is adsorbed on the surface of the membrane by the adsorption performance of the modified hollow fiber membrane to form a biofilm. The hydraulic retention time of biofilm formation stage is 0.5-1h, pH is 7.2±1, and temperature is room temperature; 3) After the biofilm formation stage ends at 15±2 days, the acclimatization stage begins to treat slightly polluted water. After the slightly polluted water enters the reactor, the nitrate nitrogen in the water undergoes a denitrification reaction with iron under the action of denitrifying bacteria, and the nitrate nitrogen is reduced to nitrogen gas, thereby achieving the removal of nitrate nitrogen; During the acclimation phase, the hydraulic retention time was 8-10 h, the pH was 7.2 ± 1, and the temperature was room temperature; The bacteria involved in iron autotrophic denitrification are mainly denitrifying Campylobacter, Dechlorocampylobacter, Dechloromonas, Brevundimonas, Chitinophora, and Pseudomonas; This method does not require an external carbon source.

7. The method according to claim 6, characterized in that Anaerobic environment, dissolved oxygen (DO) is less than 0.25 mg / L.

8. The method according to claim 6, characterized in that The modification method of PVDF is as follows: first prepare a 0.1 mol / L, pH=8.5 tris(hydroxymethyl)aminomethane-hydrochloric acid buffer solution, then dissolve levodopa in a Tris-HCl buffer solution, prepare a 2 g / L DOPA solution, immerse the PVDF membrane in the DOPA solution, shake it in the dark for 24 hours, take it out, wash it with deionized water until the washing liquid is clear and colorless, use filter paper to absorb the moisture on the surface of the membrane fiber, and dry it naturally to obtain a modified PVDF membrane.

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