Method for culturing denitrification mixed flora under sulfate condition

Abstract

The invention provides a method for culturing denitrification mixed flora under a sulfate condition, and belongs to the technical field of sludge culture. The method comprises the following steps: inoculating activated sludge and anaerobic digestion sludge in a reactor, taking simulated wastewater as inlet water, adding a trace element I solution at the same time, operating to be stable, increasing the concentration of ammonia nitrogen and nitrite nitrogen in a gradient manner, and operating for 14-30 days at each gradient; when certain conditions are met, the water inlet substrate is replaced, and meanwhile the microelement II solution is added; the concentration of ammonia nitrogen and sulfate is increased in a gradient mode, the operation time of each gradient is 30-45 days, and the operation is conducted till the ammonia nitrogen removal rate of effluent of the reactor is stabilized to be 90% or above. According to the method disclosed by the invention, the ammonia nitrogen in the high-concentration sulfate wastewater can be removed under the conditions of not increasing aeration consumption and not increasing the sulfate concentration of effluent, and the removal rate of the ammonia nitrogen can reach 90% or above.

Description

Technical Field

[0001] The present invention relates to the technical field of sludge cultivation, and particularly to a method for cultivating denitrifying mixed bacteria under sulfate conditions. Background Art

[0002] Nitrogen pollution is a global problem in current water environment treatment. According to the statistical data of the Ministry of Ecology and Environment, the proportion of sections with excessive total nitrogen in surface water in China has been maintained above 20% for a long time. Among them, ammonia nitrogen, as the main existing form, the excessive concentration directly threatens the ecological safety of water bodies.

[0003] Although traditional ammonia nitrogen removal technologies such as stripping method and breakpoint chlorination method have certain treatment effects, they have significant application bottlenecks. Taking the most widely used steam stripping method as an example, treating 1 ton of ammonia nitrogen requires consuming 120 - 150 tons of steam, and at the same time, ammonia-containing tail gas is generated and an additional acid solution absorption device needs to be configured, and the comprehensive operation cost is as high as 80 - 100 yuan / kg. Although the ion exchange method can achieve ammonia nitrogen recovery, the high-salt wastewater (Cl- concentration can reach 5 - 8 g / L) generated during the resin regeneration process is likely to cause secondary pollution. These physical and chemical methods generally have technical and economic defects such as high energy consumption and difficult disposal of by-products, and it is difficult to meet the sustainable development requirements of modern wastewater treatment.

[0004] Biological nitrogen removal technology has become the mainstream choice due to its advantages of environmental friendliness and low operation cost. The traditional nitrification-denitrification process gradually oxidizes ammonia nitrogen to nitrate by nitrite bacteria and nitrifying bacteria, and then reduces it to nitrogen gas by denitrifying bacteria under anoxic conditions. This process requires strict partitioned dissolved oxygen control and sufficient carbon source supply. However, in typical high ammonia nitrogen wastewater such as coking, pharmaceutical, and landfill leachate, there is a common phenomenon of coexistence of high-concentration sulfate. The competitive utilization of carbon source by sulfate-reducing bacteria and denitrifying bacteria will significantly inhibit the denitrification efficiency. At the same time, the accumulation of sulfide has biological toxicity to nitrifying bacteria, resulting in a 40% - 60% decrease in the system's nitrogen removal efficiency. This contradiction is particularly prominent in the treatment of coal chemical wastewater. The actual operation data of a coking plant shows that when the concentration of SO4 2- exceeds 1500 mg / L, the total nitrogen removal rate drops sharply from 85% to 52%.

[0005] Existing biological nitrogen removal systems mostly adopt sulfur autotrophic denitrification process to deal with high sulfate wastewater. However, this technology needs to completely nitrify ammonia nitrogen and then carry out nitrate reduction driven by sulfide, which not only increases the aeration energy consumption but also generates a large amount of sulfate radicals, exacerbating the salinity of the effluent. In addition, in some methods, biological membranes and biological inducers need to be added, and the biological membranes need to be replaced during the later operation and maintenance, resulting in high operation costs. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for cultivating denitrifying mixed bacteria under sulfate conditions, aiming to solve at least one of the problems raised in the above background art.

[0007] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0008] The present invention provides a method for culturing a mixed denitrifying bacterial community under sulfate conditions, comprising the following steps:

[0009] Step 1: The reactor was inoculated with activated sludge and anaerobic digested sludge, and simulated wastewater containing ammonia nitrogen, nitrite nitrogen, sulfate, inorganic salts, trace elements and sodium bicarbonate was used as the influent matrix. Trace element I solution was also added. The reactor was operated under anaerobic conditions with a temperature of 33°C and a constant hydraulic retention time until it stabilized.

[0010] Step 2: Gradient increase in the concentrations of ammonia nitrogen and nitrite nitrogen, with each gradient running for 14 to 30 days;

[0011] Step 3: When the removal rates of ammonia nitrogen and nitrite nitrogen are stable at above 85%, the ratio of nitrate nitrogen in the effluent to ammonia nitrogen in the influent = 0.26, and the ratio of nitrite nitrogen consumed to ammonia nitrogen consumed = 1.32, replace the influent matrix and add trace element II solution at the same time;

[0012] Step 4: Gradiently increase the concentrations of ammonia nitrogen and sulfate, with each gradient running for 30 to 45 days, until the ammonia nitrogen removal rate of the reactor effluent stabilizes at more than 90%; at this point, the cultivation of the denitrifying mixed bacterial community in the presence of sulfate is completed.

[0013] Furthermore, the simulated wastewater includes wastewater A and wastewater B. The simulated wastewater used in step 1 is wastewater A, and the influent matrix is replaced with wastewater B in step 3.

[0014] Furthermore, the composition of the wastewater A is: 55-200 mg / L ammonia nitrogen, 30-200 mg / L nitrite nitrogen, 0.32 g / L MgCl2·6H2O, 0.48 g / L CaCl2, 0.044 g / L KH2PO4, 1.6 g / L NaHCO3, and 0.02 g / L FeSO4·7H2O.

[0015] Furthermore, the composition of the wastewater B is: 100-200 mg / L ammonia nitrogen, 266.67-533.33 mg / L sulfate, 0.2 g / L MgCl2·6H2O, 0.138 g / L CaCl2, 0.027 g / L KH2PO4, and 0.5 g / L NaHCO3.

[0016] Further, the trace element I solution contains: 430 mg / L of MgSO4·7H2O, 240 mg / L of CoCl2·6H2O, 220 mg / L of NaMoO4·H2O, 190 mg / L of NiCl2·6H2O, 210 mg / L of MgSeO4·10H2O, 15 mg / L of H3BO4, 10 mg / L of FeCl3, and 5000 mg / L of EDTA.

[0017] Further, the trace element II solution contains: 250 mg / L of CuSO4·5H2O, 240 mg / L of CoCl2·6H2O, 220 mg / L of NaMoO4·H2O, 190 mg / L of NiCl2·6H2O, 430 mg / L of ZnSO4·7H2O, 15 mg / L of H3BO4, 990 mg / L of MnCl2·4H2O, and 5000 mg / L of EDTA.

[0018] Further, the inoculation ratio of the activated sludge to the anaerobic digested sludge is 1:1; the concentration of volatile suspended solids in the activated sludge and the anaerobic digested sludge is 6.95 g / L.

[0019] Further, in Step 2, the ammonia nitrogen concentration after each increase is 1 to 2 times the ammonia nitrogen concentration before the increase, and the nitrite nitrogen concentration after each increase is 1.6 to 2 times the nitrite nitrogen concentration before the increase.

[0020] Further, in Step 4, the ammonia nitrogen concentration after each increase is 1 to 2 times the ammonia nitrogen concentration before the increase, and the sulfate concentration after each increase is 1 to 2 times the sulfate concentration before the increase.

[0021] The present invention also provides an application of the method for culturing a denitrifying mixed flora under sulfate conditions as described in the above technical solution in a denitrification system.

[0022] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0023] In the method for culturing a denitrifying mixed flora in the presence of sulfate in the present invention, it greatly improves the sulfur autotrophic denitrification process commonly used in the biological denitrification system of high-concentration sulfate wastewater. It can remove ammonia nitrogen in high-concentration sulfate wastewater without increasing aeration consumption and without increasing the sulfate concentration in the effluent, and the ammonia nitrogen removal rate can reach more than 90%. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a gradient domestication situation diagram when using Wastewater A for the simulated wastewater in Example 1 of the present invention;

[0025] Figure 2This is the graph of gradient acclimation when simulated wastewater B is used in Example 1 of the present invention. Detailed implementation mode

[0026] The present invention provides a method for culturing a denitrifying mixed flora under sulfate conditions, comprising the following steps:

[0027] Step 1: Inoculate activated sludge and anaerobic digestion sludge in a reactor, use simulated wastewater containing ammonia nitrogen, nitrite nitrogen, sulfate, inorganic salts, trace elements and sodium bicarbonate as the influent substrate, and at the same time add trace element I solution, and operate under anaerobic conditions at a temperature of 33°C with a constant hydraulic retention time until stable;

[0028] Step 2: Gradually increase the concentrations of ammonia nitrogen and nitrite nitrogen, and the operation time for each gradient is 14 - 30 days;

[0029] Step 3: When the removal rates of ammonia nitrogen and nitrite nitrogen are stable above 85%, the ratio of nitrate nitrogen in the effluent to ammonia nitrogen in the influent = 0.26, and the ratio of consumed nitrite nitrogen to consumed ammonia nitrogen = 1.32, replace the influent substrate, and at the same time add trace element II solution;

[0030] Step 4: Gradually increase the concentrations of ammonia nitrogen and sulfate, and the operation time for each gradient is 30 - 45 days, and operate until the ammonia nitrogen removal rate in the reactor effluent is stable above 90%; thus, the cultivation of the denitrifying mixed flora under the condition of the presence of sulfate is completed.

[0031] In the present invention, the simulated wastewater includes wastewater A and wastewater B. The simulated wastewater used in Step 1 is wastewater A, and the influent substrate is replaced with wastewater B in Step 3.

[0032] In the present invention, the composition of wastewater A is: 55 - 200 mg / L of ammonia nitrogen, 30 - 200 mg / L of nitrite nitrogen, 0.32 g / L of MgCl2·6H2O, 0.48 g / L of CaCl2, 0.044 g / L of KH2PO4, 1.6 g / L of NaHCO3, 0.02 g / L of FeSO4·7H2O.

[0033] In the present invention, the composition of wastewater B is: 100 - 200 mg / L of ammonia nitrogen, 266.67 - 533.33 mg / L of sulfate, 0.2 g / L of MgCl2·6H2O, 0.138 g / L of CaCl2, 0.027 g / L of KH2PO4, 0.5 g / L of NaHCO3.

[0034] In the present invention, the trace element I solution contains: 430 mg / L of MgSO4·7H2O, 240 mg / L of CoCl2·6H2O, 220 mg / L of NaMoO4·H2O, 190 mg / L of NiCl2·6H2O, 210 mg / L of MgSeO4·10H2O, 15 mg / L of H3BO4, 10 mg / L of FeCl3, and 5000 mg / L of EDTA.

[0035] In the present invention, the trace element II solution contains: 250 mg / L of CuSO4·5H2O, 240 mg / L of CoCl2·6H2O, 220 mg / L of NaMoO4·H2O, 190 mg / L of NiCl2·6H2O, 430 mg / L of ZnSO4·7H2O, 15 mg / L of H3BO4, 990 mg / L of MnCl2·4H2O, and 5000 mg / L of EDTA.

[0036] In the present invention, the inoculation ratio of the activated sludge to the anaerobic digested sludge is 1:1; the concentration of volatile suspended solids in the activated sludge and the anaerobic digested sludge is 6.95 g / L.

[0037] In the present invention, in step 2, the ammonia nitrogen concentration after each increase is 1 to 2 times that before the increase, and the nitrite nitrogen concentration after each increase is 1.6 to 2 times that before the increase.

[0038] In the present invention, in step 4, the ammonia nitrogen concentration after each increase is 1 to 2 times that before the increase, and the sulfate concentration after each increase is 1 to 2 times that before the increase.

[0039] The present invention also provides the application of the method for culturing denitrifying mixed flora under sulfate conditions as described in the above technical solution in a denitrification system.

[0040] In the present invention, unless otherwise specified, the raw materials required for preparation are all commercially available products well-known to those skilled in the art.

[0041] The following describes in detail the technical solutions provided by the present invention with reference to the embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0042] Example 1

[0043] (1) A 5.0 L upflow helical bed reactor was taken, inoculated with activated sludge and anaerobic digested sludge at a ratio of 1:1. The concentration of volatile suspended solids was 6.95 g / L. The dominant phyla were Pateschbacteria (21.21%), Synergistota (20.61%), and Proteobacteria (11.69%), followed by Chloroflexi (10.90%), Bacteroidota (10.60%), and Desulfobacterota (6.42%);

[0044] Using simulated wastewater as the influent, the reactor was operated under anaerobic conditions at a constant temperature of 33 ± 1 °C with a hydraulic retention time of 24 h to 48 h;

[0045] At this time, wastewater A was used as the simulated wastewater. The composition of wastewater A was as follows: 55 - 200 mg / L ammonia nitrogen, 30 - 200 mg / L nitrite nitrogen, 0.32 g / L of MgCl₂·6H₂O, 0.48 g / L of CaCl₂, 0.044 g / L of KH₂PO₄, 1.6 g / L of NaHCO₃, 0.02 g / L of FeSO₄·7H₂O, and trace element I solution was added. The ionic concentration ratio of ammonia nitrogen to nitrite nitrogen was 1 - 1.83, and ammonia nitrogen and nitrite nitrogen were provided by NH₄Cl and NaNO₂ respectively;

[0046] Among them, the trace element I solution contained: 430 mg / L of MgSO₄·7H₂O, 240 mg / L of CoCl₂·6H₂O, 220 mg / L of NaMoO₄·H₂O, 190 mg / L of NiCl₂·6H₂O, 210 mg / L of MgSeO₄·10H₂O, 15 mg / L of H₃BO₄, 10 mg / L of FeCl₃, and 5000 mg / L of EDTA.

[0047] At this time, the reactor adopted a gradient acclimation strategy. After the effluent removal rates of ammonia nitrogen and nitrite nitrogen remained above 80% and the reactor had been operating stably for several days, the concentration was increased. The whole process was divided into 3 stages, a total of 90 days. The influent substrate concentration of the simulated wastewater when using wastewater A was as shown in Table 1;

[0048] Table 1 Influent substrate concentration of wastewater A

[0049] Running time (d) <![CDATA[NH4 + -N (mg / L)]]> <![CDATA[NO2 - -N (mg / L)]]> HRT (d) 1～30 55 30 24 31～45 60 50 24 46～60 100 100 24 61～90 200 200 24

[0050] The gradient acclimation of the simulated wastewater when using wastewater A was as Figure 1 shown, Figure 1 where (a) was the influent and effluent concentrations, and (b) was the substrate removal rate. Based on Figure 1 it can be seen that:

[0051] Stage Ⅰ (1 - 13 d) is the active lag phase. Since the 1st day of startup, the concentration of NH4 + -N in the effluent has been lower than that in the influent. The average removal amount of NH4 + -N is 2.40 mg / L. After that, the removal amount surges and stabilizes at about 40.76 mg / L. The corresponding removal rate rises from 1.66% to 72.92%. The removal amount of NO2 - -N decreases from 30.04 mg / L on the 1st day to 22.24 mg / L on the 7th day, and the corresponding removal rate decreases from 96.46% to 67.29%. The concentration of NO3 - -N in the effluent of this stage is very low and almost no NO3

[0052] Stage Ⅱ (14 - 39 d) is the active promotion phase. The concentration of NH4 + -N in the influent of this stage is 55 - 60 mg / L. On the 25th day of the reaction operation, the removal amount of NH4 + -N decreases from 55.27 mg / L to 37.50 mg / L, and the corresponding removal rate decreases from 97.01% to 62.59%. In contrast to Stage Ⅰ, the removal amount of NO2 - -N increases to 52.14 mg / L, and the removal rate has been stable at an efficient level of 94.42%. At the same time, the generation amount of NO3 - -N shows a progressive growth trend and stabilizes at 2.79 mg / L - 5.80 mg / L;

[0053] Stage Ⅲ (40 - 90 d) is the stable operation phase. Both NH4 + -N and NO2 - -N in the influent increase steadily. When the highest reaches 200 mg / L, the concentrations of NH4 + -N and NO2 - -N in the effluent still remain stable. The generation amount of NO3 - -N also gradually increases and remains stable, increasing from 2.86 mg / L at the end of Stage Ⅱ to 42.86 mg / L. At this time, the ratio of nitrite nitrogen consumed to ammonia nitrogen consumed during the system operation cycle is 1.32, and the ratio of nitrate nitrogen in the effluent to ammonia nitrogen in the influent is 0.24. In the later stage of the reactor operation, both the ratio of nitrite nitrogen consumed to ammonia nitrogen consumed and the ratio of nitrate nitrogen in the effluent to ammonia nitrogen in the influent reach 1.32 and 0.26;

[0054] At this time, the microbial community structure undergoes significant adjustment. The dominant phyla are Pateschbacteria (26.25%) and Chloroflexi (23.29%), among which the proportions of Proteobacteria (11.46%) and Bacteroidota (5.14%) decrease.

[0055] (2) When the following conditions are all met: ① The removal rates of ammonia nitrogen and nitrite nitrogen are stable; ② The nitrate nitrogen in the effluent: the ammonia nitrogen in the influent = 0.26; the consumed nitrite nitrogen: the consumed ammonia nitrogen = 1.32; change the influent substrate to Wastewater B, and increase the concentrations of ammonia nitrogen and sulfate in gradients. The composition of Wastewater B is: 100 - 200 mg / L of ammonia nitrogen, 266.67 - 533.33 mg / L of sulfate, 0.2 g / L of MgCl2·6H2O, 0.138 g / L of CaCl2, 0.027 g / L of KH2PO4, 0.5 g / L of NaHCO3, and add Trace Element II solution. The nitrogen-sulfur element ratio of ammonia nitrogen and sulfate is controlled at 2. Ammonia nitrogen and sulfate are provided by NH4Cl and Na2SO4 respectively. Among them, the Trace Element II solution contains: 250 mg / L of CuSO4·5H2O, 240 mg / L of CoCl2·6H2O, 220 mg / L of NaMoO4·H2O, 190 mg / L of NiCl2·6H2O, 430 mg / L of ZnSO4·7H2O, 15 mg / L of H3BO4, 990 mg / L of MnCl2·4H2O, 5000 mg / L of EDTA;

[0056] The reactor still adopts the gradient acclimation strategy. The whole process is divided into 2 stages, a total of 108 days. When using Wastewater B, its influent substrate concentration is shown in Table 2;

[0057] Table 2 Influent Substrate Concentration Table of Wastewater B

[0058] Running time (d) <![CDATA[NH4 + -N(mg / L)]]> <![CDATA[SO4 2- (mg / L)]]> HRT (h) 1～30 100 266.67 24 31～76 100 266.67 48 77～108 200 533.33 48

[0059] When the simulated wastewater uses Wastewater B, the gradient acclimation situation is as Figure 2 shown. Based on Figure 2 it can be known that: in the first stage, the influent NH4 + -N substrate concentration is 100 ± 3.04 mg / L. The influent NH4 + -N substrate concentration is always higher than the influent, but the removal amount of NH4 + -N is relatively reduced compared with Anammox. On the 7th day of operation, the removal amount is as low as 41.38 mg / L, but with the increase of operation time, the removal amount gradually increases. On the 30th day of the reaction, the removal amount is 88.73 mg / L. In the second stage, keeping other operating parameters unchanged and increasing the hydraulic retention time to 48 h, it can be found that the removal amount of NH4 + -N increases slightly compared with the previous stage. The removal amount of NH4 + -N reaches a maximum of 99.52 mg / L, and the removal rate reaches a maximum of 99.75%. It can be found that at this time, the SO4 2- concentration does not change significantly.

[0060] At this time, the dominant phyla of microorganisms are Chloroflexi (22.54%) and Proteobacteria (15.76%), while the abundance of Planctomycetota has increased significantly to 11.27%, and the proportions of Acidobacteria (11.61%) and Bacteroidota (8.97%) have also increased. Considering the effluent ammonia nitrogen situation, it shows that a mixed bacterial community for nitrogen removal under the condition of sulfate presence has been cultivated.

[0061] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A method for culturing a denitrifying mixed flora under sulfate conditions, characterized in that, It includes the following steps: Step 1: Inoculate activated sludge and anaerobic digestion sludge into the reactor. Use simulated wastewater containing ammonia nitrogen, nitrite nitrogen, sulfate, inorganic salts, trace elements, and sodium bicarbonate as the influent substrate. At the same time, add trace element I solution. Operate under anaerobic conditions at a temperature of 33°C with a constant hydraulic retention time until it reaches stability. Step 2: Gradually increase the concentrations of ammonia nitrogen and nitrite nitrogen. The operating time for each gradient is 14 - 30 days. Step 3: When the removal rates of ammonia nitrogen and nitrite nitrogen are stable above 85%, and the ratio of nitrate nitrogen in the effluent to ammonia nitrogen in the influent is 0.26, and the ratio of consumed nitrite nitrogen to consumed ammonia nitrogen is 1.32, replace the influent substrate and add trace element II solution at the same time. Step 4: Gradually increase the concentrations of ammonia nitrogen and sulfate. The operating time for each gradient is 30 - 45 days. Operate until the ammonia nitrogen removal rate in the reactor effluent is stable above 90%. Thus, the cultivation of the denitrifying mixed flora under the condition of the presence of sulfate is completed.

2. The method for culturing a denitrifying mixed flora under sulfate conditions according to claim 1, wherein The simulated wastewater includes wastewater A and wastewater B. The simulated wastewater used in Step 1 is wastewater A, and the influent substrate is replaced with wastewater B in Step 3.

3. The method for culturing denitrifying mixed flora under sulfate conditions according to claim 2, wherein The composition of wastewater A is: 55 - 200 mg / L of ammonia nitrogen, 30 - 200 mg / L of nitrite nitrogen, .32 g / L of MgCl2·6H2O, 0.48 g / L of CaCl2, 0.044 g / L of KH2PO4, 1.6 g / L of NaHCO3, and 0.02 g / L of FeSO4·7H2O.

4. The method for culturing denitrifying mixed bacteria under sulfate conditions according to claim 2, wherein The composition of wastewater B is: 100 - 200 mg / L of ammonia nitrogen, 266.67 - 533.33 mg / L of sulfate, 0.2 g / L of MgCl2·6H2O, 0.138 g / L of CaCl2, 0.027 g / L of KH2PO4, and 0.5 g / L of NaHCO3.

5. The method for culturing a denitrifying mixed bacterial community under sulfate conditions according to claim 1, wherein The trace element I solution contains: 430 mg / L of MgSO4·7H2O, 240 mg / L of CoCl2·6H2O, 220 mg / L of NaMoO4·H2O, 190 mg / L of NiCl2·6H2O, 210 mg / L of MgSeO4·10H2O, 15 mg / L of H3BO4, 10 mg / L of FeCl3, 5000 mg / L of EDTA.

6. The method for culturing a denitrifying mixed flora under sulfate conditions according to claim 1, wherein The trace element II solution contains: 250 mg / L of CuSO4·5H2O, 240 mg / L of CoCl2·6H2O, 220 mg / L of NaMoO4·H2O, 190 mg / L of NiCl2·6H2O, 430 mg / L of ZnSO4·7H2O, 15 mg / L of H3BO4, 990 mg / L of MnCl2·4H2O, 5000 mg / L of EDTA.

7. The method for culturing a denitrifying mixed flora under sulfate conditions according to claim 1, characterized in that, The inoculation ratio of the activated sludge and anaerobic digestion sludge is 1:1; the concentration of volatile suspended solids in the activated sludge and anaerobic digestion sludge is 6.95 g / L.

8. The method for culturing a denitrifying mixed flora under sulfate conditions according to claim 1, wherein In the step 2, the ammonia nitrogen concentration after each increase is 1 to 2 times that before the increase, and the nitrite nitrogen concentration after each increase is 1.6 to 2 times that before the increase.

9. The method for culturing a denitrifying mixed flora under sulfate conditions according to claim 1, characterized in that, In the step 4, the ammonia nitrogen concentration after each increase is 1 to 2 times that before the increase, and the sulfate concentration after each increase is 1 to 2 times that before the increase.

10. Application of the method for culturing a denitrifying mixed flora under sulfate conditions according to any one of claims 1 to 9 in a denitrification system.

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