CN117904095A - Method for fixing nitrifying bacteria by sodium alginate-polyurethane filler - Google Patents

Abstract

The invention discloses a method for fixing nitrifying bacteria with sodium alginate-polyurethane filler, and belongs to the technical field of water treatment. Sodium alginate is first mixed with nitrifying bacteria, and then polyurethane filler is added to the mixed solution of sodium alginate and nitrifying bacteria, and after being fully stirred, it is placed in a calcium chloride solution for cross-linking and fixation. The advantage of the present invention is that nitrifying bacteria are directly fixed by using sodium alginate and porous polyurethane filler, avoiding the defect of cultivating nitrifying bacteria by prolonging the activated sludge retention time and hydraulic retention time in the traditional sewage biological denitrification process, and at the same time, the immobilized nitrifying bacteria carrier has high activity, which can directionally increase the nitrification rate and reduce the aeration tank capacity, providing technical support for low-carbon and high-efficiency denitrification in the sewage treatment industry.

Description

A method for fixing nitrifying bacteria using sodium alginate-polyurethane filler

Technical Field

The invention belongs to the technical field of water treatment, and in particular relates to a method for fixing nitrifying bacteria with sodium alginate-polyurethane filler.

Background technique

Excessive nitrogen discharge is one of the key factors leading to eutrophication of water bodies. Biological denitrification of sewage mainly relies on the nitrification and denitrification of microorganisms, among which nitrification reaction is the rate-limiting step. This is due to the slow growth rate and long generation cycle of nitrifying bacteria in the activated sludge system of the sewage treatment plant, as well as the small number and small proportion of nitrifying bacteria in the activated sludge. The nitrification rate of nitrifying bacteria in sewage treatment plants is low, resulting in a longer hydraulic retention time in the nitrification process to completely remove ammonia nitrogen, which in turn leads to a larger area occupied by sewage treatment facilities with denitrification functions. In the case of extremely tight urban land resources, it is of great significance to save land resources if the nitrification rate of nitrifying bacteria can be increased.

Immobilized microbial technology relies on physical or chemical methods to restrict microorganisms to a certain spatial area. This technology can increase the number of microorganisms in the system, reduce the loss of microorganisms, and reduce the interference of external factors on microorganisms. At present, the most common fixation method for immobilized microorganism technology is the embedding method. The materials used in the embedding method are divided into two categories: synthetic high molecular organic matter and natural high molecular organic matter. Among them, polyvinyl alcohol is one of the most common carriers in synthetic high molecular organic matter, but polyvinyl alcohol and polyvinyl alcohol cross-linking agent boric acid are toxic to microorganisms and will reduce the activity of microorganisms. At the same time, polyvinyl alcohol as an immobilized material has poor mass transfer performance. Sodium alginate is one of the most common carriers in natural high molecular organic matter. Compared with polyvinyl alcohol, sodium alginate has the advantages of good mass transfer performance and non-toxicity to microorganisms. In addition, sodium alginate reacts quickly with the cross-linking agent calcium chloride during the microbial immobilization process, and the time required is short, which can reduce the use time of the immobilization process. However, the mechanical properties of sodium alginate are also poor and the service life is short.

Therefore, it is of great practical significance to develop an immobilized microorganism method with excellent mechanical properties, long service life, low toxicity to microorganisms, good mass transfer performance, simple preparation process and low price, which can achieve efficient nitrification, shorten the hydraulic retention time of the nitrification process for biological denitrification in sewage treatment, and save the floor space of sewage treatment facilities and equipment.

Summary of the invention

To achieve the above object, the present invention provides a method for fixing nitrifying bacteria using sodium alginate-polyurethane filler, comprising the following steps:

(1) preparing sodium alginate into sodium alginate hydrogel;

(2) centrifuging the liquid nitrifying bacteria agent, discarding the supernatant, adding the precipitate to the sodium alginate hydrogel, and stirring evenly to obtain a nitrifying bacteria hydrogel mixture;

(3) adding a polyurethane filler into the nitrifying bacteria hydrogel mixture and stirring to obtain a polyurethane filler containing a mixture of sodium alginate and nitrifying bacteria (the polyurethane filler contains a mixture of sodium alginate and nitrifying bacteria), adding the filler into a calcium chloride solution for cross-linking and fixing, and washing after the cross-linking and fixing are completed.

The advantages of the method of the present invention are that it reduces the damage to bacteria during the immobilization process and improves the mechanical properties of the bacterial immobilization carrier. At the same time, nitrifying bacteria with high nitrification performance are fixed on the polyurethane filler, which can directionally increase the nitrification rate and shorten the hydraulic retention time of the sewage denitrification and nitrification process, providing technical support for efficient denitrification in the sewage treatment industry.

The liquid nitrifying bacteria agent of the present invention can also be replaced with a powdered nitrifying bacteria agent. If it is a powdered nitrifying bacteria agent, it needs to be dissolved in pure water and then centrifuged. If it is a liquid agent, it can be directly centrifuged.

Furthermore, in step (1), the preparation method of the sodium alginate hydrogel is as follows: sodium alginate is added to water, heated and stirred to fully dissolve, and then allowed to stand and cool at room temperature to obtain the sodium alginate hydrogel; further, the preparation method of the sodium alginate hydrogel is as follows: sodium alginate is added to a beaker filled with pure water, heated and stirred to obtain a sodium alginate solution, and then allowed to stand and cool at room temperature to obtain the sodium alginate hydrogel.

Furthermore, in step (1), the mass fraction of the sodium alginate solution is 1%. The method for preparing the sodium alginate hydrogel is preferably as follows: sodium alginate is added to a beaker filled with purified water, heated and stirred for 1 hour to obtain a sodium alginate solution with a mass fraction of 1%, and the solution is allowed to stand at room temperature for 1 hour to obtain the sodium alginate hydrogel. The mass fraction of the sodium alginate solution will affect the nitrification rate of the fixed nitrifying bacteria, and the mass fraction of the sodium alginate solution defined in the present invention is an appropriate concentration after screening.

Furthermore, in step (1), the temperature of heating and stirring is 70° C. and the rotation speed is 120 r/min.

Further, in step (2), the nitrifying bacteria in the liquid nitrifying bacteria agent are the dominant bacteria, and their relative abundance is 28.39%. The mixed liquid suspended solid concentration of the nitrifying bacteria agent is 3.7 g/L ("mixed liquid suspended solid concentration" represents the concentration of microorganisms in the agent). Further, Nitrosomonas and Nitrosospirillum are the dominant bacteria in the liquid nitrifying bacteria agent, and their relative abundances are 27.96% and 0.43%, respectively.

Furthermore, in step (2), the mass fraction of the nitrifying bacteria agent in the nitrifying bacteria hydrogel mixture is 0.37% based on dry weight.

Furthermore, in step (2), the rotation speed during centrifugation is 6000 r/min, preferably centrifugation for 10 min.

Furthermore, in step (3), the specification of the polyurethane filler is 1×1×1 cm, and the pore density is 20 ppi.

Furthermore, in step (3), the cross-linking fixation time is 2 hours, and the cleaning is to rinse with pure water 3 times after the cross-linking fixation is completed.

Furthermore, in step (3), the mass fraction of the calcium chloride solution is 2%.

Compared with the prior art, the present invention has the following advantages and technical effects:

(1) The immobilization material sodium alginate used in the present invention is a natural high molecular organic matter, which is non-toxic and harmless to microorganisms during the immobilization process, and has good mass transfer performance, which is conducive to improving the treatment effect of microorganisms on pollutants. In addition, the reaction speed of sodium alginate and the cross-linking agent calcium chloride is fast, making the immobilization preparation process simple and fast;

(2) The polyurethane filler used in the present invention provides excellent mechanical support for sodium alginate, which can extend its service life. The sodium alginate-polyurethane filler fixed nitrifying bacteria particles added to the reactor can be stably used for more than 60 days;

(3) The number of nitrifying bacteria in the immobilized microorganisms of the present invention is large and the proportion is high. The nitrification rate of nitrifying bacteria fixed by sodium alginate-polyurethane filler can reach 9.32 mg/(g MLSS·h). When the nitrifying bacteria particles fixed by sodium alginate-polyurethane filler are added to the reactor, the reactor startup time is short, and the ammonia nitrogen can be completely oxidized and removed under a short hydraulic retention time.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

FIG1 is a physical picture of sodium alginate-polyurethane filler immobilizing nitrifying bacteria in Example 1;

FIG2 shows the effluent of the reactor in Application Example 1 with an influent ammonia nitrogen concentration of 30 mg/L;

FIG3 shows the effluent of the reactor in Application Example 1 with an influent ammonia nitrogen concentration of 50 mg/L;

FIG4 shows the effluent of the reactor in Application Example 1 when the ammonia nitrogen concentration in the inlet water is 100 mg/L;

FIG5 shows the treatment effect of polyvinyl alcohol-sodium alginate immobilized nitrifying bacteria on ammonia nitrogen in Comparative Example 1;

FIG. 6 shows the morphological changes of the polyvinyl alcohol-sodium alginate-immobilized nitrifying bacteria particles in Comparative Example 1.

Detailed ways

Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

It should be understood that the terms described in the present invention are only for describing special embodiments and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. The intermediate value in any stated value or stated range, and each smaller range between any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and/or materials associated with the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to those skilled in the art. The present invention description and examples are exemplary only.

The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

The liquid nitrifying bacteria agent in the embodiment of the present invention was purchased from Shanghai Pro Biotechnology Co., Ltd., wherein the relative abundances of Nitrosomonas and Nitrosospira were 27.96% and 0.43%, respectively, and the suspended solid concentration of the mixed liquid of the nitrifying bacteria agent was 3.7 g/L.

Example 1

(1) 20 g of sodium alginate was added to 2 L of pure water, heated and stirred at 70 ° C and 120 r/min for 1 h, and then allowed to stand and cool for 1 h to obtain a sodium alginate hydrogel;

(2) taking 2 L of liquid nitrifying bacteria agent, centrifuging (at a speed of 6000 r/min) for 10 min, discarding the supernatant, pouring the precipitate into the sodium alginate hydrogel, and stirring evenly to obtain a nitrifying bacteria hydrogel mixture, wherein the mass fraction of the nitrifying bacteria agent in the nitrifying bacteria hydrogel mixture is 0.37% based on dry weight;

(3) Add the polyurethane filler (with a specification of 1×1×1 cm and a pore density of 20 ppi) to the nitrifying bacteria hydrogel mixture, and stir continuously to allow the nitrifying bacteria hydrogel to fully penetrate into the polyurethane filler. Then, add the polyurethane filler (containing a mixture of sodium alginate and nitrifying bacteria) into 10 L of a 2% calcium chloride solution, cross-link and fix for 2 h, and rinse with pure water 3 times after the cross-linking is completed to obtain sodium alginate-polyurethane filler-fixed nitrifying bacteria particles with a specification of 1×1×1 cm, as shown in FIG1 .

Example 2

(1) 1 g of sodium alginate was added to a beaker containing 100 mL of purified water, heated and stirred at 70° C. for 1 h at a speed of 120 r/min, and then allowed to stand at room temperature for 1 h to obtain a sodium alginate hydrogel;

(2) taking 100 mL of nitrifying bacteria agent, centrifuging (at a speed of 6000 r/min) for 10 min, discarding the supernatant, pouring the precipitate into the sodium alginate hydrogel, and stirring evenly to obtain a nitrifying bacteria hydrogel mixture, wherein the mass fraction of the nitrifying bacteria agent in the nitrifying bacteria hydrogel mixture is 0.37% based on dry weight;

(3) Adding a polyurethane filler (with a specification of 1×1×1 cm and a pore density of 20 ppi) to the nitrifying bacteria hydrogel mixture, stirring continuously to allow the nitrifying bacteria hydrogel to fully penetrate into the polyurethane filler, and then adding the polyurethane filler (containing a mixture of sodium alginate and nitrifying bacteria) to 500 mL of a 2% calcium chloride solution, cross-linking and fixing for 2 hours, and rinsing with pure water three times after the cross-linking is completed, to obtain sodium alginate-polyurethane filler-fixed nitrifying bacteria particles with a specification of 1×1×1 cm.

The sodium alginate-polyurethane filler-immobilized nitrifying bacteria particles prepared in step (3) were placed in 300 mL of the prepared test solution (the substances and substance concentrations in the test solution were as follows: NH ₄ Cl: 382 mg/L, MgSO ₄ ·7H ₂ O: 4 mg/L, KH ₂ PO ₄ : 10 mg/L, FeSO ₄ ·7H ₂ O: 4 mg/L, MnCl ₂ ·4H ₂ O: 0.2 mg/L, CuSO ₄ ·5H ₂ O: 0.2 mg/L), saturated sodium bicarbonate solution was added dropwise thereto, the pH was adjusted to 7-8, the dissolved oxygen concentration in the solution was maintained at 4 mg/L, and then placed in a constant temperature shaker at a speed of 200 rpm and a temperature of 25°C. Take a water sample once before placing it in a shaker, the timing is 0h, and then take a water sample every 0.5h. Mix thoroughly before each sampling, and take samples 6 times in succession, respectively 0.5h, 1h, 1.5h, 2h, 2.5h, 3h, each sampling volume is 5mL, filter with a 0.45um filter membrane immediately after sampling, and keep the filtrate in a sealed manner for the determination of ammonia nitrogen, nitrite nitrogen, and nitric nitrogen in the filtrate. After each sampling, adjust the pH of the solution in the conical flask to 7-8 with a saturated sodium carbonate solution, and then place it in a constant temperature shaker to continue the reaction. The filtrate is stored at 4°C before the determination.

Plot the graph with sampling time t (h) as the horizontal axis and the residual ammonia nitrogen mass concentration p (mg/L) of the test solution as the vertical axis (in N), fit the data points in the graph (p = r·t+b), and obtain the absolute value r of the slope of the straight line in mg NH ₄ ⁺ -N/(L·h). The nitrification rate P of the nitrifying bacteria agent in mg NH ₄ ⁺ -N/(g MLSS·h) is calculated according to the following formula:

Where: M-the concentration of nitrifying bacteria agent, the unit is mg/L, M=3761mg/L;

V ₁ - volume of fixed nitrifying bacteria agent, in mL, V ₁ = 100 mL;

V ₂ - volume of test solution, in mL, V ₂ = 300 mL;

r - the absolute value of the slope of the fitting straight line of the sample measurement experiment, in mg NH ₄ ⁺ -N/(L·h).

Through the above calculation method, it is obtained that the nitrification rate of nitrifying bacteria fixed on sodium alginate-polyurethane filler can reach 9.32 mg/(gMLSS·h). This result shows that nitrifying bacteria fixed on sodium alginate-polyurethane filler have good nitrification performance.

Application Example 1 Treatment effect on simulated wastewater

The sodium alginate-polyurethane filler immobilized nitrifying bacteria particles obtained in Example 1 were added to the reactor, and the filling ratio (volume ratio of immobilized particles to influent water) was set to 40%. The diameter of the reactor was 160 mm, the height was 320 mm (height to diameter ratio was 2), the effective volume was 6 L, and the reactor material was plexiglass.

The reactor inlet water is artificially distributed. The reactor is equipped with a 30L water inlet bucket. The water components in the water inlet bucket are NH ⁺ ₄ -N, KH ₂ PO ₄ (nitrogen-phosphorus ratio of 5), CaCl ₂ (111 mg/L), MgSO ₄ (15 mg/L), FeSO ₄ ·7H ₂ O (11.1 mg/L), and NaCl (50 mg/L). The hydraulic retention time is adjusted by a timer, and aeration is carried out using an aeration pump and an aeration head. The dissolved oxygen concentration in the reactor is controlled by a gas flow meter.

The reactor was operated under the conditions of influent ammonia nitrogen concentration of 30 mg/L, 50 mg/L and 100 mg/L, the temperature was set to 25°C, the pH was controlled at 7-8, the dissolved oxygen was controlled at 4-5 mg/L, and the changes of ammonia nitrogen, nitrite nitrogen and nitric nitrogen in the effluent were monitored every day.

As can be seen from Figure 2, on the first day of startup, the ammonia nitrogen removal rate of the reactor can reach 100%, which shows that the reactor operates well and can achieve rapid startup. For the simulated wastewater with an inlet ammonia nitrogen concentration of 30 mg/L, the ammonia nitrogen can be completely removed at an HRT of 2h; as can be seen from Figure 3, for the simulated wastewater with an inlet ammonia nitrogen concentration of 50 mg/L, the ammonia nitrogen removal rate fluctuates between 90% and 100% when the HRT is 2h; as can be seen from Figure 4, for the simulated wastewater with an inlet ammonia nitrogen concentration of 100 mg/L, extending the HRT can enhance the removal effect of ammonia nitrogen. When the HRT is 6h, the ammonia nitrogen removal rate can reach more than 80%.

In summary, the reactor built based on the sodium alginate-polyurethane filler fixed nitrifying bacteria particles of the present invention has a short startup time, a short required hydraulic retention time, and a stable and efficient removal effect of ammonia nitrogen. In addition, the sodium alginate-polyurethane filler fixed nitrifying bacteria particles not only maintain efficient nitrification performance after 60 days of use, but also do not break the particles, and the particles have a long service life.

Application Example 2: Treatment effect on actual domestic sewage

The sodium alginate-polyurethane filler immobilized nitrifying bacteria particles obtained in Example 1 were added to the reactor, and the filling ratio (volume ratio of immobilized particles to influent water) was set to 40%. The diameter of the reactor was 160 mm, the height was 320 mm (height to diameter ratio was 2), the effective volume was 6 L, and the reactor material was plexiglass.

The reactor inlet water is actual domestic sewage, and the inlet ammonia nitrogen concentration fluctuates between 60 and 90 mg/L. The reactor is equipped with a 30L inlet bucket. The hydraulic retention time is adjusted by a timer, and aeration is performed using an aeration pump and an aeration head. The dissolved oxygen concentration in the reactor is controlled by a flow meter.

The reactor temperature was set at 25°C, the dissolved oxygen was controlled at 4-5 mg/L, and the changes in ammonia nitrogen, nitrite nitrogen, and nitrate nitrogen in the effluent were monitored every day.

When the hydraulic retention time of the reactor is 4h and the concentration of ammonia nitrogen in the influent fluctuates greatly, the reactor operates stably and the removal rate of ammonia nitrogen can reach 80%. This shows that the sodium alginate-polyurethane filler fixed nitrifying bacteria particles also have a high efficiency in treating ammonia nitrogen in actual domestic sewage.

Comparative Example 1 (without polyurethane filler as support)

Take 200g of polyvinyl alcohol, add it to a beaker filled with 2L of pure water, soak it for 24h, put it in a constant temperature water bath and heat it (70℃) for 2h to obtain polyvinyl alcohol hydrogel, take 16g of sodium alginate, add it to the polyvinyl alcohol hydrogel, heat and stir at 70℃ for 1h, the speed is 120r/min, and then stand for 1h to obtain polyvinyl alcohol-sodium alginate hydrogel;

Take 2 L of nitrifying bacteria inoculum, centrifuge for 10 min, remove and discard the supernatant, pour the precipitate into the polyvinyl alcohol-sodium alginate hydrogel, stir evenly, and prepare a nitrifying bacteria hydrogel mixture;

The nitrifying bacteria hydrogel mixture is added dropwise into 20 L of a cross-linking agent (the cross-linking agent is composed of 4% by mass boric acid and 2% by mass calcium chloride) by injection, cross-linked and fixed for 16 hours, and rinsed with pure water 3 times after the cross-linking is completed, so as to obtain polyvinyl alcohol-sodium alginate fixed nitrifying bacteria particles;

Polyvinyl alcohol-sodium alginate fixed nitrifying bacteria particles are added to the reactor, and the filling ratio (volume ratio of immobilized particles to influent water) is set to 40%. The diameter of the reactor is 160 mm, the height is 320 mm (height to diameter ratio is 2), the effective volume is 6 L, and the reactor material is plexiglass.

The reactor inlet water is artificially distributed. The reactor is equipped with a 30L water inlet bucket. The water components in the water inlet bucket are NH ⁺ ₄ -N, KH ₂ PO ₄ (nitrogen-phosphorus ratio of 5), CaCl ₂ (111mg/L), MgSO ₄ (15mg/L), FeSO ₄ ·7H ₂ O (11.1mg/L), and NaCl (50mg/L). The hydraulic retention time is adjusted by a timer, and aeration is carried out using an aeration pump and an aeration head. The dissolved oxygen concentration in the reactor is controlled by a flow meter.

The temperature was set at 25°C, the pH was controlled at 7-8, the dissolved oxygen was controlled at 4-5 mg/L, the hydraulic retention time was 4 h, and the changes in ammonia nitrogen, nitrite nitrogen, and nitrate nitrogen in the effluent were monitored every day.

As shown in Figure 5, when the influent ammonia nitrogen concentration is 50 mg/L and the hydraulic retention time is 4 hours, the removal rate of ammonia nitrogen by the reactor based on polyvinyl alcohol-sodium alginate fixed nitrifying bacteria particles is about 90%. However, as shown in Figure 6, as the use time of polyvinyl alcohol-sodium alginate fixed nitrifying bacteria particles increases, the polyvinyl alcohol-sodium alginate fixed nitrifying bacteria particles become soft and dissolve.

Compared with polyvinyl alcohol-sodium alginate fixed nitrifying bacteria particles, the reactor built based on sodium alginate-polyurethane filler fixed nitrifying bacteria particles has a better removal effect on ammonia nitrogen at the same hydraulic retention time and a longer service life.

The above are only preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims (9)

1. A method for fixing nitrifying bacteria using sodium alginate-polyurethane filler, characterized in that it comprises the following steps: (1) preparing sodium alginate into sodium alginate hydrogel; (2) centrifuging the liquid nitrifying bacteria agent, discarding the supernatant, adding the precipitate to the sodium alginate hydrogel, and stirring evenly to obtain a nitrifying bacteria hydrogel mixture; (3) adding a polyurethane filler into the nitrifying bacteria hydrogel mixture and stirring to obtain a polyurethane filler containing a mixture of sodium alginate and nitrifying bacteria, cross-linking and fixing with a calcium chloride solution, and washing after the cross-linking and fixing is completed. 2. The method for fixing nitrifying bacteria on sodium alginate-polyurethane filler according to claim 1, characterized in that in step (1), the preparation method of the sodium alginate hydrogel is as follows: sodium alginate is added to water, heated and stirred, and allowed to stand and cool at room temperature to obtain the sodium alginate hydrogel. 3. The method for fixing nitrifying bacteria using sodium alginate-polyurethane filler according to claim 2, characterized in that in step (1), the mass fraction of the sodium alginate solution is 1%. 4. The method for fixing nitrifying bacteria with sodium alginate-polyurethane filler according to claim 2, characterized in that in step (1), the temperature of heating and stirring is 70°C and the rotation speed is 120r/min. 5. The method for fixing nitrifying bacteria with sodium alginate-polyurethane filler according to claim 1, characterized in that, in step (2), nitrifying bacteria are dominant bacteria in the liquid nitrifying bacteria agent, and their relative abundance is 28.39%. 6. The method for fixing nitrifying bacteria on sodium alginate-polyurethane filler according to claim 5, characterized in that Nitrosomonas and Nitrosospirillum are dominant bacteria in the liquid nitrifying bacteria agent, and their relative abundances are 27.96% and 0.43%, respectively. 7. The method for fixing nitrifying bacteria using sodium alginate-polyurethane filler according to claim 1, characterized in that, in step (2), the mass fraction of the nitrifying bacteria agent in the nitrifying bacteria hydrogel mixture is 0.37% based on dry weight. 8. The method for fixing nitrifying bacteria using sodium alginate-polyurethane filler according to claim 1, characterized in that in step (2), the rotation speed during centrifugation is 6000 r/min. 9. The method for fixing nitrifying bacteria using sodium alginate-polyurethane filler according to claim 1, characterized in that in step (3), the specification of the polyurethane filler is 1×1×1 cm, and the pore density is 20 ppi.