A reactor and method for cultivating denitrifying granular sludge

Abstract

This invention discloses a reactor and method for cultivating denitrifying granular sludge. The reactor includes a denitrification stirred tank and an aerobic aeration tank. Pre-granulated seed sludge is added to both the denitrification stirred tank and the aerobic aeration tank. The denitrification stirred tank is equipped with a stirring device and has an inlet on its top side. The bottom of the denitrification stirred tank is connected to the bottom of the aerobic aeration tank via a sludge-water circulation pipe. The bottom of the aerobic aeration tank is connected to a sludge discharge pipe, and the top side has an outlet. An aeration device is located at the bottom of the aerobic aeration tank and is connected to an air pump. An air-lift sludge device is located above the aeration device. The air-lift sludge device includes an air collection hood and an upflow pipe connected to the center of the top of the air collection hood. The top of the upflow pipe is connected to the top of the denitrification stirred tank via an upper sludge-water circulation pipe. The reactor and method for cultivating denitrifying granular sludge of this invention create suitable environmental conditions for the rapid cultivation of denitrifying AGS in a continuous flow process.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, and in particular to a reactor and method for cultivating denitrifying granular sludge. Background Technology

[0002] Aerobic granular sludge (AGS) has the characteristics of excellent settling performance, strong shock resistance and high denitrification efficiency, but it also has limitations such as long cultivation cycle, difficult cultivation and harsh cultivation conditions.

[0003] Currently, the technology for cultivating AGS is mainly based on the sequencing batch reactor (SBR) process. For example, under SBR process conditions, AGS has been successfully cultivated by alternating between sludge feeding and starvation to increase hydraulic shear force, washing flocculent sludge, and adding flocculants and crystal nuclei.

[0004] However, in actual engineering practice, the application of SBR technology is far less widespread than that of continuous flow technology because the requirements of SBR technology for the operation and management of wastewater treatment plants are quite stringent.

[0005] Traditional continuous flow processes struggle to create alternating environments of sludge saturation and starvation, cannot selectively wash flocculent sludge, and suffer from low hydraulic shear forces. Furthermore, the need for pumps to crush granular sludge during sludge recirculation leads to a significant lag in AGS cultivation technology compared to SBR processes. In particular, technologies for cultivating AGS with specific functions, such as denitrification AGS, are even more lacking in continuous flow processes.

[0006] Therefore, it is of great significance to find a way to rapidly cultivate denitrified AGS under a continuous flow process. Summary of the Invention

[0007] The purpose of this invention is to provide a reactor for cultivating denitrifying granular sludge and its usage method, so as to solve the problems existing in the prior art and provide support for the rapid cultivation of denitrifying granular sludge under continuous flow processes.

[0008] The objective of this invention is achieved as follows: A reactor for cultivating denitrified granular sludge includes a denitrification mixing tank and an aerobic aeration tank. Pre-granulated seed sludge is added to both the denitrification mixing tank and the aerobic aeration tank. The denitrification mixing tank is equipped with a stirring device and an inlet on its top side. The bottom of the denitrification mixing tank is connected to the bottom of the aerobic aeration tank via a lower sludge-water circulation pipe. The bottom of the aerobic aeration tank is connected to a sludge discharge pipe, and the top side is equipped with an outlet. An aeration device is installed at the bottom of the aerobic aeration tank and is connected to an air pump. An air-lift sludge device is installed above the aeration device. The air-lift sludge device includes an air collection hood and an upflow pipe connected to the center of the top of the air collection hood. The top of the upflow pipe is connected to the top of the denitrification mixing tank via an upper sludge-water circulation pipe.

[0009] The reactor for cultivating denitrifying granular sludge of this invention, through a partitioned design of a denitrification stirring tank and an aerobic aeration tank, combined with a lower sludge-water circulation pipe, an air-lift sludge device, and an upper sludge-water circulation pipe, constructs a sludge-water circulation system. This system allows for alternating "saturated" and "starved" environments for sludge in the denitrification tank and the aerobic tank in a continuous flow process, solving the problem that traditional continuous flow processes cannot create alternating environments. Furthermore, by utilizing the sedimentation zone formed in the aerobic aeration tank, selective pressure is applied to sludge with different settling properties, allowing poorly settling flocculent sludge to be washed out of the reactor, while retaining granular sludge with good settling properties. During the air-lift sludge-water process, hydraulic shear force is increased, and the sludge rotates regularly. The stirring device and aeration device provide suitable conditions for denitrification and nitrification reactions, respectively. The air-lift sludge device uses the air-lift force generated by aeration to drive sludge recirculation, minimizing the crushing and breaking of granular sludge during sludge-water circulation, thus ensuring the continuous flow cultivation of denitrifying AGS.

[0010] As a further improvement of the present invention, the upflow pipe adopts a spiral design, which can prolong the residence time of sludge and water in the pipe, enhance hydraulic shear force, and promote the formation and compaction of granular sludge; the connection end of the upper sludge and water circulation pipe is set above the designed liquid level to ensure that the sludge can be effectively returned to the denitrification mixing tank; the sludge and water flow velocity is controlled at more than 20 m / h, and the high flow velocity can enhance the mixing of sludge and water and the collision between particles, accelerating the granulation process; the height-to-diameter ratio of the aerobic aeration tank is greater than 3, and the resulting slender tank structure is conducive to the formation of a stable upflow during aeration, improving mass transfer efficiency and hydraulic shear force, and further optimizing the cultivation conditions of granular sludge. These designs together improve sludge circulation efficiency and granulation effect.

[0011] As a further improvement of this invention, the dissolved oxygen (DO) in the high dissolved oxygen zone of the aerobic aeration tank (i.e., the area below the gas collection hood) is controlled above 2.0 mg / L to provide a sufficient oxygen environment for nitrifying bacteria, ensuring that ammonia nitrogen can be efficiently converted into nitrate. In the denitrification stirred tank, the DO is controlled below 0.5 mg / L to create a strictly anoxic environment, promoting the reduction of nitrate to nitrogen gas using organic matter by denitrifying bacteria, thus achieving efficient nitrogen removal. By precisely controlling the dissolved oxygen concentration in both zones, an environment suitable for the growth of different functional microorganisms is created, enhancing the continuity and efficiency of the nitrogen removal reaction and ensuring the functional specificity of the denitrification AGS.

[0012] As a further improvement of this invention, the MLSS (mixed liquor suspended solids concentration) in the high dissolved oxygen zone of the aerobic aeration tank is controlled above 5000 mg / L. The higher sludge concentration increases the number of microorganisms, providing a sufficient biomass basis for nitrification and improving ammonia nitrogen removal efficiency. Simultaneously, the high concentration of sludge forms denser sludge flocs under aeration, which is beneficial for the aggregation and growth of granular sludge, accelerating the granulation process and ensuring that the cultivated denitrifying AGS has good settling performance and stable denitrification function.

[0013] As a further improvement of this invention, the volume ratio of the denitrification mixing tank to the aerobic aeration tank is controlled between 1:3 and 1:6, which is beneficial for optimizing the distribution of sludge retention time in the saturated (denitrification mixing tank) and starved (aerobic aeration tank) stages. A smaller denitrification tank volume is conducive to maintaining a higher substrate concentration (saturated), while a larger aerobic tank volume prolongs the time of sludge in a low substrate, high shear environment (starved), more effectively simulating the selectivity pressure of the SBR and promoting particle formation and stabilization.

[0014] This invention also provides a method for cultivating denitrifying granular sludge, using the aforementioned reactor for cultivating denitrifying granular sludge, characterized by comprising the following steps: (1) In the denitrification mixing tank and the aerobic aeration tank, pre-granulated seed sludge is added, and clean water is introduced into the denitrification mixing tank until the liquid level in the aerobic aeration tank reaches the design liquid level. This provides a basic microbial carrier for subsequent sludge cultivation. Pre-granulated seed sludge is more conducive to the rapid formation of granular sludge than ordinary sludge. Adjusting the liquid level to the design liquid level is to ensure that the reactor has suitable volume and water level conditions during subsequent operation, so that the sludge and water can carry out reaction and circulation operations within a reasonable space.

[0015] (2) Turn on the air pump of the aerobic aeration tank and the stirring system of the denitrification mixing tank. Without introducing wastewater to be treated, aerate for 48-72 hours to activate the seed sludge. Aeration restores the activity of aerobic bacteria (especially nitrifying bacteria), while stirring maintains the suspended state of the sludge in the denitrification tank, allowing the sludge to adapt to the hydraulic shear flow generated by the airlift circulation in advance. During this process, residual organic matter is consumed to remove impurities, and a partitioned environment with high dissolved oxygen (>2mg / L) in the aerobic zone and low dissolved oxygen (<0.5mg / L) in the denitrification zone is initially established, laying the foundation for the subsequent directional cultivation of denitrification granular sludge.

[0016] (3) After activation, the wastewater to be treated is fed into the denitrification mixing tank in a continuous flow manner. The sludge is circulated between the aerobic aeration tank and the denitrification mixing tank by using the air-lift sludge device, the upper sludge-water circulation pipe and the lower sludge-water circulation pipe. In the aerobic aeration tank, nitrifying microorganisms are cultivated and starvation conditions are created. In the denitrification mixing tank, denitrifying microorganisms are cultivated and sludge saturation conditions are created. Through the circulation of sludge and water between different tanks, the sludge alternates between the saturation stage and the starvation stage.

[0017] The method for cultivating denitrifying granular sludge of the present invention provides a core framework for granulation by adding pre-granulated seed sludge, thus shortening the cultivation cycle; the sludge activation step can restore sludge activity and improve the metabolic capacity of microorganisms; under continuous flow influent conditions, the reactor's circulation system is used to achieve alternation between sludge saturation and starvation stages, simulating the advantages of the SBR process while avoiding its demanding operation and management problems; denitrifying and nitrifying microorganisms are cultivated separately in different areas to form functional zones, enabling the denitrifying AGS to simultaneously possess efficient denitrification and nitrification capabilities, achieving the goal of rapidly cultivating AGS with specific denitrification functions under continuous flow processes, and has strong engineering practicality.

[0018] As a further improvement to the method of the present invention, in step (1), the pre-granulated seed sludge is made by squeezing sludge from a municipal wastewater treatment plant and soaking it in clean water for 1-2 weeks, with intermittent stirring during the soaking process. The amount of pre-granulated seed sludge added is such that when the reactor is started up, it can ensure that the MLSS (mixed liquor suspended solids concentration) in the aerobic aeration tank is above 5000 mg / L. The raw materials are widely available and inexpensive; the soaking process can promote the gradual coagulation of flocculent substances in the sludge, and the intermittent stirring simulates the hydraulic shear force, initially forming granular rudiments and improving the granulation degree of the seed sludge; the amount added ensures that the MLSS in the high dissolved oxygen zone of the aerobic aeration tank is above 5000 mg / L, providing sufficient microbial mass and initial granular cores for the subsequent cultivation of granular sludge, so that the reactor has a good sludge foundation when it is started up, and further shortens the cultivation time of denitrification AGS.

[0019] As a further improvement to the method of the present invention, in step (3), the ratio of BOD:N:P in the wastewater to be treated is 100:10:1. Sufficient carbon source (BOD) ensures the supply of electron donors in the denitrification stage, high nitrogen ratio (BOD:N=100:10) ensures the balance of substrates for nitrification and denitrification, and suitable phosphorus element (BOD:P=100:1) supports microbial synthesis. The three factors work together to optimize the denitrification efficiency, while the gradient distribution of carbon source between the two tanks strengthens the "saturation-starvation" selective pressure, promoting the formation and stability of dense denitrification granular sludge.

[0020] As a further improvement to the method of the present invention, in step (4), the sludge-water recirculation ratio in the reactor is greater than 200%, which can enhance the material exchange between the two zones, so that the nitrogen generated by denitrification can be discharged in time, and at the same time, the nitrified liquid (nitrate nitrogen and nitrite nitrogen) in the aerobic tank can be fully recirculated to the denitrification tank to improve the denitrification efficiency; the time ratio of the sludge saturation stage and the starvation stage is controlled between 1:3 and 1:6, which optimizes the selection pressure, and the longer starvation period is conducive to promoting particle stability; the total sludge age (SRT) of the reactor is controlled at more than 15 days, which is conducive to the enrichment and retention of slow-growing nitrifying bacteria and core microorganisms of granular sludge, and maintains the long-term stability and high efficiency of denitrification AGS.

[0021] As a further improvement to the method of this invention, the sludge return ratio or hydraulic shear force can be increased by increasing the aeration rate, reducing the inner diameter of the riser pipe, and / or increasing the vertical height h1 of the riser pipe below the liquid surface, as needed, and vice versa. Similarly, the flocculent sludge washing efficiency can be improved by reducing the vertical height h2 of the air-lift sludge device below the liquid surface, and vice versa. By providing flexible and adjustable operating methods, this invention can adapt to different influent water qualities and operational objectives, exhibiting greater adaptability and the ability to optimize cultivation conditions according to actual conditions, thereby improving the cultivation effect and stability of denitrified AGS. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the reactor for cultivating denitrifying granular sludge according to the present invention.

[0023] The components include: 1. Denitrification mixing tank; 2. Aerobic aeration tank; 3. Mixing device; 4. Inlet; 5. Lower sludge-water circulation pipe; 6. Sludge discharge pipe; 7. Outlet; 8. Aeration device; 9. Air pump; 10. Air collection hood; 11. Upflow pipe; 12. Upper sludge-water circulation pipe; 13. Guide plate; and 14. Sampling port. Detailed Implementation

[0024] like Figure 1The reactor for cultivating denitrifying granular sludge shown includes a denitrification mixing tank 1 and an aerobic aeration tank 2. Pre-granulated seed sludge is added to both tanks. The denitrification mixing tank 1 is equipped with a stirring device 3, and an inlet 4 is located at its top. The bottom of the denitrification mixing tank 1 is connected to the bottom of the aerobic aeration tank 2 via a sludge circulation pipe 5. The bottom of the aerobic aeration tank 2 is connected to a sludge discharge pipe 6, and the top has an outlet 7. An aeration device 8 is located at the bottom of the aerobic aeration tank 2 and is connected to an air pump 9. An air-lift sludge device is located above the aeration device 8. The air-lift sludge device includes a gas collection hood 10 and an upflow pipe 11 connected to the center of the top of the gas collection hood 10. The upflow pipe 11 extends upwards in a spiral shape, and the connection end of the upper sludge circulation pipe 12 to the upflow pipe 11 is positioned above the designed liquid level in the aerobic aeration tank 2. The top of the upflow pipe 11 is connected to the top of the denitrification mixing tank 1 via the sludge and water circulation pipe 12. The top of the aeration device 8 is equipped with a water distributor, and a guide plate 13 is provided between the air collection hood 10 and the inner wall of the aerobic aeration tank 2. A sampling port 14 is provided in the middle of the aerobic aeration tank 2.

[0025] In this embodiment, there is one air-lift sludge device. However, in actual implementation, multiple air-lift sludge devices can be installed in a large-scale aerobic aeration tank.

[0026] In this embodiment, the reactor for cultivating denitrifying granular sludge, the denitrification mixing tank 1, and the aerobic aeration tank 2 serve the following functions: In the denitrification mixing tank 1: A) The wastewater to be treated (nutrient substrate) enters the denitrification mixing tank and is thoroughly mixed with the sludge in the tank to provide a saturated environment for the sludge; B) Using the carbon source in the wastewater to be treated and the nitrification liquid from the aerobic aeration tank, denitrifying microorganisms are cultivated under anoxic conditions and denitrification is carried out.

[0027] In aerobic aeration tank 2: A) The nutrient substrate in the wastewater to be treated is oxidized and decomposed, creating a starved environment for the sludge in the tank; B) Utilizing the air-lift sludge device to guide the gas flow, the aerobic aeration tank is divided into a high dissolved oxygen zone and a low dissolved oxygen zone, with the gas collection hood as the boundary, the area below and above which are the high dissolved oxygen zone and the low dissolved oxygen zone (sedimentation zone), respectively; C) In the high dissolved oxygen zone, ammonia nitrogen in the wastewater to be treated is used as the oxidation substrate to cultivate nitrifying microorganisms; D) In ​​the low dissolved oxygen zone, the selective pressure on sludge with different settling properties in the low dissolved oxygen zone is used to wash out flocculent sludge with poor settling properties from the reactor, leaving sludge with good settling properties; E) During the air-lift sludge-water process, the rapid spiral ascent of the sludge-water in the upflow pipe creates high hydraulic shear force and achieves regular rotation of the sludge. The circulation of sludge-water between the denitrification mixing tank and the aerobic aeration tank creates an alternating saturated and starved environment for the sludge in the continuous flow reactor. Furthermore, by employing airlift and gravity flow, the sludge is circulated between different tanks, thus avoiding the crushing and grinding of granular sludge by water pumps in traditional continuous flow reactors.

[0028] The start-up and operation method of the reactor for cultivating denitrifying granular sludge in this embodiment is as follows: (1) In the denitrification mixing tank 1 and the aerobic aeration tank 2, pre-granulated seed sludge is added. Clean water is introduced into the denitrification mixing tank 1 until the liquid level in the aerobic aeration tank 2 reaches the design liquid level. The pre-granulated seed sludge is made by squeezing sludge from the municipal sewage treatment plant and soaking it in clean water for 1 to 2 weeks, during which intermittent stirring is carried out. The amount of pre-granulated seed sludge added is such that when the reactor is started up and running, it can ensure that the MLSS (mixed liquor suspended solids concentration) in the high dissolved oxygen zone of the aerobic aeration tank 2 is above 5000 mg / L. (2) Turn on the air pump of the aerobic aeration tank and the stirring system of the denitrification mixing tank, and aerate for 48-72 hours without adding wastewater to be treated to activate the seed sludge; (3) After activation, the wastewater to be treated is continuously fed into the denitrification mixing tank. The BOD:N:P ratio in the wastewater is 100:10:1. The sludge is circulated between the aerobic aeration tank 2 and the denitrification mixing tank 1 using the air-lift sludge device, the upper sludge-water circulation pipe 12, and the lower sludge-water circulation pipe 5. In the aerobic aeration tank 2, nitrifying microorganisms are cultivated and starvation conditions are created, while in the denitrification mixing tank 1, denitrifying microorganisms are cultivated and sludge is saturated. Through the circulation of sludge and water between different tanks, the sludge alternates between the saturation and starvation stages. The treated wastewater is discharged from the outlet 7 of the aerobic aeration tank 2. During reactor operation, the liquid level in the aerobic aeration tank is always maintained at the designed liquid level.

[0029] This method utilizes an air-lift sludge device to guide the gas from the aerobic aeration tank 2 to a level above the designed liquid surface for release. During this process, the density difference created within the air-lift device powers the sludge-water mixture from the aerobic aeration tank 2 to the denitrification mixing tank 1. Under the influence of the water level difference, the sludge-water mixture from the denitrification mixing tank 1 flows by gravity to the aerobic aeration tank. Through the air-lift of the sludge-water mixture from the aerobic aeration tank 2 and the gravity flow of the sludge-water mixture from the denitrification mixing tank 1, a circulating flow of sludge is achieved between the denitrification mixing tank 1 and the aerobic aeration tank 2.

[0030] To achieve optimal nitrogen removal granular sludge cultivation, during reactor operation, the sludge-water flow velocity in the upflow pipe 11 should be controlled above 20 m / h, and the height-to-diameter ratio of the aerobic aeration tank 2 should be above 3 (ideally around 4). The dissolved oxygen (DO) in the high dissolved oxygen zone of the aerobic aeration tank 2 (i.e., the area below the gas collection hood 10) should be controlled above 2.0 mg / L, while the DO in the denitrification mixing tank 1 should be controlled below 0.5 mg / L. The mixed liquor suspended solids (MLSS) in the high dissolved oxygen zone of the aerobic aeration tank 2 should be controlled above 5000 mg / L. The sludge-water recirculation ratio within the reactor should be greater than 200% (ideally around 300%), and the time ratio between the sludge saturation and starvation stages should be controlled between 1:3 and 1:6 (this can be achieved by controlling the volume ratio of the denitrification mixing tank 1 and the aerobic aeration tank 2). The total sludge retention time (SRT) of the reactor should be controlled above 15 days.

[0031] During operation, when it is necessary to increase the sludge return ratio or hydraulic shear force, measures such as increasing the aeration rate, reducing the inner diameter of the riser pipe, and increasing the vertical height h1 of the riser pipe below the liquid surface can be taken to accelerate the flow velocity of sludge and water in the riser pipe and increase the total amount of sludge and water lifted by the air-lift sludge device. Conversely, the opposite is also true. When it is necessary to improve the washing efficiency of flocculent sludge, the vertical height h2 of the air-lift sludge device below the liquid surface should be reduced, and vice versa.

[0032] The reactor and method for cultivating denitrifying granular sludge in this embodiment, under a continuous flow process, create suitable environmental conditions for the rapid cultivation of denitrifying AGS through the following approaches: A) Creating an environment of alternating sludge saturation and starvation through the circulation of sludge between the aerobic aeration tank 2 and the denitrification stirred tank 1; B) Cultivating nitrifying microorganisms and denitrifying microorganisms in the aerobic aeration tank 2 and the denitrification stirred tank 1, respectively; C) Utilizing the vertical height of the low dissolved oxygen zone above the gas collection hood 10, selective pressure is applied to sludge with different settling properties, washing out flocculent sludge with poor settling properties from the reactor and retaining sludge with good settling properties; D) Increasing the hydraulic shear force and achieving regular rotation of the sludge through the rapid ascent of the sludge-water mixture in the spiral rising flow pipe 11; E) Avoiding the crushing and breaking of sludge granules by the air pump during the circulation of sludge between different tanks through air lifting and gravity flow of the sludge-water mixture; F) Establishing appropriate operating parameters for the continuous flow reactor, etc.

[0033] This invention is not limited to the above embodiments. Based on the technical solutions disclosed in this invention, those skilled in the art can make some substitutions and modifications to some of the technical features without creative effort, and all such substitutions and modifications are within the protection scope of this invention.

Claims

1. A reactor for cultivating denitrifying granular sludge, characterized in that: The system includes a denitrification mixing tank and an aerobic aeration tank. Pre-granulated seed sludge is added to both tanks. The denitrification mixing tank is equipped with a mixing device and has an inlet at its top. The bottom of the denitrification mixing tank is connected to the bottom of the aerobic aeration tank via a sludge circulation pipe. The aerobic aeration tank has a sludge discharge pipe connected to its bottom and an outlet at its top. An aeration device is located at the bottom of the aerobic aeration tank and is connected to an air pump. An air-lift sludge device is installed above, which includes an air collection hood and an upflow pipe connected to the center of the top of the air collection hood. The upflow pipe extends upward in a spiral shape, and the top of the upflow pipe is connected to the top of the denitrification mixing tank through an upper sludge-water circulation pipe. The connection end of the upper sludge-water circulation pipe and the upflow pipe is located above the designed liquid level of the aerobic aeration tank. The MLSS in the high dissolved oxygen zone of the aerobic aeration tank is controlled above 5000 mg / L, and the sludge-water internal recirculation ratio in the reactor is greater than 200%.

2. The reactor for cultivating denitrifying granular sludge according to claim 1, characterized in that: The sludge-water flow velocity in the upflow pipe is controlled at 20 m / h or higher, and the height-to-diameter ratio of the aerobic aeration tank is 3 or higher.

3. The reactor for cultivating denitrifying granular sludge according to claim 1, characterized in that: The dissolved oxygen (DO) level in the high dissolved oxygen zone of the aerobic aeration tank is controlled above 2.0 mg / L, and the DO level in the denitrification stirred tank is controlled below 0.5 mg / L.

4. The reactor for cultivating denitrifying granular sludge according to claim 1, characterized in that: The volume ratio of the denitrification mixing tank to the aerobic aeration tank is controlled at 1:3 to 1:

6.

5. A method for cultivating denitrifying granular sludge, using a reactor for cultivating denitrifying granular sludge as described in any one of claims 1-4, characterized in that, Includes the following steps: (1) Add pre-granulated seed sludge to the denitrification mixing tank and the aerobic aeration tank, and pass clean water into the denitrification mixing tank until the liquid level in the aerobic aeration tank reaches the design liquid level. (2) Turn on the air pump of the aerobic aeration tank and the stirring system of the denitrification mixing tank, and aerate for 48-72 hours without adding wastewater to be treated to activate the seed sludge; (3) After activation, the wastewater to be treated is fed into the denitrification mixing tank in a continuous flow manner. The sludge is circulated between the aerobic aeration tank and the denitrification mixing tank by using the air-lift sludge device, the upper sludge-water circulation pipe and the lower sludge-water circulation pipe. Nitrifying microorganisms are cultivated and starvation conditions are created in the aerobic aeration tank, and denitrifying microorganisms are cultivated and sludge saturation conditions are created in the denitrification mixing tank. Through the circulation of sludge and water between different tanks, the sludge alternates between the saturation stage and the starvation stage.

6. The method according to claim 5, characterized in that: In step (1), the pre-granulated seed sludge is made by squeezing sludge from urban sewage treatment plants and soaking it in clean water for 1 to 2 weeks, with intermittent stirring during the process. The dosage ensures that the MLSS in the high dissolved oxygen zone of the aerobic aeration tank is above 5000 mg / L.

7. The method according to claim 5, characterized in that: In step (3), the ratio of BOD:N:P in the wastewater to be treated is 100:10:

1.

8. The method according to claim 5, characterized in that: The time ratio of the sludge saturation stage to the starvation stage is controlled between 1:3 and 1:6, and the total sludge age of the reactor is controlled to be more than 15 days.

9. The method according to claim 5, characterized in that: Increase the sludge return ratio or hydraulic shear force by increasing the aeration rate, reducing the inner diameter of the riser pipe, and / or increasing the vertical height of the riser pipe below the liquid surface; improve the efficiency of flocculent sludge washing by reducing the vertical height of the air-lift sludge device below the liquid surface.

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