CN110342638B - Low-carbon-nitrogen-ratio sewage denitrification device and method based on double reflux and gradient oxygen limitation - Google Patents

Low-carbon-nitrogen-ratio sewage denitrification device and method based on double reflux and gradient oxygen limitation Download PDF

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CN110342638B
CN110342638B CN201910655492.0A CN201910655492A CN110342638B CN 110342638 B CN110342638 B CN 110342638B CN 201910655492 A CN201910655492 A CN 201910655492A CN 110342638 B CN110342638 B CN 110342638B
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reaction chamber
sewage
gradient oxygen
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CN110342638A (en
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张砺彦
傅木星
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HANGZHOU DILI ECOLOGY CYCLE ECONOMIC ENGINEERING CO LTD
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HANGZHOU DILI ECOLOGY CYCLE ECONOMIC ENGINEERING CO LTD
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    • CCHEMISTRY; METALLURGY
    • C02TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02FTREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F3/00Biological treatment of water, waste water, or sewage
    • C02F3/30Aerobic and anaerobic processes
    • C02F3/302Nitrification and denitrification treatment
    • CCHEMISTRY; METALLURGY
    • C02TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02FTREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F2301/00General aspects of water treatment
    • C02F2301/04Flow arrangements
    • C02F2301/046Recirculation with an external loop

Abstract

The invention discloses a low-carbon-nitrogen-ratio sewage denitrification device based on double reflux and gradient oxygen limitation and a method thereof, belonging to the technical field of biological sewage treatment denitrification in environmental engineering. The device comprises: a regulating tank, a water distribution tank, a biological rotating disk denitrification reactor, a gradient oxygen-limiting biochemical tank and a sedimentation tank. When the device is used, sewage to be treated sequentially flows through the regulating tank, the water distribution tank, the biological rotating disk denitrification reactor, the gradient oxygen-limiting biochemical tank and the sedimentation tank. Wherein sewage at the end part of the gradient oxygen-limited biochemical pool respectively flows back to the front end of the water distribution tank and the front end of the gradient oxygen-limited biochemical pool, and partial sludge in the sedimentation pool flows back to the same position to form a double-backflow system. The invention has the advantages of no need of a large amount of aeration and an external carbon source, low operation cost, high treatment efficiency and stable effluent quality, and can meet the requirements of the first-class A standard in the discharge Standard of pollutants for municipal wastewater treatment plants (GB18918-2002) and the discharge Standard of clean water in Zhejiang province on ammonia nitrogen and total nitrogen.

Description

Low-carbon-nitrogen-ratio sewage denitrification device and method based on double reflux and gradient oxygen limitation
Technical Field
The invention belongs to the technical field of biological sewage treatment and denitrification in environmental engineering, and particularly relates to a method for denitrifying sewage with a low carbon-nitrogen ratio by autotrophic nitrification-denitrification based on double reflux and gradient oxygen limitation.
Background
In recent decades, the economic society of China is rapidly developed, the scale of cities and towns is gradually enlarged, and the discharge amount of sewage in cities and towns is rapidly increased, but the existing urban sewage treatment plants have the problems of defects and the like due to technical processes, so that a large amount of nitrogen elements and the like enter water bodies, and the water environment pollution condition is gradually worsened. Meanwhile, the national requirements on the quality of the effluent water of a sewage treatment plant are more and more strict, and the first-level A standard in the pollutant discharge standard of the urban sewage treatment plant (GB18918-2002) requires that the ammonia nitrogen concentration of the effluent water is less than 5mg/L and the total nitrogen concentration is less than 15 mg/L. The clean water discharge is provided in Zhejiang province, the ammonia nitrogen concentration is less than 3mg/L, the total nitrogen concentration is less than 12mg/L, and the standard discharge of surface water IV is achieved. At present, the discharge standards of a plurality of urban sewage treatment plants are still in the first-level B standard, some discharge standards even cannot reach the standard stably, and the effluent quality needs to be improved urgently.
At present, the sewage denitrification method can be divided into a physical and chemical method and a biological method, wherein the biological method is widely applied to the treatment of urban sewage due to high efficiency and economic feasibility. The traditional biological sewage denitrification technology comprises three processes of ammoniation, nitrification and denitrification: ammoniation means that organic nitrogen in a water body is converted into ammoniacal nitrogen under the action of ammoniation bacteria; the nitrification is that ammonia nitrogen is oxidized into nitrite nitrogen by ammonia oxidizing bacteria, and then the nitrite oxidizing bacteria oxidize the nitrite nitrogen into nitrate nitrogen; denitrification, namely nitrite nitrogen and nitrate nitrogen are reduced into nitrogen by denitrifying bacteria under the condition of oxygen deficiency or anaerobism. A large amount of aeration is needed in the nitration process, and the power consumption is large; the denitrification process requires a large amount of carbon source. The carbon-nitrogen ratio of urban sewage in China is generally lower than 4, the carbon source is insufficient, and in actual operation, the carbon source needs to be added, so that the operation cost is increased. Meanwhile, the additional carbon source can also increase the amount of excess sludge, and increase the cost and difficulty of sludge treatment in sewage treatment plants.
Therefore, on the basis of the existing basic research, the method breaks through the traditional process, and has important significance in developing the urban sewage biological denitrification treatment technology with high treatment efficiency and low operating cost, wherein the effluent quality meets the first class A of discharge standards of pollutants for urban sewage treatment plants (GB 18918-2002).
Disclosure of Invention
The invention aims to overcome the defects that a large amount of aeration is needed in the nitrification process and an additional carbon source is needed in the denitrification process when the low carbon-nitrogen ratio sewage is treated by the existing sewage biological denitrification technology, and provides a method for efficiently denitrifying the low carbon-nitrogen ratio sewage through autotrophic nitrification and denitrification based on double reflux and gradient oxygen limitation. The method provided by the invention does not need a large amount of aeration and an external carbon source, has low operation cost, high treatment efficiency and stable effluent quality, the concentration of ammonia nitrogen is less than 3mg/L, and the concentration of total nitrogen is less than 12mg/L, and can meet the requirements of the first-class A standard in the discharge standard of pollutants for municipal wastewater treatment plants (GB18918-2002) and the discharge standard of clean water in Zhejiang province on ammonia nitrogen and total nitrogen.
In order to achieve the purpose of the invention, the invention adopts the following technical scheme:
a low carbon nitrogen ratio sewage denitrification device based on double reflux and gradient oxygen limitation comprises a regulating tank, a water distribution tank, a biological rotating disc denitrification reactor, a gradient oxygen limitation biochemical tank and a sedimentation tank; the regulating tank, the water distribution tank, the biological rotating disc denitrification reactor, the gradient oxygen-limiting biochemical tank and the sedimentation tank are sequentially communicated end to end through water distribution pipelines; the front end of the regulating tank is provided with a water inlet; the gradient oxygen-limiting biochemical pool comprises four reaction chambers which are sequentially communicated in series, and inlet water sequentially passes through a first reaction chamber, a second reaction chamber, a third reaction chamber and a fourth reaction chamber in a plug flow manner; each reaction chamber is internally provided with a stirring device and an aeration device; the tail end of the gradient oxygen-limiting biochemical pool is provided with a sewage backflow pipeline which is used for respectively returning partial sewage to the front end of the water distribution tank and the front end of the first reaction chamber of the gradient oxygen-limiting biochemical pool; the tail end of the sedimentation tank is provided with a water outlet and a sludge discharge port; the sludge discharge port is arranged at the bottom of the sedimentation tank and is externally connected with a sludge pipeline for discharging the sludge in the sedimentation tank, and simultaneously part of the sludge respectively flows back to the front end of the water distribution tank and the front end of the first reaction chamber of the gradient oxygen-limiting biochemical tank.
Preferably, the disks of the biological rotating disk denitrification reactor are of a three-dimensional net structure, and the porosity is more than 95-98%.
Preferably, the upper part of the rotating disc of the biological rotating disc denitrification reactor is provided with a semicircular glass fiber reinforced plastic cover.
Preferably, the rotating speed of the biological rotating disc denitrification reactor can be adjusted.
The invention also aims to provide a method for denitrifying sewage with low carbon-nitrogen ratio based on double reflux and gradient oxygen limitation by using the treatment device in any one of the schemes, which comprises the following steps:
1) after passing through the regulating tank, sewage is mixed with return liquid at the tail end of the gradient oxygen-limited biochemical tank and return sludge of the sedimentation tank and enters a water distribution tank, the sulfur pollutants are decomposed by microorganisms to eliminate peculiar smell, and meanwhile, the reflux ratio is regulated according to the quality of inlet water to ensure that the concentration of ammonia nitrogen entering the biological rotating disc denitrification reactor is lower than 200 mg/L;
2) introducing the effluent of the water distribution tank into a biological rotating disk denitrification reactor, controlling the concentration of dissolved oxygen in the biological rotating disk denitrification reactor to be 0.8-1.0 mg/L by adjusting the rotating speed of a rotating disk, realizing partial removal of nitrogen and carbon pollutants by utilizing the biological phase on a biological membrane, and optimizing the water quality entering a gradient oxygen-limiting biochemical pool;
3) introducing the effluent of the biological rotating disk denitrification reactor into a gradient oxygen-limiting biochemical tank, and controlling aeration devices to sequentially decrease aeration amounts in four reaction chambers along the water flow direction so as to realize deep removal of sewage nitrogen; sewage at the end part of the fourth reaction chamber of the gradient oxygen-limiting biochemical pool flows back to the water distribution tank and the front end of the first reaction chamber of the gradient oxygen-limiting biochemical pool;
4) introducing the effluent of the gradient oxygen-limiting biochemical tank into a sedimentation tank, discharging clear liquid after mud-water separation, refluxing partial sludge to the front ends of the water distribution tank and the gradient oxygen-limiting biochemical tank respectively, and discharging the rest sludge.
Preferably, the concentration of sludge in the rotating biological disk denitrification reactor in the step 2) is 10000-30000 mg/L when the rotating biological disk denitrification reactor works.
Preferably, the molar ratio of the effluent nitrite nitrogen to the ammonia nitrogen in the water distribution tank is 1.0-1.4: 1.
preferably, the dissolved oxygen concentrations of the first reaction chamber, the second reaction chamber, the third reaction chamber and the fourth reaction chamber in the gradient oxygen-limited biochemical pool are respectively controlled to be below 0.8-0.6 mg/L, 0.6-0.4 mg/L, 0.4-0.2 mg/L and 0.2 mg/L.
Preferably, the sewage in the step 1) is town sewage.
Compared with the prior art, the beneficial results of the invention are as follows:
1. the invention has high denitrification efficiency, the final effluent ammonia nitrogen concentration is less than 3mg/L, the total nitrogen concentration is less than 12mg/L, and the invention meets the requirements of the first-class A standard in the discharge Standard of pollutants for municipal wastewater treatment plant (GB18918-2002) and the discharge Standard of clean water in Zhejiang province on ammonia nitrogen and total nitrogen.
2. The invention has low energy consumption, the dissolved oxygen concentration is lower than 1.0mg/L in the whole treatment process, the aeration quantity is small, and a large amount of energy consumption is saved.
3. The method does not need an additional carbon source, saves the cost of the additional carbon source, and reduces the production amount of excess sludge, compared with an activated sludge method, the production amount of the excess sludge is reduced by 40-50%.
Drawings
FIG. 1 is a process flow diagram of the present invention;
the reference numbers are as follows: a regulating tank 1, a water distribution tank 2, a biological rotating disk denitrification reactor 3, a gradient oxygen-limiting biochemical tank 4 and a sedimentation tank 5.
Detailed Description
The invention is explained in further detail below with reference to the figures. These examples are intended to illustrate the invention and are not intended to limit the scope of the invention.
As shown in fig. 1, the low carbon-nitrogen ratio sewage denitrification apparatus based on double reflux and gradient oxygen limitation in the present embodiment includes a regulating tank 1, a water distribution tank 2, a rotating biological disk denitrification reactor 3, a gradient oxygen limitation biochemical tank 4 and a sedimentation tank 5; the adjusting tank 1, the water distribution tank 2, the biological rotating disk denitrification reactor 3, the gradient oxygen-limiting biochemical tank 4 and the sedimentation tank 5 are sequentially communicated end to end through water distribution pipelines. The front end of the regulating tank 1 is provided with a water inlet from which sewage to be treated is input. The gradient oxygen-limiting biochemical pool 4 comprises four reaction chambers which are sequentially communicated in series, and the inlet water sequentially passes through the first reaction chamber, the second reaction chamber, the third reaction chamber and the fourth reaction chamber in a plug flow mode. Each reaction chamber is provided with an independent stirring device and an independent aeration device, and the dissolved oxygen concentration of the four reaction chambers can be respectively controlled by adjusting the aeration rate and the stirring speed. The tail end of the gradient oxygen-limiting biochemical pool 4 is provided with a sewage backflow pipeline which is used for respectively returning partial sewage to the front end of the water distribution tank 2 and the front end of the first reaction chamber of the gradient oxygen-limiting biochemical pool 4. The tail end of the sedimentation tank 5 is provided with a water outlet and a sludge discharge port, and the water outlet is externally connected with a water outlet pipe; the sludge discharge port is arranged at the bottom of the sedimentation tank 5 and is externally connected with a sludge pipeline for discharging the sludge in the sedimentation tank 5 and respectively returning partial sludge to the front end of the water distribution tank 2 and the front end of the first reaction chamber of the gradient oxygen-limiting biochemical tank 4.
In the embodiment, the disks of the biological rotating disk denitrification reactor 3 are in a three-dimensional net structure, the three-dimensional net structure of the disks is formed by processing net fibers, and the porosity is more than 95-98%. The upper part of the rotating disk of the biological rotating disk denitrification reactor 3 is provided with a semicircular glass fiber reinforced plastic cover. The rotating speed of the biological rotating disk denitrification reactor 3 can be adjusted, and the system can be in different dissolved oxygen states by adjusting the rotating speed of the rotating disk.
The low-carbon-nitrogen-ratio sewage denitrification method based on the treatment device and based on double reflux and gradient oxygen limitation comprises the following steps:
1) after passing through the regulating tank 1, sewage is mixed with return liquid at the tail end of the gradient oxygen-limited biochemical tank 4 and return sludge of the sedimentation tank 5 and enters the water distribution tank 2, sulfur pollutants are decomposed by microorganisms to eliminate peculiar smell, and meanwhile, the reflux ratio is regulated according to the quality of inlet water to ensure that the concentration of ammonia nitrogen entering the biological rotating disc denitrification reactor 3 is lower than 200 mg/L;
2) introducing the effluent of the water distribution tank 2 into a biological rotating disk denitrification reactor 3, controlling the dissolved oxygen concentration in the biological rotating disk denitrification reactor 3 to be 0.8-1.0 mg/L by adjusting the rotating speed of a rotating disk, realizing partial removal of nitrogen and carbon pollutants by utilizing the biological phase on a biological membrane, and optimizing the water quality entering a gradient oxygen-limiting biochemical pool 4;
3) leading the effluent of the biological rotating disk denitrification reactor 3 into a gradient oxygen-limiting biochemical tank 4, and controlling aeration devices to sequentially decrease aeration quantities in four reaction chambers along the water flow direction so as to realize deep removal of sewage nitrogen; sewage at the end part of the fourth reaction chamber of the gradient oxygen-limiting biochemical pool 4 flows back to the water distribution tank 2 and the front end of the first reaction chamber of the gradient oxygen-limiting biochemical pool 4;
4) the effluent of the gradient oxygen-limiting biochemical tank 4 is introduced into a sedimentation tank 5, the clear liquid is discharged after mud-water separation, part of sludge respectively flows back to the water distribution tank 2 and the front end of the gradient oxygen-limiting biochemical tank 4, and the rest of sludge is discharged.
According to the invention, reflux liquid at the tail end of a gradient oxygen-limited biochemical pool, reflux sludge of a sedimentation pool and outlet water of an adjusting pool are mixed, the concentration of dissolved oxygen of mixed liquid is 1.0mg/L by adjusting reflux ratio, the mixed liquid enters a water distribution tank and stays for 2-4 h, and microorganisms such as bacillus subtilis and the like in the reflux sludge are used for decomposing sulfide in the mixed liquid, so that peculiar smell is eliminated, and meanwhile, the concentration of the dissolved oxygen of the mixed liquid is reduced to 0.8-1.0 mg/L.
Introducing the water discharged from the water distribution tank into a biological rotating disc denitrification reactor, and maintaining the concentration of dissolved oxygen in the reactor at 0.8-1.0 mg/L for 8-12 h by adjusting the rotating speed of the rotating disc. When the biological rotating disk denitrification reactor 3 works, the sludge concentration is 10000-30000 mg/L. Water in contact with the surface of the disc mainly undergoes a nitrosation reaction under the action of microorganisms such as bacillus subtilis and the like to convert ammonia nitrogen into nitrite nitrogen, and because the dissolved oxygen concentration in the disc is lower, denitrification reaction mainly occurs to reduce nitrate nitrogen and nitrite nitrogen into nitrogen to be removed, and COD of sewage is reduced. Controlling the ratio of nitrite nitrogen to ammonia nitrogen in the effluent of the water distribution tank to be 1.0-1.4: 1, a good water inlet condition is created for subsequent deep denitrification.
The effluent of the biological rotating disk denitrification reactor is introduced into a gradient oxygen-limiting biochemical pool and passes through the reaction chambers one by one in a plug flow mode. The dissolved oxygen concentration of each reaction chamber in the gradient oxygen-limiting biochemical pool is different and gradually reduced along the advancing direction of the plug flow. The dissolved oxygen concentration of the four reaction chambers, namely the first reaction chamber, the second reaction chamber, the third reaction chamber and the fourth reaction chamber is controlled to be below 0.8-0.6 mg/L, 0.6-0.4 mg/L, 0.4-0.2 mg/L and 0.2mg/L respectively by adjusting the aeration rate and the stirring speed. The reaction chamber with high dissolved oxygen at the front end of the biochemical pool mainly performs nitrosation reaction, and the reaction chamber with low dissolved oxygen at the back mainly performs anaerobic ammonia oxidation and other reactions. And the sewage at the tail end of the gradient oxygen-limited biochemical tank partially flows back to the front end of the reaction tank for retreatment, and the sludge in the sedimentation tank partially flows back to the front end of the biochemical tank for supplementing the loss of microorganisms.
Sewage at the end part of the gradient oxygen-limited biochemical pool respectively flows back to the front end of the water distribution tank and the front end of the gradient oxygen-limited biochemical pool, and partial sludge of the sedimentation pool flows back to the same position to form a double-backflow system, so that the water quality of inlet and outlet water of each reactor is adjusted, the microbial concentration of each reactor is ensured, and the deep removal of nitrogen elements in the sewage is facilitated.
The effects of the present invention will be described below with reference to specific examples based on the above-described apparatus and process. In the following embodiment, raw water of a certain urban sewage treatment plant is used as sewage for treatment, wherein COD of the sewage is 180-250 mg/L, ammonia nitrogen concentration is 20-50 mg/L, and total nitrogen is 21-60 mg/L.
Example 1
Raw water of the municipal sewage treatment plant is taken as treated sewage and enters a regulating tank, the effluent of the regulating tank is mixed with terminal reflux of a gradient oxygen-limited biochemical tank and reflux sludge of a sedimentation tank, mixed liquid with dissolved oxygen concentration of 1.0mg/L is formed and enters a water distribution tank, the mixed liquid stays for 2 hours, and microorganisms are utilized to decompose sulfur pollutants to eliminate peculiar smell; introducing the water outlet of the water distribution tank into a biological rotating disk denitrification reactor, controlling the concentration of dissolved oxygen in the biological rotating disk denitrification reactor to be 1.0mg/L by adjusting the rotating speed of a rotating disk, and keeping the treatment residence time to be 10 h; considering the water quality fluctuation of the biological rotating disk denitrification reactor, setting the reflux ratio (adjusting tank effluent: the sewage which flows back to the water distribution tank from the tail end of the gradient oxygen-limiting biochemical tank) to be 3, wherein about 50 percent of ammonia nitrogen and total nitrogen can be removed; introducing the effluent of a biological rotating disk denitrification reactor into a gradient oxygen-limiting biochemical pool, controlling the dissolved oxygen concentration of reaction chambers by adjusting the aeration rate and the stirring speed in each reaction chamber, controlling the first reaction chamber to be 0.8mg/L, controlling the second reaction chamber to be 0.6mg/L, controlling the third reaction chamber to be 0.4mg/L, controlling the fourth reaction chamber to be 0.2mg/L, controlling the retention time of each reaction chamber to be 2h, controlling the total retention time to be 8h, refluxing the sewage at the tail end of the fourth reaction chamber of the gradient oxygen-limiting biochemical pool to the front end of the first reaction chamber of the gradient oxygen-limiting biochemical pool, and setting the reflux ratio (effluent of the biological rotating disk denitrification reactor: refluxing the sewage at the tail end of the gradient oxygen-limiting biochemical pool to the front end of the biochemical pool) to be 3; and introducing the effluent of the biological rotating disk denitrification reactor into a sedimentation tank, standing for 1h for removal, returning part of sludge to the front ends of a water distribution system and the gradient oxygen-limiting biochemical tank, and discharging the rest sludge. The COD concentration of the final effluent is 40.5mg/L, the ammonia nitrogen concentration is 2.8mg/L, and the total nitrogen concentration is 11.7 mg/L.
Comparative example 1
In this comparative example, the gradient oxygen limited biochemical tank mixed liquor and settling tank sludge flowback was closed, and the remaining conditions were the same as in example 1. The COD concentration of the final effluent is 73.4mg/L, the ammonia nitrogen concentration is 8.5mg/L, and the total nitrogen concentration is 22.7 mg/L.
Compared with the embodiment 1, the comparative embodiment closes the double-reflux system, on one hand, no sludge is refluxed to the front ends of the water tank and the oxygen-limited reaction tank, so that the concentration of microorganisms in the water tank is reduced, and the treatment efficiency is reduced, on the other hand, the tail end water of the gradient oxygen-limited biochemical tank is directly fed into the sedimentation tank without being refluxed, so that the COD (chemical oxygen demand), ammonia nitrogen and total nitrogen concentration of the effluent are increased, the quality of the effluent is deteriorated, and the first-class A standard requirements in pollutant discharge Standard of municipal wastewater treatment plant (GB18918-2002) cannot be met.
Comparative example 2
In this comparative example, the dissolved oxygen in all four reaction chambers of the gradient oxygen-limited biochemical pond was controlled to 0.5mg/L, and the remaining conditions were the same as in example 1. The COD concentration of the final effluent is 45.8mg/L, the ammonia nitrogen concentration is 9.4mg/L, and the total nitrogen concentration is 25.3 mg/L.
Compared with the example 1, the contrast implementation has the advantages that the gradient oxygen limitation is not implemented in the gradient oxygen limitation biochemical pool, the concentration of dissolved oxygen in each reaction chamber is controlled to be 0.5mg/L, the sewage cannot be nitrosated in the gradient oxygen limitation biochemical pool under different dissolved oxygen conditions, so that the nitrosation efficiency is reduced, the quality of the anaerobic ammonia oxidation and denitrification inlet water cannot be optimized, the COD, ammonia nitrogen and total nitrogen concentration of the outlet water are increased, the quality of the outlet water is poor, and the requirement of the first-level A standard in pollutant discharge standard of municipal wastewater treatment plant (GB18918-2002) cannot be met.
Example 2
In this example, the dissolved oxygen in the biological rotating disk denitrification reactor is controlled to be 0.9mg/L, the dissolved oxygen in the first reaction chamber in the gradient oxygen-limited biochemical pool is controlled to be 0.7mg/L, the dissolved oxygen in the second reaction chamber is controlled to be 0.5mg/L, the dissolved oxygen in the third reaction chamber is controlled to be 0.3mg/L, the dissolved oxygen in the fourth reaction chamber is controlled to be 0.1mg/L, and the rest conditions are the same as those in the example. The COD concentration of the final effluent is 42.8mg/L, the ammonia nitrogen concentration is 2.5mg/L, and the total nitrogen concentration is 11.4 mg/L.
Example 3
In this example, the dissolved oxygen in the biological rotating disk denitrification reactor is controlled to be 0.8mg/L, the dissolved oxygen in the first reaction chamber in the gradient oxygen-limited biochemical pool is controlled to be 0.6mg/L, the dissolved oxygen in the second reaction chamber is controlled to be 0.4mg/L, the dissolved oxygen in the third reaction chamber is controlled to be 0.2mg/L, the dissolved oxygen in the fourth reaction chamber is controlled to be 0.05mg/L, and the rest conditions are the same as those in the example. The COD concentration of the final effluent is 45.3mg/L, the ammonia nitrogen concentration is 2.8mg/L, and the total nitrogen concentration is 11.8 mg/L.
The above-described embodiments are merely preferred embodiments of the present invention, which should not be construed as limiting the invention. Various changes and modifications may be made by one of ordinary skill in the pertinent art without departing from the spirit and scope of the present invention. Therefore, the technical scheme obtained by adopting the mode of equivalent replacement or equivalent transformation is within the protection scope of the invention.

Claims (8)

1. A low carbon nitrogen ratio sewage denitrification device based on double reflux and gradient oxygen limitation is characterized by comprising an adjusting tank (1), a water distribution tank (2), a biological rotating disc denitrification reactor (3), a gradient oxygen limitation biochemical tank (4) and a sedimentation tank (5); the adjusting tank (1), the water distribution tank (2), the biological rotating disc denitrification reactor (3), the gradient oxygen-limiting biochemical tank (4) and the sedimentation tank (5) are sequentially communicated end to end through water distribution pipelines; the front end of the regulating tank (1) is provided with a water inlet; the gradient oxygen-limiting biochemical pool (4) comprises four reaction chambers which are sequentially communicated in series, and inlet water sequentially passes through the first reaction chamber, the second reaction chamber, the third reaction chamber and the fourth reaction chamber in a plug flow manner; each reaction chamber is internally provided with a stirring device and an aeration device; a sewage backflow pipeline is arranged at the tail end of the gradient oxygen-limiting biochemical pool (4) and is used for respectively returning partial sewage to the front end of the water distribution tank (2) and the front end of the first reaction chamber of the gradient oxygen-limiting biochemical pool (4); the tail end of the sedimentation tank (5) is provided with a water outlet and a sludge discharge port; the sludge discharge port is arranged at the bottom of the sedimentation tank (5), is externally connected with a sludge pipeline and is used for discharging sludge in the sedimentation tank (5) and simultaneously respectively reflowing partial sludge to the front end of the water distribution tank (2) and the front end of the first reaction chamber of the gradient oxygen-limiting biochemical tank (4); the aeration quantities of the first reaction chamber, the second reaction chamber, the third reaction chamber and the fourth reaction chamber in the gradient oxygen-limiting biochemical tank (4) are sequentially reduced along with the water flow direction, and the dissolved oxygen concentrations of the first reaction chamber, the second reaction chamber, the third reaction chamber and the fourth reaction chamber are respectively controlled to be below 0.8-0.6 mg/L, 0.6-0.4 mg/L, 0.4-0.2 mg/L and 0.2 mg/L.
2. The sewage denitrification device with low carbon-nitrogen ratio based on double reflux and gradient oxygen limitation as claimed in claim 1, wherein: the disc of the biological rotating disc denitrification reactor (3) is of a three-dimensional net structure, and the porosity is more than 95-98%.
3. The sewage denitrification device with low carbon-nitrogen ratio based on double reflux and gradient oxygen limitation as claimed in claim 1, wherein: the upper part of the rotating disc of the biological rotating disc denitrification reactor (3) is provided with a semicircular glass reinforced plastic cover.
4. The sewage denitrification device with low carbon-nitrogen ratio based on double reflux and gradient oxygen limitation as claimed in claim 1, wherein: the rotating speed of the biological rotating disk denitrification reactor (3) can be adjusted.
5. The method for denitrifying the sewage with the low carbon-nitrogen ratio based on double reflux and gradient oxygen limitation by using the device as claimed in any one of claims 1 to 4 is characterized by comprising the following steps:
1) after passing through the regulating tank (1), sewage is mixed with return liquid at the tail end of the gradient oxygen-limited biochemical tank (4) and return sludge of the sedimentation tank (5) and enters the water distribution tank (2), the sulfur pollutants are decomposed by microorganisms to eliminate peculiar smell, and meanwhile, the reflux ratio is regulated according to the quality of inlet water to ensure that the concentration of ammonia nitrogen entering the biological rotating disc denitrification reactor (3) is lower than 200 mg/L;
2) introducing the effluent of the water distribution tank (2) into a biological rotating disk denitrification reactor (3), controlling the concentration of dissolved oxygen in the biological rotating disk denitrification reactor (3) to be 0.8-1.0 mg/L by adjusting the rotating speed of a rotating disk, realizing partial removal of nitrogen and carbon pollutants by utilizing the biological phase on a biological membrane, and optimizing the water quality entering a gradient oxygen-limiting biochemical pool (4);
3) the effluent of the biological rotating disk denitrification reactor (3) is introduced into a gradient oxygen-limiting biochemical tank (4), and aeration quantities in four reaction chambers are sequentially reduced along the water flow direction by controlling an aeration device, so that the deep removal of sewage nitrogen is realized; sewage at the end part of a fourth reaction chamber of the gradient oxygen-limiting biochemical pool (4) flows back to the water distribution tank (2) and the front end of the first reaction chamber of the gradient oxygen-limiting biochemical pool (4);
4) the effluent of the gradient oxygen-limiting biochemical tank (4) is introduced into a sedimentation tank (5), the clear liquid is discharged after mud-water separation, part of sludge respectively flows back to the water distribution tank (2) and the front end of the gradient oxygen-limiting biochemical tank (4), and the rest of sludge is discharged.
6. The method for denitrifying sewage with low carbon-nitrogen ratio based on dual reflux and gradient oxygen limitation as claimed in claim 5, wherein: the sludge concentration of the biological rotating disk denitrification reactor (3) in the step 2) is 10000-30000 mg/L when the reactor works.
7. The method for denitrifying sewage with low carbon-nitrogen ratio based on dual reflux and gradient oxygen limitation as claimed in claim 5, wherein: the molar ratio of effluent nitrite nitrogen to ammonia nitrogen in the water distribution tank (2) is 1.0-1.4: 1.
8. the method for denitrifying sewage with low carbon-nitrogen ratio based on dual reflux and gradient oxygen limitation as claimed in claim 5, wherein: the dissolved oxygen concentrations of the first reaction chamber, the second reaction chamber, the third reaction chamber and the fourth reaction chamber in the gradient oxygen-limiting biochemical pool (4) are respectively controlled to be below 0.8-0.6 mg/L, 0.6-0.4 mg/L, 0.4-0.2 mg/L and 0.2 mg/L.
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