Limit denitrification system and method for sewage with low carbon nitrogen ratio

Abstract

The invention discloses an ultimate denitrification system and method for sewage with a low carbon-nitrogen ratio, the system is composed of two sections of denitrification parts, the first section of denitrification part adopts a traditional biological denitrification module, and the second section of denitrification part adopts a gradient dissolved oxygen reactor; an anaerobic zone, an anoxic zone and a mud-water separation zone are arranged in the gradient dissolved oxygen reactor from bottom to top, the anaerobic zone is used for sludge disintegration and anaerobic fermentation to release organic matters, the anoxic zone is used for denitrification nitrogen removal by using the anaerobic released organic matters, and the mud-water separation zone is used for separating the denitrified sludge from water. Through two-stage denitrification, for raw domestic sewage with the carbon-nitrogen ratio COD / TN being smaller than 4.5, the total nitrogen concentration of inlet water is 30-40 mg / L, the total nitrogen concentration of outlet water without external carbon sources can be as low as 2 mg / L, the optimal state can reach 1 mg / L, limited efficient denitrification of sewage with the low carbon-nitrogen ratio is achieved, and meanwhile the process has the advantages of being safe and reliable in operation, easy to maintain and low in cost.

Description

Technical Field

[0001] The present invention relates to the technical field of sewage treatment, and in particular to a system and method for extreme denitrification of low carbon-nitrogen ratio sewage. Background Art

[0002] In the process of urban sewage treatment, the use of biological methods is very effective, and the treatment cost is low and widely used. In the process of biological treatment of sewage, microorganisms continue to proliferate and produce a large amount of sludge mainly composed of organic matter. This sludge is difficult to be absorbed in the sewage treatment process and eventually becomes residual sludge. The high cost of treating and disposing of residual sludge has always troubled the sewage treatment industry. Traditional residual sludge treatment is mostly based on concentration-dehydration-drying as the basic treatment procedure, and finally landfilled or incinerated or used as building materials. The treatment and disposal process consumes huge amounts of chemicals and energy, and the treatment cost is high. A sewage treatment plant with a daily sewage treatment capacity of 100,000 tons often produces tens of tons of wet residual sludge (typical moisture content is about 80%) every day. The sludge treatment process not only consumes huge energy and material consumption, but the final sludge generally needs to be transported for disposal, and the treatment and disposal cost often reaches hundreds of yuan per ton of dry mud. The high treatment and disposal costs bring huge economic burdens to enterprises.

[0003] At the same time, the sewage treatment process based on the traditional denitrification method requires a large amount of carbon source to maintain the effect of denitrification and phosphorus removal. However, many municipal sewage treatment plants are short of carbon sources (especially those in the south), and they need to purchase carbon sources to make up for the problem of insufficient carbon sources in the process. Therefore, if the organic matter in the sludge can be converted into an available carbon source, it can not only greatly reduce the amount of residual sludge and achieve sludge reduction, but also solve the problem of insufficient carbon source for sewage treatment. However, the organic matter in the sludge is difficult to be utilized by the denitrification process. Therefore, how to convert the organic matter in the sludge into a carbon source that can be used for biological denitrification is of great significance. In order to solve this problem, many researchers have set their sights on sludge cracking to release part of the organic matter in the sludge, such as acid-base cracking, ultrasound, mechanical crushing, hot-alkali combined treatment, anaerobic microbial methods, etc. However, these methods have problems such as low utilization rate of sludge organic matter release rate and high treatment cost, and it is difficult to enter the market. Summary of the invention

[0004] In order to simultaneously solve the above-mentioned problems of difficulty in sludge treatment and disposal and insufficient carbon source for sewage, the present invention provides a low carbon-nitrogen ratio sewage extreme denitrification system and method, which utilizes a specific gradient dissolved oxygen reactor to replace the traditional secondary sedimentation tank, so that the organic matter in the sludge can be fully utilized by the denitrification process to complete the extreme denitrification, and realize the effective separation of high-concentration sludge and water, as well as the smooth discharge of sludge residue.

[0005] The present invention achieves the above-mentioned purpose through the following technical solutions:

[0006] A low carbon-nitrogen ratio sewage extreme denitrification system, the system consists of two denitrification parts, wherein the first denitrification part adopts a traditional biological denitrification module, and the second denitrification part adopts a gradient dissolved oxygen reactor;

[0007] The gradient dissolved oxygen reactor comprises an anaerobic zone, an anoxic zone and a mud-water separation zone from top to bottom. The dissolved oxygen concentrations at different positions in the reactor are different and distributed in a gradient. The anaerobic zone is used for sludge cracking and anaerobic fermentation to release organic matter. The anoxic zone uses the organic matter released from the anaerobic zone to perform denitrification and denitrification. The mud-water separation zone is used to separate the denitrified sludge and water.

[0008] A further improvement is that the first biological denitrification module is a conventional biological denitrification method, including but not limited to A 2 / O, oxidation ditch, SBR, AO.

[0009] A further improvement is that the anaerobic zone, the anoxic zone and the mud-water separation zone in the gradient dissolved oxygen reactor are an integrated structure without any physical separation measures.

[0010] A further improvement is that the gradient dissolved oxygen reactor adopts one of a membrane bioreactor, a sieve filter reactor, and a suspended sludge layer reactor.

[0011] A further improvement is that the horizontal cross-section of the gradient dissolved oxygen reactor is circular or regular polygonal, a stirring mechanism is provided inside the gradient dissolved oxygen reactor for stirring to form a horizontal annular flow field, the slag discharge port of the gradient dissolved oxygen reactor is located at the center of the bottom, and the bottom surface of the reactor adopts a smooth surface with a slope of 2-7%.

[0012] A further improvement is that the membrane bioreactor is a type of gradient dissolved oxygen reactor drainage method, the main structure is a gradient dissolved oxygen reactor, the mud and water separation method of the effluent is ultrafiltration or microfiltration membrane, so a microfiltration membrane or an ultrafiltration membrane is used to intercept suspended sludge particles, and the mud and water separation areas in the sieve filter reactor and the suspended sludge layer reactor are isolated by a filter, and the mesh number of the filter of the sieve filter reactor is 280-350 mesh, and the mesh number of the filter of the suspended sludge layer reactor is 30-50 mesh.

[0013] The present invention also provides a method for extreme denitrification of low carbon-nitrogen ratio sewage, which comprises the following two steps:

[0014] Step 1: introduce the sewage into the biological denitrification module for primary treatment;

[0015] Step 2: The sewage and sludge after the primary treatment are introduced into a gradient dissolved oxygen reactor with an anaerobic zone, an anoxic zone and a mud-water separation zone for secondary treatment, wherein the sewage is introduced from the middle of the reactor and directly enters the anoxic zone; under the action of gravity, part of the sludge in the anoxic zone can fall into the anaerobic zone, and the sludge is broken and anaerobically fermented in the anaerobic zone to release organic matter; the organic matter released by anaerobes enters the anoxic zone through diffusion, and the sludge uses the organic matter for denitrification and denitrification in the anoxic zone, and finally the denitrified sludge and water are separated in the mud-water separation zone.

[0016] A further improvement is that the inlet carbon-nitrogen ratio COD / TN of the sewage is less than 4.5, the outlet total nitrogen concentration is ≤2 mg / L, and the sludge concentration MLSS of the sewage when entering the sludge-water separation zone reaches a maximum of 30,000-50,000 mg / L.

[0017] A further improvement is that the mud-water separation zone separates the denitrified sludge and water by using one of membrane filtration, screen filtration or suspended sludge layer interception separation methods.

[0018] A further improvement is that, during the secondary treatment of the sewage, stirring treatment is applied in the gradient dissolved oxygen reactor so that a horizontal annular secondary flow field is formed at the bottom of the gradient dissolved oxygen reactor.

[0019] A further improvement is that ball mill beads of appropriate specifications are added to the gradient dissolved oxygen reactor to further break down the sludge cells to increase the release amount and release rate of organic matter in the anaerobic zone.

[0020] The present invention adds a special gradient dissolved oxygen reactor on the basis of the traditional denitrification process, and cancels the secondary sedimentation tank at the same time. Based on the structure and operation characteristics of the reactor, the sludge is cracked in the reactor and anaerobic fermentation is taken into account, a large amount of organic matter that can be used for denitrification is released, and the anoxic zone of the reactor is denitrified, so that the organic matter of the sludge can be utilized and denitrification can be achieved. However, the basic requirement for the reactor to achieve denitrification is that the sludge maintains a high concentration and a short hydraulic retention time (i.e., a higher SRT but a shorter HRT), which leads to the failure of the mud-water separation of the traditional method (i.e., the traditional secondary sedimentation tank cannot be used), so membrane filtration, sieve filtration or suspended sludge layer is used to achieve high-concentration sludge and water separation. At the same time, the sludge residue after anaerobic fermentation-cracking, the main component of which is inorganic slag with very fine particle size, needs to be discharged from the reactor. For this reason, the effect of the secondary flow field can effectively separate the extremely fine slag, so an annular flow field along the horizontal direction is formed at the bottom of the reactor by stirring.

[0021] The beneficial effects of the present invention are as follows: the present invention uses two-stage denitrification, and for domestic sewage raw water with a carbon-nitrogen ratio COD / TN <4.5, the total nitrogen concentration of the inlet water is 30-40 mg / L, and the total nitrogen concentration of the effluent without the addition of an external carbon source can be as low as 2 mg / L, and can reach 1 mg / L in the best state, thereby achieving extremely efficient denitrification of low carbon-nitrogen ratio sewage. At the same time, the process has the characteristics of safe and reliable operation, easy maintenance and low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a system principle diagram of the present invention. DETAILED DESCRIPTION

[0023] The present application is further described in detail below in conjunction with the accompanying drawings. It is necessary to point out here that the following specific implementation methods are only used to further illustrate the present application and cannot be understood as limiting the scope of protection of the present application. Technical personnel in this field can make some non-essential improvements and adjustments to the present application based on the above application content.

[0024] like Figure 1 As shown, a low carbon-nitrogen ratio wastewater extreme denitrification system is composed of two denitrification parts, wherein the first denitrification part adopts a traditional biological denitrification module, such as A 2 / O, oxidation ditch, SBR, AO, the second stage denitrification part adopts a gradient dissolved oxygen reactor, the return sludge is at the top of the gradient dissolved oxygen reactor, and the remaining sludge is discharged at the bottom of the gradient dissolved oxygen reactor. The gradient dissolved oxygen reactor can replace the traditional secondary sedimentation tank, and can effectively separate the biological sludge from the water under the condition of higher hydraulic load, and can also perform denitrification and denitrification, anaerobic fermentation of sludge to release organic matter under low hydraulic load conditions. It is a multifunctional reactor, which adopts an integrated structure, and an anaerobic zone, an anoxic zone and a mud-water separation zone are formed inside. The anaerobic zone is used for sludge cracking and anaerobic fermentation to release organic matter, the anoxic zone is used for denitrification and denitrification using organic matter, and the mud-water separation zone is used to separate the denitrified sludge from water.

[0025] In addition, the horizontal cross-section of the gradient dissolved oxygen reactor is circular or regular polygonal. The method for controlling the sludge concentration in the gradient dissolved oxygen reactor is to adjust the flow rate of the return sludge. A stirring mechanism is arranged in the gradient dissolved oxygen reactor for stirring to form a horizontal annular flow field. The slag discharge port of the gradient dissolved oxygen reactor is located at the center of the bottom, and the bottom surface of the reactor adopts a smooth surface with a slope of 2-7%.

[0026] In the present invention, the gradient dissolved oxygen reactor is the core, and a membrane bioreactor, a sieve filter reactor, or a suspended sludge layer reactor can be used as one of the mud-water separation methods. The details are as follows:

[0027] Membrane bioreactor: uses microfiltration membrane or ultrafiltration membrane to intercept suspended sludge particles, and the sewage penetrates the membrane to separate the sludge from the water;

[0028] Screen-filter reactor: The mud-water separation zone is isolated by a filter with a mesh size of 280 to 350. The sludge zone needs to be stirred, and the stirring direction must be perpendicular to the horizontal plane and present a circular shape in the plane direction. The water flow velocity at the filter in the mud-water separation zone should be controlled at 0.50 to 1.5 m / s. Too high a flow rate will lead to increased energy consumption and increase the risk of sludge breakage. Too low a flow rate will affect the equipment's processing capacity and increase the risk of clogging. Due to the agglomeration property, biological sludge has large particles (the particle size can generally reach 0.2-0.5mm; the particle size of broken sludge can generally reach 0.05-0.1mm or more), and the Brownian motion is weak. When maintaining a certain flow rate, the sludge movement trajectory at the edge of the agitator is to move forward along the tangent direction of the water flow, and it cannot pass through the mesh of the screen whose aperture is smaller than the sludge diameter and the penetration direction is perpendicular to the water flow direction; while water molecules are small, from a microscopic point of view, the molecular diffusion speed is fast, although there is movement along the tangent direction of the agitator, it can still easily pass through the 280-350 mesh screen, with fast penetration speed, small resistance, and large penetration flow. That is, under very low pressure difference, sewage can pass through the net, but sludge is difficult to pass through the net.

[0029] Suspended sludge layer reactor: Its mud-water separation area is isolated by a filter screen with a mesh size of 30 to 50. There is a large amount of denitrified suspended sludge within a certain range on the reactor surface. When the sludge is denitrified, a large number of microbubbles adhere to the sludge, and the viscosity of the sludge increases. The sludge can form large pieces of suspended state floating on the surface of the pool. The large flocculent sludge is porous and can intercept fine granular sludge, while allowing water to flow through the gaps between the sludge, so it has the function of mud-water separation.

[0030] In addition, the principle of the reactor is: (1) In the reactor, biological sludge is prone to hydrolysis and acidification in an anaerobic environment, releasing the organic matter in the sludge into the water in a dissolved state, and the organic matter can basically be used for denitrification. (2) In the existing low carbon-nitrogen ratio activated sludge process, the main item that exceeds the standard in the effluent is total nitrogen, and the total nitrogen mainly comes from nitrate nitrogen. The amount of residual sludge produced by the sewage treatment plant can basically meet the carbon source demand of denitrification in the sewage treatment plant (unless the carbon-nitrogen ratio of the inlet water is extremely low). (3) Under the action of the secondary flow field, fine-grained inorganic matter can gather at the center of the bottom of the reactor in the annularly stirred reactor. (4) Small spherical beads can break up the sludge under friction. (5) The ammonia released by the sludge breaking can be oxidized to nitrite in a shorter time at a lower dissolved oxygen concentration (0.5 mg / L). (6) Under medium pH conditions, the ammonia in the water is mainly NH4 + The activated sludge surface is negatively charged and has a negative charge on the surface of NH4+ There is a certain adsorption effect, so for municipal sewage, when the original ammonia concentration is only a few milligrams per liter, the effluent ammonia nitrogen concentration can be easily controlled at a very low level. After the nitrate and nitrite nitrogen are fully reduced, the concentration of total dissolved nitrogen in the water is very low.

[0031] Based on the above principle, the present invention provides a method for extreme denitrification of low carbon-nitrogen ratio sewage, which consists of the following two steps:

[0032] Step 1: introduce the sewage into the biological denitrification module for primary treatment;

[0033] Step 2: The sewage after primary treatment is introduced into a gradient dissolved oxygen reactor with an anaerobic zone, an anoxic zone and a mud-water separation zone for secondary treatment, wherein the sewage enters from the middle of the reactor, and denitrification and denitrification are carried out using organic matter in the anoxic zone. Part of the sludge enters the anaerobic zone at the bottom of the reactor under the action of gravity and diffusion, and the sludge is broken and anaerobically fermented in the anaerobic zone to release organic matter. After entering the reactor, the sludge gradually moves upward, and finally the denitrified sewage and sludge are separated in the mud-water separation zone.

[0034] The above process route is: water inlet → traditional biological denitrification process → gradient dissolved oxygen reactor → water outlet. Traditional biological denitrification can adopt currently commonly used process methods, such as oxidation ditch, SBR, A 2 / O, AO, etc. The key is to replace the secondary sedimentation tank of the traditional method with a gradient dissolved oxygen reactor, which is equivalent to mixing the effluent (containing a certain amount of nitrate nitrogen) and the residual sludge (containing a certain amount of organic matter) of the sewage plant into the reactor according to the actual discharge ratio of the sewage plant. The sludge is retained anaerobically for a long time in the reactor, so the sludge is anaerobically digested. On the one hand, the sludge cells rupture to release a large amount of cell contents. On the other hand, in the anaerobic environment, the ruptured fragments can continue to undergo hydrolysis and acidification reactions and continue to release dissolved organic matter. Since the reactor is an integrated operation, anaerobic and anoxic are not physically separated (spatial separation exists, and mass transfer is carried out through diffusion and precipitation), and the dissolved organic matter released by anaerobes can be quickly utilized by denitrifying bacteria to achieve biological denitrification. Sludge accumulates and retains in the system in large quantities, and the sludge concentration will continue to increase. The sludge concentration MLSS can be controlled to reach a maximum of 30,000 to 50,000 mg / L. The separation of high-concentration sludge and water is carried out by screening, membrane filtration and suspended sludge layer. The principle of sieving is that when the sludge containing particles moves in a circular motion, the particles have a kinetic energy greater than the potential energy of the liquid level difference on both sides of the screen, so water molecules can easily pass through the screen, while the granular sludge is difficult to penetrate the net and is not easy to stay near the net. It keeps moving in a circular motion with the movement of the water flow. Experiments have shown that the effluent quality of this method is better than that of the traditional secondary sedimentation tank. Membrane filtration uses ultrafiltration membranes or microfiltration membranes to intercept suspended sludge. The principle of the suspended sludge layer is that when the sludge undergoes anoxic denitrification, the sludge can be suspended to the water surface and the sludge adhesion performance is greatly enhanced. The sludge and bubbles form a porous substance like flocs, which can be intercepted by a screen with a smaller mesh number, and the sewage can flow out through the gap, thereby achieving mud and water separation.

[0035] At the same time, the fine sand and ultra-fine sand with higher density in the sludge are continuously circulated in the pool (the stirring cannot be very violent, the anaerobic zone is stirred with medium intensity, and the anoxic zone is stirred slowly). Under the action of the secondary flow field, they can gradually gather to the center of the pool bottom (the experiment verifies the feasibility of this method, and by controlling appropriate hydraulic conditions, the particles of the sand slag are very fine). They are discharged from the reactor with the slag, and the discharged fine slag is used as secondary residual sludge. The organic matter in the sludge is effectively utilized as a resource, and the sludge volume and quality are also significantly reduced.

[0036] In summary, it can be seen that the gradient dissolved oxygen reactor of the present invention has four main functions in the process: (1) biological denitrification, nitrate is converted into nitrogen gas; (2) sludge is anaerobic fermented and converted into organic matter; (3) granular sludge is separated from water to maintain clear effluent; (4) sludge is broken down and residue is discharged.

[0037] In fact, if this reactor uses a very high hydraulic load, it can be directly used as a secondary sedimentation tank of the traditional activated sludge process to separate mud and water (that is, only the mud and water separation function is maintained at high load). While completing the mud and water separation, if the sludge in the pool is greatly increased to a maximum of 30,000-50,000 mg / L, and the anaerobic environment at the bottom of the pool is maintained, while reducing the hydraulic load, increasing the hydraulic retention time, and controlling the appropriate dissolved oxygen conditions, the nitrate nitrogen in the effluent can also be removed, and at the same time, the remaining sludge can be effectively recycled, so it has economic and technical advantages.

[0038] Further preferably, when the carbon-nitrogen ratio of the raw water is particularly low (COD / TN<4), ball mill beads of appropriate specifications can be added to the bottom of the reactor to further promote the crushing of anaerobic microorganisms to improve the recovery rate of organic matter. In this case, the bottom of the reactor needs to be made of a particularly wear-resistant material, and the bottom needs to be flat, and it is not advisable to maintain a slope.

[0039] Further preferably, the anoxic zone of the gradient dissolved oxygen reactor needs to maintain a low DO concentration (DO about 0.3-0.5 mg / L) rather than being completely anaerobic. Under low DO concentration conditions, both denitrification and denitrification are ensured and ammonia oxidation is required.

[0040] Further preferably, the anaerobic zone of the gradient dissolved oxygen reactor needs to maintain completely anaerobic conditions (DO should be substantially zero, with the measured value of 0 mg / L being optimal). If there is oxygen in the anaerobic zone, the release rate of organic matter will be severely inhibited.

[0041] The following takes the treatment test of the water source of the Hefei sewage plant as an example to introduce the operation of three specific embodiments:

[0042] Example 1: "A 2 / O+MBR”

[0043] Process route: water inlet → A 2 / O→MBR→Effluent. The dissolved oxygen concentration in A1 pool is controlled at <0.01mg / L, the dissolved oxygen concentration in A2 pool is controlled at 0.2-0.5mg / L, the dissolved oxygen concentration in O pool is controlled at 1.8-2.5mg / L, the nitrification liquid return ratio is controlled at 250%, the sludge return ratio (return from the membrane pool) is 16%, the MLSS of the two A pools and O pool is controlled in the range of 4000-5000mg / L, and the MLSS concentration of the membrane pool is controlled in the range of 35000-45000mg / L. The dissolved oxygen concentration at the bottom of the membrane pool is 0, and the dissolved oxygen concentration in the middle and upper parts is 0.3-0.6mg / L. According to the test, the influent carbon-nitrogen ratio COD / TN of the sewage is 4.4, the influent total nitrogen concentration is 33.1mg / L, and the average effluent total dissolved nitrogen concentration after treatment is 1.7mg / L.

[0044] Example 2: "SBR + sieve filter reactor"

[0045] The process route is: inlet → SBR → filter reactor → outlet. The residence time of the SBR pool is controlled according to anaerobic 1.5h, anoxic 4h, and aerobic 10.5h; the dissolved oxygen concentration in the anaerobic, anoxic and aerobic reactions is controlled at <0.01mg / L, <0.4mg / L, 2-2.5mg / L, respectively, and the process water filling ratio is controlled at 0.25. The effluent and sludge of the SBR pool enter the filter reactor, and the HRT of the filter reactor is 4h. The MLSS concentrations of the SBR pool and the filter tank are controlled at 5000-7000mg / L and 30000-35000mg / L, respectively. The dissolved oxygen concentration at the bottom of the filter tank is 0.0mg / L, and the dissolved oxygen concentration in the middle and upper parts is 0.3-0.6mg / L. After testing, the inlet carbon-nitrogen ratio COD / TN of the sewage was 4.2, the inlet total nitrogen concentration was 31.7 mg / L, and the average total dissolved nitrogen concentration of the effluent after treatment was 1.9 mg / L.

[0046] Example 3: "Oxidation ditch + suspended sludge layer reactor"

[0047] The process route is: inlet → oxidation ditch → suspended sludge layer reactor → outlet. The oxidation ditch operates in AO mode, with anoxic and aerobic sections in the ditch, with a ratio of about 1:3, a total hydraulic retention time of 20h, DO concentration in the aerobic section of the ditch controlled at 1.0-1.8mg / L, anoxic section controlled at 0.3-0.6mg / L, and the rest is a transition section, MLSS in the oxidation ditch controlled at 4500-6000mg / L; HRT of the sludge layer reactor is 6h, sludge concentration is controlled in the range of 22000-26000mg / L, and sludge return ratio is 20%. The dissolved oxygen concentration at the bottom of the suspended sludge layer reactor is 0.0mg / L, and the dissolved oxygen concentration in the middle and upper parts is 0.3-0.5mg / L. After testing, the inlet carbon-nitrogen ratio COD / TN of the sewage was 4.0, the inlet total nitrogen concentration was 35.1 mg / L, and the average total dissolved nitrogen concentration of the effluent after treatment was 2.4 mg / L.

[0048] The above-described embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the patent scope of the present invention. All technical methods that divide the denitrification process into two stages (the first stage is the traditional denitrification method, and the second stage is deep denitrification), and the second stage denitrification is integrated with mud-water separation to form a new type of deep denitrification, regardless of the mud-water separation method, all belong to the protection scope of the present invention. It should be pointed out that for ordinary technicians in this field, without departing from the concept of the present invention, several variations and improvements can be made, which all belong to the protection scope of the present invention.

Claims

1. A low carbon-nitrogen ratio sewage extreme denitrification system, characterized in that: The system consists of two denitrification sections, wherein the first denitrification section adopts a biological denitrification module, and the second denitrification section adopts a gradient dissolved oxygen reactor; The gradient dissolved oxygen reactor comprises an anaerobic zone, an anoxic zone and a mud-water separation zone. The dissolved oxygen concentrations at different positions in the reactor are different and distributed in a gradient. The anaerobic zone is used for sludge cracking and anaerobic fermentation to release organic matter. The anoxic zone uses the organic matter released by anaerobism to perform denitrification and denitrification. The mud-water separation zone is used to separate the denitrified sludge and water.

2. A low carbon-nitrogen ratio sewage extreme denitrification system according to claim 1, characterized in that: The first biological denitrification module is A 2 / O, oxidation ditch, SBR, AO.

3. The low carbon-nitrogen ratio sewage extreme denitrification system according to claim 1 is characterized in that: The anaerobic zone, the anoxic zone and the mud-water separation zone in the gradient dissolved oxygen reactor are an integrated structure.

4. A low carbon-nitrogen ratio sewage extreme denitrification system according to claim 3, characterized in that: The gradient dissolved oxygen reactor is one of a membrane bioreactor, a sieve filter reactor and a suspended sludge layer reactor.

5. A low carbon-nitrogen ratio sewage extreme denitrification system according to claim 4, characterized in that: The horizontal cross-section of the gradient dissolved oxygen reactor is circular or regular polygonal. A stirring mechanism is arranged inside the gradient dissolved oxygen reactor for stirring the mixed liquid to form a horizontal annular flow field. The slag discharge port of the gradient dissolved oxygen reactor is located at the center of the bottom, and the bottom surface of the reactor is a smooth surface with a slope of 2-7%.

6. A low carbon-nitrogen ratio sewage extreme denitrification system according to claim 4, characterized in that: The membrane bioreactor uses a microfiltration membrane or an ultrafiltration membrane to intercept suspended sludge particles. The mud-water separation zone in the sieve filter reactor and the suspended sludge layer reactor is isolated by a filter screen. The mesh number of the filter screen in the sieve filter reactor is 280-350 meshes, and the mesh number of the filter screen in the suspended sludge layer reactor is 30-50 meshes.

7. A method for extreme denitrification of low carbon-nitrogen ratio sewage, characterized in that: The method consists of the following two steps: Step 1: introduce the sewage into the biological denitrification module for primary treatment; Step 2: The sewage and sludge after the primary treatment are introduced into a gradient dissolved oxygen reactor with an anaerobic zone, an anoxic zone and a mud-water separation zone for secondary treatment, wherein the sewage is introduced from the middle of the reactor and directly enters the anoxic zone; under the action of gravity, part of the sludge in the anoxic zone can fall into the anaerobic zone, and the sludge is broken and anaerobically fermented in the anaerobic zone to release organic matter; the organic matter released by anaerobes enters the anoxic zone through diffusion, and the sludge uses the organic matter for denitrification and denitrification in the anoxic zone, and finally the denitrified sludge and water are separated in the mud-water separation zone.

8. The method for extreme denitrification of low carbon-nitrogen ratio sewage according to claim 7, characterized in that: The inlet carbon-nitrogen ratio COD / TN of the sewage is less than 4.5, the outlet total nitrogen concentration is ≤2 mg / L, and the sludge concentration MLSS of the sewage when entering the sludge-water separation zone reaches 30,000-50,000 mg / L.

9. The method for extreme denitrification of low carbon-nitrogen ratio sewage according to claim 7, characterized in that: The mud-water separation zone separates the denitrified sludge and water by using one of membrane filtration, screen filtration or suspended sludge layer interception methods.

10. The method for extreme denitrification of low carbon-nitrogen ratio sewage according to claim 7, characterized in that: During the secondary treatment of the sewage, stirring treatment is applied in the gradient dissolved oxygen reactor to form a horizontal annular flow field in the gradient dissolved oxygen reactor.

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