Method for treating low C / N sewage by anaerobic-anoxic-anoxic sulfur autotrophic continuous flow A3 process

Abstract

The invention belongs to the technical field of biological nitrogen removal of sewage, and provides a method for treating low C / N (carbon / nitrogen) sewage by an anaerobic-anoxic-anoxic sulfur autotrophic continuous flow A3 process, which comprises the following steps: sequentially connecting an anaerobic tank, an anoxic tank, a degassing tank and an anoxic sulfur autotrophic tank. Synchronous nitrification and denitrification nitrogen removal is enhanced through the synergistic effect of autotrophic nitrification, heterotrophic denitrification, heterotrophic nitrification aerobic denitrification and sulfur autotrophic denitrification. The method has the advantages that no additional carbon source is needed, and the residual sludge generation amount is small; nitrification liquid backflow is not needed, the sludge external backflow ratio is low, the aeration rate is low, and energy consumption is effectively reduced. Compared with a single sulfur autotrophic denitrification process, the process is low in sulfate yield, low in external alkalinity and high in impact load resistance, and can be stably operated at the temperature of 10 DEG C or above. The process provides a new thought for economical and efficient treatment of low C / N wastewater, and has important practical application value and popularization prospect.

Description

Technical Field

[0001] The present invention belongs to the technical field of biological nitrogen removal from sewage, and relates to the research on an anaerobic - anoxic - anoxic sulfur autotrophic process, and particularly to the research on the application of biological nitrogen and phosphorus removal from low C / N sewage. Background Art

[0002] Nitrogen pollution not only causes water eutrophication, leads to a large number of algae blooms, and destroys the aquatic ecosystem, but may also affect human health through the food chain. Effectively controlling the nitrogen content in the effluent of sewage treatment plants is one of the key tasks in the field of sewage treatment. Sewage treatment plants often use processes such as A2O and MUCT for biological nitrogen removal. However, when the influent has a low C / N ratio, external organic carbon sources are required for biological nitrogen removal, resulting in a significant increase in operating costs. Moreover, processes such as A2O and MUCT require a large proportion of nitrified liquid reflux, the dissolved oxygen concentration in the aeration tank is higher than 2 mg / L, the operation is complex, the energy consumption is high, and the emissions of greenhouse gases such as CO2 and N2O are also the drawbacks of such processes. Therefore, developing a new type of biological nitrogen removal process with high efficiency, stability, low energy consumption and energy saving for low C / N sewage is an important research direction in the field of sewage treatment.

[0003] Simultaneous nitrification and denitrification (SND) technology is to regulate the microbial community structure under the condition of low dissolved oxygen concentration in the same environment, so that nitrifying bacteria and denitrifying bacteria act synergistically to convert ammonia nitrogen into nitrogen gas and discharge it from the system. It has the advantages of low required aeration volume, saving floor area, reducing energy consumption, reducing sludge production, etc., and mostly occurs in processes such as activated sludge method, biological contact oxidation, and biological moving bed. Further promoting the occurrence of simultaneous nitrification and denitrification in the sewage treatment system and using simultaneous nitrification and denitrification to improve the nitrogen removal performance are effective methods to reduce the energy consumption of the sewage treatment plant's treatment process and are worthy of expansion and in - depth exploration.

[0004] Sulfur autotrophic denitrification uses a low - cost reduced sulfur source as an electron donor for denitrification and nitrogen removal. When treating low C / N ratio sewage, no external carbon source is required, the sludge production is low, and compared with heterotrophic denitrification processes such as A2O and MUCT, the greenhouse gas emissions can be reduced by 95 - 100%. It is an economical green nitrogen removal process. However, at present, the research and development of sulfur autotrophic denitrification technology by domestic and foreign research institutions mostly adopts the form of filters, and there are problems such as the consumption of the electron donor leading to a decrease in the filter material particle size and the easy blockage of the filter bed. When the electron donor is insufficient, adding filter material requires the process to stop, the operation is complex, and the maintenance cost is high. In addition, sulfur autotrophic denitrification technology has low tolerance to low temperatures, and the nitrogen removal effect is easily affected by temperature. Therefore, how to enable sulfur autotrophic denitrification technology to be efficiently and conveniently applied to the nitrogen removal field is worthy of exploration.

[0005] For low C / N wastewater, the present invention constructs an anaerobic-anoxic-anoxic sulfur autotrophic continuous flow A3 process. This process removes nitrogen through autotrophic nitrification, heterotrophic denitrification, heterotrophic nitrification, aerobic denitrification, and sulfur autotrophic denitrification to enhance simultaneous nitrification and denitrification. The A2 tank requires low aeration, significantly reducing energy consumption and the amount of sulfur autotrophic electron donor required for the subsequent anoxic sulfur autotrophic tank. The A3 process primarily comprises a biochemical tank and a secondary sedimentation tank. The biochemical tank includes an anaerobic tank (A1 tank), an anoxic tank (A2 tank), and an anoxic sulfur autotrophic tank (A3 tank), all connected in sequence. When treating low-C / N wastewater, the A3 process significantly reduces energy consumption and operating costs compared to heterotrophic denitrification processes such as A2O and MUCT. Its advantages include: requiring no external carbon source, resulting in low excess sludge production; requiring no nitrification solution return, simplifying operation; a low sludge return ratio, which can be reduced to 80%; low aeration requirements; only the anoxic tank A2 requires a controlled dissolved oxygen concentration of 0.2-0.5 mg / L; neither the A1 nor A3 tanks require aeration, resulting in low energy consumption; and the sulfur autotrophic denitrification process produces no CO2, with N2O emissions approximately one-fifth that of heterotrophic denitrification, resulting in low greenhouse gas emissions. Compared to single sulfur autotrophic denitrification technologies, the A3 process produces less sulfate, requires less added alkalinity, and is highly resistant to shock loads, maintaining stable operation above 10°C. Summary of the Invention

[0006] The present invention aims to provide an anaerobic-anoxic-anoxic sulfur autotrophic continuous flow A3 biological denitrification process for treating low C / N sewage and its sewage treatment method, which combines autotrophic nitrification and heterotrophic denitrification, heterotrophic nitrification and aerobic denitrification, and sulfur autotrophic denitrification to strengthen simultaneous nitrification and denitrification, and enhances the phosphorus removal effect of the process by anaerobic phosphorus release and denitrification and phosphorus removal. The anaerobic-anoxic-anoxic sulfur autotrophic continuous flow A3 process adopts a plug flow method and mainly includes an anaerobic tank (A1 tank), an anoxic tank (A2 tank), an anoxic sulfur autotrophic tank (A3 tank) and a secondary sedimentation tank. The process completes anaerobic phosphorus release in the A1 tank, and completes denitrification and phosphorus removal using nitrate as an electron acceptor in the A2 tank. Denitrification and phosphorus removal can simultaneously remove nitrogen and phosphorus. Because the reactor is a plug flow type, simultaneous nitrification and denitrification and sulfur autotrophic denitrification can all occur in the A1 tank, the A2 tank, and the A3 tank. The organic carbon source content in the influent of pool A1 is slightly higher, so heterotrophic nitrification and denitrification can occur, completing partial denitrification of return sludge and sewage; pool A2 mainly undergoes autotrophic nitrification and heterotrophic denitrification, completing simultaneous nitrification and denitrification under low dissolved oxygen conditions; pool A3 mainly undergoes sulfur autotrophic denitrification, and when the organic carbon source in the influent is exhausted, it completes the removal of residual nitrogen without the need for an external carbon source, ensuring that the denitrification of the A3 process meets the standards.

[0007] Specifically, after the sewage enters the A1 tank, in the presence of organic carbon sources, the removal of organic matter and heterotrophic denitrification are achieved. Meanwhile, under anaerobic conditions, polyphosphate-accumulating organisms release the intracellular polyphosphate and store organic matter such as PHA. Subsequently, the effluent from the A1 tank enters the A2 tank, where the simultaneous nitrification and denitrification process is completed through the synergistic action of nitrifying bacteria, heterotrophic denitrifying bacteria, and sulfur autotrophic denitrifying bacteria under low dissolved oxygen conditions, oxidizing NH4 + -N to NO X , then reducing it to N2 and discharging it, while removing NH4 + -N and NO X , achieving efficient removal of total nitrogen and providing nitrate nutrients for the subsequent A3 tank. Finally, the effluent from the A2 tank enters the degassing tank and the A3 tank, where, without the addition of external carbon sources, using reduced sulfur as an electron donor, the residual NO3 - -N and NO2 - -N in the sewage are used as electron donors to reduce NO X to N2 and discharge it, completing sulfur autotrophic denitrification and further reducing the total nitrogen concentration of the effluent.

[0008] The technical solution of the present invention:

[0009] A method for treating low C / N sewage by an anaerobic-anoxic-anoxic sulfur autotrophic continuous flow A3 process, the steps are as follows:

[0010] The plug flow type is adopted, mainly consisting of an anaerobic tank (A1 tank), an anoxic tank (A2 tank), an anoxic sulfur autotrophic tank (A3 tank) and a secondary sedimentation tank connected in sequence. Inoculated sludge is added to enrich and culture autotrophic nitrifying bacteria, heterotrophic denitrifying bacteria, heterotrophic nitrifying aerobic denitrifying bacteria and sulfur autotrophic denitrifying bacteria. Heterotrophic nitrification-aerobic denitrification, heterotrophic denitrification and sulfur autotrophic denitrification can occur in the anaerobic tank (A1 tank), anoxic tank (A2 tank) and anoxic sulfur autotrophic tank (A3 tank), and there are certain differences in the main denitrification processes in different tanks. Mechanical stirrers are installed in both the anaerobic tank and the anoxic sulfur autotrophic tank to ensure uniform mixing of the mud and water. Aeration is carried out in the anoxic tank through a blower, a flowmeter and an aeration head. The return sludge pump returns the sludge from the secondary sedimentation tank to the anaerobic tank. In the anaerobic-anoxic-anoxic sulfur autotrophic continuous flow A3 process, the sewage first enters the anaerobic tank. In the presence of organic carbon sources, heterotrophic denitrification is achieved. At the same time, under the action of polyphosphate-accumulating organisms, the intracellular polymerized phosphate is released to achieve anaerobic phosphorus release. The effluent from the anaerobic tank enters the anoxic tank, where phosphorus is absorbed by denitrifying phosphorus-removing bacteria. Under low dissolved oxygen conditions, through the synergistic action of nitrifying bacteria, heterotrophic denitrifying bacteria and sulfur autotrophic denitrifying bacteria, simultaneous nitrification and denitrification are completed, and ammonia nitrogen and nitrogen oxides are removed at the same time to achieve efficient removal of total nitrogen. The effluent from the anoxic tank enters the degassing tank and the anoxic sulfur autotrophic tank. Under the condition that the organic carbon source is exhausted, using reduced sulfur as an electron donor, the residual nitrogen removal without adding external carbon sources is completed to ensure the denitrification compliance of the A3 process. When the ammonia nitrogen concentration in the sewage is high, the degassing tank and the anoxic sulfur autotrophic tank can remove part of the ammonia nitrogen through the sulfur autotrophic denitrification process.

[0011] The DO concentration in the anoxic tank is adjusted by the flowmeter so that its dissolved oxygen concentration is 0.2 - 0.5 mg / L, and simultaneous nitrification and denitrification and denitrifying phosphorus removal are achieved in the anoxic environment.

[0012] The sludge return ratio of the sludge from the secondary sedimentation tank to the anaerobic tank is 80%.

[0013] The anoxic sulfur autotrophic tank regularly supplements the reduced sulfur electron donor and alkalinity according to the sulfur-nitrogen ratio of 2.5 to maintain stable sulfur autotrophic denitrification.

[0014] The beneficial effects of the present invention: The A3 process of the present invention strengthens simultaneous nitrification and denitrification for nitrogen and phosphorus removal through sulfur autotrophic denitrification. The required aeration volume in the A2 tank is low, reducing the dosage of the sulfur autotrophic denitrification electron donor in the A3 tank. It does not require external carbon sources, does not require nitrification liquid reflux, has a low sludge return ratio, less surplus sludge, saves energy and reduces consumption, and can operate stably above 10 °C. It can achieve economical and efficient deep denitrification for low C / N sewage. Description of the Drawings

[0015] Figure 1 It is a device diagram of the anaerobic-anoxic-anoxic sulfur autotrophic continuous flow A3 process.

[0016] Figure 2 Schematic diagram of nitrogen removal performance of A3 process

[0017] Figure 3 SND efficiency diagram of A3 process

[0018] Figure 4 SND rate diagram of A3 process

[0019] Figure 5 Flow chart of A3 process

[0020] In the figure: 1 - influent tank, 1.1 - influent pump, 2 - biochemical tank, 2.1 - anaerobic tank, 2.2 - anoxic tank, 2.3 - degassing tank, 2.4 - anoxic sulfur autotrophic tank, 2.5 - mechanical stirrer, 2.6 - blower, 2.7 - flowmeter, 2.8 - aeration strip, 2.9 - dissolved oxygen detector, 3 - secondary sedimentation tank, 3.1 - return sludge pump, 4 - effluent tank Specific implementation manners

[0021] The following further describes the specific implementation manners of the present invention in conjunction with the accompanying drawings and technical solutions

[0022] Example 1

[0023] As Figure 1 shown, an anaerobic - anoxic - anoxic sulfur autotrophic continuous - flow A3 biochemical reaction system for treating low C / N sewage provided in this example: In this example, the anaerobic tank 2.1, anoxic tank 2.2, degassing tank 2.3 and anoxic sulfur autotrophic tank 2.4 are adjacent and connected in sequence, separated by partition walls. The anoxic tank 2.2 includes an aeration first corridor, an aeration second corridor, an aeration third corridor, an aeration fourth corridor and an aeration fifth corridor. The passage of each corridor is in an S shape, and the water flow is in a zigzag state. The dissolved oxygen content in the anoxic tank 2.2 is controlled at 0.2 - 0.5 mg / L to promote simultaneous nitrification and denitrification and denitrifying phosphorus removal in an anoxic environment; Reduced sulfur electron donors and alkalinity are regularly added to the top of the anoxic sulfur autotrophic tank 2.4 to maintain a stable sulfur autotrophic denitrification process. The influent is pumped from the influent tank 1 into the biochemical tank 2 through the influent pump 1.1, and the effluent of the biochemical tank 2 is discharged into the secondary sedimentation tank 3. A part of the sludge in the secondary sedimentation tank 3 is refluxed to the anaerobic tank 2.1 at the front end of the biochemical tank 2 by the return sludge pump 3.1, and the excess sludge is regularly discharged from the secondary sedimentation tank 3. The supernatant of the secondary sedimentation tank 3 is discharged into the effluent tank 4. The biochemical tank 2 mainly includes an anaerobic tank 2.1, an anoxic tank 2.2 and an anoxic sulfur autotrophic tank 2.4. Among them, the anaerobic tank 2.1, anoxic tank 2.2 and anoxic sulfur autotrophic tank 2.4 are stirred by the mechanical stirrer 2.5 to make the mud - water mixture evenly mixed. The anoxic tank 2.2 is aerated by the blower 2.6 and the aeration strip 2.8, and its dissolved oxygen concentration is adjusted by the flowmeter 2.7, and the dissolved oxygen concentration of the biochemical tank 2 is monitored in real time by the detector 2.8

[0024] In this embodiment, domestic sewage is used as the system influent: the COD mass concentration is 200 - 300 mg / L, the NH4 + -N mass concentration is 70 - 120 mg / L, the TP mass concentration is 5 mg / L, and pH = 7.5.

[0025] The domestic sewage and the returned sludge from the sedimentation tank 3 first enter the anaerobic tank 2.1 of the biochemical tank. The influent is diluted to a certain extent under the action of the returned sludge. The nitrate nitrogen carried in the returned sludge undergoes heterotrophic denitrification under anaerobic conditions. The phosphorus - releasing bacteria in the sludge release intracellular polyphosphate in the anaerobic tank, resulting in anaerobic phosphorus release. Part of the ammonia nitrogen in the influent undergoes heterotrophic nitrification in the anaerobic tank. The effluent from the anaerobic tank enters the anoxic tank 2.2. An aeration strip with tiny aeration holes is installed at the bottom of the anoxic tank 2.2 to make the dissolved oxygen distribution uniform. Adjust the flowmeter 2.7 so that the dissolved oxygen content does not exceed 0.5 mg / L. Under low dissolved oxygen conditions, through the synergistic action of nitrifying bacteria, heterotrophic denitrifying bacteria, and sulfur - autotrophic denitrifying bacteria, simultaneous nitrification and denitrification occur to generate nitrogen gas, completing the removal of most of the total nitrogen. And part of the remaining organic matter in the sewage is used for the growth and reproduction of microorganisms, and part is used for heterotrophic denitrification. At the same time, the denitrifying phosphorus - removing bacteria use the nitrate in the water as an electron acceptor under anoxic conditions, reduce it to nitrogen gas, and absorb phosphate for nitrogen and phosphorus removal. The remaining dissolved oxygen in the effluent from the anoxic tank is removed under the action of the mechanical stirrer in the degassing tank 2.3, providing an anoxic environment for the subsequent anoxic sulfur - autotrophic tank 2.4, and denitrification also occurs in this area. The effluent from the degassing tank enters the anoxic sulfur - autotrophic tank. Under the condition of no additional carbon source, using reduced sulfur as an electron donor and residual nitrate nitrogen as an electron acceptor, sulfur - autotrophic denitrification occurs to further reduce the total nitrogen concentration of the effluent. Finally, in the secondary sedimentation tank 3, the sludge is sedimented, the supernatant is discharged, and part of the sludge at the bottom of the tank body is refluxed to the anaerobic tank 2.1 through the sludge return pipeline.

[0026] Calculation of the theoretical SND rate and SND reaction rate:

[0027] Let ΔNO x - be the increase in the mass concentration of NO x - (NO3 - -N and NO2 - -N) before and after the anoxic tank (2.2); ΔNH4 + be the decrease in the mass concentration of NH4 + before and after the anoxic tank 2.2; MLVSS be the activated sludge concentration; T be the hydraulic retention time of the anoxic tank 2.2, then there are:

[0028]

[0029]

[0030] In this embodiment, nitrification, heterotrophic denitrification, and sulfur autotrophic denitrification are used to enhance simultaneous nitrification and denitrification, and the removal of nutrients is completed by combining denitrifying phosphorus removal. As Figure 3 and Figure 4 shown, after the A3 system operates stably, the TN removal rate is above 90%, the SND efficiency is 48% - 70%, and the SND rate is 13 - 45 mg / (g VSS) / d. At the same time, when operating under low-temperature conditions (above 10°C), the TN removal rate is above 85%. This indicates that the A3 process has good shock load resistance performance and can treat high-ammonia-nitrogen wastewater under low-temperature conditions. By analyzing the microbial flora, autotrophic nitrifying bacteria, heterotrophic denitrifying bacteria, sulfur autotrophic denitrifying bacteria, and heterotrophic nitrifying aerobic denitrifying bacteria are enriched in each partition of the system. The enrichment of related bacterial genera ensures that the A3 system can stably treat low C / N wastewater.

[0031] The above are the specific embodiments of the present invention, which are convenient for those skilled in the art of this technology to better understand and apply the present invention.

[0032] Comparative Example 1

[0033] Aerobic tank, secondary sedimentation tank, and sulfur autotrophic denitrification filter are set after the anaerobic tank. The filler in the sulfur autotrophic denitrification filter is a mixture of sulfur particles and limestone particles. This process performs nitrogen and phosphorus removal through anaerobic phosphorus release, aerobic excessive phosphorus uptake, nitrification, and sulfur autotrophic denitrification, and the TN removal rate reaches above 80%. This process requires the effluent of the sulfur autotrophic denitrification filter to be refluxed to the anaerobic tank, and the reflux ratio needs to be controlled at 300 - 500%; the aeration volume required for the aerobic tank is also relatively high, and it needs to be controlled at 2 - 4 mg / L, so the energy consumption is high; the sulfur autotrophic denitrification tank is in the form of a filter, the filler is easy to clog, requires backwashing, and the operation is complex, and the maintenance cost is high. Therefore, it is difficult for this process to economically, effectively, and stably meet strict emission standards. Compared with this process, in the A3 process, not only the aeration volume required for the A2 tank is low, only 0.2 - 0.5 mg / L, saving energy consumption, but also the A3 tank uses direct addition of sulfur autotrophic electron donors for denitrification and nitrogen removal, with simple operation and low cost.

[0034] Comparative Example 2

[0035] An anoxic tank, a low-oxygen aeration tank and a degassing tank are arranged behind the anaerobic tank, and sulfur element is not used for denitrification in the anoxic tank. In this scheme, large-proportion reflux channels are arranged at the liquid outlet end and the liquid inlet end of the low-oxygen aeration tank, and a nitrite nitrogen reflux channel is arranged between the degassing tank and the anoxic tank. This process combines simultaneous nitrification and denitrification and anaerobic ammonia oxidation reactions for nitrogen removal. The process mainly removes nitrogen and phosphorus through anaerobic phosphorus release, aerobic excessive phosphorus uptake, ammonia nitrogen nitrification and anoxic sulfur autotrophic denitrification. In this process, simultaneous nitrification and denitrification and anaerobic ammonia oxidation are realized through large-proportion internal circulation and nitrite nitrogen reflux, and the required energy consumption is relatively high; this process combines simultaneous nitrification and denitrification and anaerobic ammonia oxidation reactions for nitrogen removal, but an external carbon source still needs to be added; anaerobic ammonia oxidizing bacteria have high growth temperature requirements, high energy consumption, slow proliferation rate, and long system startup time. Compared with this process, the A3 process only needs sludge reflux, and the sludge reflux ratio is as low as 80%. No external carbon source is required. By adding a sulfur autotrophic electron donor, simultaneous nitrification and denitrification are further strengthened for nitrogen and phosphorus removal, ensuring the effluent quality, saving energy and reducing consumption, and realizing economic and efficient nitrogen removal treatment for low C / N sewage.

[0036] Comparative Example 3

[0037] An aerobic tank and a sulfur autotrophic anoxic tank are arranged behind the anaerobic tank. Nitrogen and phosphorus are removed through anaerobic phosphorus release, aerobic excessive phosphorus uptake, nitrification, sulfur autotrophic denitrification and iron autotrophic denitrification, and the effluent reaches the national first-class A discharge standard. The aerobic tank of this process requires a relatively high amount of aeration, and the sulfur autotrophic anoxic tank adopts the form of a filter tank, filling 2-3 mm pyrite gravel in the sponge filler and placing it in the anoxic area. Therefore, it has the disadvantages of easy clogging of the filler, difficult maintenance, and high cost, and cannot economically and efficiently treat low C / N ratio sewage. Compared with this process, the A3 process requires a low amount of aeration, only 0.2-0.5 mg / L, saving energy and reducing consumption, and the anoxic sulfur autotrophic tank adopts the form of directly adding an electron donor, which is convenient for operation, easy to control, and has a low cost. Therefore, for the green, economic and stable treatment of low C / N sewage, the anaerobic-anoxic-anoxic sulfur autotrophic continuous-flow A3 process is superior to this process.

Claims

1. A method for treating low C / N sewage by an anaerobic-anoxic-anoxic sulfur autotrophic continuous flow A3 process, characterized in that, The steps are as follows: Adopt the plug-flow type, which is mainly composed of an anaerobic tank, an anoxic tank, an anoxic sulfur autotrophic tank and a secondary sedimentation tank connected in sequence; add inoculated sludge to enrich and culture autotrophic nitrifying bacteria, heterotrophic denitrifying bacteria, heterotrophic nitrifying aerobic denitrifying bacteria and sulfur autotrophic denitrifying bacteria; mechanical stirrers are installed in both the anaerobic tank and the anoxic sulfur autotrophic tank to ensure uniform mixing of mud and water, aerate the anoxic tank through a blower, a flow meter and an aeration head, and a return sludge pump returns the sludge in the secondary sedimentation tank to the anaerobic tank.

2. The method for treating low C / N sewage by anaerobic-anoxic-anoxic sulfur autotrophic continuous flow A3 process according to claim 1, characterized in that, Heterotrophic nitrification-aerobic denitrification, heterotrophic denitrification and sulfur autotrophic denitrification can all occur in the anaerobic tank, anoxic tank and anoxic sulfur autotrophic tank.

3. The method for treating low C / N sewage by anaerobic-anoxic-anoxic sulfur autotrophic continuous flow A3 process according to claim 1, characterized in that, The DO concentration in the anoxic tank is adjusted by the flow meter so that the dissolved oxygen concentration is 0.2 - 0.5 mg / L, and simultaneous nitrification and denitrification and denitrifying phosphorus removal are achieved under an anoxic environment.

4. The method for treating low C / N sewage by the anaerobic-anoxic-anoxic sulfur autotrophic continuous flow A3 process according to claim 1, characterized in that The sludge return ratio of the sludge in the secondary sedimentation tank returned to the anaerobic tank is 80%.

5. The method for treating low C / N sewage by the anaerobic-anoxic-anoxic sulfur autotrophic continuous flow A3 process according to claim 1, characterized in that The anoxic sulfur autotrophic tank regularly supplements the reduced sulfur electron donor and alkalinity according to a sulfur-nitrogen ratio of 2.5 to maintain stable sulfur autotrophic denitrification.

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