A sewage treatment system and a sewage treatment method

Abstract

The present invention relates to a sewage treatment system and a sewage treatment process, which includes a pretreatment unit, a pump, a biological treatment unit, and a deep treatment unit connected in sequence. The biological treatment unit includes an anoxic tank, an aerobic tank, and a clarifier connected in sequence. The water outlet of the pretreatment unit is connected to the anoxic tank, and the water outlet of the clarifier is connected to the deep treatment unit. The sewage treatment system of the present invention has a compact structure, can save floor area, save investment, reduce operating costs, and improve treatment effects.

Description

Technical Field

[0001] The present invention relates to a sewage treatment system and a sewage treatment method, belonging to the field of water treatment. Background Art

[0002] With the acceleration of the rural modernization and urbanization process and the booming development of township enterprises, for the construction of township sewage treatment facilities, how to make the operation and maintenance of related facilities simple, miniaturized, modularized, standardized, combined with landscaping, and achieve high efficiency and energy conservation is an urgent problem to be solved. Summary of the Invention

[0003] Aiming at the deficiencies of the prior art, one of the purposes of the present invention is to provide a sewage treatment system with a compact structure and good treatment effect; the second purpose of the present invention is to provide a sewage treatment method.

[0004] To solve the above technical problems, the technical solution of the present invention is as follows:

[0005] A sewage treatment system includes a pretreatment unit, a pump, a biological treatment unit, and a deep treatment unit connected in sequence. The biological treatment unit includes an anoxic tank, an aerobic tank, and a clarification tank connected in sequence. The water outlet of the pretreatment unit is connected to the anoxic tank, and the water outlet of the clarification tank is connected to the deep treatment unit.

[0006] In this way, the sewage to be treated is pretreated and then transported from the pretreatment unit to the biological treatment unit (anoxic tank, aerobic tank, clarification tank) through a pump. The pollution factors in the sewage are removed by microorganisms and then flow into the deep treatment unit by gravity. After deep treatment, it can meet the discharge standards.

[0007] Further, it further includes a blower and an aeration pipe arranged in the aerobic tank, and the blower is connected to the aeration pipe.

[0008] Further, it further includes a first reflux mechanism for refluxing the sludge in the clarification tank to the anoxic tank and the aerobic tank.

[0009] Further, it further includes a second reflux mechanism for refluxing the mixed liquid in the aerobic tank to the anoxic tank. Optionally, the second reflux mechanism is a non-powered reflux mechanism.

[0010] Further, the anoxic tank, the aerobic tank, and the clarification tank are integrated in a tank body to form a modular structure, which is convenient for transportation, disassembly, and assembly.

[0011] The biological treatment unit can remove pollutants such as COD, ammonia nitrogen, SS, and phosphorus in the sewage through microorganisms. The effluent quality can reach the first-class A standard for COD, BOD, and ammonia nitrogen, and the total nitrogen and total phosphorus can reach the first-class B standard.

[0012] Further, the advanced treatment unit is a constructed wetland. In this way, the sewage flowing into the advanced treatment unit can be further purified of pollutants in the sewage by the animal and plant community composed of microorganisms and plant roots and the high-efficiency phosphorus-accumulating filler, realizing advanced treatment. Optionally, the constructed wetland is a modular constructed wetland.

[0013] Through the constructed wetland, the COD, BOD, total nitrogen, SS, and phosphorus in the sewage can be further removed by the animal and plant community composed of microorganisms and plant roots and the high-efficiency phosphorus-accumulating filler. It has the dual functions of activated sludge and ecology, and the effluent quality reaches better than Class A standard. At the same time, it replaces the conventional advanced treatment process with chemical agents (high-efficiency coagulation and sedimentation process) and the high-energy-consuming denitrification process (deep bed denitrification process), reducing the operating cost by 0.1 - 0.2 yuan / ton of sewage.

[0014] Further, the pretreatment unit includes a tank body and a grille tank arranged at the top of the tank body. The inner cavity of the tank body is divided into an adjustment area, a grit chamber, and a sludge storage area arranged in sequence along the horizontal direction by two partition plates. The grille tank is connected with a water inlet pipe and a sewage pipe. The lower end of the sewage pipe extends to the bottom of the grit chamber. A number of water passing holes are respectively arranged on the two partition plates, and the water passing holes are located above the lower end of the sewage pipe. The pretreatment unit further includes a drainage mechanism communicated with the adjustment area and a sludge inlet pipe communicated with the sludge storage area. The outlet end of the drainage mechanism is connected with a water outlet pipe, and the water outlet pipe is connected with the inlet end of a pump. The upper end of the sludge inlet pipe is connected with a clarifier, and the lower end of the sludge inlet pipe extends to the bottom of the sludge storage area. An aeration mechanism and a third reflux mechanism are arranged in the sludge storage area, and the output end of the third reflux mechanism is connected with the adjustment area and the grit chamber.

[0015] The pretreatment unit combines functions such as slag separation, grit removal, water quality and quantity adjustment, etc., and is integrated. It saves 20% of the floor area compared with conventional pretreatment equipment, can improve the shock resistance of the subsequent treatment unit, and is convenient for transportation, disassembly and assembly.

[0016] Further, a flow guide plate is arranged in the grille tank. The bottom surface of the flow guide plate is fixed on the bottom surface of the grille tank, and one side end surface of the flow guide plate is fixed on the side wall of the grille tank, so that a C-shaped flow channel is formed in the grille tank. A grille is arranged in the C-shaped flow channel, and the grille is located in the flow channel section between the water inlet pipe and the sewage pipe.

[0017] Further, the inner cavity of the tank body is divided into an adjustment area, a grit chamber, and a sludge storage area arranged in sequence by a first partition plate and a second partition plate that are parallel to each other. The position of the water passing hole on the second partition plate is higher than the position of the water passing hole on the first partition plate.

[0018] A sewage treatment method is carried out by using the sewage treatment system as described above, and includes the following steps:

[0019] The sewage to be treated is introduced into the pretreatment unit. After pretreatment, the sewage is pumped into the biological treatment unit. After passing through anoxic treatment and aerobic treatment in sequence, it undergoes sedimentation and separation in the clarifier to obtain supernatant and excess sludge with upper and lower layers.

[0020] The supernatant is input into the advanced treatment unit. After advanced treatment, it is discharged up to standard or reused.

[0021] 20 - 30 wt% of the excess sludge is refluxed to the anoxic tank, 20 - 30 wt% of the excess sludge is refluxed to the aerobic tank, and part of the mixed liquid in the aerobic tank is refluxed to the anoxic tank; 40 - 60 wt% of the excess sludge is refluxed to the pretreatment unit.

[0022] Through the combined action of the pretreatment unit, biological treatment unit and advanced treatment unit of the present application, there is no need to set up conventional high - efficiency coagulation sedimentation units and deep - bed denitrification and phosphorus removal and nitrogen removal units.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] (1) The sewage treatment system of the present invention has a compact structure, can save floor area, save investment, reduce operating costs, improve treatment effects, and the effluent indexes are better than the first - level A standard of the "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants" (GB18918 - 2002), and even reach the surface water class IV standard. It can achieve better than the standard of "Quality of Reclaimed Water for Urban Miscellaneous Use" (GB / T18920 - 2002), with effluent COD: < 30 mg / L; BOD: < 10 mg / L; NH3 - N: < 5 mg / L; SS: < 10 mg / L.

[0025] (2) The pretreatment unit, biological treatment unit and constructed wetland are respectively modularized designs, with simple and convenient installation and high flexibility.

[0026] (3) While treating sewage, the sludge of the sewage treatment system of the present invention is synchronously treated, and the organic excess sludge is greatly reduced, and nearly zero discharge can be achieved.

[0027] (3) The sewage treatment system of the present invention has low operation energy consumption and few operation actions, is easy to realize automatic control and unattended operation, and the comprehensive operation energy consumption is 20 - 30% more energy - saving than the conventional MBR process.

[0028] (4) The whole system is simplified to the greatest extent, making the design structure and layout of the sewage treatment plant simple, making the operation and maintenance of sewage treatment in small and medium - sized towns simple, miniaturized, modularized, standardized, combined with landscape, and achieving high efficiency and energy - saving, with broad application prospects. Description of the Drawings

[0029] Figure 1It is a schematic structural diagram of the sewage treatment system according to the first embodiment of the present invention.

[0030] Figure 2 It is a top view of the sewage pretreatment tank according to the first embodiment of the present invention.

[0031] Figure 3 It is along Figure 2 A sectional view taken along line F-F in

[0032] Figure 4 It is along Figure 2 A sectional view taken along line G-G in

[0033] Figure 5 It is along Figure 2 A sectional view taken along line H-H in

[0034] Figure 6 It is along Figure 2 A sectional view taken along line D-D in

[0035] Figure 7 It is along Figure 2 A sectional view taken along line E-E in

[0036] In the figure, A - pretreatment unit, B - biological treatment unit, B1 - anoxic tank, B2 - aerobic tank, B3 - clarifier, B4 - blower, B5 - aeration pipe, B6 - first reflux mechanism, B7 - second reflux mechanism, C - constructed wetland, 1 - tank body, 2 - regulation area, 3 - grit chamber, 4 - sludge storage area, 5 - first inspection port, 6 - grille channel, 7 - second inspection port, 8 - third inspection port, 9 - grille, 10 - baffle plate, 11 - sewage pipe, 12 - sludge inlet pipe, 13 - aeration inlet pipe, 14 - drainage mechanism, 1401 - first submersible pump, 1402 - pump outlet pipe, 15 - inlet pipe, 16 - outlet pipe, 17 - first partition plate, 18 - second partition plate, 19 - water passing hole A, 20 - water passing hole B, 21 - first reflux pipe, 22 - perforated aeration pipe, 23 - second reflux pipe, 24 - third reflux mechanism, 2401 - second submersible pump, 2402 - main reflux pipe. Specific embodiments

[0037] The following description depicts alternative embodiments of the present invention to instruct those of ordinary skill in the art on how to implement and reproduce the present invention. To teach the technical solutions of the present invention, some conventional aspects have been simplified or omitted. Those of ordinary skill in the art should understand that variations or substitutions derived from these embodiments will fall within the scope of protection of the present invention. Those of ordinary skill in the art should understand that the following features can be combined in various ways to form multiple variations of the present invention. Thus, the present invention is not limited to the following alternative embodiments but is defined only by the claims and their equivalents. For the convenience of description, words such as "upper", "lower", "left", and "right" hereinafter only indicate the same directions as the upper, lower, left, and right directions of the accompanying drawings themselves and do not limit the structure.

[0038] See Figure 1 , a sewage treatment system, comprising a pretreatment unit A, a pump 25, a biological treatment unit B, and a advanced treatment unit C connected in sequence. The biological treatment unit B includes an anoxic tank B1, an aerobic tank B2, and a clarifier B3 connected in sequence. The outlet of the pretreatment unit A is connected to the anoxic tank B1, and the outlet of the clarifier B3 is connected to the advanced treatment unit C.

[0039] It further includes a blower B4 and an aeration pipe B5 disposed in the aerobic tank B2, and the blower B4 is connected to the aeration pipe B5.

[0040] It further includes a first reflux mechanism B6 for refluxing the sludge in the clarifier B3 to the anoxic tank B1 and the aerobic tank B2.

[0041] It further includes a second reflux mechanism B7 for refluxing the mixed liquid in the aerobic tank 1 to the anoxic tank B1.

[0042] The anoxic tank B1, the aerobic tank B2, and the clarifier B3 are integrated in a single tank body.

[0043] The advanced treatment unit C is an artificial wetland.

[0044] The pretreatment unit A includes a tank body 1 and a grille tank 6 arranged at the top of the tank body. The inner cavity of the tank body 1 is divided into an adjustment area 2, a grit chamber 3, and a sludge storage area 4 arranged in sequence along the horizontal direction by two partition plates. An inlet pipe 15 and a sewage pipe 11 are connected to the grille tank 6. The lower end of the sewage pipe 11 extends into the bottom of the grit chamber 3. A number of water passing holes are respectively arranged on the two partition plates, and the water passing holes are located above the lower end of the sewage pipe 11. The pretreatment unit A further includes a drainage mechanism 14 communicated with the adjustment area 2 and a sludge inlet pipe 12 communicated with the sludge storage area 4. The outlet end of the drainage mechanism 14 is communicated with an outlet pipe 16, and the outlet pipe 16 is communicated with the inlet end of a pump 25. The upper end of the sludge inlet pipe 12 is communicated with a clarification tank B3, and the lower end of the sludge inlet pipe 12 extends into the bottom of the sludge storage area 4. An aeration mechanism and a third reflux mechanism are arranged in the sludge storage area 4, and the output end of the third reflux mechanism is communicated with the adjustment area 2 and the grit chamber 3.

[0045] In the pretreatment unit, the grille tank, the adjustment area, the grit chamber, and the sludge storage area are integrated into one, with a compact structure. The sewage to be treated enters the grille tank through the inlet pipe. After removing large-particle impurities, the sewage enters the grit chamber through the sewage pipe for sedimentation separation. The finer-particle impurities settle to the bottom of the grit chamber. At the same time, the clear water in the grit chamber enters the adjustment area through the water passing holes to adjust the water volume and water quality. The water discharged from the adjustment area can be sent to the subsequent biological treatment unit through the outlet pipe for treatment. In addition, the excess sludge generated by the biological treatment unit can be sent into the sludge storage area. Part of the sludge is aerated and digested by the aeration mechanism to reduce the sludge volume. At the same time, the third reflux mechanism can partially reflux the activated sludge generated in the sludge storage area to the adjustment area and the grit chamber, playing a role in hydrolysis acidification, reducing the load of subsequent biochemical treatment, and improving the effluent quality. In addition, the supernatant in the sludge storage area can flow into the grit chamber through the water passing holes on the partition plate. It is only necessary to regularly clean the impurities and sludge in the grille tank, the grit chamber, and the sludge storage area.

[0046] The setting of the sludge storage area, on the one hand, the excess sludge from the subsequent biological treatment can be input into the sludge storage area through the fourth reflux mechanism B8, avoiding the cost increase caused by separately setting a sludge storage structure or a sludge storage tank. On the other hand, 50wt% of the activated sludge generated in the sludge storage area can be conveniently refluxed to the grit chamber and the adjustment area. Through the hydrolysis acidification effect, the non-dissolved organic matter in the sewage is transformed into dissolved organic matter, and the difficult-to-biodegradable organic matter is transformed into easily biodegradable organic matter, improving the biodegradability of the wastewater, reducing the load of subsequent biochemical treatment, and enhancing the treatment effect and efficiency.

[0047] A flow guide plate 10 is arranged in the grid groove 6. The bottom surface of the flow guide plate 10 is fixed on the bottom surface of the grid groove 6, and one end face of the flow guide plate 10 is fixed on the side wall of the grid groove 6, so that a C-shaped flow channel is formed in the grid groove 6. A grid 9 is arranged in the C-shaped flow channel, and the grid 9 is located in the flow channel section between the water inlet pipe 15 and the sewage pipe 11. The grid 9 is inclined along the water flow direction, and the included angle between the grid and the horizontal plane is 75°. In this way, through the action of the C-shaped flow channel and the grid, the kinetic energy of the sewage can be weakened, so that the sewage flows into the grit chamber at a stable and appropriate speed, avoiding unnecessary disturbance and improving the treatment effect. In addition, the grid can remove large-size sundries in the sewage and prevent pipeline blockage.

[0048] The inner cavity of the tank body is divided into an adjustment area 2, a grit chamber 3 and a sludge storage area 4 arranged in sequence by a first partition plate 17 and a second partition plate 18 which are parallel to each other. The position of the water passing hole A19 on the second partition plate 18 is higher than the position of the water passing hole B20 on the first partition plate 17. In this way, the supernatant in the sludge storage area flows into the grit chamber through the water passing hole, and the supernatant in the grit chamber flows into the adjustment area through the water passing hole, showing a one-way flow as a whole. The water passing hole is a rectangular hole with a size of L*H = 0.3*0.08 mm.

[0049] The lower end of the sewage pipe 11 is communicated with a water distribution pipe 1101. The water distribution pipe 1101 is horizontally arranged to form an inverted T-shaped structure. In this way, the sewage flows into the grit chamber 3 in the horizontal direction, which can effectively reduce the adverse disturbance caused by the inflowing sewage to the bottom layer of the grit chamber 3.

[0050] The drainage mechanism 14 includes a first submersible pump 1401 and a pump outlet pipe 1402 which are connected in sequence. The first submersible pump 1401 is fixed at the bottom of the adjustment area 2, and the pump outlet pipe is communicated with the outlet pipe 16. Optionally, a pump outlet pipe is arranged inside the adjustment area, and the pump outlet pipe is connected with the outlet pipe through the first inspection port at the upper part of the tank body.

[0051] The third reflux mechanism 24 includes a second submersible pump 2401 arranged at the bottom of the sludge storage area. The output end of the second submersible pump 2401 is communicated with a main reflux pipe 2402. A first reflux pipe 21 and a second reflux pipe 23 are connected in parallel on the main reflux pipe 2402. The first reflux pipe 21 extends into the adjustment area 2, and the lower end of the second reflux pipe 23 extends into the bottom of the grit chamber 3.

[0052] The aeration mechanism includes an aeration inlet pipe 13 and multiple perforated aeration pipes 22 communicated with the aeration inlet pipe. The aeration inlet pipe 13 is communicated with a blower arranged outside the tank body. Optionally, the aeration volume (DO) is 0.6 - 1 mg / L. The multiple perforated aeration pipes 22 are evenly distributed at the bottom of the sludge storage area. By micro-aeration for aerobic digestion of part of the sludge, the amount of surplus sludge in the biochemical section is greatly reduced, the sludge disposal cost is reduced, and thus the operation cost is reduced. At the same time, anaerobic digestion of the surplus sludge is prevented, and the release of phosphorus and the generation of odor are avoided.

[0053] It further includes a first inspection port 5, a second inspection port 7 and a third inspection port 8. The first inspection port 5, the second inspection port 7 and the third inspection port 8 are respectively arranged at the tops of the regulation area 2, the grit chamber area 3 and the sludge storage area 4 to facilitate regular inspection of the operation of pipelines and facilities to ensure the normal operation of the pretreatment tank. One side of the first inspection port is connected to the outlet pipe, and the other side is connected to the grille channel and is located above the regulation area; the second inspection port is connected to the other side of the grille channel and is located above the grit chamber area; the third inspection port is connected to the other side of the second inspection port and is located above the sludge storage area. A flip cover is arranged inside the inspection port. When maintenance or cleaning is required, the flip cover can be opened for operation.

[0054] During operation, sewage enters the grille channel 6 from the inlet pipe 15 and flows through the grille 9 through the guide plate 10 in the grille channel 6. The grille 9 can intercept the coarser particulate impurities in the sewage to avoid blocking pipelines and subsequent treatment facilities. After passing through the grille 9, the sewage enters the grit chamber area 3 through the sewage pipe 12. The grit chamber area 3 can remove the finer particulate impurities in the sewage, effectively reduce the SS of the sewage, improve the water quality, and enhance the subsequent treatment capacity. The sewage treated by the grit chamber area 3 overflows into the regulation area 2 through the water passing holes on the first partition plate 17. The regulation area 2 can regulate the water volume and water quality of the sewage to ensure that subsequent treatment facilities are not affected by the peak flow or concentration change of the sewage. The effluent of the regulation area 2 is connected to the outlet pipe 16 through the water pump outlet pipe 1402 and enters the subsequent biological treatment unit. The surplus sludge obtained by the biological treatment unit enters the sludge storage area 4 through the sludge pipe 12. An aeration inlet pipe 13 and a perforated aeration pipe 21 are arranged in the sludge storage area 4. Anaerobic digestion of the surplus sludge is prevented by micro-aeration, and the release of phosphorus and the generation of odor are avoided. The supernatant in the sludge storage area 4 flows into the grit chamber area 3 through the water passing holes on the second partition plate 18. The sundries, sludge, etc. in the grille 9, the grit chamber area 3 and the sludge storage area 4 need to be regularly cleaned. At the same time, the operation of pipelines and facilities is regularly inspected through the inspection ports to ensure the normal operation of the system. The supernatant in the clarifier of the biological treatment unit flows into the constructed wetland by itself. After advanced treatment, it can be reused or discharged up to the standard.

[0055] The applicant applied the above sewage treatment system to a relatively centralized rural sewage treatment project in Hunan, with a water volume of 200 t / d, an influent COD of 400 mg / L, SS of 220 mg / L, BOD of 160 mg / L, and TN of 33 mg / L. The wastewater passes through the pretreatment unit, where large particulate pollutants are removed in the grille area and then enter the regulation area. Subsequently, it enters the biological treatment unit to remove most of the COD, ammonia nitrogen, and TN in the wastewater. Part of the sludge at the bottom of the clarifier is pumped into the sludge storage area of the pretreatment unit, and the supernatant enters the subsequent advanced treatment unit. The animal and plant community composed of microorganisms and plant roots in the constructed wetland and the high-efficiency phosphorus-accumulating filler further remove the pollutants in the sewage to achieve advanced treatment. The project operates stably, and the removal rates of COD, BOD, and TN are 92.3%, 95.0%, and 70.5% respectively. The effluent stably meets the first-class A standard of the "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants" (GB18918-2002).

[0056] The content clarified in the above embodiments should be understood that these embodiments are only used to illustrate the present invention more clearly, rather than to limit the scope of the present invention. After reading the present invention, various equivalent forms of modification of the present invention by those skilled in the art all fall within the scope defined by the appended claims of this application.

Claims

1. A sewage treatment system, characterized in that, It includes a pretreatment unit (A), a pump (25), a biological treatment unit (B), and a advanced treatment unit (C) that are connected in sequence. The biological treatment unit (B) includes an anoxic tank (B1), an aerobic tank (B2), and a clarifier (B3) that are connected in sequence. The water outlet of the pretreatment unit (A) is connected to the anoxic tank (B1), and the water outlet of the clarifier (B3) is connected to the advanced treatment unit (C); The pretreatment unit (A) includes a tank body (1) and a grille tank (6) arranged on the top of the tank body. The inner cavity of the tank body (1) is divided into an adjustment area (2), a grit chamber (3), and a sludge storage area (4) arranged in sequence along the horizontal direction by parallel first partition plate (17) and second partition plate (18). An inlet pipe (15) and a sewage pipe (11) are connected to the grille tank (6). The lower end of the sewage pipe (11) extends into the bottom of the grit chamber (3). A number of water passing holes are respectively arranged on the two partition plates, and the water passing holes are located above the lower end of the sewage pipe (11). The position of the water passing holes on the second partition plate (18) is higher than the position of the water passing holes on the first partition plate (17); The pretreatment unit (A) further includes a drainage mechanism (14) connected to the adjustment area (2) and a sludge inlet pipe (12) connected to the sludge storage area (4). The outlet end of the drainage mechanism (14) is connected to an outlet pipe (16), and the outlet pipe (16) is connected to the inlet end of the pump (25). The upper end of the sludge inlet pipe (12) is connected to the clarifier (B3), and the lower end of the sludge inlet pipe (12) extends into the bottom of the sludge storage area (4); An aeration mechanism and a third reflux mechanism are arranged in the sludge storage area (4). The output end of the third reflux mechanism is connected to the adjustment area (2) and the grit chamber (3); The sludge storage area (4) is connected to the clarifier (B3) through a fourth reflux mechanism (B8); It further includes a blower (B4) and an aeration pipe (B5) arranged in the aerobic tank (B2), and the blower (B4) is connected to the aeration pipe (B5); The anoxic tank (B1), the aerobic tank (B2), and the clarifier (B3) are integrated in one tank body.

2. The sewage treatment system according to claim 1, wherein It further includes a first reflux mechanism (B6) for returning the sludge in the clarifier (B3) to the anoxic tank (B1) and the aerobic tank (B2).

3. The sewage treatment system according to claim 1, characterized in that It further includes a second reflux mechanism (B7) for returning the mixed liquid in the aerobic tank (B2) to the anoxic tank (B1).

4. The sewage treatment system according to claim 1, wherein The advanced treatment unit (C) is an artificial wetland.

5. The sewage treatment system according to claim 1, characterized in that, A flow guide plate (10) is arranged in the grille tank (6). The bottom surface of the flow guide plate (10) is fixed on the bottom surface of the grille tank (6), and one side end surface of the flow guide plate (10) is fixed on the side wall of the grille tank (6), so that a C-shaped flow channel is formed in the grille tank (6). A grille (9) is arranged in the C-shaped flow channel, and the grille (9) is located in the flow channel section between the inlet pipe (15) and the sewage pipe (11).

6. A sewage treatment method, characterized in that, It is carried out by using the sewage treatment system according to any one of claims 1-5, and includes the following steps: The sewage to be treated is introduced into the pretreatment unit (A). After pretreatment, the sewage is pumped into the biological treatment unit (B). After passing through anoxic treatment and aerobic treatment in sequence, it undergoes sedimentation and separation in the clarifier (B3), obtaining supernatant and excess sludge with upper and lower layers; The supernatant is input into the advanced treatment unit (C). After advanced treatment, it is discharged up to standard or reused; 20 - 30 wt% of the excess sludge is refluxed to the anoxic tank (B1), 20 - 30 wt% of the excess sludge is refluxed to the aerobic tank (B2), and part of the mixed liquid in the aerobic tank (B2) is refluxed to the anoxic tank (B1); 40 - 60 wt% of the excess sludge is refluxed to the pretreatment unit (A).

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