Method for treating aged leachate from a landfill

Abstract

The application provides a landfill aged leachate treatment method and relates to the technical field of domestic waste treatment, which comprises the following steps: injecting aged leachate into a garbage pit in a multi-point rotation mode, mixing the aged leachate with the original leachate in the garbage pit and carrying out anaerobic fermentation pretreatment; collecting the inflow original leachate into a collection pool, filtering through a hub grating machine, sending into a pre-sedimentation tank and flowing to a regulating tank, realizing water quality homogenization by compressed air stirring; uniformly lifting the liquid from the regulating tank to an anaerobic tank for anaerobic degradation treatment; after precipitation, the liquid sequentially enters a denitrification tank and a nitrification tank for biochemical denitrification and COD removal treatment; the biochemical treatment effluent is sequentially subjected to ultrafiltration, nanofiltration and reverse osmosis membrane system deep treatment, the pH value of the ultrafiltration effluent is adjusted, the final effluent meets the discharge standard or is reused, the treatment is completed, the problems of additional homogenization mixing pool and lack of special characteristic regulation and control standard are solved, and the technical effect of efficiently treating aged leachate without additional operation cost is achieved.

Description

Technical Field

[0001] This invention relates to the field of municipal solid waste treatment technology, and in particular to a method for treating old leachate from landfills. Background Technology

[0002] The leachate generated from municipal solid waste landfills after years of storage generally has the characteristics of low COD, high ammonia nitrogen, and poor biodegradability. When it is directly sent to the leachate treatment system of a municipal solid waste incineration power plant and treated using conventional processes, problems such as insufficient biochemical denitrification efficiency and effluent quality exceeding standards are likely to occur. In addition, the amount of reagents added increases significantly, which significantly raises the water treatment operating cost.

[0003] Current treatment methods primarily involve pre-treating landfill leachate and incinerator leachate separately before transporting them to a dedicated homogenization tank for thorough mixing. The mixture then sequentially enters the biological treatment unit and the membrane advanced treatment unit for purification. This approach utilizes the existing biological and membrane treatment lines at the incinerator, eliminating the need for constructing a new water treatment production line and reducing infrastructure investment to some extent. Furthermore, it avoids the need for large-scale addition of acids, alkalis, and flocculants to adjust water quality.

[0004] However, existing technologies require separate pretreatment facilities for landfill leachate before it enters the site, and additional homogenization mixing tanks are needed, which significantly increases civil engineering investment, equipment operation and maintenance, and labor workload. Furthermore, there are no standardized operating control parameters for the entire process based on the characteristics of old leachate, and there is a lack of control standards for anaerobic, nitrification-denitrification, and membrane treatment sections, resulting in a lack of technical solutions for fluctuations in operating conditions. Summary of the Invention

[0005] The purpose of this invention is to provide a method for treating old leachate from landfills, so as to alleviate the technical problems of existing technologies that require additional homogenization mixing tanks and lack specific characteristic control standards.

[0006] The method for treating old leachate from landfills provided by this invention is implemented according to the following steps: Step 1: Inject the old leachate into the landfill pit in a multi-point rotation manner, mix it with the original leachate of the landfill pit and carry out anaerobic fermentation pretreatment to obtain the first mixed liquid; Step 2: The first mixed liquid is collected and flows into the original leachate collection tank. After being filtered by the rotary bar screen, it is sent to the pre-sedimentation tank and flows by gravity to the equalization tank. Compressed air is used to stir the water to achieve water homogenization and obtain the equalization liquid. Step 3: The conditioning liquid is uniformly lifted from the conditioning tank to the anaerobic tank for anaerobic degradation treatment to obtain the intermediate liquid; Step 4: After sedimentation, the intermediate liquid is sequentially fed into the denitrification tank and the nitrification tank for biological denitrification and COD removal treatment to maintain the stability of the carbon-nitrogen ratio and operating parameters of the biological system. Step 5: The biochemically treated effluent is then subjected to ultrafiltration, nanofiltration and reverse osmosis membrane systems for further treatment. The pH value of the ultrafiltration permeate is adjusted, and the final permeate meets the discharge or reuse standards, completing the entire treatment process.

[0007] Furthermore, the specific steps of step 1 are as follows: Step 1.1: Unload the old leachate into the leachate unloading collection pool of the landfill, and distribute it to multiple injection points in the landfill pit through the conveying pipeline. The valve spacing between adjacent injection points is 15-20m. Step 1.2: Open the valves at each injection point in sequence every 3 to 5 days to ensure that the two types of leachate are mixed evenly. The daily injection volume of the old leachate in the first mixture should not exceed the daily production volume of the original leachate in the landfill pit.

[0008] Furthermore, in step 1.1, each injection point is equipped with an extension tube with a length of not less than 3 to 5 m.

[0009] Furthermore, in step 2, the rotating bar screen and its flushing valve are interlocked with the inlet water flow rate. When the inlet water flow rate is greater than 10 m³, the bar screen and flushing program are automatically started.

[0010] Furthermore, in step 2, an S-shaped stirring tube is used for aeration and stirring in the equalization tank.

[0011] Furthermore, in step 3, the operating parameters of the intermediate liquid in the anaerobic tank are controlled as follows: temperature 35-38℃, pH value 7.9-8.1, upward flow velocity not less than 0.5m / h, and sludge concentration in the reaction zone not less than 30g / L.

[0012] Furthermore, step 3 also includes sludge discharge during the operation of the anaerobic tank. The sludge discharge from the anaerobic tank continues until the temperature inside the sludge discharge pipe is the same as the temperature of the intermediate liquid inside the anaerobic tank.

[0013] Furthermore, the specific steps of step 4 are as follows: Step 4.1: After sedimentation in the anaerobic sedimentation tank, the intermediate liquid enters the denitrification tank. Based on the carbon-nitrogen ratio of the anaerobic effluent, a portion of the conditioning liquid is directly sent to the biological treatment tank through the bypass pipeline to maintain the BOD to total nitrogen ratio of the biological treatment system at no less than 10:1. Step 4.2: Nitrogen and COD removal are completed in the denitrification tank and nitrification tank. The dissolved oxygen concentration in the nitrification tank is controlled at 4 mg / L, the sludge concentration is 16-20 g / L, the nitrate return ratio is not less than 6:1, the water temperature is 31-33℃, and the pH value is not less than 7.0. If the pH is insufficient, liquid alkali is added for adjustment.

[0014] Furthermore, in step 4.1, the settled sludge from the anaerobic sedimentation tank is periodically returned to the anaerobic tank, and the remaining sludge is periodically discharged from the treatment system. In step 4.2, the aeration equipment in the biological treatment tank is equipped with a cooling system to cool the aeration device online in order to maintain a stable water temperature.

[0015] Furthermore, in step 5, the reflux ratio of the return water is increased during the ultrafiltration process to assist in stabilizing the nitrification and denitrification reactions of the biological system; The pH value of the ultrafiltration permeate is controlled at 6.5–7.5. When the ultrafiltration membrane tube becomes clogged, an online unclogging device is used.

[0016] Beneficial effects: The method for treating aged landfill leachate provided by this invention utilizes existing landfill pits as mixing and anaerobic fermentation pretreatment sites. Through multi-point rotational injection, the aged leachate is thoroughly mixed with the primary leachate, eliminating the need for dedicated landfill leachate pretreatment facilities and homogenization tanks, thus saving on infrastructure investment costs. This allows landfill leachate to be directly treated in conjunction with existing biochemical and membrane treatment processes at incineration power plants, avoiding the need for large amounts of acid-base regulators, flocculants, and coagulants required for separate treatment. This significantly reduces reagent consumption and operating costs while ensuring treatment effectiveness.

[0017] Furthermore, this invention, through the implementation of specific control standards for the entire treatment process (anaerobic intermediate liquid upflow velocity, sludge concentration in the anaerobic reaction zone, dissolved oxygen concentration in the nitrification tank, sludge concentration in the nitrification tank, nitrate reflux ratio, nitrification tank water temperature, nitrification tank pH value, carbon-nitrogen ratio of the biological system, pH value of ultrafiltration permeate, control of the daily total influent load of the anaerobic tank, and control of the daily total injection volume of old leachate), not only enhances the degradation efficiency of pollutants and prevents pipe scaling and equipment fouling, but also ensures the smooth progress of nitrification and denitrification reactions in the biological system and the stable operation of the membrane system. The final permeate, after deep treatment by ultrafiltration, nanofiltration, and reverse osmosis, consistently meets discharge standards or reuse requirements. Attached Figure Description

[0018] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0019] Figure 1 A schematic flowchart illustrating a method for treating old leachate from landfills provided in an embodiment of the present invention; Figure 2 A schematic diagram of the distribution of landfill pits in step 1 of the method for treating old leachate from landfills provided in this embodiment of the invention; Figure 3This is a schematic diagram showing the distribution of the equalization tank in step 2 of the method for treating old leachate from a landfill provided in an embodiment of the present invention.

[0020] Icons: 1-Landfill pit; 2-Pure leachate collection tank; 3-Leachate unloading collection tank; 4-Transfer pipe; 5-Equalization tank; 6-Compressed air pipe; 7-S-type stirring pipe; 8-Pre-sedimentation tank. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0022] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0023] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0024] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of this invention is in use. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention. In addition, the terms "first," "second," "third," etc., are only used to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0025] Furthermore, terms such as "horizontal," "vertical," and "sag" do not imply that components must be absolutely horizontal or suspended, but rather that they can be slightly tilted. For example, "horizontal" simply means that its direction is more horizontal relative to "vertical," and does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0026] In the description of this invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0027] The following detailed description of some embodiments of the present invention is provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0028] like Figure 1 As shown, the method for treating old landfill leachate provided by the present invention is implemented according to the following steps: Step 1: Inject the old leachate into landfill 1 in a multi-point rotation manner, mix it with the original leachate of landfill 1 and carry out anaerobic fermentation pretreatment to obtain the first mixed liquid; like Figure 2 As shown, in step 1.1, the old leachate is unloaded into the leachate unloading collection pool 3 of the landfill, and distributed to multiple injection points in the landfill pit 1 via the conveying pipeline 4. The valve spacing between adjacent injection points is 20m; each injection point is equipped with a forward extension pipe with a length of not less than 5m.

[0029] Step 1.2: Open the valves at each injection point in sequence every 3 days to ensure that the two types of leachate are mixed evenly. The daily injection volume of the old leachate in the first mixture should not exceed the daily production volume of the original leachate in landfill 1.

[0030] Specifically, the old leachate transported from the landfill is first unloaded into the landfill leachate unloading and collection pool 3 for temporary storage. It is then distributed to multiple leachate injection points in the waste pit 1 via the delivery pipe 4. The spacing between the control valves of adjacent injection points is uniformly set at 20m. The multi-point distributed pipeline can avoid local water quality imbalance caused by concentrated single-point addition of leachate. In actual operation, the valves of each injection point are opened in rotation every 3 days according to the pipeline layout sequence, and the leachate spray position is continuously changed to ensure that the old leachate from the landfill is mixed with the original leachate in the waste pit 1 on a large scale and in all directions. At the same time, the total daily injection volume of old leachate is controlled to ensure that its value does not exceed the total daily natural production of original leachate in the waste pit 1. The abundant carbon source of the original leachate is used to neutralize the water quality defects of old leachate, such as low COD, high ammonia nitrogen, and poor biodegradability. All injection points are equipped with forward-extending spray pipes with a length of not less than 5m to evenly spray leachate onto the surface of domestic waste in the garbage pit 1, preventing leachate from falling directly to the bottom of the pit along the concrete pool wall, extending the contact time between leachate and domestic waste, and simultaneously completing in-situ anaerobic fermentation pretreatment in the garbage pit 1 to pre-degrade some of the difficult-to-degrade pollutants, and balancing the overall water quality to obtain the first mixed liquid.

[0031] No separate homogenizing mixing tank or pretreatment structure is required. It can be implemented by simply modifying the existing pipelines in the plant area, which greatly reduces civil engineering investment, stabilizes the influent water quality from the source, and avoids the subsequent biological system from being impacted by high ammonia nitrogen leachate.

[0032] Step 2, as follows Figure 3 As shown, the first mixed liquid is collected and flows into the original leachate collection tank 2, filtered by the rotary bar screen machine and sent to the pre-sedimentation tank 8, and then flows by gravity to the equalization tank 5. The water quality is homogenized by using compressed air to agitate and obtain the equalization liquid. In step 2, the rotary bar screen and its flushing valve are interlocked with the influent flow rate. When the influent flow rate exceeds 10 m³, the bar screen and flushing program are automatically activated. S-shaped stirring pipes 7 are used for aeration and mixing in the equalization tank 5.

[0033] Specifically, the first mixed liquid formed at the bottom of the garbage pit 1 flows into the original leachate collection tank 2. The lifting pump in the tank continuously transports the mixed liquid to the rotating bar screen. The bar screen body, high-pressure flushing valve and water inlet flow meter are set with interlocking automatic control logic. When the instantaneous water inlet flow in the pipeline is greater than 10m³, the system automatically starts the bar screen rotation and high-pressure flushing to promptly intercept and flush away solid particles such as plastic debris, silt, and kitchen waste in the leachate. This effectively prevents the bar screen gaps and leachate delivery pipe 4 in the collection tank from becoming clogged, ensuring continuous and stable leachate delivery.

[0034] The filtered water flows by gravity into the pre-sedimentation tank 8 and the leachate equalization tank 5. The entire tank is covered with S-shaped stirring pipes 7 (which transmit compressed air through compressed air pipes 6) to enhance the mixing of landfill leachate and incinerator raw leachate in the equalization tank 5, so as to achieve uniform water quality. At the same time, the stirring device can be used to stir the sludge in the leachate evenly and prevent it from settling, thereby reducing the accumulation of sludge at the bottom of the equalization tank 5.

[0035] Step 3: The conditioning liquid is uniformly lifted from the conditioning tank 5 to the anaerobic tank for anaerobic degradation treatment to obtain intermediate liquid. The operating parameters of the anaerobic tank are controlled as follows: temperature 37℃, pH value 8, upward flow velocity not less than 0.5m / h, and sludge concentration in the reaction zone not less than 30g / L. This also includes sludge discharge during anaerobic tank operation, which continues until the temperature inside the sludge discharge pipe is the same as the temperature of the intermediate liquid in the anaerobic tank.

[0036] Specifically, the conditioning liquid first flows by gravity into the pre-sedimentation tank 8 and the conditioning tank 5 to complete the initial sedimentation of large particulate suspended solids. Then, it is evenly and continuously transported to the anaerobic tank throughout the day by a booster pump. The total daily influent load of the anaerobic tank is controlled to not exceed 80% of the rated design capacity of the equipment to prevent the high load operation from causing the decline of anaerobic microbial activity.

[0037] The anaerobic digester is operated under controlled conditions throughout the entire process (maintaining a stable reaction temperature of 37℃, pH value of 8.0, upward flow velocity of not less than 0.5m / h, and sludge concentration in the reaction zone of not less than 30g / L). To maintain the continuous temperature of the anaerobic digester within this range, the circulating liquid in the anaerobic digester needs to be circulated and heated. In this invention, a control valve can be added downstream of the anaerobic circulation pump, and a heat exchanger can be set up in parallel with the control valve. The low-temperature leachate from the anaerobic digester circulation pump end is first introduced into the heat exchanger as the cold end. At the same time, another control valve is added downstream of the cooling pump outlet of the nitrification tank in step 4, and this control valve is also connected in parallel with the heat exchanger. The high-temperature leachate from the nitrification tank is introduced into the heat exchanger as the hot end. Utilizing the temperature difference between the two leachates, heat is fully transferred and exchanged through the stainless steel partition of the compartment to achieve the system's temperature equilibrium point. This method achieves temperature and energy balance and energy saving through temperature control, greatly reducing the amount of steam used for heating the anaerobic tank and the energy consumption and water loss of the equipment used for cooling the nitrification tank. It also provides supplementary heating and cooling equipment.

[0038] In addition, if scaling or flow reduction occurs in the circulation pipes, acid washing devices for the anaerobic pond circulation pipes and landfill leachate treatment equipment (existing technologies, which will not be elaborated on here) can be used for online acid washing and unblocking to ensure the normal circulation flow of the anaerobic pond and maintain the smooth progress of the anaerobic reaction in the anaerobic pond.

[0039] The anaerobic tank undergoes standardized sludge removal operations, continuously discharging aged anaerobic sludge from the bottom of the tank. The temperature of the leachate in the sludge removal pipe is monitored in real time until the temperature of the discharged liquid is consistent with the temperature of the intermediate liquid inside the anaerobic tank before the sludge removal valve is closed. This ensures that the deposited sludge in the anaerobic tank sludge removal pipe is completely removed, ensuring that the bacterial community in the anaerobic tank is in an optimal active state. After the hydrolysis, acidification, and methanogenic degradation of macromolecular organic matter in the anaerobic tank, the recalcitrant pollutants are transformed into small-molecule, easily biodegradable substrates, resulting in an intermediate liquid with significantly improved biodegradability. This provides a good water quality foundation for subsequent nitrification and denitrification processes.

[0040] Step 4: After sedimentation, the intermediate liquid is sequentially fed into the denitrification tank and the nitrification tank for biological denitrification and COD removal treatment to maintain the stability of the carbon-nitrogen ratio and operating parameters of the biological system. Step 4.1: After sedimentation in the anaerobic sedimentation tank, the intermediate liquid enters the denitrification tank. Based on the carbon-nitrogen ratio of the anaerobic effluent, a portion of the conditioning liquid is directly sent to the biological treatment tank through the bypass pipeline to maintain the BOD to total nitrogen ratio of the biological treatment system at no less than 10:1. In Step 4.1, the sedimented sludge from the anaerobic sedimentation tank is periodically returned to the anaerobic tank, and the remaining sludge is periodically discharged from the treatment system. Step 4.2: Nitrogen and COD removal are completed in the denitrification and nitrification tanks. The dissolved oxygen concentration in the nitrification tank is controlled at 4 mg / L, the sludge concentration at 16 g / L, the nitrate return ratio at no less than 6:1, the water temperature at 32℃, and the pH value at no less than 7.0. If the pH is insufficient, liquid alkali is added for adjustment. In Step 4.2, the aeration equipment in the biological treatment tank is equipped with a cooling system to provide online cooling of the aeration devices to maintain stable water temperature.

[0041] Specifically, the intermediate liquid from the overflow of the anaerobic tank flows by gravity into the anaerobic sedimentation tank to complete the sludge-water separation. The anaerobic sludge accumulated in the sedimentation tank is returned to the anaerobic tank at fixed intervals to continuously replenish the functional anaerobic microbial community in the tank. The aged excess sludge is periodically discharged for dewatering and disposal. The carbon-to-nitrogen ratio of the effluent from the anaerobic sedimentation tank is monitored in real time. Once the BOD / TN ratio is lower than 10:1, the bypass pipeline is immediately opened to extract a portion of the conditioning liquid and send it directly into the biological treatment tank to supplement the carbon source. This forces the maintenance of the BOD / TN ratio of the biological treatment system to be no less than 10:1, solving the problems of insufficient carbon source and hindered denitrification reaction in old leachate.

[0042] The supernatant flows sequentially through the denitrification tank and the nitrification tank to complete nitrogen and COD removal. Throughout the process, the dissolved oxygen concentration in the nitrification tank is controlled at 4 mg / L, the sludge concentration in the biological treatment tank is 16 g / L, the nitrate return ratio is not less than 6:1, the system water temperature is kept stable at 32℃, and the pH value in the nitrification tank is not lower than 7.0 throughout the process. If the pH value is insufficient, liquid alkali is automatically added to adjust it, ensuring the survival environment of nitrifying bacteria. At the same time, a cooling system and aeration device (existing technology, which will not be described in detail here) can be used for online cooling to offset the heat generated by aeration and prevent high temperature from inhibiting the activity of nitrifying bacteria.

[0043] This invention enables the system to efficiently oxidize ammonia nitrogen and reduce nitrates through multi-parameter coordinated control, significantly reducing total nitrogen and residual COD in leachate, stabilizing the operating conditions of the biochemical system, and preventing problems such as sludge bulking and decreased denitrification efficiency when disposing of old leachate.

[0044] Step 5: The biochemically treated effluent is then subjected to ultrafiltration, nanofiltration and reverse osmosis membrane systems for further treatment. The pH value of the ultrafiltration permeate is adjusted, and the final permeate meets the discharge or reuse standards, thus completing the treatment of old leachate.

[0045] In step 5, the reflux ratio of the return water is increased during the ultrafiltration process to help stabilize the nitrification and denitrification reactions of the biological system; the pH value of the ultrafiltration permeate is controlled to 7; and a tubular ultrafiltration membrane cleaning device is used for online cleaning when the ultrafiltration membrane tubes become clogged.

[0046] Specifically, the nitrified effluent from the biological treatment tank is transported to the ultrafiltration unit for sludge-water separation. During operation, the ultrafiltration concentrate reflux ratio is actively increased, and the reflux liquid is sent back to the upstream biological treatment tank to continuously replenish the sludge concentration of the biological treatment system and stabilize the nitrification and denitrification reaction environment. The pH value of the ultrafiltration permeate is precisely controlled to a stable 7 by a hydrochloric acid dosing metering device to prevent the acid and alkaline environment from excessively affecting the operation of the downstream nanofiltration and reverse osmosis membrane elements and causing scaling tendency.

[0047] It should be noted that if the ultrafiltration membrane tube becomes clogged or the transmembrane pressure difference increases during operation, a tubular ultrafiltration membrane unblocking device (an existing technical means, which will not be elaborated on here) can be used to carry out physical and mechanical unblocking, which can improve employee work efficiency, shorten equipment downtime, and improve continuous operation efficiency.

[0048] The ultrafiltration solution sequentially enters the nanofiltration and reverse osmosis membrane deep purification units. Nanofiltration removes residual large-molecule recalcitrant organic matter and trace heavy metals from the water, while reverse osmosis further removes salt, residual total nitrogen, and trace pollutants. The multi-stage membranes work together to ensure that the final reverse osmosis permeate meets the standards in the long term. The permeate can be directly discharged or reused in the circulating cooling water system of the incineration power plant, realizing the recycling of water resources.

[0049] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for treating old leachate from a landfill, characterized in that, Includes the following steps: Step 1: Inject the old leachate into the landfill pit in a multi-point rotation manner, mix it with the original leachate of the landfill pit and carry out anaerobic fermentation pretreatment to obtain the first mixed liquid; Step 2: The first mixed liquid is collected and flows into the original leachate collection tank. After being filtered by the rotary bar screen, it is sent to the pre-sedimentation tank and flows by gravity to the equalization tank. Compressed air is used to stir the water to achieve water homogenization and obtain the equalization liquid. Step 3: The conditioning liquid is uniformly lifted from the conditioning tank to the anaerobic tank for anaerobic degradation treatment to obtain intermediate liquid; Step 4: After sedimentation, the intermediate liquid is sequentially fed into the denitrification tank and the nitrification tank for biological denitrification and COD removal treatment to maintain the stability of the carbon-nitrogen ratio and operating parameters of the biological system. Step 5: The biochemically treated effluent is then subjected to ultrafiltration, nanofiltration and reverse osmosis membrane systems for further treatment. The pH value of the ultrafiltration permeate is adjusted, and the final permeate meets the discharge or reuse standards, completing the entire treatment process.

2. The method for treating old leachate from landfills according to claim 1, characterized in that, The specific steps of step 1 are as follows: Step 1.1: Unload the old leachate into the leachate unloading collection pool of the landfill, and distribute it to multiple injection points in the landfill pit through the conveying pipeline. The valve spacing between adjacent injection points is 15-20m. Step 1.2: Open the valves at each injection point in sequence every 3 to 5 days to ensure that the two types of leachate are mixed evenly. The daily injection volume of the old leachate in the first mixture shall not exceed the daily production volume of the original leachate in the landfill pit.

3. The method for treating old leachate from landfills according to claim 2, characterized in that, In step 1.1, each injection point is equipped with a protruding tube with a length of not less than 3 to 5 m.

4. The method for treating old leachate from landfills according to claim 1, characterized in that, In step 2, the rotating bar screen machine and its flushing valve are interlocked with the inlet water flow rate. When the inlet water flow rate is greater than 10m³, the bar screen machine and flushing program are automatically started.

5. The method for treating old leachate from landfills according to claim 1, characterized in that, In step 2, an S-shaped stirring tube is used for aeration and stirring in the regulating tank.

6. The method for treating old leachate from landfills according to claim 1, characterized in that, In step 3, the intermediate liquid temperature in the anaerobic tank is controlled at 35-38℃, the pH value is 7.9-8.1, the upward flow velocity is not less than 0.5m / h, and the sludge concentration in the reaction zone is not less than 30g / L.

7. The method for treating old landfill leachate according to claim 1, characterized in that, Step 3 also includes sludge discharge during the operation of the anaerobic tank. The sludge discharge from the anaerobic tank continues until the temperature inside the sludge discharge pipe is the same as the temperature of the intermediate liquid inside the anaerobic tank.

8. The method for treating old leachate from landfills according to claim 1, characterized in that, The specific steps of step 4 are as follows: Step 4.1: After sedimentation in the anaerobic sedimentation tank, the intermediate liquid enters the denitrification tank. Based on the carbon-nitrogen ratio of the anaerobic effluent, a portion of the conditioning liquid is directly sent to the biological treatment tank through the bypass pipeline to maintain the BOD to total nitrogen ratio of the biological treatment system at no less than 10:

1. Step 4.2: Nitrogen and COD removal are completed in the denitrification tank and nitrification tank. The dissolved oxygen concentration in the nitrification tank is controlled at 4 mg / L, the sludge concentration is 16-20 g / L, the nitrate return ratio is not less than 6:1, the water temperature is 31-33℃, and the pH value is not less than 7.

0. If the pH is insufficient, liquid alkali is added for adjustment.

9. The method for treating old landfill leachate according to claim 8, characterized in that, In step 4.1, the sludge settled in the anaerobic sedimentation tank is periodically returned to the anaerobic tank, and the remaining sludge is periodically discharged from the treatment system. In step 4.2, the aeration equipment in the biological tank is equipped with a cooling system to cool the aeration device online in order to maintain a stable water temperature.

10. The method for treating old landfill leachate according to claim 9, characterized in that, In step 5, the reflux ratio of the return water is increased during the ultrafiltration process to help stabilize the nitrification and denitrification reactions of the biological system. The pH value of the ultrafiltration permeate is controlled at 6.5–7.

5. When the ultrafiltration membrane tube becomes clogged, an online unclogging device is used.

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