A method for treating low carbon-nitrogen ratio landfill leachate

Abstract

The present invention discloses a method for treating low-carbon-nitrogen ratio landfill leachate, comprising the following steps: S1: after the landfill leachate passes through a regulating tank to adjust the water quality, it enters a stripping unit; S2: the effluent from the stripping unit sequentially enters a bio-rotating disc unit and an AOOA reactor; S3: part of the effluent from the AOOA reactor flows back to the bio-rotating disc unit and the AOOA reactor, and part of it enters an MBR unit; S4: the water produced from the MBR unit enters a coagulation and sedimentation unit, and part of the sludge from the MBR unit flows back to the bio-rotating disc unit and the AOOA reactor unit, and part of it enters a coagulation and sedimentation unit; S5: the water produced from the coagulation and sedimentation unit sequentially enters a primary Fenton unit, a BAF unit, and a secondary Fenton unit for deep oxidation; S6: the effluent from the secondary Fenton unit enters a buffer tank to adjust the water quality for discharge. The treatment method of the present invention is mainly applicable to the treatment of stock landfill leachate and aged leachate with a low carbon-nitrogen ratio. It has a reasonable design and ensures that the aged landfill leachate can meet discharge standards through the coordination of multiple processes, with low treatment costs.

Description

Technical Field

[0001] The invention relates to the technical field of landfill leachate treatment, and in particular to a method for treating landfill leachate with a low carbon-nitrogen ratio. Background Art

[0002] With the increase of my country's urban population, the expansion of urban scale and the improvement of residents' living standards, the amount of domestic waste generated is also increasing day by day. At present, the average daily garbage production per person in my country is 0.8-1.1 kg, and it is increasing at a rate of 10% each year. It is expected to reach 409 million tons by 2030 and 528 million tons by 2050.

[0003] During the landfill treatment process of garbage, leachate is produced due to anaerobic fermentation, organic matter decomposition, leaching and erosion by precipitation, and groundwater infiltration. It is a high-concentration wastewater with a high content of organic pollutants, complex properties, and difficulty in treatment. The leachate contains a large amount of difficult-to-degrade organic matter, heavy metal ions, high ammonia nitrogen, and a variety of toxic and harmful pollutants, which will pose long-term potential hazards to the environment, animals, plants, and humans. The development of a method for collecting and efficiently treating leachate has become a water treatment problem that needs to be urgently solved in my country.

[0004] Currently, the more mature methods for treating landfill leachate include physicochemical and biological methods. Physicochemical methods include coagulation and sedimentation, adsorption, stripping, membrane separation, and advanced oxidation technologies. Biological methods are generally divided into anaerobic and aerobic methods. A small number of treatment methods use recharge or constructed wetlands.

[0005] The domestic leachate treatment industry largely relies on membrane separation technology, but this inevitably creates issues with concentrates. Previously, membrane concentrates were often re-injected into landfills or transported to sewage treatment plants for treatment. However, the Ministry of Ecology and Environment issued a notice soliciting public opinion on the national standard, "Pollution Control Standards for Municipal Solid Waste Landfills (Draft for Comment)," which explicitly mandates that concentrates generated from leachate treatment be disposed of separately and not re-injected into municipal solid waste landfills or enter centralized sewage treatment facilities. Furthermore, if other concentrate treatment methods, such as evaporation and advanced oxidation, are employed, the costs of membrane separation technology will inevitably increase significantly.

[0006] In this case, the advantages of biological treatment of landfill leachate are greatly increased. However, due to the complex water quality of landfill leachate, a single biological process is difficult to achieve standard discharge. Among them, anaerobic biological treatment is mainly suitable for leachate with high COD, but its ability to remove ammonia nitrogen in the leachate is poor. Aerobic biological treatment has a good degradation effect on ammonia nitrogen and biodegradable organic matter, but most sanitary landfills in China are currently closed, and the biodegradability of aged leachate is poor. If traditional aerobic biological treatment is to be used to treat landfill leachate to achieve standard total nitrogen discharge, a large amount of carbon source must be added. The increase in carbon source will also significantly increase the overall treatment cost, which is contrary to the original intention of using biological treatment for landfill leachate.

[0007] In view of this, the inventors of the present application have invented a method for treating low carbon-nitrogen ratio landfill leachate. Summary of the Invention

[0008] The purpose of the present invention is to provide a treatment method for low carbon-nitrogen ratio landfill leachate with reasonable design, low cost and good treatment effect.

[0009] To achieve the above object, the present invention adopts the following technical solution: a method for treating low carbon-nitrogen ratio landfill leachate, comprising the following steps:

[0010] S1: After the leachate passes through the regulating tank to adjust the water quality, it enters the stripping unit to remove part of the ammonia nitrogen and sulfide;

[0011] S2: The effluent from the stripping unit enters the biological rotary unit and the AOOA reactor in sequence, where a short-range nitrification and denitrification reaction occurs to remove suspended solids and organic matter in the water;

[0012] S3: Part of the effluent from the AOOA reactor flows back to the biological rotary unit and the AOOA reactor, and part of it flows into the MBR unit for mud-water separation;

[0013] S4: The water produced by the MBR unit enters the coagulation and sedimentation unit. Part of the sludge from the MBR unit flows back to the biological rotary unit and the AOOA reactor unit, and part of it enters the coagulation and sedimentation unit to remove colloidal particles.

[0014] S5: The water produced by the coagulation and sedimentation unit enters the primary Fenton unit, BAF unit, and secondary Fenton unit for deep oxidation in sequence;

[0015] S6: The effluent from the secondary Fenton unit enters the buffer tank to regulate the water quality for discharge.

[0016] Furthermore, regulating the water quality of the regulating tank in step S1 means that the conductivity of the effluent from the regulating tank is less than 3000 S / m.

[0017] Furthermore, the stripping unit includes a primary reaction tank, a primary sedimentation tank, a stripping tower, a secondary reaction tank, and a secondary sedimentation tank connected in sequence. Lime is added to the primary reaction tank to remove metal ions, and the pH value of the effluent from the primary sedimentation tank is adjusted to 8.0-9.0 and the water temperature is 20-30°C. The pH value of the water in the secondary reaction sedimentation tank is adjusted to 6.5-7.5.

[0018] Furthermore, the bio-rotor unit includes a water distribution tank, a bio-rotor connected to the water distribution tank, and a carbon source adding device. The rotation speed of the bio-rotor is 2.0-4.0 r / min.

[0019] Furthermore, the AOOA reactor includes four aeration tanks and an aeration device. The four aeration tanks are a primary anoxic tank, a primary aerobic tank, a secondary aerobic tank and a secondary anoxic tank connected in sequence. The primary anoxic tank is connected to a biological rotary disc and a carbon source dosing device. The aeration device supplies oxygen to the four aeration tanks, and part of the liquid in the secondary anoxic tank flows back to the water distribution tank and the primary anoxic tank.

[0020] Furthermore, the MBR unit includes an MBR membrane pool and a hollow fiber ultrafiltration membrane assembly located in the MBR membrane pool. The sludge outlet of the MBR unit is connected to the water distribution tank and the first-level anoxic tank through the MBR return pipe, and part of the sludge from the MBR unit flows back to the water distribution tank and the first-level anoxic tank.

[0021] Furthermore, the MBR unit also includes a backwash tank and a cleaning water tank, both of which are connected to the water production port of the hollow fiber ultrafiltration membrane assembly, and the hollow fiber ultrafiltration membrane assembly is cleaned through the backwash tank or the cleaning water tank.

[0022] Furthermore, the MBR unit also includes a water supply pipe, which is connected to the backwash tank, the cleaning water tank and the water production port of the hollow fiber ultrafiltration membrane assembly. The backwash tank or the cleaning water tank is replenished with water through the water supply pipe, and the vacuum in the hollow fiber ultrafiltration membrane assembly is assisted by the water supply pipe.

[0023] Furthermore, the coagulation sedimentation includes a coagulation reaction tank and a coagulation sedimentation tank connected to each other, and polyferric sulfate is added to the coagulation reaction tank to remove colloidal particles.

[0024] Furthermore, step S6 of adjusting the water quality of the buffer tank refers to adjusting the pH value of the effluent from the buffer tank to 7.0-8.0.

[0025] After adopting the above technical solution, the present invention has the following advantages compared with the prior art:

[0026] The treatment method of the present invention is mainly applicable to the treatment of existing landfill leachate and aged leachate with a low carbon-nitrogen ratio. It has a reasonable design and the average degradation rates of COD, ammonia nitrogen and total nitrogen are 68%, 91% and 85% respectively. Through the coordination of multiple processes, it is ensured that the aged landfill leachate can meet the discharge standards and the treatment cost is low. By connecting the biological turntable unit, AOOA reactor and MBR unit in sequence, the landfill leachate short-range nitrification and denitrification process can reduce the amount of carbon source added, lower aeration energy consumption and higher treatment efficiency per unit tank volume compared to the traditional aerobic activated sludge method. Through the design of the primary anoxic tank, primary aerobic tank, secondary aerobic tank and secondary anoxic tank, the dissolved oxygen in the aerobic tank and the mixed liquid reflux in the secondary anoxic tank are strictly controlled to avoid the impact of the aerobic liquid reflux on the front-end denitrification (biological turntable and primary anoxic tank) process. At the same time, the front-denitrification unit can also make full use of the carbon source. The MBR unit structure has the characteristics of easy cleaning and disassembly of the membrane components. The backwash tank and the cleaning water tank are equipped with a cleaning function. It is more suitable for the treatment of landfill leachate with complex components, easy membrane fouling, and the need for frequent cleaning of membrane components. It has strong anti-pollution ability. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Flow chart of the processing method of the present invention;

[0028] Figure 2 Schematic diagram of the stripping tower of the present invention;

[0029] Figure 3 Schematic diagram of the bio-rotating disc unit and AOOA reactor of the present invention;

[0030] Figure 4 、 Figure 5 for Figure 3 A partial enlarged view of

[0031] Figure 6 Schematic diagram of the MBR unit of the present invention;

[0032] Figure 7 、 Figure 8 for Figure 6 A partial enlarged view of

[0033] Figure 9 This is a schematic diagram of biofilm formation on a rotating biodisc in an experiment according to an embodiment of the present invention;

[0034] Figure 10 The data are as follows: The degradation effect of the bio-rotating disc unit and the AOOA reactor on the COD of landfill leachate in the experiment of the embodiment of the present invention;

[0035] Figure 11 The bio-rotating disc coupled AOOA process is used to treat NH4 + -N degradation effect statistics;

[0036] Figure 12 The statistical results of the degradation effect of the bio-rotating disc coupled with AOOA process on landfill leachate TN in the experiment of the present invention are shown in Figure 2.

[0037] Figure 13 The average pollutant removal rate of each unit is compared with the influent in the test of the embodiment of the present invention;

[0038] Figure 14 The average pollutant removal rate of each unit in the test of the embodiment of the present invention is the previous unit as the control;

[0039] Figure 15 Schematic diagram of the relationship between temperature and pollutant removal rate in the experiment of the embodiment of the present invention. DETAILED DESCRIPTION

[0040] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0041] It should be noted that in the present invention, the terms "up", "down", "left", "right", "vertical", "horizontal", "inside", "outside", etc. are all based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element of the present invention must have a specific orientation, and therefore cannot be understood as a limitation on the present invention.

[0042] Example

[0043] Cooperate Figures 1 to 8 As shown, the present invention discloses a method for treating low-carbon-nitrogen ratio landfill leachate, which is mainly applicable to the treatment of existing landfill leachate and aged leachate with low carbon-nitrogen ratio. It can effectively reduce the amount of carbon source added and the aeration energy consumption is also lower.

[0044] A method for treating low carbon-nitrogen ratio landfill leachate comprises the following steps:

[0045] S1: After the leachate passes through the regulating tank to adjust the water quality, it enters the stripping unit to remove part of the ammonia nitrogen and sulfide;

[0046] S2: The effluent from the stripping unit enters the biological rotary unit and the AOOA reactor in sequence, where a short-range nitrification and denitrification reaction occurs to remove suspended solids and organic matter in the water;

[0047] S3: Part of the effluent from the AOOA reactor flows back to the biological rotary unit and the AOOA reactor, and part of it flows into the MBR unit for mud-water separation;

[0048] S4: The water produced by the MBR unit enters the coagulation and sedimentation unit. Part of the sludge from the MBR unit flows back to the biological rotary unit and the AOOA reactor unit, and part of it enters the coagulation and sedimentation unit to remove colloidal particles.

[0049] S5: The water produced by the coagulation and sedimentation unit enters the primary Fenton unit, BAF unit, and secondary Fenton unit for deep oxidation in sequence;

[0050] S6: The effluent from the secondary Fenton unit enters the buffer tank to regulate the water quality for discharge.

[0051] In step S1, the landfill leachate to be treated (hereinafter referred to as the leachate) first enters a regulating tank. The regulating tank regulates the water quality by ensuring that the conductivity of the effluent is less than 3000 S / m. Specifically, sodium bicarbonate is added until the ammonia nitrogen / alkalinity ratio in the leachate is 1 / 5. The regulating tank also provides a buffering effect, ensuring a stable flow of water into the stripping unit.

[0052] Cooperate Figure 2 As shown, the stripping unit includes a primary reaction tank, a primary sedimentation tank, a stripping tower, a secondary reaction tank, and a secondary sedimentation tank which are connected in sequence.

[0053] The stripping unit includes a primary reaction tank, a primary sedimentation tank, a stripping tower, a secondary reaction tank, and a secondary sedimentation tank connected in sequence. Lime is added to the primary reaction tank to remove metal ions. The pH value of the effluent from the primary sedimentation tank is adjusted to 8.0-9.0 and the water temperature is adjusted to 20-30°C. The pH value of the water in the secondary reaction sedimentation tank is adjusted to 6.5-7.5.

[0054] The effluent from the regulating tank enters the primary reaction tank, where the liquid reacts with lime to form insoluble salts of metal ions. This salt is then precipitated in the primary sedimentation tank. The effluent (supernatant) from the primary sedimentation tank enters the stripping tower. The pH of the effluent is adjusted to 8.0-9.0 (by adding sodium hydroxide or other suitable substances in the primary reaction tank) and the water temperature is maintained at 20-30°C (with appropriate heating). This denitrification process is carried out by the multi-faceted hollow packing balls within the stripping tower, which maintain a gaseous ammonia concentration below the equilibrium concentration under these conditions. This allows dissolved ammonia in the wastewater to cross the gas-liquid interface and enter the gas phase for removal. The ammonia-containing waste gas is then absorbed in saturated sulfuric acid in a crystallizer to produce ammonium sulfate. The deammoniated liquid enters the secondary reaction tank and secondary sedimentation tank, where acid is added to adjust the pH to 6.5-7.5, preferably 7. Preferably, when the ammonia nitrogen in the stripping tower is 3000 mg / L, the water temperature is 25°C, the pH is 8.5, the gas-liquid ratio is 10000, and the stripping time is 2h, ammonia stripping pretreatment can achieve the best removal effect under relatively energy-saving and environmentally friendly conditions, and the ammonia nitrogen removal rate can reach 45%, and the sulfide removal rate can reach 80%.

[0055] Cooperate Figures 3 to 8As shown, the effluent from the secondary sedimentation tank enters the bio-rotor unit, AOOA reactor and MBR unit. The bio-rotor unit and AOOA reactor are integrated into one device, which is convenient for transfer and flexible in use.

[0056] Cooperate Figures 3 to 5 As shown, the bio-rotor unit includes a water distribution tank, a bio-rotor connected to the water distribution tank, and a carbon source dosing device. The bio-rotor includes a water tank and a rotating disk body partially immersed in the water tank. The carbon source dosing device is connected to the water distribution tank.

[0057] After processing in the stripping unit, the liquid has been partially stripped of ammonia, nitrogen, and sulfides. The pH of the liquid is adjusted to approximately 7 before entering the distribution tank of the bio-disc unit. A carbon source is added to the distribution tank to increase the carbon-nitrogen ratio of the leachate, making it biodegradable. Specifically, the carbon source is added until the BOD / TN ratio reaches 2. The distribution tank is also equipped with a stirring shaft and an online pH sensor. The stirring shaft has stirring blades, which rotate to thoroughly mix the leachate and carbon source in the distribution tank. The online pH sensor monitors the pH value of the leachate in the distribution tank in real time.

[0058] Wherein, the carbon source adding device includes a medicine dispensing box, the medicine dispensing box is provided with a medicine dosing port, and the medicine dispensing box is connected to a water supply pipe, and a stirring shaft is provided in the medicine dispensing box. Glucose or sodium acetate or other suitable substances are added to the medicine dispensing box as a carbon source through the medicine dosing port, and water is added to the medicine dispensing box through the water supply pipe to dissolve the carbon source. The stirring shaft is provided with stirring blades, and the rotation of the stirring shaft accelerates the dissolution of the substances in the medicine dispensing box. The liquid with the carbon source dissolved in the medicine dispensing box is output to the water distribution tank or the primary anoxic tank mentioned later through a dosing pump. In addition, the carbon source can also be added to the regulating tank in step S1, and added to the feed liquid at BOD / TN=2. At this time, no carbon source is added to the water distribution tank.

[0059] The bio-disc consists of a water tank and a rotating disc partially submerged in the tank. Liquid from the distribution tank flows into the bio-disc, partially submerging the disc. As the disc rotates, it alternately comes into contact with the liquid and air in the tank. After a period of rotation, a biofilm forms on the disc. While in the liquid, the biofilm absorbs organic pollutants from the wastewater and absorbs dissolved oxygen from the water film surrounding the biofilm, breaking down the organic matter and enabling microbial growth. When the disc is removed from the wastewater, it comes into contact with air, which continuously dissolves into the water film, increasing the dissolved oxygen content. During this process, the biofilm attached to the disc, the wastewater, and the air not only transfer organic matter (BOD, COD) and oxygen, but also other substances such as CO2 and NH3. This creates a continuous process of adsorption, oxidation, decomposition, and oxygen absorption, resulting in continuous purification of the wastewater.

[0060] The bio-rotating disc process is a type of biofilm wastewater biological treatment technology, which is an artificial enhancement of wastewater irrigation and land treatment. This treatment method allows bacteria, fungi, and micro-animals such as protozoa to grow and reproduce on the bio-rotating disc filler carrier, and can also produce a certain amount of Bacillus, which is a type of Gram-negative bacteria and one of the most tolerant cells in nature. These microorganisms combine together to form a film-like biological sludge - biofilm.

[0061] Experimental data showed that COD removal reached a high level when the bio-disc speed was approximately 2.0 ≤ n ≤ 4.0 r / min. The COD removal efficiency reached its peak at n = 3.5 r / min, with a removal rate of 56%. When the bio-disc speed was 2.0 ≤ n ≤ 5.5 r / min, the ammonia nitrogen removal rate reached 48%, and at n = 3.2 r / min, the ammonia nitrogen removal rate reached 50%. When the bio-disc speed was set at 1.5 ≤ n ≤ 3.0 r / min, the total nitrogen removal rate reached 35%, and at n = 2.5 r / min, the total nitrogen removal rate was the highest, reaching 40%. Observation of biofilm formation on the bio-disc during this stage revealed a dense, brownish-yellow biofilm approximately 0.5 mm thick, indicating healthy microbial growth.

[0062] The AOOA reactor includes four aeration tanks and an aeration device. The four aeration tanks are a primary anoxic tank, a primary aerobic tank, a secondary aerobic tank and a secondary anoxic tank connected in sequence. The primary anoxic tank is connected to a biological rotary disk and a carbon source dosing device. The aeration device includes four aeration pipes that extend into the four aeration tanks in sequence. The secondary anoxic tank is connected to a water distribution tank and a primary anoxic tank through an internal reflux pipe.

[0063] All four aeration tanks are microaerobic, arranged in an "AOOA" pattern (anoxic, aerobic, aerobic, and anoxic) based on aeration volume. These are the primary anoxic tank, the primary aerobic tank, the secondary aerobic tank, and the secondary anoxic tank, respectively. Short-term nitrification and denitrification reactions occur within these four aeration zones. The AOOA pattern prevents the backflow of aerobic feed from impacting the front-end denitrification process (the bio-rotating disc and the primary anoxic tank). The pre-denitrification unit also fully utilizes the carbon source. The dissolved oxygen concentration in the primary and secondary aerobic tanks is less than 1 mg / L.

[0064] Each aeration tank has an effective volume of 10m³. The aeration system provides oxygen to each tank, and a plug-flow agitator is installed at the bottom of each tank to ensure thorough mixing and reaction of the liquid in the tank. Each aeration tank is equipped with an online DO (dissolved oxygen) sensor and an online pH sensor to monitor the dissolved oxygen and pH values ​​in each tank in real time.

[0065] After treatment by the bio-rotating disc, the slurry enters the primary anoxic tank, where it is sequentially pumped through the entire reactor via a centrifugal pump. Short-cut nitrification and denitrification reactions occur in the primary anoxic tank, the primary aerobic tank, and the secondary aerobic tank. The slurry is then buffered in the secondary anoxic tank, forming an anoxic slurry reflux. Part of the slurry in the secondary anoxic tank flows through a pipeline to the back-end MBR for treatment, while the remaining portion flows back through an internal reflux pipeline to the water distribution tank and the primary anoxic tank. This slurry (anoxic slurry) facilitates the denitrification process in the bio-rotating disc and the primary anoxic tank. The reflux ratio of the slurry returning to the primary anoxic tank is 500%.

[0066] Nitrification (aerobic) and denitrification (anoxic) biological treatment is increasingly being used in the treatment of high-concentration organic wastewater. Biological treatment through nitrification and denitrification can remove COD, BOD, and NH3-N through biodegradation. Short-term nitrification and denitrification involves the oxidation of ammonium ions to nitrite ions, which are then directly reduced to nitrogen gas through denitrification.

[0067] The complete nitrification and denitrification process is as follows:

[0068]

[0069] The short-range nitrification and denitrification process is as follows:

[0070]

[0071] It can be seen from the above reaction formula that short-term nitrification and denitrification reduces the steps of converting nitrite into nitrate and reducing nitrate to nitrite compared with full-process nitrification and denitrification, and short-term nitrification saves 25% of oxygen compared with complete nitrification, reducing energy consumption; shortening the reaction process and saving the volume of the reaction tank; achieving denitrification under low C / N conditions, less carbon source consumption, and reducing costs; reducing sludge production and saving sludge disposal costs.

[0072] After the reaction in AOOA reactor, COD can be further removed Cr , ammonia nitrogen, total nitrogen, chroma, TDS, etc., can significantly remove suspended solids and organic matter in wastewater, with an average COD removal rate of 40%-50%, a suspended COD removal rate of 60%-80%, and an effluent suspended solids concentration of <50mg / L; providing excellent influent water quality (i.e. improving the biodegradability of wastewater) conditions and improving the efficiency of aerobic treatment.

[0073] Cooperate Figures 6 to 8 As shown, the MBR unit includes an MBR membrane pool connected to the secondary anoxic tank, a hollow fiber ultrafiltration membrane assembly is provided in the MBR membrane pool, and the sludge outlet of the MBR unit is connected to the water distribution tank and the primary anoxic tank through the MBR return pipe.

[0074] The feed from the secondary anoxic tank flows through a pipeline into the MBR membrane tank, where it is filtered through hollow fiber ultrafiltration modules to achieve sludge-water separation. The filtered permeate enters the next stage of coagulation and sedimentation treatment. The filtered sludge is partially returned to the water distribution tank (bio-rotating unit) and the primary anoxic tank (AOOA reactor) through the MBR return pipe, while the remaining portion is returned to the sludge tank for filtration by a plate and frame filter press. Part of the sludge is returned to the bio-rotating unit and AOOA reactor to maintain the sludge concentration at the front end of the system, ensuring smooth reaction progress. The return from the MBR membrane tank to the AOOA reactor ensures a sludge return ratio of 330% at the front end of the system.

[0075] In this embodiment, the MBR membrane tanks include two parallel-arranged tanks, one of which serves as a backup to ensure that the system will not stop working due to a problem with one MBR membrane tank. Each MBR membrane tank is equipped with a liquid level sensor and a pH online sensor.

[0076] In this embodiment, the MBR unit further includes a backwash tank, which is connected to the water production port of the hollow fiber ultrafiltration membrane module.

[0077] The backwash tank primarily directs the filtered water from the hollow fiber ultrafiltration (HFU) membrane assembly back into the HFU membrane assembly, cleaning the membrane surface and removing contaminants. This ensures full utilization of the produced water and conserves water resources. During backwashing, the produced water from the HFU membrane assembly enters the backwash tank and is then pumped back into the HFU membrane assembly. This bypass of the backwash tank is not continuously operated; it operates periodically, depending on the contamination of the HFU membrane assembly. When the backwash tank is not operating, the produced water from the HFU membrane assembly directly enters the next coagulation and sedimentation process.

[0078] The MBR unit further comprises a cleaning water tank, which is provided with a dosing port and is connected to a water production port of the hollow fiber ultrafiltration membrane module. The cleaning water tank is provided with a stirring shaft.

[0079] The functions and working principles of the cleaning water tank and the backwash tank are similar. When the water produced in the backwash tank alone is no longer able to clean the membrane surface of the hollow fiber ultrafiltration membrane assembly, the hollow fiber ultrafiltration membrane assembly is cleaned by the liquid in the cleaning water tank.

[0080] Cleaning agent is added to the cleaning water tank through the tank's dosing port. The agent dissolves in the water and is then pumped into the hollow fiber ultrafiltration membrane assembly by a pressure pump to clean the membrane assembly. The stirring shaft has blades whose rotation accelerates the dissolution of the cleaning agent. The cleaning agent is a highly oxidizing agent, such as sodium hypochlorite. During cleaning, the system switches to the standby MBR, drains the unused MBR feed liquid to the front end, injects clean water, and then injects the cleaning agent. Aeration and flushing are carried out for 2 hours, and the cleaning liquid is drained. The system then enters standby mode after cleaning is complete.

[0081] The MBR unit further includes a water supply pipe, which is connected to the backwash tank, the cleaning water tank and the water production port of the hollow fiber ultrafiltration membrane assembly.

[0082] The water supply pipe is the same pipe connected to the carbon source dosing device mentioned above and is actually an external tap water pipe. The water supply pipe is connected to the backwash tank to replenish the backwash tank water source; the water supply pipe is connected to the cleaning water tank to provide water for dissolving cleaning agents and cleaning membrane components; the water supply pipe is connected to the water production port of the hollow fiber ultrafiltration membrane component. When the hollow fiber ultrafiltration membrane component is first used, it needs to be vacuumed. First, water is injected into the interior through the water supply pipe, and then the water is drained to complete the vacuuming, so that the feed liquid can be sucked into the MBR membrane tank.

[0083] The MBR unit structure features easy cleaning and disassembly of the membrane assembly, making it more suitable for treating landfill leachate with complex components, prone to membrane fouling, and requiring frequent cleaning of the membrane assembly. It also boasts strong anti-pollution capabilities: the inner supporting membrane filaments are made of PVDF material, with a tensile strength of >300N and no breakage. The membrane is resistant to repeated air scrubbing and chemical cleaning, and has excellent durability. The membrane surface pore size is uniform, ensuring good effluent quality, with a produced water turbidity of <0.6 NTU.

[0084] The water produced by the MBR unit enters the coagulation and sedimentation unit for coagulation and sedimentation treatment. The coagulation and sedimentation unit includes a coagulation reaction tank and a coagulation sedimentation tank connected in sequence.

[0085] Polyferric sulfate (PFS) is added to the coagulation reaction tank to remove colloidal particles. Polyferric sulfate is a superior inorganic polymer coagulant that provides a multi-component core-hydroxy complex, exerting a variety of coagulation effects on colloidal particles in water. High-valent complex ions with smaller relative molecular weights are attracted to the negatively charged colloid particles and suspended matter in the feed solution and enter the compacted layer, compressing the colloid particles' double layer and lowering their zeta potential, leading to rapid destabilization and coagulation. This increases the relative molecular weight of the inorganic polymer coagulant, leading to greater elongation, increased contact points, and enhanced interparticle adsorption. PFS provides a large number of macromolecular complexes and hydrophobic hydroxide aggregates in the solution, providing excellent adsorption.

[0086] The various core hydroxyl complexes in PFS solution differ from organic polymer flocculants in that their relative molecular weights are much smaller than those of organic flocculants. Due to their molecular size and structural characteristics, these complex ions possess a strong adsorption and neutralization effect during coagulation. Therefore, the primary contribution of the high-valent macromolecular complex ions in PFS solution to coagulation is adsorption and neutralization of the charge of the colloid particles, as well as interparticle agglomeration. PFS flocs have a large surface area and high surface energy, a compact and dense structure, and a certain strength. During sedimentation, they adsorb a large amount of colloidal particles, exhibiting an adsorption coprecipitation effect and prone to sweeping sedimentation. The precipitate has a small volume and a fast settling rate, significantly enhancing the coagulation effect of PFS. Pollutants such as macromolecular colloids and humic acid in the system are further precipitated and removed, reducing COD and color. Optimally, the optimal coagulation effect is achieved when the pH is 5 and the PFS dosage is 1.2‰.

[0087] After reacting with polyferric sulfate, the feed liquid enters the coagulation sedimentation tank for precipitation, and the effluent (supernatant) of the coagulation sedimentation tank enters the primary Fenton unit, BAF unit, and secondary Fenton unit for deep oxidation in sequence.

[0088] The first-stage electro-Fenton and the second-stage electro-Fenton use the Fenton reaction principle. Power is supplied to the Fenton water treatment equipment to electrolyze and produce Fe 2+ and H2O2, generating hydroxyl radicals that maintain continuous high activity, which can oxidize various toxic and difficult-to-degrade organic compounds to achieve the purpose of removing pollutants. It is particularly suitable for the oxidation treatment of organic wastewater such as landfill leachate that is difficult to biodegrade or difficult to be oxidized by general chemical oxidation. The factors affecting the treatment of landfill leachate by Fenton method are mainly pH, the amount of H2O2 added and the amount of iron salt added. It can further reduce COD and remove part of organic nitrogen. Preferably, when (H2O2 / Fe 2+ ) molar ratio is 3 and the FeCl3 dosage is 1‰, the COD removal rate can reach 50%, the total nitrogen removal rate can reach 35%, and the total phosphorus removal rate can reach 90%.

[0089] The BAF unit includes a BAF tank, or biological aerated filter (BAF), a high-load, submerged, fixed-biofilm three-phase reactor. It combines the advantages of the activated sludge and biofilm processes, integrating biochemical reactions and physical filtration in a single reactor. The BAF tank consists of a granular biofill bed, an aeration system, and a backwash system. The feed enters the BAF tank, and the aeration system allows air to enter from the bottom of the granular biofill bed. The leachate comes into contact with the granular biofill bed, where aerobic conditions allow a reaction. Pollutants are trapped on the granular material of the granular biofill bed. The treated leachate flows into a drainage tank and, after testing and compliance, is discharged. The backwash system regularly backwashes the granular biofill bed.

[0090] The BAF tank uses granular fillers and their attached biofilm as the treatment medium, leveraging biological metabolism, physical filtration, physical adsorption by the membrane and its combination with the fillers, and multi-stage predation within the reactor to remove pollutants within the same unit reactor. The aerated zeolite biofilter combines the excellent selective ion exchange properties of zeolite with the biochemical functions of the aerated biofilter, simultaneously achieving filtration and adsorption, ion exchange, biological oxidation, and biological regeneration. This ensures that the final effluent ammonia nitrogen and total nitrogen levels consistently meet standards.

[0091] The secondary Fenton effluent enters a buffer tank, where it is adjusted for water quality and then discharged directly. Adjusting the water quality in the buffer tank means adjusting the effluent pH to 7.0-8.0. Preferably, the effluent pH is 7.5.

[0092] The method for treating low carbon-nitrogen ratio landfill leachate in the present application also includes a sludge treatment process, in which the sludge in the MBR membrane tank (part), coagulation sedimentation tank, primary Fenton and secondary Fenton are filtered through a plate and frame filter press.

[0093] The following is a further explanation of this application through a specific real-time method (experiment):

[0094] 1. Test water quality

[0095] The inoculated sludge used in the reactor operation was taken from the biochemical pool for nitrification and denitrification treatment of leachate from a waste incineration plant in the north. The wastewater treated in the experiment was leachate from the regulating pool of a domestic waste landfill in the north. The water quality is shown in the following table:

[0096] Table 1-1 Experimental raw water quality

[0097]

[0098] 2. Reactor device and operation

[0099] The above-mentioned process and corresponding equipment for treating low carbon-nitrogen ratio landfill leachate of the present application are adopted.

[0100] 3. Test methods

[0101] The instruments and testing equipment required for this test are shown in Table 3-1.

[0102] Table 3-1 Instruments and testing equipment required for the test

[0103]

[0104] During the whole experiment, a certain amount of mud-water mixture was collected from each reaction unit every day to measure the NH4 + -N, NO2 - -N, NO3- -N、COD cr The measurement of each indicator is based on the "Water and Waste Monitoring and Analysis Methods (Fourth Edition)", in which NH4 + -N using Nessler's reagent spectrophotometry, NO2 - -N uses Hach fast reagent (ferrous sulfate method), NO3 - -N hash fast reagent (cadmium reduction method), COD cr The rapid digestion method using a Hach kit was used. Alkalinity was determined by potentiometric titration, MLSS and MLVSS by gravimetric methods, pH by a portable pH meter, and DO by a Shanghai Yidian portable dissolved oxygen meter, JPBJ610L.

[0105] 4. Test results

[0106] In order to control the higher short-term nitrification rate in the system and achieve a higher nitrite accumulation rate (NAR), the reference literature and laboratory pilot results were used to control the dissolved oxygen in the primary aerobic tank and the secondary aerobic tank to <1 mg / L. At the same time, by adding alkalinity and carbon sources, the C / N ratio was controlled to 2:1, the influent flow rate was 200 L / d, and the reflow was 1400 L / d. The total hydraulic retention time (HRT) was controlled at around 9.85 days, and the reflow ratio was 700%. Day 0 to Day 45 of the entire experiment was the sludge acclimation period. Prior to this, sludge inoculation and biofilm formation on the bio-rotating disc were mainly carried out (with an ammonia nitrogen removal rate of 99% at the designed retention time as the node). Day 46 to Day 190 was the experimental exploration stage, during which the main research was on the stability of short-term nitrification and denitrification under different carbon-nitrogen ratios, influent water quality fluctuations and low temperatures (<15°C). The bio-rotating disc film formation is shown in the figure below. Figure 9 shown.

[0107] Since the activated sludge inoculated is full-process nitrification and denitrification sludge, DO is controlled at 1mg / L in the initial stage of the experiment. In a low-oxygen environment, short-term nitrification bacteria are gradually screened and domesticated. Taking the ammonia nitrogen removal rate as a reference, when there is a high nitrite accumulation rate at a high ammonia nitrogen removal rate, it is considered that the reactor has achieved a short-term nitrification and denitrification process. During long-term operation, affected by the fluctuation of on-site water quality, the nitrification rate of the reactor decreased within a certain period of time, and it was readapted to the water quality by reducing the load. At the end of the experiment, the on-site temperature dropped significantly. The suitable temperature for short-term nitrification and denitrification is 28-40℃, but in a low-temperature environment, the ammonia nitrogen removal rate was less affected. The total nitrogen removal rate dropped by about 10%, but the nitrite accumulation rate also dropped by about 10%. It can be considered that after gradual domestication, short-term nitrification also has a certain tolerance to low temperature. Figure 10As shown, the average COD of the influent leachate is about 8157 mg / L, the average COD of the effluent is about 2500 mg / L, and the average COD degradation rate is about 68%. In the early stage of the reactor operation (day 1-50), the influent COD is relatively stable. When 6 kg of carbon source is added per ton of water, it is basically maintained at 6000-9000 mg / L, and the effluent COD is maintained at 2000-3000 mg / L. However, in the middle of the experiment, even if the amount of added carbon source is reduced, the influent COD still rises sharply, reaching a maximum of 13800 mg / L, which is mainly caused by the fluctuation of raw water COD. The raw water COD rises from about 4000 mg / L to 8000 mg / L. The COD did not return to normal levels until the 86th day of the experiment. Figure 10 It can be seen that when the influent COD fluctuates slightly, the effluent COD remains at around 2200 mg / L. This may be because the carbon and nitrogen content of the raw leachate water is relatively low, and most of the biodegradable COD may have been consumed in the biological rotary unit and the primary anoxic tank. The remaining ones are mostly difficult to biodegrade organic matter. In addition, as the nitrite accumulation rate increases, the measured COD value will increase when detecting COD under high nitrite concentration conditions.

[0108] like Figure 11 As shown, since the inoculum sludge was derived from a fully nitrified and denitrified system and had undergone two days of aeration and five days of intermittent water inflow prior to the experiment, it initially had a certain capacity for ammonia nitrogen degradation and gradually adapted to the influent conditions. Even with a shortened residence time, the removal rate maintained an upward trend. However, midway through the experiment, nitrification was significantly inhibited by fluctuating water quality (due to the influx of fly ash leachate from the on-site regulating tank). Reduction of the load, increasing the carbon-nitrogen ratio, and adjusting the dissolved oxygen did not restore nitrification. After three days of aeration, the concentration of free ammonia in the system decreased. The inflow was then gradually increased at 20% of the design load, ultimately restoring the ammonia nitrogen removal rate to over 95%. However, since the discharge of fly ash leachate into the regulating tank ceased at the end of the adjustment period, it is unclear whether this was due to the system's gradual adaptation to the water quality or to improved water quality.

[0109] Since the conditions for total nitrogen detection were not available on site at the beginning of the experiment, total nitrogen data were collected starting from the 20th day. The total recirculation ratio of this experiment was designed to be 700%, so the theoretical denitrification efficiency should be around 87% (considering only the removal of total nitrogen by the denitrification unit). Figure 12As can be seen, from days 45 to 80, the total nitrogen removal rate exhibited the same fluctuating trend, limited by the ammonia nitrogen removal rate. After stabilization, the denitrification efficiency was largely above the theoretical value, confirming that a short-term nitrification process was indeed occurring within the system, with nitrite-based denitrification being the primary driver. Subsequently, a significant drop in system temperature resulted in incomplete denitrification. Consequently, the TN degradation efficiency gradually decreased. When the added carbon source was 3 kg per ton of water, the measured BOD5 / TN ratio was 2:1. Generally, the optimal carbon-nitrogen ratio for full-scale nitrification and denitrification is between 5 and 6, allowing for complete denitrification within this range. This also demonstrates that short-term nitrification and denitrification require a relatively low carbon source.

[0110] Depend on Figure 13 As can be seen, pollutant removal rates are higher in the distribution tank and primary anoxic tank. The return fluid dilutes pollutants, reducing ammonia nitrogen concentrations and preventing the biotoxicity of excessive FA. However, there is still a certain gap compared to the return ratio, due to the presence of certain concentrations of pollutants in the return fluid. The overall ammonia nitrogen removal rate is high, generally close to the theoretical value (80%). However, the concentrations of total nitrogen and COD in the return fluid are relatively high.

[0111] like Figure 14As shown, if the pollutant concentration in the previous unit is used as a control, the distribution tank still has a dilution effect. The bio-rotor also has a certain degradation effect on ammonia nitrogen. This is likely due to the fact that when the rotor rotates above the liquid level, oxygen in the air is transferred through gas-liquid mass transfer, allowing nitrifying bacteria to carry out some nitrification. Combined with the COD and total nitrogen data, the bio-rotor plays a certain role in denitrification. The primary anoxic tank is an anoxic environment, and theoretically there should be no ammonia nitrogen removal effect. This should be due to dilution caused by the sludge return flow, resulting in an average return ratio of 225%. Considering the total nitrogen concentration in the return flow, the average total nitrogen removal rate due to dilution should be approximately 19%, with approximately 10% contributed by denitrification, indicating that total nitrogen in the system is primarily degraded in the bio-rotor and the primary anoxic tank. The primary aerobic tank and the secondary aerobic tank mainly undergo short-range nitrification reactions in a microaerobic environment, and the ammonia nitrogen removal rate is relatively high. However, the removal rate of the primary aerobic tank is higher than that of the secondary aerobic tank. This should be related to the substrate concentration. From the perspective of sludge load, the overall ammonia nitrogen sludge load of the system is about 0.02 kgNH4 / kgMLSS·d, and the sludge load of conventional biochemical processes is generally 0.04kgNH4 / kgMLSS·d, indicating that the reactor still has great treatment potential and the load can be further increased. The primary and secondary aerobic tanks also achieved some total nitrogen removal, suggesting simultaneous nitrification and denitrification, possibly due to uneven mass transfer. The primary aerobic tank still contained some biodegradable organic matter, leading to a moderate COD removal rate. The headphone aerobic tank, however, appears to be an endogenous denitrification process. Nitrite nitrogen accumulation rates reached 80% in both the primary and secondary aerobic tanks during stable operation, a slight difference from the previous laboratory pilot (>90%), likely due to the inherent characteristics of the bacterial strain. Even during periods of fluctuation and low temperatures, the rates remained above 60%, further demonstrating the achievement of a stable, short-term nitrification and denitrification process. The low ammonia nitrogen removal rate in the secondary anoxic tank likely stems from incomplete oxygen utilization in the secondary aerobic tank, leading to nitrification there. The denitrification process was limited by a carbon source, resulting in a low total nitrogen removal rate. The MBR unit has a high removal rate for ammonia nitrogen, total nitrogen and COD. The degradation of ammonia nitrogen should be related to the sludge concentration and dissolved oxygen; while total nitrogen and COD may be part of the organic pollutants intercepted by the MBR, thus showing the ability to remove these two.

[0112] In the later stage of the experiment, due to the significant drop in ambient temperature, we attempted to explore the stability of short-range nitrification and denitrification under low temperature conditions. Figure 15 As shown, even at the end of the experiment, when the water temperature was around 10°C, the nitrification process was slightly affected, with the removal rate decreasing by about 2%. However, the total nitrogen removal rate decreased by about 10%. This may be because the optimum temperature for denitrification is higher than that for nitrification, and denitrification is significantly inhibited at low temperatures.

[0113] 5. Test conclusion

[0114] This experiment achieved the goal of treating old landfill leachate with the "bio-rotating disc coupled AOOA" process by strictly limiting the dissolved oxygen in the primary aerobic tank and the secondary aerobic tank units, and realized a short-range nitrification and denitrification process.

[0115] Under a carbon-nitrogen ratio of 2:1, a residence time of 9.8 days, and a total reflow ratio of 700%, the reactor maintained dissolved oxygen levels in both the primary and secondary aerobic tanks below 1 mg / L. Average degradation rates for COD, ammonia nitrogen, and total nitrogen were 68%, 91%, and 85%, respectively. During stable operation, ammonia nitrogen removal rates reached 99%, and total nitrogen removal rates reached 90%. The effluent COD concentration of the treated landfill leachate was 2000-2500 mg / L, ammonia nitrogen below 25 mg / L, and total nitrogen below 200 mg / L. Considering the ammonia nitrogen sludge loading, the reactor still has considerable treatment potential.

[0116] Judging from the removal rates of each unit, excluding the dilution effect of the backflow from the water distribution tank and the primary anoxic tank, nitrification primarily occurs in the primary aerobic tank, the secondary aerobic tank, and the MBR unit; denitrification primarily occurs in the primary anoxic tank and the bio-rotating disc unit. Due to carbon source limitations, the secondary anoxic tank contributes less total nitrogen removal, suggesting an endogenous denitrification process. Furthermore, due to factors such as uneven DO mass transfer, simultaneous nitrification and denitrification occur within some units. Later experiments demonstrated that, even under low-temperature conditions, after appropriate acclimatization and inoculation, a stable short-range nitrification process can be achieved.

[0117] In general, due to the different tolerances of AOB and NOB to environmental factors, various methods can be used to inhibit NOB activity, limiting the nitrification process to the nitrite stage, thereby achieving a short-term nitrification and denitrification process. Based on previous pilot plant results and literature, short-term nitrification and denitrification are more easily achieved in systems with high ammonia nitrogen, high alkalinity / pH, and low carbon-nitrogen ratios.

[0118] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the present invention should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. A method for treating low carbon-nitrogen ratio landfill leachate, characterized by: The following steps are involved: S1: After the leachate passes through the regulating tank to adjust the water quality, it enters the stripping unit to remove part of the ammonia nitrogen and sulfide; S2: The effluent from the stripping unit enters the biological rotary unit and the AOOA reactor in sequence, where a short-range nitrification and denitrification reaction occurs to remove suspended solids and organic matter in the water; S3: Part of the effluent from the AOOA reactor flows back to the biological rotary unit and the AOOA reactor, and part of it flows into the MBR unit for mud-water separation; S4: The water produced by the MBR unit enters the coagulation and sedimentation unit. Part of the sludge from the MBR unit flows back to the biological rotary unit and the AOOA reactor unit, and part of it enters the coagulation and sedimentation unit to remove colloidal particles. S5: The water produced by the coagulation and sedimentation unit enters the primary Fenton unit, BAF unit, and secondary Fenton unit for deep oxidation in sequence; S6: The effluent from the secondary Fenton unit enters the buffer tank to regulate water quality for discharge; The stripping unit includes a primary reaction tank, a primary sedimentation tank, a stripping tower, a secondary reaction tank, and a secondary sedimentation tank connected in sequence. Lime is added to the primary reaction tank to remove metal ions. The pH value of the effluent from the primary sedimentation tank is adjusted to 8.0-9.0 and the water temperature is adjusted to 20-30°C. The pH value of the water in the secondary reaction tank is adjusted to 6.5-7.

5. The bio-rotor unit includes a water distribution tank, a bio-rotor connected to the water distribution tank, and a carbon source dosing device. The rotation speed of the bio-rotor is 2.0-4.0 r / min; The AOOA reactor includes four aeration tanks and an aeration device. The four aeration tanks are a primary anoxic tank, a primary aerobic tank, a secondary aerobic tank, and a secondary anoxic tank connected in sequence. The primary anoxic tank is connected to a biological rotary disc and a carbon source dosing device. The aeration device supplies oxygen to the four aeration tanks. Part of the liquid in the secondary anoxic tank is returned to the water distribution tank and the primary anoxic tank. The reflux ratio of the liquid returned to the primary anoxic tank is 500%. The MBR unit includes an MBR membrane pool and a hollow fiber ultrafiltration membrane assembly located in the MBR membrane pool. The sludge outlet of the MBR unit is connected to the water distribution tank and the first-level anoxic tank through the MBR return pipe. Part of the sludge from the MBR unit is returned to the water distribution tank and the first-level anoxic tank; the MBR membrane pool is returned to the AOOA reactor to ensure that the sludge return ratio at the front end of the system is 330%.

2. The method for treating low carbon-nitrogen ratio landfill leachate according to claim 1, wherein: In step S1, regulating the water quality of the regulating tank means that the conductivity of the water discharged from the regulating tank is less than 3000S / m.

3. The method for treating low carbon-nitrogen ratio landfill leachate according to claim 1, wherein: The MBR unit further includes a backwash tank and a cleaning water tank, both of which are connected to the water production port of the hollow fiber ultrafiltration membrane assembly, and the hollow fiber ultrafiltration membrane assembly is cleaned through the backwash tank or the cleaning water tank.

4. The method for treating low carbon-nitrogen ratio landfill leachate according to claim 1, wherein: The coagulation and sedimentation unit comprises a coagulation reaction tank and a coagulation and sedimentation tank connected to each other. Polyferric sulfate is added to the coagulation reaction tank to remove colloidal particles.

5. The method for treating low carbon-nitrogen ratio landfill leachate according to claim 1, wherein: Step S6: adjusting the water quality of the buffer tank refers to adjusting the pH value of the effluent from the buffer tank to 7.0-8.0.

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