External tubular membrane anaerobic digestion wastewater treatment system and wastewater treatment method

Abstract

The application provides an external tubular membrane anaerobic digestion wastewater treatment system and a wastewater treatment method, and the external tubular membrane anaerobic digestion wastewater treatment system comprises an anaerobic digestion biological reactor, an external tubular membrane biological reactor unit, a sludge backflow pipeline and a biogas heat energy recovery system. The anaerobic digestion biological reactor is used for anaerobic digestion treatment of organic wastewater, and the produced sludge-water mixture enters the external tubular membrane biological reactor unit for solid-liquid separation; the intercepted active sludge is backflowed to the anaerobic digestion biological reactor through the sludge backflow pipeline for deep digestion and reduction; the biogas is subjected to biogas heat energy recovery and system heat balance regulation and control; the system has the characteristics of good effluent water quality, high integration degree and low operation cost; and deep denitrification can be realized in combination with an anaerobic ammonia oxidation process.

Description

Technical Field

[0001] This invention belongs to the field of high-concentration organic wastewater treatment and resource utilization technology. Specifically, it relates to an external tubular membrane anaerobic digestion wastewater treatment system and wastewater treatment method. Background Technology

[0002] Traditional treatment of high-concentration organic wastewater often employs a series of anaerobic digestion and aerobic treatment processes. Anaerobic digestion technology can convert most organic pollutants into biogas through microbial action under anaerobic conditions, achieving energy recovery. However, its effluent typically contains high levels of residual chemical oxygen demand (COD), suspended solids, and ammonia nitrogen, making it unsuitable for direct discharge or affecting subsequent aerobic treatment processes. Conventional anaerobic digestion sludge-water separation processes utilize secondary sedimentation tanks, which suffer from problems such as large footprint, sensitivity to load fluctuations, high sludge production, and expensive disposal costs.

[0003] Membrane bioreactor (MBR) technology, especially external tubular membrane reactors, utilizes membrane separation to replace secondary sedimentation tanks, resulting in high-quality effluent and a compact system. Currently, anaerobic digestion typically requires an aerobic process before connecting it in series with the membrane reactor. For wastewater with high pollutant concentrations and high suspended solids, anaerobic digestion sedimentation processes use sedimentation tanks, lacking coupling and integration with tubular membrane bioreactors, thus missing the internal circulation and synergy of matter and energy. In view of this, the present invention is hereby proposed. Summary of the Invention

[0004] The primary objective of this invention is to provide an external tubular membrane anaerobic digestion wastewater treatment system, which aims to solve the problems of low solid-liquid separation efficiency, poor system integration, and insufficient energy utilization in traditional processes.

[0005] The second objective of this invention is to provide a wastewater treatment method based on the above-mentioned external tubular membrane anaerobic digestion wastewater treatment system, aiming to achieve synergistic effects of efficient pollutant removal and resource recovery.

[0006] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: An external tubular membrane anaerobic digestion wastewater treatment system includes: Anaerobic digestion bioreactor, external tubular membrane bioreactor unit, sludge return pipeline, biogas heat recovery system; The external tubular membrane bioreactor unit is connected to the outlet of the anaerobic digestion bioreactor via a pipe. The sludge return pipe connects the sludge outlet of the external tubular membrane bioreactor unit to the sludge inlet of the anaerobic digestion bioreactor. The biogas heat recovery system is connected to the biogas outlet of the anaerobic digestion bioreactor.

[0007] This invention integrates an anaerobic digestion bioreactor, an external tubular membrane bioreactor unit, a sludge return pipeline, and a biogas heat recovery system into a single unit, constructing a closed-loop system architecture for material and energy circulation. The external tubular membrane bioreactor unit uses membrane separation instead of traditional sedimentation for solid-liquid separation, reducing the system's footprint while producing high-quality effluent with extremely low suspended solids. The sludge return pipeline returns the activated sludge retained in the unit to the anaerobic digestion bioreactor, allowing the returned sludge to be further digested and decomposed in an anaerobic environment as a supplementary substrate and microbial inoculum, effectively reducing the amount of sludge discharged externally. The biogas heat recovery system reuses the biogas heat energy generated from anaerobic digestion to maintain the reactor's operating temperature, reducing external energy consumption. The synergistic effect of these three subsystems solves the problems of high energy consumption, large sludge production, low resource recovery rate, and high suspended solids in anaerobic effluent associated with traditional processes.

[0008] Preferably, as a further specific embodiment, the external tubular membrane bioreactor unit adopts a high-speed cross-flow filtration method; This invention further defines the operation mode and structural configuration of the external tubular membrane bioreactor unit. It operates using a high-speed cross-flow filtration method, utilizing cross-flow shear force to continuously flush the membrane surface to prevent contaminant adhesion and accumulation, thereby delaying membrane fouling. The external tubular membrane bioreactor unit is equipped with a cross-flow filtration circulation pump. The return water from this pump, while supplying feed to the membrane filtration system, also returns concentrated sludge to the anaerobic digestion bioreactor, driving continuous material flow within the reactor. This simultaneously achieves sludge return and anaerobic circulation within the same cycle, thereby enhancing material mixing and substrate mass transfer within the system without requiring additional stirring devices.

[0009] Preferably, as a further specific embodiment, the biogas heat energy recovery system includes a desulfurization and dehydration device and a heat exchange device; The heat exchange device prioritizes the use of heat energy generated from biogas combustion to maintain the mesophilic conditions of the anaerobic digestion bioreactor, while excess heat is introduced into the circulation pipeline of the external tubular membrane bioreactor unit.

[0010] This invention further defines the composition and heat distribution logic of the biogas heat recovery system. After desulfurization and dehydration purification, biogas enters a heat exchanger for combustion and heat release. The generated heat is preferentially used to maintain the mesophilic operating conditions of the anaerobic digestion bioreactor to ensure the metabolic activity of methanogenic bacteria. After meeting the insulation requirements of the anaerobic reactor, excess heat is introduced into the circulation pipeline of the external tubular membrane bioreactor unit to maintain the filtration performance of the membrane separation system under low-temperature conditions. Through this priority allocation, the system achieves tiered utilization of heat energy and self-regulation of heat balance.

[0011] Preferably, as a further specific embodiment, the biogas heat energy recovery system includes a desulfurization and dehydration device and a biogas boiler or cogeneration device, and is connected to the anaerobic digester and the external tubular membrane bioreactor unit respectively through heat exchange pipelines.

[0012] This invention provides an alternative implementation of a biogas heat recovery system. After desulfurization and dehydration, the biogas is fed into a biogas boiler or a combined heat and power (CHP) unit for utilization. Using a biogas boiler allows for efficient heat production; using a CHP unit generates both heat and electricity to power the system's equipment. The generated heat and / or electricity are transported via heat exchange pipelines to the anaerobic digester and the external tubular membrane bioreactor unit, respectively, enabling independent temperature regulation and precise distribution of the two core units, ensuring both are maintained within their respective suitable operating temperature windows.

[0013] The present invention also provides a wastewater treatment method for the aforementioned external tubular membrane anaerobic digestion wastewater treatment system, comprising the following steps: The pretreated organic wastewater is introduced into the anaerobic digestion bioreactor, where organic pollutants are converted into biogas by anaerobic microorganisms and then collected. The mud-water mixture produced by the anaerobic digestion bioreactor is introduced into the external tubular membrane bioreactor unit for solid-liquid separation. The purified water that meets the discharge standards will then enter the subsequent treatment process. The activated sludge retained in the external tubular membrane bioreactor unit is partially or completely returned to the anaerobic digestion bioreactor for deep digestion and sludge reduction. The heat energy generated by the combustion of biogas is recycled through a biogas heat recovery system to maintain the process operating temperature of the anaerobic digestion bioreactor.

[0014] This invention provides a treatment method that integrates four steps in a sequential manner: anaerobic digestion, tubular membrane solid-liquid separation, sludge recirculation and digestion, and biogas heat energy recovery. Pretreated organic wastewater first enters an anaerobic digestion bioreactor, where most organic pollutants are converted into biogas by anaerobic microorganisms and collected. The sludge-water mixture discharged from the anaerobic digestion reactor then enters an external tubular membrane bioreactor unit for solid-liquid separation, obtaining high-quality effluent with extremely low suspended solids, which can be directly discharged in compliance with standards or used as influent for subsequent advanced treatment. The activated sludge retained in the external tubular membrane bioreactor unit is partially or completely returned to the anaerobic digestion bioreactor, where it is further digested and decomposed in an anaerobic environment, reducing the amount of sludge discharged from the system. The collected biogas is burned, and the heat is recovered through a heat energy recovery system to maintain the process operating temperature of the anaerobic digestion bioreactor. The synergy of these steps achieves integrated high-efficiency pollutant removal, in-situ sludge reduction, and biogas energy recovery.

[0015] Preferably, as a further specific embodiment, the recirculation ratio of the activated sludge is 50%-80%.

[0016] This invention further limits the recirculation ratio of activated sludge. 50%-80% of the activated sludge retained by the external tubular membrane bioreactor unit is continuously recirculated back to the anaerobic digestion bioreactor. This allows the sludge, rich in active microorganisms, to be continuously fed into the anaerobic environment for deep digestion, effectively reducing the amount of net sludge discharged from the entire system. Simultaneously, this ratio range does not excessively impact the existing hydraulic conditions and microbial community within the anaerobic reactor, ensuring the treatment stability of the anaerobic digestion system.

[0017] Preferably, as a further specific embodiment, the control conditions of the anaerobic digestion bioreactor are: mesophilic digestion range of 33℃-37℃, pH value of 6.8-7.5, hydraulic retention time of 2-5 days, and organic loading rate of 5-15 kg COD / (m³). 3 (d) Biogas yield 0.35-0.45 Nm³ 3 / kgCOD removal.

[0018] This invention further defines the key operating parameters of the anaerobic digestion bioreactor. The temperature is controlled within a mesophilic range of 33-37℃, and the pH value is maintained between 6.8 and 7.5 to provide suitable environmental conditions for the survival and metabolism of methanogenic bacteria, ensuring efficient anaerobic digestion and maintaining a stable biogas yield. The hydraulic retention time is set to 2-5 days, and the organic loading rate is controlled at 5-15 kg COD / (m³). 3 Within the range of ·d), the system is capable of treating high-concentration organic wastewater, while maintaining a stable treatment effect even when the influent organic load fluctuates.

[0019] Preferably, as a further specific embodiment, the subsequent treatment process includes an anaerobic ammonia oxidation (AAO) denitrification process. The purified water enters the AAO reactor and undergoes short-cut nitrification under oxygen-limited conditions (DO 0.3-1.0 mg / L). Nitrogen gas is then generated by the action of anaerobic ammonia oxidizing bacteria. The effluent from the AAO reactor enters the external tubular membrane bioreactor unit for solid-liquid separation. This invention further specifies a concrete scheme for using anaerobic ammonia oxidation as a subsequent denitrification process when the total nitrogen in wastewater is high. The low-suspended-solids clarified liquid produced by the external tubular membrane bioreactor unit enters the anaerobic ammonia oxidation reactor. Under oxygen-limited conditions, ammonia-oxidizing bacteria first convert some ammonia nitrogen into nitrite. Subsequently, the anaerobic ammonia-oxidizing bacteria utilize the remaining ammonia nitrogen to react with the produced nitrite to produce nitrogen gas. The entire process requires no external organic carbon source and has extremely low sludge production, significantly reducing aeration energy consumption and operating costs compared to traditional nitrification-denitrification denitrification methods. The effluent from the anaerobic ammonia oxidation reactor enters the external tubular membrane bioreactor unit for solid-liquid separation. The complete retention effect of the tubular membrane retains the slowly proliferating anaerobic ammonia-oxidizing bacteria within the system, preventing the loss of functional bacteria with the effluent, thus ensuring the long-term stable maintenance of denitrification efficiency and solving the engineering problem of the difficulty in cultivating and retaining anaerobic ammonia-oxidizing bacteria.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) Compared with the traditional anaerobic treatment system that uses sedimentation tanks for sludge-water separation, this invention uses membrane separation instead of sedimentation, which significantly reduces the system's footprint and obtains high-quality effluent with extremely low suspended solids concentration. By returning the sludge retained by the membrane unit to the anaerobic digestion bioreactor for secondary digestion, the amount of excess sludge discharged by the system is greatly reduced, thus lowering sludge disposal costs. By recovering biogas heat energy to maintain the system's operating temperature, external energy consumption is reduced. The combined effect of these three factors enables the system to achieve a unified approach of compact layout, sludge reduction, and energy self-sufficiency.

[0021] (2) The wastewater treatment method of the present invention achieves efficient removal of organic pollutants, effective control of suspended solids in effluent, source reduction of residual sludge and on-site recovery of biogas energy in the same process flow through the orderly coordination of four steps, thus transforming the wastewater treatment process from simple pollutant disposal to a multi-objective treatment process with resource recovery function. Detailed Implementation

[0022] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] To more clearly illustrate the technical solutions in this invention, specific embodiments are described below.

[0024] In the following embodiments, the detection methods for indicators such as COD, ammonia nitrogen, total nitrogen, and SS are all performed in accordance with national standard methods. Biogas production is measured using a wet gas flow meter, and biogas composition is determined by gas chromatography.

[0025] Example 1 This embodiment takes the treatment of wastewater from a large-scale pig farm as an example to provide a detailed description of the external tubular membrane anaerobic digestion wastewater treatment system and wastewater treatment method of the present invention. Wastewater from pig farms (raw water COD approximately 15000 mg / L, TN approximately 800 mg / L, NH4+) + -N approximately 600 mg / L) After solid-liquid separation to remove large suspended particles, it enters an effective volume of 800 m³. 3 A completely mixed anaerobic reactor. Anaerobic digested sludge is inoculated into the reactor at a rate of 30% of the effective volume. The temperature is strictly controlled at 35℃ via a heat exchange system connected to a combined heat and power (CHP) unit. The pH is automatically adjusted between 6.8 and 7.5, the hydraulic retention time is set to 4 days, and the organic loading rate is controlled at 10-12 kgCOD / (m³). 3 ·d).

[0026] The anaerobic digestion liquid enters the anaerobic ammonia oxidation reactor. This reactor has an effective volume of 300 m³, and a dissolved oxygen gradient is created within a single reactor through aeration and stirring design. The dissolved oxygen level is controlled at 0.5 ± 0.2 mg / L in the micro-aerobic zone at the top of the reactor. The absolutely anoxic zone at the bottom of the reactor and within the granular sludge is dominated by enriched anaerobic ammonia-oxidizing bacteria: NH₄⁺ + + 1.32 NO2 - → 1.02 N2+ 0.26 NO3 - .

[0027] The effluent from the anaerobic ammonia oxidation reactor enters the external tubular membrane bioreactor unit. This unit uses a PVDF tubular membrane with a pore size of 0.02 μm and a membrane area of ​​300 m². 2 The reactor mixture was circulated and filtered at a cross-flow velocity of 3.2 m / s, with the membrane flux maintained at 12-18 L / (m²). 2 ·h).

[0028] The activated sludge retained by the external tubular membrane bioreactor unit is continuously returned to the anaerobic digestion bioreactor via a sludge return pipeline at a 70% return ratio. The return water from the cross-flow filtration circulation pump simultaneously realizes the material circulation within the anaerobic digestion reactor during the sludge return process. Finally, the clear effluent after membrane filtration is obtained from the product water section of the external tubular membrane bioreactor unit.

[0029] The biogas produced by anaerobic digestion is desulfurized and dehydrated before entering a combined heat and power (CHP) unit. The electricity is used to drive equipment within the system (including circulating pumps, aeration devices, control systems, etc.), while waste heat (hot water) is used to simultaneously insulate the anaerobic digester and the anaerobic ammonia oxidation reactor via coils. The heat exchanger prioritizes the use of the heat energy generated from biogas combustion to maintain the mesophilic conditions (35°C) in the anaerobic digester, and excess heat is introduced into the circulation pipeline of the external tubular membrane bioreactor unit.

[0030] Example 2 This embodiment takes the treatment of food processing wastewater as an example, using a biogas boiler to recover heat energy without coupling an anaerobic ammonia oxidation process.

[0031] Food processing wastewater (raw water COD approximately 12000 mg / L, BOD5 approximately 6000 mg / L, SS approximately 5000 mg / L) is treated by a screen to remove suspended solids and adjust water quality and quantity before entering a 600 m³ / h effluent. 3 A completely mixed anaerobic reactor was constructed. Anaerobic digested sludge was inoculated into the reactor at a rate of 30% of its effective volume. The temperature was controlled at 35±1℃ via a heat exchange system connected to a biogas boiler. The pH was automatically adjusted between 6.8 and 7.5, the hydraulic retention time was set to 3 days, and the organic loading rate was controlled at 8~10 kgCOD / (m³). 3 ·d).

[0032] The sludge-water mixture produced by the anaerobic digester enters the external tubular membrane bioreactor unit via pipelines. This unit uses a PVDF tubular membrane with a pore size of 0.03 μm and a membrane area of ​​300 m². 2 The reactor mixture was circulated and filtered at a cross-flow velocity of 3.0 m / s, with the membrane flux maintained at 15–20 L / (m²). 2 ·h).

[0033] The activated sludge retained by the external tubular membrane bioreactor unit is continuously returned to the anaerobic digestion bioreactor via a sludge return pipe at a return ratio of 60%. Finally, the clear effluent after membrane filtration is obtained from the product water section of the external tubular membrane bioreactor unit.

[0034] The biogas produced by anaerobic digestion is desulfurized and dehydrated before entering the biogas boiler for combustion. The heat generated is used to maintain the mesophilic conditions (35±1℃) of the anaerobic digestion bioreactor through a heat exchange device. Excess heat is introduced into the circulation pipeline of the external tubular membrane bioreactor unit.

[0035] Example 3 This embodiment takes the treatment of food processing wastewater as an example.

[0036] Food processing wastewater (raw water COD approximately 12000 mg / L, BOD5 approximately 6000 mg / L, SS approximately 5000 mg / L) enters a completely mixed anaerobic reactor with an effective volume of 500 m³ after pretreatment. 3 The temperature is controlled at 33℃ through a heat exchange system, the pH value is controlled at 6.8~7.2, the hydraulic retention time is set to 2 days, and the organic loading rate is controlled at 14~15 kgCOD / (m³). 3 ·d).

[0037] The sludge-water mixture from the anaerobic digester enters an external tubular membrane bioreactor unit. The external tubular membrane bioreactor unit uses a PES tubular membrane with a pore size of 0.05 μm and a membrane area of ​​250 m². 2 Operating at a crossflow velocity of 3.8 m / s, the membrane flux was maintained at 18–25 L / (m²). 2 ·h).

[0038] The activated sludge retained by the external tubular membrane bioreactor unit is returned to the anaerobic digestion bioreactor at a 50% return ratio. Finally, the clear effluent after membrane filtration is obtained from the product water section of the external tubular membrane bioreactor unit.

[0039] The biogas produced by anaerobic digestion is desulfurized and dehydrated before entering the heat exchange device. The heat energy is preferentially used to maintain the temperature of the anaerobic digestion bioreactor (33℃), and the excess heat is introduced into the circulation pipeline of the external tubular membrane bioreactor unit.

[0040] Example 4 This embodiment takes the treatment of wastewater from a large-scale pig farm as an example.

[0041] Wastewater from pig farms (raw water COD approximately 15000 mg / L, TN approximately 800 mg / L, NH4+) + -N (approximately 600 mg / L) is pretreated before entering an upflow anaerobic sludge blanket reactor with an effective volume of 400 m³. 3 The heat exchange system of the combined heat and power (CHP) unit controls the temperature at 37℃, the pH value at 7.0-7.5, the hydraulic retention time at 5 days, and the organic load rate at 5-6 kgCOD / (m³). 3 ·d).

[0042] The anaerobic digestion effluent enters the anaerobic ammonia oxidation reactor, with DO controlled at 0.6-0.8 mg / L. The effluent from the anaerobic ammonia oxidation reactor enters an external tubular membrane bioreactor unit. This unit uses a PVDF tubular membrane with a pore size of 0.01 μm and a membrane area of ​​200 m². 2 The crossflow velocity was 2.5 m / s, and the membrane flux was maintained at 20-30 L / (m²). 2 ·h).

[0043] The activated sludge retained by the external tubular membrane bioreactor unit is returned to the anaerobic digestion bioreactor at a return ratio of 80%, and finally the clear effluent after membrane filtration is obtained from the product water section of the external tubular membrane bioreactor unit.

[0044] The biogas produced by anaerobic digestion is desulfurized and dehydrated before entering a combined heat and power (CHP) unit. The electricity is used to drive the equipment in the system, and the waste heat is used to maintain the system's operating temperature.

[0045] Example 5 This embodiment takes the treatment of biopharmaceutical wastewater as an example.

[0046] Biopharmaceutical wastewater (raw COD approximately 20,000 mg / L, TN approximately 1,200 mg / L) is pretreated before entering an upflow anaerobic sludge blanket reactor with an effective volume of 1000 m³. 3 The biogas heat recovery system controls the temperature at 35±1℃, the pH value at 6.8-7.5, the hydraulic retention time at 4 days, and the organic load rate at 10-12 kgCOD / (m³). 3 ·d).

[0047] The anaerobic digestion effluent enters the anaerobic ammonia oxidation reactor, with dissolved oxygen (DO) controlled at 0.4-0.6 mg / L. The effluent from the anaerobic ammonia oxidation reactor then enters an external tubular membrane bioreactor unit for solid-liquid separation. This unit uses a polysulfone tubular membrane with a pore size of 0.04 μm and a membrane area of ​​400 m². 2 The crossflow velocity was 3.5 m / s, and the membrane flux was maintained at 10-15 L / (m²). 2 ·h).

[0048] The activated sludge retained by the external tubular membrane bioreactor unit is returned to the anaerobic digestion bioreactor at a 75% return ratio. Finally, the clear effluent after membrane filtration is obtained from the product water section of the external tubular membrane bioreactor unit.

[0049] The biogas produced by anaerobic digestion is desulfurized and dehydrated before entering the biogas boiler. The heat energy is used to maintain the temperature of the anaerobic digester (35±1℃), and the excess heat is introduced into the circulation pipeline of the external tubular membrane bioreactor unit.

[0050] Example 6 This embodiment takes the treatment of biopharmaceutical wastewater as an example.

[0051] Biopharmaceutical wastewater (raw water COD approximately 20,000 mg / L, TN approximately 1,200 mg / L) enters a completely mixed anaerobic reactor with an effective volume of 1,200 m³ after pretreatment including oil removal and solid-liquid separation. 3The temperature is controlled at 35±1℃, the pH value at 6.8-7.5, the hydraulic retention time at 3 days, and the organic load rate at 7-8 kgCOD / (m³) through the biogas boiler heat exchange system. 3 ·d).

[0052] The anaerobic digestion effluent enters the anaerobic ammonia oxidation reactor, with DO controlled at 0.5-0.8 mg / L. The effluent from the anaerobic ammonia oxidation reactor enters an external tubular membrane bioreactor unit. This unit uses a PVDF tubular membrane with a pore size of 0.03 μm and a membrane area of ​​500 m². 2 The crossflow velocity was 3.0 m / s, and the membrane flux was maintained at 14-20 L / (m²). 2 ·h).

[0053] The activated sludge retained by the external tubular membrane bioreactor unit is returned to the anaerobic digestion bioreactor at a 65% return ratio. Finally, the clear effluent after membrane filtration is obtained from the product water section of the external tubular membrane bioreactor unit.

[0054] The biogas produced by anaerobic digestion is desulfurized and dehydrated before entering the biogas boiler. The heat energy is preferentially used to maintain the temperature of the anaerobic digestion reactor, and the excess heat is introduced into the circulation pipeline of the external tubular membrane bioreactor unit.

[0055] Comparative Example 1 This comparative example uses the traditional "anaerobic digestion + sedimentation tank" process to treat food processing wastewater from the same source as in Example 2, without setting up an external tubular membrane bioreactor unit or sludge return pipeline.

[0056] The wastewater source and pretreatment method are the same as in Example 2. The effective volume of the anaerobic digester, inoculation method, temperature, pH value, hydraulic retention time, and organic loading rate are all consistent with those in Example 2. The effluent from the anaerobic digester enters an effective volume of 200 m³. 3 The vertical flow sedimentation tank has a surface loading rate of 0.8 m³. 3 / (m 2 The sedimentation time is 4 hours. The biogas produced by anaerobic digestion is desulfurized and dehydrated before entering the biogas boiler. The heat energy is used to maintain the mesophilic operating conditions of the anaerobic digester (same as in Example 2). Finally, the effluent is obtained from the overflow outlet of the supernatant in the sedimentation tank.

[0057] Comparative Example 2 The wastewater source and treatment method in this comparative example are the same as in Example 2, except that an anaerobic digestion bioreactor is not used. The membrane material, membrane area, cross-flow velocity, and membrane flux of the external tubular membrane bioreactor unit are all the same as in Example 2. Finally, the clarified effluent after membrane filtration is obtained from the product water section of the external tubular membrane bioreactor unit.

[0058] Experiment Example 1 Performance Testing The effluent solutions obtained in Examples 1-6 and Comparative Examples 1-2 were measured. COD (Chemical Oxygen Demand): Determined according to the "Determination of Chemical Oxygen Demand in Water - Dichromate Method" (HJ 828) or the "Determination of Chemical Oxygen Demand in Water - Rapid Digestion Spectrophotometric Method" (HJ / T 399).

[0059] Ammonia nitrogen (NH4⁺-N): Determined according to Nessler's reagent spectrophotometric method for the determination of ammonia nitrogen in water quality (HJ 535) or salicylic acid spectrophotometric method for the determination of ammonia nitrogen in water quality (HJ 536).

[0060] Total nitrogen (TN): The determination was performed according to the "Determination of Total Nitrogen in Water by Alkaline Potassium Persulfate Digestion Ultraviolet Spectrophotometric Method" (HJ 636).

[0061] SS (suspended solids): Determined according to the "Determination of Suspended Solids in Water - Gravimetric Method" (GB / T 11901).

[0062] Biogas production: Measured using a wet gas flow meter.

[0063] Biogas components (CH4, CO2, H2S, etc.): determined by gas chromatography (refer to "Determination of Methane and Carbon Dioxide in Biogas by Gas Chromatography").

[0064] The final results are shown in Table 1 below.

[0065] Table 1

[0066] As can be seen from Table 1: All effluent SS values ​​in the embodiments were "not detected," indicating that the external tubular membrane bioreactor unit can effectively retain suspended solids, resulting in clear effluent. The effluent CODs of Examples 1 and 2 were 62 mg / L and 58 mg / L, respectively; the effluent COD of Example 3 was 92 mg / L; the effluent COD of Example 4 was 42 mg / L, the lowest among all examples; and the effluent CODs of Examples 5 and 6 were 72 mg / L and 68 mg / L, respectively.

[0067] Examples 1, 4, 5, and 6 coupled with an anaerobic ammonia oxidation process, achieving deep removal of both ammonia nitrogen and total nitrogen. Example 4 showed the best denitrification effect, with effluent total nitrogen of 12 mg / L and ammonia nitrogen of 4 mg / L; Examples 1 and 6 were next, with effluent total nitrogen of 22 mg / L and ammonia nitrogen of 7 mg / L; Example 5 had effluent total nitrogen of 26 mg / L and ammonia nitrogen of 8 mg / L. Examples 2 and 3, without coupled anaerobic ammonia oxidation, produced effluent ammonia nitrogen of 52 mg / L and 58 mg / L, and total nitrogen of 68 mg / L and 72 mg / L, respectively, indicating that anaerobic digestion alone cannot effectively remove ammonia nitrogen and total nitrogen, thus confirming the necessity of coupled anaerobic ammonia oxidation for deep denitrification.

[0068] Comparative Example 1 used a traditional sedimentation tank for solid-liquid separation, resulting in effluent SS of 120 mg / L and COD of 425 mg / L, significantly higher than in the other examples, indicating that traditional sedimentation tanks cannot effectively remove fine suspended particles. Comparative Example 2, although using membrane separation, showed no detectable SS in the effluent; however, due to the lack of an anaerobic digestion step, dissolved organic matter was not degraded, resulting in an effluent COD as high as 9350 mg / L, failing to meet discharge standards. The comparison between Examples 1-6 and Comparative Examples 1 and 2 demonstrates that the coupling of the anaerobic digestion bioreactor with the external tubular membrane bioreactor unit is crucial for achieving efficient organic matter degradation and high-quality effluent.

[0069] All embodiments achieved stable biogas energy recovery due to the inclusion of anaerobic digestion bioreactors. The biogas yield in Example 4 was 0.44 Nm³. 3 / kgCOD removal and methane content of 68% were the highest among all examples; the biogas yields of Examples 1 and 6 were 0.41 and 0.40 Nm³, respectively. 3 / kgCOD removal and methane content were 63% and 62%, respectively; the biogas yield in Example 3 was 0.37 Nm³. 3 The COD removal rate was ≥83% per kg, and the methane content was 58%, the lowest among all examples. Example 4 had the highest COD removal rate (90%), while Example 3 had the lowest (83%).

[0070] Activated sludge was retained and returned to an anaerobic digestion bioreactor for deep digestion using an external tubular membrane bioreactor unit. In all embodiments, in-situ sludge reduction was achieved. Based on Example 2 (food wastewater, HRT=3d, OLR=8~10, return ratio 60%), sludge production was significantly reduced in all embodiments.

[0071] In Comparative Example 1 (which included anaerobic digestion but lacked an external tubular membrane bioreactor unit and sludge return pipeline), the sedimentation tank could not effectively retain activated sludge, resulting in the loss of a large number of microorganisms with the effluent. The COD removal rate was only 73%, lower than that of the other examples, and the biogas production rate was 0.34 Nm³. 3 / kgCOD removal, methane content 54%, and residual sludge production approximately 2.5 times that of Example 2.

[0072] In Comparative Example 2 (no anaerobic digestion, only tubular membrane), due to the lack of an anaerobic digestion step, the COD removal rate was only 64% (COD removal only by membrane interception of suspended solids), which was much lower than that of the embodiments; and the system could not produce biogas, could not achieve energy recovery, and the remaining sludge production was about 3.0 times that of Example 2.

[0073] In summary, the coupling of the anaerobic digestion bioreactor and the external tubular membrane bioreactor unit is key to achieving efficient organic matter removal, energy recovery, and sludge reduction in this invention; the synergistic effect of both is indispensable. The effluent quality in each embodiment of this invention is excellent, biogas energy recovery is stable, and sludge production is significantly reduced, verifying the effectiveness and superiority of the technical solution of this invention.

[0074] 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. An external tubular membrane anaerobic digestion wastewater treatment system, characterized in that, include: Anaerobic digestion bioreactor, external tubular membrane bioreactor unit, sludge return pipeline, biogas heat recovery system; The external tubular membrane bioreactor unit is connected to the outlet of the anaerobic digestion bioreactor via a pipe. The sludge return pipe connects the sludge outlet of the external tubular membrane bioreactor unit to the sludge inlet of the anaerobic digestion bioreactor. The biogas heat recovery system is connected to the biogas outlet of the anaerobic digestion bioreactor.

2. The external tubular membrane anaerobic digestion wastewater treatment system according to claim 1, characterized in that, The external tubular membrane bioreactor unit employs a high-speed cross-flow filtration method; The external tubular membrane bioreactor unit is equipped with a cross-flow filtration circulation pump, the return water of which is used to achieve sludge return and anaerobic circulation.

3. The external tubular membrane anaerobic digestion wastewater treatment system according to claim 1, characterized in that, The biogas heat energy recovery system includes a desulfurization and dehydration device and a heat exchange device; The heat exchange device prioritizes the use of heat energy generated from biogas combustion to maintain the mesophilic conditions of the anaerobic digestion bioreactor, while excess heat is introduced into the circulation pipeline of the external tubular membrane bioreactor unit.

4. The external tubular membrane anaerobic digestion wastewater treatment system according to claim 1, characterized in that, The biogas heat recovery system includes a desulfurization and dehydration device and a biogas boiler or cogeneration device, which are connected to the anaerobic digester and the external tubular membrane bioreactor unit respectively through heat exchange pipelines.

5. A wastewater treatment method for an external tubular membrane anaerobic digestion wastewater treatment system as described in any one of claims 1-4, characterized in that, Includes the following steps: The pretreated organic wastewater is introduced into the anaerobic digestion bioreactor, where organic pollutants are converted into biogas by anaerobic microorganisms and then collected. The mud-water mixture produced by the anaerobic digestion bioreactor is introduced into the external tubular membrane bioreactor unit for solid-liquid separation. The purified water that meets the discharge standards will then enter the subsequent treatment process. The activated sludge retained in the external tubular membrane bioreactor unit is partially or completely returned to the anaerobic digestion bioreactor for deep digestion and sludge reduction. The heat energy generated by the combustion of biogas is recycled through a biogas heat recovery system to maintain the process operating temperature of the anaerobic digestion bioreactor.

6. The wastewater treatment method according to claim 5, characterized in that, The recirculation ratio of the activated sludge is 50%-80%.

7. The wastewater treatment method according to claim 5, characterized in that, The control conditions for the anaerobic digestion bioreactor are as follows: mesophilic digestion range of 33℃-37℃, pH value of 6.8-7.5, hydraulic retention time of 2-5 days, and organic loading rate of 5-15 kg COD / (m³). 3 (d) Biogas yield 0.35-0.45 Nm³ 3 / kgCOD removal.

8. The wastewater treatment method according to claim 5, characterized in that, The subsequent treatment process includes an anaerobic ammonia oxidation denitrification process. The purified water enters the anaerobic ammonia oxidation reactor and undergoes short-cut nitrification under oxygen-limited conditions of DO 0.3-1.0 mg / L. Then, nitrogen gas is generated by the action of anaerobic ammonia oxidizing bacteria. The effluent from the anaerobic ammonia oxidation reactor enters the external tubular membrane bioreactor unit for solid-liquid separation.

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