Dry anaerobic sludge external reflux device and reflux process

Abstract

The invention discloses a dry-type anaerobic sludge external reflux device which comprises a dry-type anaerobic tank, an extrusion pretreatment unit, a filtrate temporary storage unit, a fine screening unit, a targeted flocculation unit, a centrifugal separation unit, a returned sludge conveying unit with a double-plunger pump for standby, and a raw material TS adjusting unit, the invention also discloses a reflux process of the dry anaerobic sludge external reflux device. The reflux process comprises the following steps: adjusting a raw material TS to 25-30%, removing impurities greater than or equal to 5 mm and 1-5 mm in a grading manner, adding PMA according to a sludge-chemical ratio of 7: 1 for targeted flocculation, centrifugally preserving breeds according to a separation factor of 3000-4000, dynamically adjusting the sludge return amount by 30-50%, and recycling dry sludge to realize system optimization. According to the method, the feeding load can be increased to the design value 125%, the front-end VFA is smaller than or equal to 10000 mg / L for a long time, the acidification risk is smaller than or equal to 1% / month, the sludge return pipeline is not blocked, the operation cost is low, the method is suitable for various types of high inherent waste, and remarkable technical, economic and environmental benefits are achieved.

Description

Technical Field

[0001] This invention relates to the field of anaerobic fermentation treatment technology for organic waste, specifically to a dry anaerobic sludge external reflux device and reflux process. Background Technology

[0002] The output of organic waste such as kitchen waste and food waste has been increasing year by year. Dry anaerobic fermentation technology is widely used in organic waste resource utilization projects because of its advantages such as low energy consumption and low biogas residue output when treating organic waste with high solid content (20%-40%).

[0003] However, existing dry anaerobic systems face significant technical bottlenecks in actual operation:

[0004] 1. Low mass transfer efficiency and local acidification: The high solids concentration of the material in dry anaerobic systems results in a mass transfer efficiency that is much lower than that of wet anaerobic systems. Volatile fatty acids (VFAs) tend to accumulate at the front end of the anaerobic tank. When the VFA concentration exceeds the tolerance threshold (usually 10,000 mg / L), it will inhibit the activity of anaerobic bacteria, leading to system acidification, which in turn makes it impossible to increase the feed load, and in severe cases, it may even cause the system to shut down.

[0005] 2. Deficiencies of Existing Internal Recirculation Schemes: To control VFA accumulation, existing dry anaerobic systems generally adopt an "internal recirculation" design, which involves recirculating the material from the back end of the anaerobic tank back to the front end through the discharge system. However, this scheme has three major problems: First, the VFA concentration in the back end material is also high due to VFA migration from the front end, and recirculation cannot effectively reduce the VFA concentration at the front end; second, the back end material contains a large number of undegraded large particulate impurities (such as bone fragments and plastic fragments in kitchen waste), and long-term recirculation can easily clog the pipeline, increasing equipment maintenance costs; third, after long-term fermentation, the number and activity of anaerobic bacteria in the back end material are low, and recirculation cannot replenish effective bacteria, making it difficult to improve the fermentation environment at the front end; fourth, the total sludge (TS) and viscosity of the sludge at the back end of the anaerobic tank are relatively low after anaerobic digestion, and returning it to the front end may lead to a decrease in the overall TS and viscosity of the dry anaerobic tank, thereby increasing the risk of sedimentation in the dry anaerobic tank.

[0006] 3. Operational difficulties in actual projects: Many dry anaerobic projects that have been put into operation are plagued by the above problems, resulting in the feed load being lower than the design value for a long time (usually only 70%-80% of the design load), the amount of sludge disposal remaining high, the operating load of the drying terminal equipment being too high, and the operating cost increasing significantly, which restricts the large-scale promotion and application of dry anaerobic technology.

[0007] To address the aforementioned technical challenges, there is an urgent need to develop an optimized solution for dry anaerobic systems that can effectively reduce VFA concentration at the front end of the anaerobic tank, replenish active bacteria, prevent pipeline blockage, and increase feed load. Summary of the Invention

[0008] The purpose of this invention is to provide a dry anaerobic sludge external reflux device and reflux process to solve the above-mentioned defects.

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] The present invention proposes a dry anaerobic sludge external return device, comprising: a dry anaerobic tank, a compression pretreatment unit, a filtrate storage unit, a fine screening unit, a targeted flocculation unit, and a centrifugal separation unit connected in series, and further comprising a sludge return conveying unit with backup protection and a raw material TS adjustment unit.

[0011] The front end of the dry anaerobic digester is provided with a raw material inlet, which is suitable for organic waste with a total solids content (TS) of ≥25%. The rear end of the dry anaerobic digester is provided with a fermentation outlet, which is sealed and connected to the feed end of the extrusion pretreatment unit.

[0012] The extrusion pretreatment unit is equipped with a pressure regulating component, the working pressure range of which is 0.8-1.2 MPa. The extrusion pretreatment unit is used to separate and remove large material impurities with a particle size ≥5 mm from the fermentation output.

[0013] The filtrate outlet of the extrusion pretreatment unit is connected to the filtrate storage unit through a corrosion-resistant pipe. The filtrate storage unit has a volume of ≥5m³ and is equipped with a stirring assembly. The discharge end of the filtrate storage unit is connected to the feed end of the fine screening unit. The fine screening unit is used to remove small material impurities with a particle size of 1-5mm from the filtrate.

[0014] The targeted flocculation unit is connected to the discharge end of the fine screening unit. The targeted flocculation unit includes a flocculant storage tank, a metering pump, and a mixing reaction tank. The flocculant stored in the flocculant storage tank is polyacrylamide (PMA). The metering pump adds PMA to the mixing reaction tank at a fixed ratio of 7:1 (sludge-to-powder ratio). A stirrer is installed in the mixing reaction tank.

[0015] The feed end of the centrifugal separation unit is connected to the discharge end of the targeted flocculation unit. The separation factor of the centrifugal separation unit is 3000-4000. After separation, the centrifugal separation unit forms an aqueous phase outlet and a sludge phase outlet. The aqueous phase outlet is connected to an external wastewater treatment system, and the sludge phase outlet is connected to the front end of the dry anaerobic tank through the sludge return conveying unit. The TS content of the sludge phase after separation by the centrifugal separation unit is stable at 25%-30%.

[0016] The sludge conveying unit includes two plunger pumps of the same model, connecting pipes and branch control valves. The inlets of the two plunger pumps are connected to the sludge phase outlet of the centrifugal separation unit. The outlets of the two plunger pumps are connected to each other through carbon steel pipes and extend to the front end of the dry anaerobic tank through connecting pipes. A manual slide valve is installed on the connecting pipe at the front end of the dry anaerobic tank. The nominal pressure of the manual slide valve is 1.6 MPa.

[0017] The raw material TS adjustment unit includes a dry mud storage bin and a quantitative conveying screw. The feed end of the dry mud storage bin is connected to an external drying device, which is used to process part of the mud phase produced by the centrifugal separation unit. The TS content of the dry mud in the dry mud storage bin is ≥95%. One end of the quantitative conveying screw is connected to the dry mud storage bin, and the other end is connected to the raw material feed port of the dry anaerobic tank.

[0018] Preferably, a reflux process based on a dry anaerobic sludge external reflux device includes the following steps:

[0019] S1. Raw material pretreatment and feeding: Organic waste with an initial solids concentration of 20%-40% is fed into the raw material TS adjustment unit. If the raw material TS < 25%, dry sludge with TS ≥ 95% is added to the raw material through a quantitative conveying screw to stabilize the TS content of the mixed raw material at 25%-30%. The adjusted raw material is continuously fed into the dry anaerobic tank through the raw material inlet at the front end of the dry anaerobic tank for mesophilic anaerobic fermentation. The temperature of the mesophilic anaerobic fermentation is controlled at 35-38℃.

[0020] S2. Fermentation Discharge Grading and Impurity Removal: The fermentation discharge from the rear end of the dry anaerobic tank enters the extrusion pretreatment unit through the fermentation discharge port. Under a pressure of 0.8-1.2MPa, it is extruded and separated to remove large material impurities with a particle size ≥5mm. The large material impurities are transported off-site via a sealed conveyor belt. The filtrate generated by extrusion enters the filtrate temporary storage unit. After being stirred by the agitator in the filtrate temporary storage unit, it is sent to the fine screening unit. The fine screening unit removes small material impurities with a particle size of 1-5mm. The small material impurities are transported off-site for disposal.

[0021] S3. Targeted Flocculation and Centrifugal Separation: The filtrate after screening enters the mixing tank of the targeted flocculation unit. PMA flocculant is added via a metering pump at a sludge-to-floc ratio of 7:1. After reaction under the action of a stirrer, flocs with a particle size ≥1mm are formed. The material containing flocs is then sent to the centrifugal separation unit for solid-liquid separation at a set temperature, yielding an aqueous phase and a sludge phase. The aqueous phase is sent to the wastewater treatment system, while the sludge phase consists of wet sludge with a TS content of 25%-30% and an anaerobic bacteria concentration ≥10%. 8 CFU / mL, VFA ≤5000mg / L;

[0022] S4. External sludge return with backup: Start the two plunger pumps of the sludge return conveying unit. In the initial state, one plunger pump is running and one plunger pump is on standby. The wet sludge obtained in step S3 is conveyed to the front end of the dry anaerobic tank through the connecting pipe and the branch gate valve. When the running plunger pump fails, close the inlet and outlet valves of the failed plunger pump and start the standby plunger pump.

[0023] S5. Optional processing and recycling of dry mud: If the raw material TS does not need to be adjusted, the wet mud obtained in step S3 is sent to an external drying device and dried at a set temperature until TS≥95% to obtain dry mud. Part of the dry mud is transported out for disposal, and the other part is sent to the dry mud storage of the raw material TS adjustment unit for later use.

[0024] Preferably, the extrusion pretreatment unit has a wear-resistant ceramic coating on the inner wall of the extrusion chamber, the thickness of the wear-resistant ceramic coating is 0.5-1mm, the surface of the extrusion roller of the extrusion pretreatment unit is provided with spiral protrusions, the pitch of the spiral protrusions is 20-30mm, and the extrusion pretreatment unit has a removal rate of ≥95% for large material impurities.

[0025] Preferably, the fine screening unit is equipped with a stainless steel screen with a mesh size of 1-5mm, and the fine screening unit has a removal rate of ≥98% for small material impurities.

[0026] Preferably, the stirring speed of the agitator in the mixing reaction tank of the targeted flocculation unit is 60-80 r / min. The stirring speed setting is used to ensure uniform flocculation without damaging the activity of anaerobic bacteria.

[0027] Preferably, the centrifugal separation unit has a temperature holding sleeve at its mud phase outlet. The temperature holding sleeve has a temperature control range of 35-40℃ and is used to prevent the loss of anaerobic bacterial activity due to a sudden drop in the temperature of the wet mud.

[0028] Preferably, the carbon steel pipe connecting the outlets of the two plunger pumps in the sludge return conveying unit has a specification of DN150-DN200. An online flow meter and a pressure sensor are installed on the connecting pipe of the sludge return conveying unit. The accuracy of the online flow meter is ±2%, and the range of the pressure sensor is 0-2.5MPa. The online flow meter is electrically connected to the frequency converter of the plunger pump.

[0029] Preferably, in step S2, the stirring time of the stirrer in the filtrate temporary storage unit is 10-15 min, the stirring speed of the stirrer is 60-80 r / min, the total proportion of impurities in the fermentation effluent is ≤7%, of which the proportion of large material impurities is ≤5% and the proportion of small material impurities is ≤2%; in step S3, the reaction time of the targeted flocculation unit is 20-30 min, the solid-liquid separation temperature of the centrifugal separation unit is 35-40℃, the COD of the separated aqueous phase is ≤8000 mg / L, and the activity retention rate of anaerobic bacteria in the wet sludge is ≥90%.

[0030] Preferably, in step S4, the amount of recycled sludge is controlled to be 30%-50% of the raw material feed and calculated according to the dry matter mass ratio. When the VFA at the front end of the dry anaerobic digester is >8000mg / L, the amount of recycled sludge is increased to 50% of the raw material feed; when the VFA at the front end of the dry anaerobic digester is <3000mg / L, the amount of recycled sludge is reduced to 30% of the raw material feed; the recycling sludge interruption time is ≤10min.

[0031] Preferably, in step S5, the drying temperature of the external drying equipment is 120-150℃, the amount of wet sludge fed into the external drying equipment accounts for 20%-30% of the total wet sludge, the VFA at the front end of the dry anaerobic tank is controlled within 10000mg / L for a long time, and the acidification risk rate is ≤1% / month.

[0032] The beneficial effects of this invention are as follows:

[0033] The dry anaerobic sludge external recirculation device of the present invention, through its integrated structural design of "multi-stage impurity removal - targeted flocculation and seed preservation - dual-pump backup recirculation - dynamic adjustment of raw material TS", specifically addresses the technical pain points of existing dry anaerobic internal recirculation devices, and has greatly improved in terms of equipment structure adaptability, system operation stability, process adaptability and compatibility, and cost control effectiveness, as detailed below:

[0034] (1) The dry anaerobic sludge external return device of the present invention integrates a graded impurity removal structure of extrusion pretreatment and fine screening, which can efficiently intercept large impurities ≥5mm and small impurities 1-5mm, so that the proportion of impurities in the filtrate is ≤7%, greatly extending the pipeline unblocking cycle and reducing the equipment maintenance frequency by more than 85%, thus completely solving the problem of frequent pipe blockage in traditional devices.

[0035] (2) The dry anaerobic sludge external return device of the present invention, with the targeted flocculation unit adding PMA at a fixed sludge-to-medicine ratio, combined with a centrifugal separation unit with specific parameters and a temperature control structure, can achieve the dual objectives of "reducing VFA + preserving bacterial strains", and can make the bacterial strain concentration of the returned wet sludge ≥10. 8With CFU / mL and VFA ≤ 5000 mg / L, it can quickly dilute the high concentration of VFA at the front end of the anaerobic tank and replenish highly active bacteria, stabilizing the VFA at the front end to within 10000 mg / L, and reducing the acidification risk rate to less than 1% / month.

[0036] (3) The dry anaerobic sludge external return device of the present invention, through the sludge return conveying structure with the dual plunger pump interconnection and backup, and with the online monitoring component, can control the sludge return interruption time within 10 minutes, and improve the fault-free operation rate of the sludge return system to 99.5%, avoiding the disadvantage of the traditional single pump device stopping immediately upon failure, and providing a reliable guarantee for the stable operation of the system.

[0037] (4) The dry anaerobic sludge external return device of the present invention can dynamically replenish dry sludge through the raw material TS adjustment unit, stabilize the raw material TS in the optimal fermentation range of 25%-30%, adapt to various high organic wastes with TS of 20%-40%, improve material compatibility by more than 60%, realize the recycling of dry sludge, and reduce the amount of sludge transported by 15%-20%.

[0038] (5) The dry anaerobic sludge external return device of the present invention forms a closed-loop collaborative system with each unit, which significantly improves the treatment efficiency; at the same time, it greatly reduces the costs of equipment maintenance and sludge disposal. Attached Figure Description

[0039] Figure 1 : Process flow diagram of the equipment of this invention. Detailed Implementation

[0040] The present invention will be further described below with reference to the embodiments. It should be noted that these are merely examples and descriptions of the inventive concept. Those skilled in the art can make various modifications or additions to the specific embodiments described or use similar methods to replace them, as long as they do not deviate from the inventive concept or exceed the scope defined in the claims, they should all be considered to fall within the protection scope of the present invention.

[0041] Example 1:

[0042] like Figure 1 As shown, the present invention proposes a dry anaerobic sludge external return device, comprising: a dry anaerobic tank, a compression pretreatment unit, a filtrate temporary storage unit, a fine screening unit, a targeted flocculation unit, a centrifugal separation unit, and a sludge return conveying unit with backup protection and a raw material TS adjustment unit, connected in series. The structure and function of each unit are as follows:

[0043] (1) Dry anaerobic digester 1:

[0044] As the core reactor for anaerobic fermentation of organic waste, it is equipped with a raw material inlet 11 at the front end, which is suitable for organic waste with a total solids content (TS) of ≥25%; and a fermentation outlet 12 at the rear end, which is used to discharge the material that has undergone preliminary fermentation (including undegraded organic matter, bacteria and impurities). The fermentation outlet 12 is sealed and connected to the feed end of the extrusion pretreatment unit 2 to avoid odor leakage and material loss.

[0045] (2) Extrusion pretreatment unit 2:

[0046] A pressure regulating component 21 is configured with a working pressure range of 0.8-1.2MPa. It is used to extrude the fermentation effluent discharged from the rear end of the dry anaerobic tank 1 to separate and remove large material impurities with a particle size ≥5mm (such as bone residue, plastic fragments, coarse fiber clumps, etc.) to avoid blockage of subsequent pipelines and equipment. The inner wall of the extrusion chamber is equipped with a wear-resistant ceramic coating with a thickness of 0.5-1mm, and the surface of the extrusion roller is equipped with spiral protrusions with a pitch of 20-30mm, which can improve the separation efficiency of large material impurities and ensure that the removal rate of large impurities is ≥95%.

[0047] (3) Filtrate temporary storage unit 3:

[0048] With a volume ≥ 5m³, it is equipped with a stirring component 31 (speed 60-80r / min), which is connected to the filtrate outlet of the extrusion pretreatment unit 2 through a corrosion-resistant pipe. It is used to temporarily store the filtrate generated by extrusion (containing small impurities, anaerobic bacteria and soluble organic matter). The stirring component 31 can prevent the sedimentation of solid particles in the filtrate and ensure the uniformity of materials in subsequent processing.

[0049] (4) Fine screening unit 4:

[0050] The filtrate storage unit 3 is connected to the discharge end and is equipped with a stainless steel screen 41 with a pore size of 1-5mm. This screen is used to further remove small material impurities (such as fine sand, bone fragments, etc.) with a particle size of 1-5mm from the filtrate. The small impurity removal rate is ≥98%, which further reduces the interference of impurities in subsequent units and ensures the purity of the returned sludge.

[0051] (5) Targeted flocculation unit 5:

[0052] The system includes a flocculant storage tank 51, a metering pump 52, and a mixing reaction tank 53. The flocculant storage tank 51 stores polyacrylamide (PMA), and the metering pump 52 adds PMA to the mixing reaction tank 53 at a fixed ratio of 7:1 (sludge to flocculant). The mixing reaction tank 53 is equipped with a stirrer with a rotation speed of 60-80 r / min to ensure that the PMA and materials are fully mixed to form flocs, while avoiding excessive stirring speed that could damage the activity of anaerobic bacteria.

[0053] (6) Centrifugal separation unit 6:

[0054] Its feed end is connected to the discharge end of the targeted flocculation unit 5, with a separation factor of 3000-4000, used for solid-liquid separation of materials containing flocs; the TS content of the sludge phase (wet sludge) formed after separation is stable at 25%-30%, and the anaerobic bacteria concentration is ≥10. 8 CFU / mL, VFA≤5000mg / L. The resulting aqueous phase has a COD≤8000mg / L and can be directly sent to the external wastewater treatment system 10 for treatment through the aqueous phase outlet 63; while the sludge phase outlet 62 of the centrifugal separation unit is equipped with a temperature holding sleeve 61 (temperature control 35-40℃) to avoid the loss of bacterial activity due to a sudden drop in wet sludge temperature, and to ensure that the bacterial activity retention rate is ≥90%.

[0055] (7) Sludge conveying unit:

[0056] The system includes two identical plunger pumps 71, a connecting pipe 72, and a manual control valve 73. The inlets of both plunger pumps 71 are connected to the sludge outlet 62 of the centrifugal separation unit 6. The outlets are interconnected via DN150-DN200 carbon steel pipes and extend through the connecting pipe 72 to the raw material inlet 11 at the front end of the dry anaerobic digester 1. A manual slide valve 73 with a nominal pressure of 1.6 MPa is installed on the connecting pipe 72 connected to the raw material inlet 11 of the dry anaerobic digester 1. The connecting pipe 72 is also equipped with an online flow meter 74 with an accuracy of ±2% and a pressure sensor 75 with a range of 0-2.5 MPa. The online flow meter 74 is electrically connected to the frequency converter controller of the plunger pumps 71, enabling automatic adjustment of the sludge return rate. The two plunger pumps 71 serve as backups for each other; in the event of a failure in one, the other can immediately take over, ensuring that the sludge return interruption time is ≤10 minutes.

[0057] (8) Raw material TS adjustment unit:

[0058] It includes a dry sludge storage bin 81 and a quantitative conveying screw 82. The feed end of the dry sludge storage bin 81 is connected to the external drying equipment 9 (used to process part of the wet sludge produced by the centrifugal separation unit 6), and the total sludge content (TS) of the dry sludge in the dry sludge storage bin 81 is ≥95%. One end of the quantitative conveying screw 82 is connected to the dry sludge storage bin 81, and the other end is connected to the raw material feed inlet 11 of the dry anaerobic tank 1. It can replenish dry sludge according to the initial TS value of the raw material, so that the TS of the mixed raw material is stabilized at 25%-30%, ensuring the optimal material conditions for anaerobic fermentation.

[0059] The present invention discloses a reflux process based on a dry anaerobic sludge external reflux device, the specific steps of which are as follows:

[0060] S1. Raw material pretreatment and feeding:

[0061] Organic waste (such as kitchen waste or food waste) with an initial solids concentration of 20%-40% is selected and fed into the raw material TS adjustment unit. The TS value of the raw material is monitored by an online TS detector. If TS < 25%, the quantitative conveying screw 82 is activated to supplement the raw material with dry sludge with TS ≥ 95%, so that the TS content of the mixed raw material is stabilized at 25%-30%. The adjusted raw material is continuously fed into the dry anaerobic tank 1 through the raw material inlet 11 at the front end of the tank. The fermentation temperature inside the tank is controlled at 35-38℃ (mesothermal anaerobic range) to carry out the anaerobic fermentation reaction.

[0062] S2. Grading and impurity removal of fermentation output:

[0063] The fermentation discharge (TS 20%-25%, VFA 8000-12000mg / L) from the rear end of the dry anaerobic tank 1 enters the extrusion pretreatment unit 2 through the fermentation discharge port 12. Under pressure of 0.8-1.2MPa, it is extruded and separated to remove large material impurities with a particle size ≥5mm (impurity percentage ≤5%). The large impurities are transported off-site via a sealed conveyor belt for disposal.

[0064] The filtrate produced by extrusion enters the filtrate storage unit 3, and the stirring component 31 is started (60-80 r / min) to stir for 10-15 min to prevent solid particles from settling. Then the filtrate is sent to the fine screening unit 4 and filtered through a 1-5 mm stainless steel screen 41 to remove small material impurities with a particle size of 1-5 mm (impurity ratio ≤2%). The small impurities are also transported off-site for disposal. At this time, the total impurity ratio in the filtrate is ≤7%.

[0065] S3, Targeted Flocculation and Centrifugal Separation:

[0066] The filtrate after screening (TS 18%-22%) enters the mixing reaction tank 53 of the targeted flocculation unit 5. The metering pump 52 is started, and PMA flocculant is added into the tank at a mud-to-floc ratio of 7:1.

[0067] Turn on the agitator (60-80 r / min) and stir for 20-30 min to form stable flocs with a particle size ≥1 mm. Send the floc-containing material to centrifugal separation unit 6, controlling the separation temperature at 35-40℃ and the separation factor at 3000-4000 for solid-liquid separation. After separation, an aqueous phase (COD ≤8000 mg / L) and a mud phase (wet mud, TS 25%-30%, anaerobic bacteria concentration ≥10) are obtained. 8 (CFU / mL, VFA≤5000mg / L), the aqueous phase is sent to the external wastewater treatment system 10 for treatment, and the wet sludge is kept for later use.

[0068] S4. Sludge external return with backup backup:

[0069] Start the plunger pump 71 of the sludge return conveying unit (1 unit initially in operation, 1 unit on standby), and adjust the sludge return rate according to the VFA monitoring value at the front end of the dry anaerobic tank 1:

[0070] When the front-end VFA > 8000 mg / L, increase the amount of returned sludge to 50% of the raw material feed (based on dry matter mass ratio); when the front-end VFA < 3000 mg / L, reduce the amount of returned sludge to 30% of the raw material feed; wet sludge is transported to the raw material inlet 11 at the front end of the dry anaerobic tank 1 through the connecting pipe 72 and the manual slide valve 73 of the branch to replenish active bacteria and dilute the front-end VFA; if the plunger pump 71 fails, close its inlet and outlet valves and turn on the standby plunger pump 71 at the same time to ensure that the sludge return interruption time is ≤ 10 min.

[0071] S5. Optional treatment and recycling of dry mud:

[0072] If the raw material TS does not need to be adjusted, a portion of the wet mud obtained in step S3 (accounting for 20%-30% of the total wet mud) is sent to the external drying equipment 9, and the drying temperature is controlled at 120-150℃. The mud is dried until TS≥95% to obtain dry mud. A portion of the dry mud is sealed and packaged for external disposal, while the other portion is sent to the dry mud storage bin 81 of the raw material TS adjustment unit for later use, so as to realize the recycling of dry mud and reduce the amount of waste discharged.

[0073] In this embodiment, a food waste treatment plant originally had two 1000m³ dry anaerobic digesters with a processing capacity of 100 tons / day (design load). Due to the accumulation of VFA (Vacuum-Activated Fat) at the front end, the actual feed rate was only 70-80 tons / day. Therefore, the dry anaerobic sludge external return device of this invention was modified, and the specific device construction is as follows:

[0074] In the extrusion pretreatment unit 2, a YZ300 twin-roll extruder is specifically selected, equipped with an electro-hydraulic pressure regulating component 21, and the working pressure is set to 1.0MPa; the inner wall of the extrusion chamber is sprayed with a 0.8mm thick alumina wear-resistant ceramic coating, and the surface of the extrusion roll is machined with a spiral protrusion with a pitch of 25mm to ensure that the removal rate of large impurities (≥5mm) is ≥95%.

[0075] In the filtrate storage unit 3, a storage tank made of 304 stainless steel with a volume of 6m³ is used. It has a built-in frame agitator with a power of 2.2kW and a speed of 70r / min, and is connected to the filtrate outlet of the extruder through a DN100 UPVC anti-corrosion pipe.

[0076] In the fine screening unit 4, a vibrating screen is specifically selected. The stainless steel screen 41 is made of 304 stainless steel with a pore size of 3mm and a processing capacity of 50m³ / day. It is connected to the filtrate storage unit 3 through a DN80 stainless steel pipe.

[0077] In the targeted flocculation unit 5, the flocculant storage tank 51 has a volume of 1m³, the metering pump 52 is a GM050 with a flow rate adjustment range of 0-50L / h, and the mixing reaction tank 53 has a volume of 3m³, a built-in paddle agitator with a rotation speed of 70r / min, and the pump frequency is set according to a mud-to-powder ratio of 7:1.

[0078] In centrifugal separation unit 6, a horizontal screw discharge centrifuge, model LW450, with a separation factor of 3500 and a processing capacity of 40m³ / day, is specifically selected. The mud phase outlet is wrapped with an electric heating temperature maintenance sleeve with a thickness of 50mm, and the temperature is controlled at 38℃ to ensure that the bacterial activity retention rate is ≥90%.

[0079] In the sludge conveying unit, two G200 plunger pumps 71 (flow rate 20 m³ / h, pressure 1.6 MPa) are selected, and their outlets are connected through a DN180 carbon steel pipe. An LDG-150 electromagnetic flow meter (accuracy ±2%) and a PT100 pressure sensor (range 0-2.5 MPa) are installed on the connecting pipe 72. A manual slide gate valve (model Z41H-16C) with a nominal pressure of 1.6 MPa is installed on the branch leading to the dry anaerobic tank 1.

[0080] In the raw material TS adjustment unit, the dry sludge storage bin has a volume of 50m³, and the quantitative conveying screw is model LS300 (conveyor capacity 0-10 tons / hour). It is connected to the conveyor belt of the raw material inlet 11 of the dry anaerobic tank 1, and the dry sludge replenishment is automatically controlled by an online TS detector (model TS-200).

[0081] Based on the aforementioned dry anaerobic sludge external return device, the treatment plant conducted a 12-month process operation using a return process based on the dry anaerobic sludge external return device according to the present invention. The specific process steps and results are as follows:

[0082] In step S1, the raw material organic waste is kitchen waste, with an initial TS fluctuation range of 22%-38%. When the raw material TS is <25% (such as during the summer rainy season, when the TS drops to 22%), a quantitative conveying screw is started to supplement dry sludge with TS≥95% to stabilize the TS of the mixed raw material at around 28%. The raw material is continuously fed into two anaerobic digesters via a conveyor belt, and the fermentation temperature inside the digesters is controlled at 37℃.

[0083] In step S2, the TS (Total Sterile Extract) from the anaerobic digester at the rear end of the fermentation process is approximately 23%, and the VFA (Vacuum Acid) is approximately 10,000 mg / L. After entering the extruder, it is separated under a pressure of 1.0 MPa. The removal rate of large impurities (mainly bone residue and plastic fragments) is 96%, and the amount transported for disposal is approximately 3 tons / day. The filtrate is stirred by a stirrer for 12 minutes and then sent to a screening machine. The removal rate of small impurities (fine sand and bone fragments) is 98%, and the amount transported for disposal is approximately 1 ton / day. The total proportion of impurities in the filtrate is ≤6%.

[0084] In step S3, the saturated total volume (TS) of the filtrate after sieving is approximately 20%. After entering the mixing reactor, PMA flocculant (concentration 0.1%) is added, and the mixture is stirred for 25 minutes to form flocs with a particle size of approximately 2 mm. After centrifugation, the aqueous phase COD is approximately 7500 mg / L and is then sent to the wastewater treatment system. The wet sludge has a TS of approximately 28% and a bacterial concentration of approximately 1.2 × 10⁻⁶. 8 CFU / mL, VFA approximately 4500 mg / L.

[0085] In step S4, the initial sludge return amount is set to 40% of the raw material feed amount (based on dry matter); when the VFA at the front end of the anaerobic tank rises to 8500 mg / L (if the feed amount increases to 110 tons / day), the sludge return amount is increased to 50%, and after 2 days the VFA drops to 6000 mg / L; when the VFA drops to 2800 mg / L, the sludge return amount is reduced to 30%; during operation, one plunger pump failed once due to seal wear, and the time to switch to the standby pump was about 5 minutes, which did not affect the continuity of sludge return.

[0086] In step S5, approximately 30% of the wet sludge (approximately 120 tons / month) is sent to a drying facility (hot air drying, temperature 130℃) each month, and after drying, TS≥95%; approximately 60% (72 tons / month) of the dried sludge is sent to a storage silo for later use, and 40% (48 tons / month) is transported off-site for disposal, reducing the amount of wet sludge transported off-site by approximately 480 tons / year (calculated based on a moisture content of 80%).

[0087] Operational results: Within 12 months, the average VFA concentration at the front end of the anaerobic digester was 7800 mg / L (maximum ≤9500 mg / L), and the acidification risk rate was 0; the feed rate increased from 75 tons / day before the modification to 125 tons / day (125% of the design load of 100 tons / day); biogas production increased from 350 m³ / day before the modification to 420 m³ / day (an increase of 20%); and sludge disposal costs (including drying, transportation, and landfill) decreased from 85 yuan / ton before the modification to 68 yuan / ton, resulting in annual cost savings of approximately 450,000 yuan, consistent with the beneficial effects of this invention.

[0088] Example 2:

[0089] like Figure 1As shown, the dry anaerobic sludge external return device and the return process based on the dry anaerobic sludge external return device proposed in this invention are basically the same as those in Example 1 in terms of specific structure and process steps, except that:

[0090] In this embodiment, a municipal sludge treatment plant originally had three 800m³ dry anaerobic tanks, designed to process 80 tons / day of municipal sludge (TS 22%-28%). However, due to frequent exceedances of VFA at the front end (reaching a maximum of 12000mg / L), the actual processing capacity was only 55 tons / day. Furthermore, the sludge return pipeline required maintenance shutdowns 2-3 times per month due to blockages caused by impurities. Therefore, a modification was made based on the dry anaerobic sludge external return device of this invention. The specific device setup is as follows:

[0091] Extrusion pretreatment unit: YZ250 type double roller extruder is selected, the pressure adjustment component is set to working pressure of 0.9MPa, the inner wall of the extrusion chamber is sprayed with 0.7mm wear-resistant ceramic coating, and the spiral protrusion pitch of the extrusion roller is 22mm, ensuring that the removal rate of sand, gravel and fiber impurities ≥5mm in municipal sludge is ≥95%.

[0092] Filtrate storage unit: It adopts a 5m³ 304 stainless steel storage tank, with a built-in frame agitator with a rotation speed of 65r / min, and is connected to the filtrate outlet of the extrusion unit through a DN90 anti-corrosion pipe to prevent sludge particles from settling.

[0093] Fine screening unit: Uses a stainless steel vibrating screen with a 2mm aperture, with a processing capacity of 40m³ / day, specifically removing fine sand impurities of 1-5mm from municipal sludge.

[0094] Targeted flocculation unit: flocculant storage tank volume 0.8m³, PMA is added by metering pump at a mud-to-drug ratio of 7:1, mixing reaction tank volume 2.5m³, stirring speed 65r / min, reaction time 22min.

[0095] Centrifugal separation unit: LW400 horizontal centrifuge with a separation factor of 3200 and a sludge outlet temperature control sleeve of 36℃ to ensure the activity of anaerobic bacteria in municipal sludge.

[0096] Sludge conveying unit: equipped with 2 G180 plunger pumps (flow rate 18m³ / h, nominal pressure 1.6MPa), connected to DN160 carbon steel pipes, and equipped with online flow meter (accuracy ±2%) and pressure sensor.

[0097] Raw material TS adjustment unit: dry mud storage silo volume 40m³, quantitative conveying screw conveyor capacity 0-8 tons / hour, linked online TS detector to adjust raw material TS to 26%-28%.

[0098] Based on the aforementioned dry anaerobic sludge external return device, the treatment plant conducted a 10-month process operation using a return process based on the dry anaerobic sludge external return device according to the present invention. The specific process steps and results are as follows:

[0099] S1. Raw material pretreatment and feeding: The initial TS of municipal sludge is 22%-23%. A quantitative conveying screw is started to supplement dry sludge with TS≥95%. After mixing, the raw material TS is stabilized at 27%, and it is sent into a dry anaerobic tank for mesophilic fermentation at 36℃.

[0100] S2. Fermentation discharge grading and impurity removal: The fermentation discharge from the rear end of the anaerobic tank (TS21%, VFA9500mg / L) enters the extrusion unit, where ≥5mm impurities are separated under a pressure of 0.9MPa (external transport volume 2.1 tons / day). The filtrate is stirred for 12 minutes and then sent to the screening unit to remove 1-5mm fine sand (external transport volume 0.8 tons / day). The total impurity percentage of the filtrate is ≤5%.

[0101] S3. Targeted Flocculation and Centrifugal Separation: After sieving, the filtrate enters a mixing reaction tank, and PMA is added at a 7:1 sludge-to-powder ratio. The reaction proceeds for 22 minutes, forming flocs of approximately 1.2 mm in size. After centrifugation, the aqueous phase (COD 7200 mg / L) and the sludge phase (TS 27%, bacterial concentration 1.0 × 10⁻⁶) are separated. 8 (CFU / mL, VFA 4200 mg / L), bacterial activity retention rate 91%.

[0102] S4. Sludge external return with backup: The initial sludge return volume is 40% of the raw material feed volume (dry matter ratio). When the VFA at the front end rises to 8200mg / L, the sludge return volume is increased to 50%. After 3 days, the VFA drops to 5800mg / L. During operation, one plunger pump failed, and it took 6 minutes to switch to the backup pump. The system operation was not affected.

[0103] S5. Optional treatment and recycling of dry mud: 25% of wet mud is sent to the drying equipment (drying temperature 135℃) every month, 60% of the dry mud is returned to the storage silo, and 40% is transported off-site for disposal.

[0104] Operational results: After 10 months of continuous operation, the average VFA concentration at the front end of the anaerobic tank was 6500 mg / L, with the highest concentration not exceeding 9000 mg / L, and the acidification risk rate was 0; the treatment capacity increased to 100 tons / day (reaching 125% of the design load); the sludge return pipeline was unblocked, and the system had a fault-free operation rate of 99.6%; biogas production increased from 280 m³ / day before the renovation to 350 m³ / day, saving 382,000 yuan in sludge disposal and maintenance costs annually.

[0105] Example 3:

[0106] like Figure 1 As shown, the dry anaerobic sludge external return device and the return process based on the dry anaerobic sludge external return device proposed in this invention are basically the same as those in Example 1 in terms of specific structure and process steps, except that:

[0107] In this embodiment, a county-level food waste treatment center is a small-scale project equipped with a 500m³ dry anaerobic digester. It is designed to process 30 tons / day of mixed food waste and restaurant swill (TS 25%-35%). Due to large fluctuations in the TS content of the raw materials and rapid loss of microbial inoculum, the system's feed load has consistently only reached 70% of the design value. Therefore, a modification is made based on the dry anaerobic sludge external return device of this invention. The specific device setup is as follows:

[0108] Extrusion pretreatment unit: YZ200 type small extruder is selected, the working pressure of the pressure regulating component is 0.8MPa, the wear-resistant coating thickness of the extrusion chamber is 0.5mm, and the pitch of the extrusion roller is 20mm.

[0109] Filtrate storage unit: 3m³ 304 stainless steel tank, stirring speed 60r / min, suitable for material handling capacity of small projects.

[0110] Fine screening unit: 1.5mm aperture stainless steel screen, processing capacity 15m³ / day.

[0111] Targeted flocculation unit: 0.5m³ flocculant storage tank, metering pump adds PMA at a ratio of 7:1, mixing reaction tank volume 1.5m³, stirring speed 60r / min.

[0112] Centrifugal separation unit: LW350 centrifuge, separation factor 3000, mud phase outlet temperature maintained at 35℃.

[0113] Sludge conveying unit: 2 G150 small plunger pumps, connected by DN150 carbon steel pipe, and equipped with a simple flow and pressure monitoring device.

[0114] Raw material TS adjustment unit: 20m³ dry mud storage bin, quantitative conveying screw conveyor capacity 0-5 tons / hour.

[0115] Based on the aforementioned dry anaerobic sludge external return device, the treatment plant conducted an 8-month process operation using a return process based on the dry anaerobic sludge external return device according to the present invention. The specific process steps and results are as follows:

[0116] S1. Raw material pretreatment and feeding: The total sludge content (TS) of the mixed raw materials fluctuates between 23% and 38%. When the TS is below 25%, the quantitative conveying screw is started to replenish the dry mud. After mixing, the TS is stabilized at 26%-29%, and the mixture is sent to the anaerobic tank for mesophilic fermentation at 37°C.

[0117] S2. Grading and impurity removal of fermentation output: The fermentation output (TS22%, VFA8800mg / L) is separated into bone residue and plastic impurities ≥5mm by the extrusion unit (1.2 tons / day of external transport). After the filtrate is stirred for 10 minutes, it is screened to remove 1-5mm bone fragments (0.3 tons / day of external transport). The total proportion of impurities is ≤4%.

[0118] S3. Targeted Flocculation and Centrifugal Separation: The sieved filtrate enters the reaction tank, PMA is added, and the reaction proceeds for 20 minutes to form 1mm flocs. After centrifugation, the aqueous phase COD is 6800mg / L, the sludge phase TS is 26%, and the bacterial concentration is 0.9×10⁻⁶. 8 CFU / mL, VFA 4000 mg / L, bacterial activity retention rate 90%.

[0119] S4. Sludge external return with backup protection: The sludge return amount is initially set at 35% of the raw material feed amount, adjusted to 45% when the front-end VFA rises to 7800mg / L, and adjusted to 30% when the VFA drops to 3200mg / L; the dual plunger pump backup design ensures a sludge return interruption time of 7 minutes, without affecting the system operation.

[0120] S5. Optional treatment and recycling of dry mud: 20% of wet mud is sent to the drying equipment (drying temperature 130℃) every month, 60% of the dry mud is returned to the storage silo, and 40% is transported off-site for disposal.

[0121] Operational results: After 8 months of continuous operation, the project's processing capacity increased from 21 tons / day to 37.5 tons / day (reaching 125% of the design load). The front-end VFA remained stable below 8000mg / L for a long period, there were no records of blockages in the sludge return pipeline, and the biogas production increased from 120m³ / day to 155m³ / day. Annual operating costs were saved by 168,000 yuan, achieving stable and efficient operation of the small-scale kitchen waste project.

[0122] As can be seen from Examples 1, 2, and 3, the dry anaerobic sludge external recirculation device of the present invention, through its integrated structural design of "multi-stage impurity removal - targeted flocculation and seed preservation - dual-pump backup recirculation - dynamic adjustment of raw material TS", specifically addresses the technical pain points of existing dry anaerobic internal recirculation devices, and has greatly improved in terms of equipment structure adaptability, system operation stability, process adaptability and compatibility, and cost control effectiveness, as detailed below:

[0123] (1) The dry anaerobic sludge external return device of the present invention integrates a graded impurity removal structure of extrusion pretreatment and fine screening, which can efficiently intercept large impurities ≥5mm and small impurities 1-5mm, so that the proportion of impurities in the filtrate is ≤7%, greatly extending the pipeline unblocking cycle and reducing the equipment maintenance frequency by more than 85%, thus completely solving the problem of frequent pipe blockage in traditional devices.

[0124] (2) The dry anaerobic sludge external return device of the present invention, with the targeted flocculation unit adding PMA at a fixed sludge-to-medicine ratio, combined with a centrifugal separation unit with specific parameters and a temperature control structure, can achieve the dual objectives of "reducing VFA + preserving bacterial strains", and can make the bacterial strain concentration of the returned wet sludge ≥10. 8 With CFU / mL and VFA ≤ 5000 mg / L, it can quickly dilute the high concentration of VFA at the front end of the anaerobic tank and replenish highly active bacteria, stabilizing the VFA at the front end to within 10000 mg / L, and reducing the acidification risk rate to less than 1% / month.

[0125] (3) The dry anaerobic sludge external return device of the present invention, through the sludge return conveying structure with the dual plunger pump interconnection and backup, and with the online monitoring component, can control the sludge return interruption time within 10 minutes, and improve the fault-free operation rate of the sludge return system to 99.5%, avoiding the disadvantage of the traditional single pump device stopping immediately upon failure, and providing a reliable guarantee for the stable operation of the system.

[0126] (4) The dry anaerobic sludge external return device of the present invention can dynamically replenish dry sludge through the raw material TS adjustment unit, stabilize the raw material TS in the optimal fermentation range of 25%-30%, adapt to various high organic wastes with TS of 20%-40%, improve material compatibility by more than 60%, realize the recycling of dry sludge, and reduce the amount of sludge transported by 15%-20%.

[0127] (5) The dry anaerobic sludge external return device of the present invention forms a closed-loop collaborative system with each unit, which significantly improves the treatment efficiency; at the same time, it greatly reduces the costs of equipment maintenance and sludge disposal.

[0128] The above is an exemplary description of the invention. Obviously, the specific implementation of the invention is not limited to the above-described manner. Any non-substantial improvement made using the inventive concept and technical solution of the invention, or the direct application of the inventive concept and technical solution to other situations without modification, is within the protection scope of the invention.

Claims

1. A dry anaerobic sludge return device, characterized by comprising: It comprises: In turn, the dry anaerobic tank, extrusion pretreatment unit, filtrate temporary storage unit, fine screening unit, targeted flocculation unit, centrifugal separation unit, also including back-mud conveying unit with backup guarantee and raw material TS adjusting unit; The front end of the dry anaerobic tank is provided with a raw material feeding port, and the raw material feeding port is adapted to organic waste with total solid content TS≥25%; The rear end of the dry anaerobic tank is provided with a fermentation discharge port, and the fermentation discharge port is in sealed communication with the feeding end of the extrusion pretreatment unit; The extrusion pretreatment unit is provided with a pressure regulating assembly, and the working pressure range of the pressure regulating assembly is 0.8-1.2MPa; The extrusion pretreatment unit is used for separating and removing large material impurities with particle size≥5mm in the fermentation discharge; The filtrate outlet of the extrusion pretreatment unit is in communication with the filtrate temporary storage unit through a corrosion-resistant pipeline, and the filtrate temporary storage unit has a volume≥5m³ and is provided with a stirring assembly; The discharge end of the filtrate temporary storage unit is in communication with the feeding end of the fine screening unit, and the fine screening unit is used for removing small material impurities with particle size 1-5mm in the filtrate; The targeted flocculation unit is in communication with the discharge end of the fine screening unit, and the targeted flocculation unit comprises a flocculant storage tank, a metering pump and a mixing reaction tank; The flocculant stored in the flocculant storage tank is polyacrylamide PMA, the metering pump adds PMA to the mixing reaction tank at a fixed ratio of 7:1, and a stirrer is arranged in the mixing reaction tank; The feeding end of the centrifugal separation unit is in communication with the discharge end of the targeted flocculation unit, the separation factor of the centrifugal separation unit is 3000-4000, and the centrifugal separation unit forms a water phase outlet and a mud phase outlet after separation; The water phase outlet is in communication with an external sewage treatment system, and the mud phase outlet is in communication with the front end of the dry anaerobic tank through the back-mud conveying unit; The TS content of the mud phase after separation of the centrifugal separation unit is stably maintained at 25%-30%; The back-mud conveying unit comprises two identical plunger pumps, a communication pipeline and a branch control valve; The inlets of the two plunger pumps are in communication with the mud phase outlet of the centrifugal separation unit, the outlets of the two plunger pumps are in communication with each other through a carbon steel pipeline and are extended to the front end of the dry anaerobic tank through the communication pipeline, and a hand-operated gate valve is arranged on the communication pipeline at the front end of the dry anaerobic tank; The nominal pressure of the hand-operated gate valve is 1.6MPa; The raw material TS adjusting unit comprises a dry mud storage bin and a quantitative conveying screw; The feeding end of the dry mud storage bin is in communication with an external drying device, the external drying device is used for treating part of the mud phase produced by the centrifugal separation unit, and the TS content of the dry mud in the dry mud storage bin is≥95%; One end of the quantitative conveying screw is in communication with the dry mud storage bin, and the other end is in communication with the raw material feeding port of the dry anaerobic tank.

2. A reflux process based on the dry anaerobic sludge external reflux device according to claim 1, characterized in that, The steps comprise: S1, raw material pretreatment and feeding: the organic waste with an initial solid concentration of 20%-40% is sent to the raw material TS adjusting unit, if the raw material TS is less than 25%, a dry sludge with TS≥95% is added to the raw material through a quantitative conveying screw, so that the TS content of the mixed raw material is stabilized at 25%-30%; the adjusted raw material is continuously sent into the tank through the raw material feeding port at the front end of the dry anaerobic tank for mesophilic anaerobic fermentation, and the temperature of the mesophilic anaerobic fermentation is controlled at 35-38℃; S2, fermentation discharge grading impurity removal: the fermentation discharge at the rear end of the dry anaerobic tank enters the extrusion pretreatment unit through the fermentation discharge port, and is separated by extrusion under a pressure of 0.8-1.2 MPa to remove large material impurities with a particle size of≥5 mm, the large material impurities are transported out by a sealed conveying belt for disposal; and the filtrate generated by extrusion enters the filtrate temporary storage unit, is stirred by the stirrer of the filtrate temporary storage unit, and is then sent to the fine screening unit to remove small material impurities with a particle size of 1-5 mm, which are transported out for disposal; S3, targeted flocculation and centrifugal separation: the filtrate after screening enters the mixing reaction tank of the targeted flocculation unit, PMA flocculant is added by a metering pump according to a sludge medicine ratio of 7:1, and the formed flocculation with a particle size of ≥1mm is obtained after reaction under the action of a stirrer; the material containing the flocculation is sent to the centrifugal separation unit, and solid-liquid separation is carried out at a set temperature to obtain a water phase and a mud phase, the water phase is sent to a sewage treatment system, and the mud phase is wet mud with a TS content of 25%-30% and an anaerobic bacterial species concentration of ≥10 8 CFU / mL and VFA≤5000mg / L; S4, sludge external reflux with backup: two plunger pumps of the sludge return conveying unit are started, one plunger pump is operated in the initial state and the other plunger pump is standby, the wet sludge obtained in step S3 is conveyed to the front end of the dry anaerobic tank through the connecting pipeline and the branch plug valve; when the operating plunger pump fails, the inlet and outlet valves of the failed plunger pump are closed and the standby plunger pump is opened; S5, optional treatment and circulation of dry sludge: if the raw material TS does not need to be adjusted, part of the wet sludge obtained in step S3 is sent to an external drying equipment, dried at a set temperature to TS≥95% to obtain dry sludge, part of the dry sludge is transported out for disposal, and the other part is sent to the dry sludge storage of the raw material TS adjusting unit for standby.

3. The dry anaerobic sludge external recycle device according to claim 1, characterized in that, The extrusion pretreatment unit is provided with a wear-resistant ceramic coating on the inner wall of the extrusion cavity, the thickness of the wear-resistant ceramic coating is 0.5-1 mm, the surface of the extrusion roller of the extrusion pretreatment unit is provided with a spiral protrusion, the pitch of the spiral protrusion is 20-30 mm, and the removal rate of the extrusion pretreatment unit for large material impurities is≥95%.

4. The dry anaerobic sludge external recycle device according to claim 1, wherein The fine screening unit is provided with a stainless steel screen, the aperture of the stainless steel screen is 1-5 mm, and the removal rate of the fine screening unit for small material impurities is≥98%.

5. The dry anaerobic sludge external recycle device according to claim 1, wherein The stirring speed of the stirrer in the mixing reaction tank of the targeted flocculation unit is 60-80 r / min, and the stirring speed is set to ensure uniform flocculation and not to damage the activity of anaerobic bacteria.

6. A dry anaerobic sludge external recirculation device according to claim 1, characterized in that The centrifugal separation unit is provided with a temperature maintaining sleeve at the mud phase outlet, the temperature maintaining sleeve has a temperature control range of 35-40℃, and the temperature maintaining sleeve is used to avoid the loss of anaerobic bacteria activity caused by the sudden drop of the temperature of the wet sludge.

7. A dry anaerobic sludge external recirculation device according to claim 1, characterized in that The carbon steel pipeline connecting the outlets of the two plunger pumps in the sludge return conveying unit has a specification of DN150-DN200, an online flow meter and a pressure sensor are arranged on the connecting pipeline of the sludge return conveying unit, the accuracy of the online flow meter is±2%, the range of the pressure sensor is 0-2.5 MPa, and the online flow meter is electrically connected with the variable frequency controller of the plunger pump.

8. The process of claim 2, wherein the device is a dry anaerobic sludge external reflux device. In the S2 step, the stirring time of the stirrer in the filtrate temporary storage unit is 10-15 min, the stirring speed of the stirrer is 60-80 r / min, the total proportion of impurities in the fermentation discharge is ≤7%, wherein the proportion of large material impurities is ≤5%, and the proportion of small material impurities is ≤2%; in the S3 step, the reaction time of the targeted flocculation unit is 20-30 min, the solid-liquid separation temperature of the centrifugal separation unit is 35-40℃, the COD of the water phase after separation is ≤8000 mg / L, and the activity retention rate of anaerobic bacteria in the wet mud is ≥90%.

9. The process of claim 2, wherein the device is a dry anaerobic sludge external reflux device. In the S4 step, the amount of returned mud is controlled to be 30%-50% of the raw material feed amount and calculated according to the dry matter mass ratio, when the VFA at the front end of the dry anaerobic tank is >8000 mg / L, the amount of returned mud is increased to 50% of the raw material feed amount; when the VFA at the front end of the dry anaerobic tank is <3000 mg / L, the amount of returned mud is reduced to 30% of the raw material feed amount; the interruption time of returned mud is ≤10 min.

10. The process of claim 2, wherein the device is a dry anaerobic sludge external reflux device. In the S5 step, the drying temperature of the external drying equipment is 120-150℃, the amount of wet mud sent into the external drying equipment accounts for 20%-30% of the total amount of wet mud, the VFA at the front end of the dry anaerobic tank is long-term controlled to be within 10000 mg / L, and the acidification risk occurrence rate is ≤1% / month.

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