Treatment and utilization method of sludge in sewage treatment system

Abstract

The invention discloses a method for treating and utilizing sludge in a sewage treatment system, which comprises the following steps of: concentrating residual sludge generated in a secondary sedimentation tank and / or sludge generated after sewage subjected to impurity removal, primary sedimentation, anaerobic treatment, anoxic treatment and aerobic treatment is discharged from the secondary sedimentation tank and is filtered, and then carrying out primary dehydration, conditioning and deep dehydration; performing variable-atmosphere pyrolysis activation on the sludge biochar; and finally, fully grinding the sludge biochar to obtain the sludge biochar. Part of the obtained sludge biochar is added as a fluidized carrier in an aerobic treatment stage or a conditioner in a sludge dewatering stage, and the rest of the sludge biochar is used as activated carbon to replace a product or sold to the outside, so that a deep product recycling system is formed, and the utilization prospect of sludge recycling products is widened.

Description

Technical Field

[0001] This invention relates to a method for disposing of and utilizing sludge in a wastewater treatment system, belonging to the field of wastewater treatment technology. Background Technology

[0002] Sludge resource utilization is an important way to achieve sludge reduction, harmlessness, and resource recovery. By exploring and practicing various methods and approaches for sludge resource utilization, effective treatment and utilization of sludge can be achieved, reducing environmental pollution and resource waste, which is of great significance for resource conservation and environmental protection. However, the preparation methods for sludge resource-based products are often complex, resulting in low product performance. The preparation process of sludge resource-based products usually involves high-temperature and high-pressure treatment, which is complex, energy-intensive, and results in high preparation costs and poor product performance (such as adsorption capacity and stability), making it difficult for sludge resource-based products to compete with commercial products such as activated carbon and sludge dewatering agents. Furthermore, the application scenarios for sludge resource-based products are limited, and the actual demand is small, preventing high-value utilization. Sludge resource-based products are mostly used in low-value-added fields (such as sludge brick making, ceramsite, and roadbed materials), where the current overall demand in the construction market is limited, hindering high-value utilization and restricting large-scale promotion. Summary of the Invention

[0003] The purpose of this invention is to provide a method for the disposal and utilization of sludge in a wastewater treatment system. The sludge generated during the wastewater treatment process is prepared into sludge biochar, part of which is reused as a fluidized carrier in the biochemical section of wastewater treatment and can also be used for emergency treatment. The remaining product can be sold as sludge biochar, thereby forming a deep recycling system and broadening the prospects for the utilization of sludge resource products.

[0004] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for disposing of and utilizing sludge in a wastewater treatment system involves concentrating, pre-dewatering, conditioning, and deeply dewatering a portion of the sludge generated in the sludge treatment system, followed by atmospheric pyrolysis activation, and finally grinding to obtain sludge biochar. Part of the obtained sludge biochar is added as a fluidizing carrier in the biochemical treatment stage or as a conditioning agent in the sludge dewatering stage, while the remainder is used as an activated carbon substitute or sold externally.

[0005] Preferably, the sludge after concentration and preliminary dewatering has a moisture content of 60-85%.

[0006] Preferably, the conditioning and deep dewatering method is as follows: 92-98 parts by weight of the concentrated and preliminarily dewatered sludge are mixed with 1-3 parts of dewatering agent, and then mechanically dewatered and dried to obtain dried sludge.

[0007] Preferably, the drying temperature is 50-150℃, and the moisture content of the mechanically dewatered sludge after drying is 10-30%.

[0008] Preferably, the dehydrating agent is any one of lime, polyferric salt, polyaluminum salt, or polyacrylamide.

[0009] Preferably, the variable atmosphere pyrolysis activation step is as follows: S1. Aerobic pyrolysis: Sludge is aerobically pyrolyzed to obtain sludge carbon residue. S2. Exhaust gas purification and blending: After removing tar from the aerobic pyrolysis exhaust gas, CO2 and water vapor are added. S3. Activation: The sludge and carbon residue are fed into the activation furnace, and the purified and blended tail gas is introduced into the activation furnace. The mixture is heated and reacted under the atmosphere of the purified and blended tail gas to obtain the product activated by CO2 and water vapor.

[0010] Preferably, in step S1, the conditions for aerobic pyrolysis are: oxygen concentration 3-10%, heating rate 5-10℃ / min, target pyrolysis temperature 350-500℃, and pyrolysis reaction time 30-50min; In step S2, the CO2 concentration in the adjusted exhaust gas is 15%-25%, and the water vapor concentration is 20%-40%. In step S3, the conditions for the heating reaction are: heating rate 10~20℃ / min, target activation temperature 750~900℃, and activation reaction time 20~40min.

[0011] Preferably, the resulting sludge biochar has a mesh size of 200-800 mesh.

[0012] Preferably, at the initial addition, the one-time addition ratio of sludge biochar as fluidizing carrier is 1.5‰~5‰ based on the volume of the biological treatment tank; during normal operation, the continuous addition ratio of sludge biochar as fluidizing carrier is 5-30 mg / L based on the influent flow rate.

[0013] The beneficial effects of this invention are as follows: Sludge biochar prepared by variable atmosphere pyrolysis activation has abundant pores, high porosity and specific surface area, making it suitable for reuse as a fluidized bed carrier in the biochemical stage of wastewater treatment, emergency treatment, and sales as a sludge biochar product. After the sludge biochar is reused, the sludge generated by the system can also be treated in the same way to prepare sludge biochar again for reuse, emergency treatment, or sale, thus forming a deep product recycling system and broadening the prospects for sludge resource utilization. Attached Figure Description

[0014] Figure 1 The image shown is a scanning electron microscope image of the sludge biochar prepared in Example 1. Detailed Implementation

[0015] This invention provides a method for the disposal and utilization of sludge in a wastewater treatment system, the specific process of which is as follows.

[0016] Primary treatment includes coarse screens, fine screens, and primary sedimentation tanks, which perform preliminary removal and sedimentation of impurities and suspended solids (SS) in the incoming water.

[0017] Secondary treatment includes anaerobic tanks, anoxic tanks, aerobic tanks, and secondary sedimentation tanks. The excess sludge produced in the secondary sedimentation tank is further concentrated in the sludge dewatering section thickening tank.

[0018] Tertiary treatment includes precision filtration facilities such as high-density sedimentation tanks, cloth filters, and denitrification filters. The sludge generated from the deep treatment is further concentrated in the sludge dewatering section thickening tank, and the wastewater is discharged after meeting the treatment standards.

[0019] Preliminary sludge dewatering: mainly includes sludge thickening tank and preliminary dewatering device. After secondary and tertiary treatment, the sludge moisture content is 97-99%. After thickening and preliminary dewatering, the sludge moisture content is reduced to 60-85%.

[0020] Sludge conditioning: 92-98 parts of sludge with a moisture content of 60-85%, and 1-3 parts of sludge dewatering agent. Sludge dewatering agents include lime, polyferric salts, polyaluminum salts, and polyacrylamide, etc. The purpose of sludge conditioning is to improve the efficiency of subsequent dewatering.

[0021] Deep sludge dewatering includes mechanical dewatering and drying processes. After deep dewatering, the moisture content of the sludge is reduced to 10-30%. Mechanical dewatering methods include belt filter press, plate and frame filter press, or vertical press. The mechanically dewatered sludge is then dried at a temperature of 50-150℃. Drying methods include low-temperature vacuum drying, heat pump drying, and waste heat drying, with waste heat drying being the preferred method. The heat source for drying is the flue gas generated by the pyrolysis activation of the sludge in a different atmosphere and the recirculation of the exhaust gas.

[0022] Sludge variable atmosphere pyrolysis activation: After drying, the sludge is first subjected to aerobic pyrolysis in a pyrolysis furnace with a slow heating rate. The oxygen concentration is 3-10%, the pyrolysis temperature is 350-500℃, the heating rate is 5-10℃ / min, and the pyrolysis time is 30-50min. The pyrolysis energy comes partly from the biomass energy released by the oxidative pyrolysis of organic matter in the sludge under oxygen participation. The product of aerobic pyrolysis of sludge is transferred to an activation furnace for rapid heating and anaerobic activation. The activation atmosphere is a combination of CO2 and water vapor (CO2 concentration 15%-25%, water vapor concentration 20%-40%), the activation temperature is 750-900℃, the heating rate is 10-20℃ / min, and the activation time is 20-40min. The final variable atmosphere pyrolysis activation product is obtained. The activation atmosphere is provided by the purified tail gas of the pyrolysis furnace.

[0023] In the aerobic pyrolysis stage, the micro-oxygen environment inside the pyrolysis furnace causes some organic matter to undergo oxidation, releasing heat to sustain the pyrolysis process and significantly reducing external energy consumption. In the tail gas purification and blending stage, tar is removed by purifying the tail gas and CO2 and water vapor are added to provide an activation atmosphere for the subsequent joint activation steps, reducing the external gas supply. In the anaerobic activation stage, the sludge pyrolysis products change from the oxygen-consuming stage to the anaerobic stage. The combined activation of CO2 and water vapor in the activation furnace continues to etch the carbon skeleton of the sludge pyrolysis products, thereby creating rich microporous and mesoporous structures. The synergistic effect of the two gaseous activators helps to form more developed interconnected channels, greatly improving the specific surface area and adsorption performance of the pyrolysis products.

[0024] Mechanical grinding: The sludge biochar activated by changing atmosphere is mechanically dry ground. Grinding equipment includes ball mills, roller mills and vertical mills. The specifications of the sludge biochar powder product after grinding are 200~800 mesh.

[0025] Sludge biochar addition: Added to the biological treatment tank (or aerobic tank) at a certain ratio as a fluidizing carrier. Simultaneously, sludge biochar can be used for emergency treatment and can also be sold as a product. After sludge biochar reuse, the sludge generated by the system can be treated in the same way to produce sludge biochar again for reuse, emergency addition, or sale, thus forming a deep product recycling system and broadening the prospects for sludge resource utilization.

[0026] During initial addition, the one-time dosage of sludge biochar accounts for 1.5‰ to 5‰ of the total volume of the biological treatment system. Under normal operating conditions (stable pollutant content in the influent), the dosage of sludge biochar as a fluidizing carrier is 5-30 mg / L (based on influent flow rate).

[0027] Example 1: A wastewater treatment plant has a daily treatment capacity of 80,000 tons / day, a biological treatment tank retention time of approximately 12 hours, and a total volume of approximately 40,000 m³. 3 The initial sludge concentration in the pool is approximately 4200 mg / L (MLVSS / MLSS≈0.6), and about 42 tons of sludge with a water content of about 80% are produced every day, which is equivalent to 8.4 tons of oven-dry sludge per day.

[0028] The sludge from the wastewater treatment plant, after initial dewatering by centrifuge, had a moisture content of 82%. Following a sludge conditioning process, 97 parts by weight of sludge and 3 parts by weight of polyaluminum salt flocculant were thoroughly mixed. After high-pressure belt filtration, the sludge moisture content decreased to 62%. Further drying using waste heat from the flue gas in the variable atmosphere pyrolysis activation section reduced the moisture content to 18%. Then, pyrolysis was performed at 400℃ (temperature rise rate 8℃ / min) under aerobic (oxygen concentration 10%) conditions for 40 minutes. Next, CO2 and steam (CO2 concentration 20%, steam concentration 30%) were introduced, and the mixture was jointly activated at 800℃ (temperature rise rate 15℃ / min) for 30 minutes. The sludge biochar residue was then crushed in a ball mill to obtain the sludge biochar product. The absolute dry sludge biochar yield was 70%, producing approximately 5.88 tons of sludge biochar powder with a particle size of approximately 300 mesh. The performance test results are shown in Table 1.

[0029] Table 1: Performance parameters of sludge biochar

[0030] Fixed carbon content analysis of sludge biochar products showed that the fixed carbon content of sludge increased from 1.08% in oven-dried sludge before pyrolysis activation to 7.64% in sludge biochar. This indicates that the organic matter in the sludge underwent sufficient thermal decomposition and carbon structure rearrangement during the treatment process, leaving a more stable carbon skeleton, which is in line with sludge biochar products.

[0031] Scanning electron microscopy was performed on the sludge biochar products. Figure 1 As can be seen, the surface of the sludge biochar has abundant pores. Combined with the main products and performance characterization results, it is shown that the prepared sludge biochar is a material with high porosity and high specific surface area. It can be used as a fluidized carrier in biochemical processes or for emergency treatment, and can also be used in sludge dewatering processes, showing broad application prospects.

[0032] Comparative Example 1: Basically the same as Example 1, except that the sludge after initial dewatering by centrifuge in this wastewater treatment plant had a moisture content of 82%. The sludge conditioning steps involved thoroughly mixing 97 parts by weight of sludge and 3 parts by weight of polyaluminum salt flocculant. After high-pressure belt filtration, the sludge moisture content was reduced to 62%. Further drying using waste heat from the flue gas in the pyrolysis activation section reduced the moisture content to 18%. Then, it was pyrolyzed for 40 minutes under aerobic (oxygen concentration 10%) pyrolysis conditions at 400℃ (temperature rise rate 8℃ / min). The sludge biochar residue was crushed by a ball mill to obtain the sludge biochar product. The performance test results are shown in Table 2.

[0033] Table 2: Performance parameters of sludge biochar

[0034] As can be seen from Example 1, Comparative Example 1, and Tables 1 and 2, the porosity and specific surface area of ​​the sludge biochar product are improved to a certain extent after aerobic pyrolysis by introducing CO2 and water vapor for anaerobic activation. This indicates that during the anaerobic activation stage, the combined activation of CO2 and water vapor introduced into the activation furnace has a positive impact on the porosity and specific surface area of ​​the sludge biochar product and can improve its adsorption performance.

[0035] Example 2: Four SBR reactors were set up, including one blank group and three control groups. The initial activated sludge concentration in the SBR reactors was 3300 mg / L. The sludge biochar prepared in Example 1 was added to the three control group SBR reactors at certain proportions of 1.5‰, 3‰, and 5‰ (based on the total volume of the SBR reactor, added to the aerobic tank). The improvement of biochemical effect of sludge biochar was observed under different influent COD loads. As shown in Table 3, it can be found that when the influent COD is low, the effluent COD of the control group with low sludge biochar addition ratio is similar to that of the blank group, and the effluent COD concentration decreases as the addition ratio increases. When the influent has a high concentration of COD (greater than 250 mg / L), the COD of the effluent from the control group with added sludge biochar is significantly lower than that of the blank group. Moreover, the higher the dosage, the better the effluent effect. This indicates that the addition of sludge biochar has certain advantages in COD removal under high load conditions, can improve the treatment effect of the biological system, enhance the stability of the biological system, and effectively cope with sudden situations such as the impact of upstream water flow on the sewage treatment plant.

[0036] Table 3: Biochemical effects of the control group and the blank control group under different influent COD loads.

[0037] Referring to the sludge biochar disposal and utilization method proposed in this invention, a circulation is initially established in the sewage treatment plant (Example 1). Based on the volume of the biological tank in the sewage treatment system, the minimum one-time addition ratio of sludge biochar is 1.5‰ (approximately 60 tons, added to the aerobic tank). After addition, the concentration of sludge and sludge biochar composite carrier in the tank is 5700 mg / L (MLVSS / MLSS≈0.44).

[0038] Since sludge biochar will be lost with the remaining sludge, to ensure the wastewater treatment effect, the continuous replenishment amount of sludge biochar after the initial addition is 20 mg / L under normal influent conditions. Based on an influent volume of 80,000 tons / day, the daily continuous replenishment amount of sludge biochar is 1.6 tons. The remaining 4.28 tons of sludge biochar can be used as a substitute for other activated carbon products, such as replacing activated carbon water purification filter media in wastewater treatment facilities, activated carbon purification and adsorption materials in exhaust gas deodorization facilities, etc., or sold externally.

Claims

1. A method for disposing of and utilizing sludge in a wastewater treatment system, characterized in that, The process involves concentrating, initially dewatering, conditioning, and deeply dewatering a portion of the sludge generated in the sludge treatment system, followed by atmospheric pyrolysis activation, and finally grinding to produce sludge biochar. Part of the obtained sludge biochar is added as a fluidizing carrier in the biochemical treatment stage or as a conditioning agent in the sludge dewatering stage, while the remainder is used as a substitute for activated carbon or sold externally.

2. The method for disposing of and utilizing sludge in a wastewater treatment system according to claim 1, characterized in that, The sludge, after concentration and preliminary dewatering, has a moisture content of 60-85%.

3. The method for disposing of and utilizing sludge in a wastewater treatment system according to claim 1, characterized in that, The conditioning and deep dewatering method is as follows: According to the mass fraction, 92-98 parts of concentrated and preliminarily dewatered sludge are mixed with 1-3 parts of dewatering agent, and then mechanically dewatered and dried to obtain dried sludge.

4. The method for disposing of and utilizing sludge in a wastewater treatment system according to claim 3, characterized in that, The drying temperature is 50-150℃, and the moisture content of the mechanically dewatered sludge after drying is 10-30%.

5. The method for disposing of and utilizing sludge in a wastewater treatment system according to claim 3, characterized in that, The dehydrating agent is any one of lime, polyferric salt, polyaluminum salt or polyacrylamide.

6. The method for disposing of and utilizing sludge in a wastewater treatment system according to claim 1, characterized in that, The steps of variable atmosphere pyrolysis activation are as follows: S1. Aerobic pyrolysis: Sludge is aerobically pyrolyzed to obtain sludge carbon residue. S2. Exhaust gas purification and blending: After removing tar from the aerobic pyrolysis exhaust gas, CO2 and water vapor are added. S3. Activation: The sludge and carbon residue are fed into the activation furnace, and the purified and blended tail gas is introduced into the activation furnace. The mixture is heated and reacted under the atmosphere of the purified and blended tail gas to obtain the product activated by CO2 and water vapor.

7. The method for disposing of and utilizing sludge in a wastewater treatment system according to claim 6, characterized in that, In step S1, the conditions for aerobic pyrolysis are: oxygen concentration 3-10%, heating rate 5-10℃ / min, target pyrolysis temperature 350-500℃, and pyrolysis reaction time 30-50min. In step S2, the CO2 concentration in the adjusted exhaust gas is 15%-25%, and the water vapor concentration is 20%-40%. In step S3, the conditions for the heating reaction are: heating rate 10~20℃ / min, target activation temperature 750~900℃, and activation reaction time 20~40min.

8. The method for disposing of and utilizing sludge in a wastewater treatment system according to claim 1, characterized in that, The resulting sludge biochar has a mesh size of 200-800 mesh.

9. The method for disposing of and utilizing sludge in a wastewater treatment system according to claim 1, characterized in that, During the initial addition, based on the volume of the biological treatment tank, the one-time addition ratio of sludge biochar as a fluidizing carrier is 1.5‰~5‰. After the biological treatment system stabilizes, the subsequent replenishment ratio of sludge biochar as a fluidizing carrier is 5-30 mg / L, based on the influent flow rate.

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