Screening regulation and control method for compact activated sludge

Abstract

The invention discloses a compact activated sludge screening regulation and control method, and relates to the technical field of sewage and sludge treatment. A sludge optimization module is additionally arranged in a sludge concentration unit of a continuous flow sewage treatment plant, densification and reinforced dehydration of residual sludge are synchronously realized, and the sludge optimization module comprises a hydrocyclone group 1 and a hydrocyclone group 2. Wherein the hydrocyclone group 1 is used for preferably selecting dense sludge, and the hydrocyclone group 2 is used for preferably selecting activated sludge which is easier to dehydrate. According to the method, the treatment and recycling ways of the excess sludge are expanded, the treatment load of a continuous flow sewage treatment plant is improved in situ by combining the hydrocyclone, and meanwhile, the dewatering performance of the excess sludge is optimized.

Description

Technical Field

[0001] This invention relates to the field of wastewater and sludge treatment technology, specifically a method for screening and regulating dense activated sludge. Background Technology

[0002] Wastewater sludge treatment and disposal is a crucial aspect of wastewater treatment plant operation. It typically involves thickening, dewatering, and stabilization before composting, incineration, or landfilling, with treatment costs accounting for 20% to 50% of the plant's operating expenses. Sludge dewatering is the primary method for reducing sludge volume, commonly employing methods such as mechanical dewatering (centrifugal dewatering, filter press dewatering, etc.), thermal dewatering (drying bed, thermal drying, etc.), and chemical conditioning dewatering. However, the stable colloidal structure and complex forms of internal water in wastewater sludge make dewatering difficult and energy-intensive. Furthermore, since wastewater sludge still contains abundant resources, its treatment and reuse hold promise for further expanding wastewater sludge treatment and disposal pathways.

[0003] Dense sludge (i.e., large sludge aggregates exceeding 150 μm in size) has been reported to simultaneously enhance sludge settling and pollutant removal performance due to its unique structure and functional properties. Even under excessive loads or high hydraulic shocks during the rainy season, it can effectively improve the flow capacity of secondary sedimentation tanks, reduce sludge loss, and thus ensure the stability of system operation during the rainy season. In most current studies, large sludge aggregates are difficult to enrich on a large scale using activated sludge systems, but enrichment can be achieved using hydrocyclones. This technology, also known as sludge densification (DAS) technology, has been applied in dozens of wastewater treatment plants worldwide.

[0004] Studies have shown that the key to DAS technology application lies in the design and operation of the hydrocyclone. The feed sludge concentration and feed velocity of the hydrocyclone play a crucial role in the sludge densification efficiency, as they affect the interaction between particles during the swirling process. Higher sludge concentrations can effectively increase the number of large-sized sludge aggregates through the hydrocyclone, thus contributing to the more stable formation of large-sized sludge aggregates at appropriate feed velocities. Research indicates that in short-sludge-age continuous flow systems for enhanced biological phosphorus removal (SRT less than 10 days), the concentration of suspended activated sludge concentrated at the bottom of the sedimentation tank can reach 10–15 g SS / L. After densification by the hydrocyclone, the proportion of large-sized sludge aggregates can exceed 50%. However, for most long-sludge-age continuous flow activated sludge processes (SRT exceeding 15 days), the settling performance of the activated sludge is poor, making it impossible to quickly concentrate it to above 10 g / L in the secondary sedimentation tank, which fails to meet the requirements for efficient sludge densification.

[0005] In the waste sludge treatment stage, sludge thickening is necessary, along with the addition of organic polymeric flocculants (such as PAM) or inorganic coagulants (such as FeCl3, PAC) to promote sludge thickening and achieve a concentration of 15–30 g SS / L. During this process, activated sludge, through charge neutralization, ion bridging, and netting / sweeping, promotes the attraction between hydrophobic sludge particles, inducing the formation of sludge aggregates and facilitating the release of water from the sludge. Therefore, it is recommended to consider using hydrocyclones to treat and reuse the thickened sludge after chemical conditioning in the waste sludge treatment stage, rather than simply focusing on densifying the sludge from the secondary treatment stage.

[0006] In the existing technology, patent CN 118978258 A discloses a short-process combined nitrogen and phosphorus removal process for in-situ expansion of wastewater treatment plants. This process effectively combines an AAO process based on sedimentation and membrane separation by using side-flow functional bacteria screening technology on the activated sludge from the secondary treatment stage. This ensures in-situ expansion of the system during the dry season, while also handling excessive expansion during the rainy season, and maintaining efficient simultaneous nitrogen and phosphorus removal. However, this patent does not cover the technical content related to large-size sludge aggregates or dense activated sludge.

[0007] Patent CN 118930002 A discloses a method for improving the yield of large bioaggregates in the densification process of side-flow sludge. This method involves setting up a sludge activity selector in the secondary treatment stage to screen out a large amount of inert material and decaying sludge from the biological treatment unit, selecting sludge with higher activity. Then, by providing a carbon source, the activated sludge utilizes its own biological characteristics to increase its organic matter metabolism capacity under anaerobic conditions, resulting in the synthesis and secretion of large amounts of EPS (extracellular polymeric substances), thereby enhancing sludge aggregation behavior. With the help of PAM or other cationic flocculants, the activated sludge rapidly adheres to each other to form large bioaggregates. However, the technical solution described in this patent is overly complex and does not address the differences in densification effects of hydrocyclones at different flow rates.

[0008] Patent CN 218931912 U discloses a wastewater treatment activated sludge screening device. This device also intercepts the excess sludge from the secondary treatment unit before it enters the sludge treatment unit, and uses a multi-stage hydrocyclone to screen out sludge with higher organic matter content, while eliminating inorganic components from the activated sludge, thus achieving a higher organic matter ratio in the sludge after hydrocyclone screening. However, this patent has a limited function, only involving the repeated use of the hydrocyclone's density screening function, and does not address the development of a multi-functional optimization method for the concentrated excess sludge in the excess sludge treatment unit. Summary of the Invention

[0009] Technical issues

[0010] The main approaches to treating and disposing of excess sludge focus on sludge reduction, harmlessness, and resource recovery and utilization of organic or metallic substances. However, existing technologies lack a method for achieving multi-functional optimization of excess sludge using hydrocyclones.

[0011] Technical solution

[0012] To expand the treatment and reuse pathways for excess sludge, this invention proposes a method for screening and regulating dense activated sludge. By installing a sludge optimization module into the excess sludge treatment unit, the concentrated excess sludge after chemical conditioning is treated. Two sets of hydrocyclones with different functions are set up: one set for efficient sludge densification and the other set for enhancing sludge dewatering performance, thereby improving the applicability of DAS technology in in-situ upgrades and expansions of wastewater treatment plants.

[0013] To achieve the above objectives, the present invention will combine Figure 1 The flowchart is explained as follows:

[0014] This invention provides a method for screening and regulating dense activated sludge, the method comprising the following steps:

[0015] Take the sludge and let it settle. When the sludge can no longer be compressed, remove the supernatant, add flocculant, and then stir to mix the flocculant and sludge thoroughly and evenly for sludge concentration. After that, pass the concentrated sludge into the hydrocyclone group.

[0016] When the hydrocyclone is running at a feed rate of 3-5 m / s, overflow sludge and underflow sludge are obtained respectively under a flow ratio of 80% / 20% for overflow / underflow. The overflow sludge is re-concentrated, and the underflow sludge is fed into the continuous flow activated sludge system.

[0017] When the hydrocyclone is running at a feed rate of 8-10 m / s, overflow sludge and underflow sludge are obtained respectively under a flow ratio of 80% / 20% for overflow / underflow. The overflow sludge is re-concentrated, while the underflow sludge is dewatered, dried, incinerated, and finally disposed of.

[0018] Furthermore, the sludge includes residual sludge in a continuous flow system of anaerobic / aerobic, anaerobic / anoxic / aerobic, and anoxic / anaerobic / aerobic processes.

[0019] Furthermore, flocculants include polymeric flocculants (PAM) or inorganic flocculants (PAC).

[0020] Furthermore, the concentration of flocculant added is 0.04–0.06 kg / m³. 3 m 3 This represents the volume of the sludge.

[0021] Furthermore, the stirring speed is 50–500 rpm / min, and the time is 1–10 minutes.

[0022] Furthermore, the concentrated sludge had an MLVSS of 8–15 g / L, an MLSS of 15–30 g / L, and a proportion of large-sized bioaggregates (wet sludge particle size exceeding 150 μm) exceeding 50%.

[0023] Furthermore, the hydrocyclone assembly consists of a column, a cone, a feed inlet, an overflow outlet, and an underflow outlet;

[0024] Install pressure gauges, flow meters, and flow control switches at the inlet, overflow, and underflow outlet, respectively;

[0025] The specific parameters of the hydrocyclone are as follows: the column diameter D is between 40 and 60 mm, and the underflow outlet diameter D... u The overflow port diameter D is between 10 and 15 mm. o Between 15 and 25 mm, the height-to-diameter ratio L1 / D of the column is 1 to 2, the height-to-diameter ratio L2 / D of the cone is 4 to 6, and the cone angle is 8 to 10°; the feed inlet has a cuboid configuration, and the inlet area S is 100 to 200 mm². 2 Between, the feed inlet tangentially enters the column; the overflow port length L o The diameter D of the column is 0.6 to 1.0, and the diameter D of the overflow outlet is... o The diameter D of the cylinder is 0.3–0.5; the underflow outlet is connected to the conical outlet, and the length of the underflow outlet Lu is equal to the diameter D of the cylinder, which is 0.6–1.0. u The diameter D of the cylinder is 0.1 to 0.3.

[0026] Furthermore, after being thickened again, the overflow sludge can be dewatered, dried, incinerated, and ultimately disposed of, or it can be reintroduced into the hydrocyclone unit for further treatment.

[0027] Furthermore, the bottom sludge is fed into the anaerobic tank of the continuous flow activated sludge system.

[0028] The present invention also provides a wastewater treatment system based on hydrocyclones for controlling the screening of dense activated sludge. The wastewater treatment system consists of an anaerobic tank, an anoxic tank, an aerobic tank, a secondary sedimentation tank, a sludge treatment unit, and a sludge optimization module.

[0029] The sludge treatment unit includes sludge thickening, sludge dewatering, drying and incineration, and sludge disposal facilities; the sludge optimization module consists of two or more sets of hydrocyclones.

[0030] In the wastewater treatment system, wastewater flows sequentially through an anaerobic tank, an anoxic tank, and an aerobic tank for treatment, and then enters a secondary sedimentation tank to achieve sludge-water separation, and is finally discharged as effluent; the excess sludge produced in the secondary sedimentation tank is partly sent to the sludge treatment unit and partly returned to the anaerobic tank.

[0031] In the sludge treatment unit, the remaining sludge is first flocculated by flocculant, and then fed into the hydrocyclone group of the sludge selection module at different feed rates for sorting.

[0032] When the hydrocyclone 1 is running at a feed rate of 3 to 5 m / s, overflow sludge 1 and underflow sludge 1 are obtained respectively under the split ratio of 80% / 20% for overflow / underflow. Overflow sludge 1 is fed into the sludge treatment unit for sludge thickening, and underflow sludge 1 is fed into the anaerobic tank.

[0033] When the hydrocyclone 2 is running at a feed rate of 8-10 m / s, overflow sludge 2 and underflow sludge 2 are obtained respectively under a flow ratio of 80% / 20% for overflow / underflow. Overflow sludge 2 is fed into the sludge treatment unit for sludge concentration, and underflow sludge 2 is fed into the sludge treatment unit for sludge dewatering, drying and incineration and sludge disposal.

[0034] Furthermore, the flocculant includes the polymeric flocculant PAM or the inorganic flocculant PAC.

[0035] Furthermore, the concentration of the flocculant added is 0.04–0.06 kg / m³. 3 m 3 This represents the volume of the remaining sludge.

[0036] The principle of this invention:

[0037] No modifications are made to the existing structures; only a sludge optimization module is added to the sludge thickening unit to treat and reuse excess sludge. First, the excess sludge is thickened using chemicals to meet the requirements for sludge concentration and the proportion of large-sized sludge aggregates. Then, by adjusting the feed flow rate to regulate the shear force and centrifugal force field inside the hydrocyclone, under low shear force, the efficient separation of hydrocyclone group 1 selects the underflow sludge 1 rich in large-sized bio-aggregates, which is then returned to the main process to enhance the system's treatment load. Under high shear force, the hydrocyclone's destructive action further selects the underflow sludge 2 with stronger dewatering capabilities, providing a dual enhancement advantage. The overflow sludge 1 and overflow sludge 2 discharged from the sludge optimization module are returned to the excess sludge thickening unit for further treatment and disposal via a booster pump.

[0038] Beneficial effects

[0039] (1) Without changing the original process structures and main process routes, the sludge optimization module can be added to process the remaining sludge in batches, which can be easily and quickly integrated into the existing treatment system.

[0040] (2) By controlling the hydrocyclone to operate at low flow rate, sludge aggregates with stronger settling performance and larger size are selected and returned to the mainstream process to achieve simultaneous enhancement of pollutant removal and settling performance.

[0041] (3) By controlling the hydrocyclone to operate at high flow rate, the sludge is broken up and the water molecules in the sludge are released, thereby selecting the bottom sludge with better dewatering performance and returning it to the sludge dewatering unit to enhance sludge dewatering. Attached Figure Description

[0042] Figure 1 This is a process design diagram of a dense activated sludge screening and control method according to the present invention;

[0043] Figure 2 The change in the proportion of large-sized bioaggregates in the feed sludge under different PAC addition amounts;

[0044] Figure 3 SVI of feed sludge under different PAC addition amounts 30 change;

[0045] Figure 4 This is a reference diagram of a hydrocyclone configuration;

[0046] Figure 5 The change in sludge particle size before and after hydrocyclone treatment at low flow rates;

[0047] Figure 6 SVI of sludge before and after hydrocyclone treatment at low flow rates 30 change;

[0048] Figure 7 The change in sludge capillary water absorption time before and after hydrocyclone treatment at low flow rates;

[0049] Figure 8 The change in sludge particle size before and after hydrocyclone treatment at high flow rates;

[0050] Figure 9 The change in capillary water absorption time of sludge before and after high-velocity hydrocyclone treatment. Detailed Implementation

[0051] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. 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.

[0052] Testing process

[0053] According to GB11901-1987, the MLSS of the obtained sludge was determined by gravimetric method.

[0054] Wet sludge particle size distribution: 1L of sludge samples were sequentially filtered using sieves with particle sizes of 200, 150, and 100μm. Based on the principle of mass conservation, the total mass of the obtained feed sludge, underflow sludge, and overflow sludge was balanced to ensure that the relative error was within ±5%.

[0055] Sludge Volume Index (SVI): To avoid the influence of sludge concentration on sludge settling performance, the sludge was diluted three times before settling performance testing. The settling ratio was recorded after 30 minutes, and the sludge volume index (SVI) was calculated. 30 ).

[0056] Capillary absorption time: Capillary absorption time was measured using a CST meter (Triton, Model 304M).

[0057] Example 1: Screening of PAC Dosage

[0058] Collected from 20000m 3 The activated sludge concentrated at the bottom of the secondary sedimentation tank of a wastewater treatment plant with a treatment capacity of / d is allowed to settle naturally. When the sludge can no longer be compressed, the supernatant is skimmed off. The MLSS of this part of the sludge is about 8-10 g / L. Among them, the number of large sludge aggregates (particle size >150μm) is only 33.0%, which is insufficient to meet the requirement of the number of large sludge aggregates (more than 50%) required to achieve efficient sludge densification.

[0059] For the remaining sludge after settling, concentrations of 0.02, 0.04, 0.06, and 0.08 kg / m³ were set sequentially. 3 The PAC addition gradient was determined by rapidly mixing the residual sludge and PAC at 300 rpm for 1 minute, followed by slow mixing at 80 rpm for 5 minutes to ensure thorough mixing. The mixed residual sludge was then analyzed, and the results are shown below. Figure 2 and 3 .

[0060] like Figure 2As shown, when the PAC dosage is greater than 0.02 kg / m³ 3 At that time, under the effect of charge neutralization, the sludge quickly adheres to form large sludge aggregates, resulting in the MLSS of the settled sludge exceeding 16 g / L, of which the proportion of large sludge aggregates exceeds 50%, which initially meets the requirements for sludge concentration and sludge aggregate size.

[0061] like Figure 3 As shown, when the PAC dosage is 0.04 kg / m³ 3 At that time, in Al 3+ Under the effect of charge neutralization, a large number of sludge flocs are formed and compacted, which improves the sludge settling performance and increases the SVI of the settled sludge. 30 At 72.41 mL / g, the sedimentation performance was the best among all PAC addition levels; as the PAC dosage continued to increase, the repulsive force between particles increased, and the sludge was significantly hindered during sedimentation and compression, resulting in SVI. 30 Increasing this amount would not align with the goal of increasing wastewater treatment capacity. Therefore, 0.04 kg / m³ is selected. 3 As the amount of PAC added.

[0062] Example 2: Installation of Hydrocyclone Assembly

[0063] Two sets of hydrocyclones, namely hydrocyclone group 1 and hydrocyclone group 2, are added to the original sewage treatment plant process.

[0064] The hydrocyclone assembly is as follows: Figure 4 As shown, it includes a column, a cone, an inlet, an overflow outlet, and an underflow outlet; a pressure gauge, a flow meter, and a flow control switch are installed at the inlet, overflow outlet, and underflow outlet, respectively; the specific parameters of the hydrocyclone are as follows: the column diameter D is between 40 and 60 mm, and the underflow outlet diameter D... u The overflow port diameter D is between 10 and 15 mm. o Between 15 and 25 mm, the height-to-diameter ratio L1 / D of the column is 1 to 2, the height-to-diameter ratio L2 / D of the cone is 4 to 6, and the cone angle is 8 to 10°; the feed inlet has a cuboid configuration, and the inlet area S is 100 to 200 mm². 2 Between, the feed inlet tangentially enters the column; the overflow port length L o The diameter D of the column is 0.6 to 1.0, and the diameter D of the overflow outlet is... o The diameter D of the cylinder is 0.3–0.5; the underflow outlet is connected to the conical outlet, and the length of the underflow outlet Lu is equal to the diameter D of the cylinder, which is 0.6–1.0. u The diameter D of the cylinder is 0.1 to 0.3.

[0065] Hydrocyclone unit 1 feeds excess sludge (PAC addition rate of 0.04 kg / m³) at a low feed velocity (3-5 m / s). 3The sludge was subjected to cyclone treatment, and overflow sludge 1 and underflow sludge 1 were obtained at a split ratio of 80% / 20%. Overflow sludge 1 was reintroduced into the sludge thickening tank, and underflow sludge 1 was introduced into the anaerobic tank. Figure 5 As shown, the proportion of large-sized sludge aggregates in the obtained bottom sludge 1 was 64.0%, exceeding that of the overflow sludge 1 by approximately 41.6%; combined with Figure 6 As shown, the SVI of bottom sludge 1 30 Approximately 66.7 mL / g, a 13.1% increase compared to the feed sludge; combined with Figure 7 As shown, the capillary water absorption time of bottom sludge 1 is 3.50 s / g VSS·L. -1 This represents an increase of approximately 5.7% compared to the feed sludge.

[0066] Hydrocyclone unit 2 feeds excess sludge (PAC addition rate of 0.04 kg / m³) at a high feed velocity (8-10 m / s). 3 The sludge is subjected to cyclone treatment, and overflow sludge 2 and underflow sludge 2 are obtained respectively at an overflow / underflow split ratio of 80% / 20%. Overflow sludge 2 is reintroduced into the sludge thickening tank, while underflow sludge 2 undergoes sludge dewatering, drying, incineration, and final disposal. Figure 8 As shown, the proportion of large-sized sludge aggregates in the obtained bottom sludge 2 was 28.2%, exceeding that of the overflow sludge 2 by approximately 12.3%; combined with Figure 9 As shown, the capillary water absorption time of the obtained bottom sludge 2 is 3.06 s / g VSS·L. -1 This represents an increase of approximately 17.7% compared to the feed sludge.

[0067] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for screening and controlling dense activated sludge, characterized in that, The method includes the following steps: Take the sludge and let it settle. When the sludge can no longer be compressed, remove the supernatant, add flocculant, and then stir to mix the flocculant and sludge thoroughly and evenly before sludge concentration. After that, pass the concentrated sludge into the hydrocyclone assembly. When the hydrocyclone is running at a feed rate of 3-5 m / s, overflow sludge and underflow sludge are obtained respectively under a flow ratio of 80% / 20% for overflow / underflow. The overflow sludge is re-concentrated, and the underflow sludge is fed into the continuous flow activated sludge system. When the hydrocyclone is running at a feed rate of 8-10 m / s, overflow sludge and underflow sludge are obtained respectively under a flow ratio of 80% / 20% for overflow / underflow. The overflow sludge is re-concentrated, while the underflow sludge is dewatered, dried, incinerated, and finally disposed of.

2. The method according to claim 1, characterized in that, The sludge includes residual sludge in continuous flow systems of anaerobic / aerobic, anaerobic / anoxic / aerobic, and anoxic / anaerobic / aerobic processes.

3. The method according to claim 1, characterized in that, Flocculants include polymeric flocculants (PAM) or inorganic flocculants (PAC); the concentration of flocculant added is 0.04–0.06 kg / m³. 3 m 3 This represents the volume of the sludge.

4. The method according to claim 1, characterized in that, The stirring speed is 50-500 rpm / min, and the time is 1-10 minutes.

5. The method according to claim 1, characterized in that, The concentrated sludge has an MLVSS of 8–15 g / L and an MLSS of 15–30 g / L, with large-sized biomass accounting for more than 50%.

6. The method according to claim 1, characterized in that, The hydrocyclone assembly consists of a column, a cone, an inlet, an overflow outlet, and an underflow outlet. Specific parameters of the hydrocyclone are as follows: the column diameter D is between 40 and 60 mm, and the underflow outlet diameter D... u The overflow port diameter D is between 10 and 15 mm. o Between 15 and 25 mm, the height-to-diameter ratio L1 / D of the column is 1 to 2, the height-to-diameter ratio L2 / D of the cone is 4 to 6, and the cone angle is 8 to 10°; the feed inlet has a cuboid configuration, and the inlet area S is 100 to 200 mm². 2 Between, the feed inlet tangentially enters the column; the overflow port length L o The diameter D of the column is 0.6 to 1.0, and the diameter D of the overflow outlet is... o The diameter D of the cylinder is 0.3–0.5; the underflow outlet is connected to the conical outlet, and the length of the underflow outlet Lu is equal to the diameter D of the cylinder, which is 0.6–1.

0. u The diameter D of the cylinder is 0.1 to 0.

3.

7. The method according to claim 1, characterized in that, The overflow sludge is thickened again before being dewatered, dried, incinerated, and finally disposed of, or it is reintroduced into the hydrocyclone unit for further treatment.

8. The method according to claim 1, characterized in that, Bottom flow sludge is fed into the anaerobic tank of the continuous flow activated sludge system.

9. A wastewater treatment system based on hydrocyclones for controlling the screening of dense activated sludge, characterized in that, The wastewater treatment system consists of an anaerobic tank, an anoxic tank, an aerobic tank, a secondary sedimentation tank, a sludge treatment unit, and a sludge optimization module. The sludge treatment unit includes sludge thickening, sludge dewatering, drying and incineration, and sludge disposal facilities; the sludge optimization module consists of two or more sets of hydrocyclones. In the wastewater treatment system, wastewater flows sequentially through an anaerobic tank, an anoxic tank, and an aerobic tank for treatment, and then enters a secondary sedimentation tank to achieve sludge-water separation, and is finally discharged as effluent; the excess sludge produced in the secondary sedimentation tank is partly sent to the sludge treatment unit and partly returned to the anaerobic tank. In the sludge treatment unit, the remaining sludge is first flocculated with a flocculant, and then fed into the hydrocyclone group of the sludge selection module at different feed rates for sorting; the flocculant includes the polymeric flocculant PAM or the inorganic flocculant PAC; the concentration of the flocculant added is 0.04-0.06 kg / m³. 3 m 3 This represents the volume of the remaining sludge. When the hydrocyclone 1 is running at a feed rate of 3 to 5 m / s, overflow sludge 1 and underflow sludge 1 are obtained respectively under the split ratio of 80% / 20% for overflow / underflow. Overflow sludge 1 is fed into the sludge treatment unit for sludge thickening, and underflow sludge 1 is fed into the anaerobic tank. When the hydrocyclone 2 is running at a feed rate of 8-10 m / s, overflow sludge 2 and underflow sludge 2 are obtained respectively under a flow ratio of 80% / 20% for overflow / underflow. Overflow sludge 2 is fed into the sludge treatment unit for sludge concentration, and underflow sludge 2 is fed into the sludge treatment unit for sludge dewatering, drying and incineration and sludge disposal.

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