Treatment method and system for low-turbidity raw water by perforated cyclone flocculation and micro-silt backflow

Abstract

The application discloses a perforated cyclone flocculation and micro-silt sludge backflow treatment method and system for low-turbidity raw water. In view of the technical problem that the alum flowers are small and the effluent turbidity is unstable due to the insufficient number of coagulation nuclei of low-turbidity raw water in a perforated cyclone flocculation tank, a micro-silt sludge selective extraction and directional backflow path is constructed between the perforated cyclone flocculation tank and the inclined-tube sedimentation tank; the micro-fine silt sludge rich in active adsorption sites at the bottom of the inclined-tube zone of the inclined-tube sedimentation tank is backflowed to the water inlet mixing zone of the flocculation tank at a volume rate of 3% to 8% of the raw water flow; in the front section of the perforated cyclone flocculation tank, the micro-silt sludge accelerates the colloidal destabilization and aggregation of the raw water in the form of coagulation nuclei; in the rear section, the flocs grow continuously in the low-shear cyclone field, and the total GT value is 2*10 4 ~1*10 5 . The effluent turbidity of the comparative example is 0.8 to 3.5 NTU, and the compliance rate of ≤1.0 NTU is about 40% to 55%; the application can make the effluent turbidity stable to reach ≤0.5 NTU during the low-turbidity period, and the PAC dosage is reduced by 40% to 55%, and is especially suitable for low-cost upgrading and reconstruction of rural water supply projects and small and medium-sized water plants.

Description

Technical Field

[0001] This invention belongs to the field of water treatment technology, specifically relating to a method and system for treating low-turbidity raw water using perforated cyclone flocculation and micro-sludge recirculation. This invention is applicable to the purification of surface water, lake / reservoir water sources, and seasonally low-turbidity water sources with turbidity of 1-20 NTU, and is particularly suitable for upgrading existing processes combining perforated cyclone flocculation tanks and inclined tube sedimentation tanks.

[0002] This invention specifically relates to a method for processing fine sludge (d) rich in active adsorption sites at the bottom of the inclined tube section of an inclined tube sedimentation tank. 50 The enhanced flocculation process, which selectively extracts 1-50 μm particles and directionally recirculates them to the higher shear zone at the front of the perforated cyclone flocculation tank, systematically solves the technical problems of difficult flocculation and unstable effluent quality in low-turbidity raw water by establishing a synergistic mechanism of "directional supply of flocculation nuclei - precise matching of gradient of perforated cyclone flow field". Background Technology

[0003] Low-turbidity raw water typically refers to surface water with a turbidity below 20 NTU, widely distributed in southern my country during the dry season, in lake and reservoir water sources, and in some plateau areas. According to the *Standards for Drinking Water Quality* (GB 5749-2022) and the *Outdoor Water Supply Design Standard* (GB 50013-2018), the turbidity of treated effluent must be ≤1 NTU, with some water supply units requiring an internal control standard of ≤0.5 NTU. Achieving stable, advanced treatment of low-turbidity raw water is a crucial technical requirement in the field of water supply engineering.

[0004] Perforated vortex flocculation tanks (also known as perforated vortex reaction tanks) are widely used hydraulic flocculation structures in small and medium-sized water treatment projects in my country. Their working principle is as follows: multiple vertical cells are arranged in series, with adjacent cells separated by perforated baffles. Water flows through the perforations, generating a high velocity, and enters the vertical cells to form a swirling flow pattern. The velocity gradient G under vortex conditions (typically within the design range of 15~70 s) is utilized. - ¹) It drives the collision and aggregation of particles in the water, completing the nucleation and growth process of flocs. The perforated vortex flocculation tank has the advantages of no moving parts, simple maintenance and management, and low cost, and is the main flocculation type for rural centralized water supply projects and small and medium-sized municipal water plants.

[0005] However, perforated cyclone flocculation tanks have significant technical limitations when treating low-turbidity raw water. The fundamental reason is that the number of colloidal particles in low-turbidity raw water is scarce, and the effective number of collisions between particles per unit time is much lower than that in medium- and high-turbidity water. Even if sufficient polyaluminum chloride (PAC) is added to destabilize the particles, it is difficult to accumulate enough collisions within the specified flocculation time. This results in small floc size and low density, leading to large fluctuations in turbidity of the effluent after inclined tube sedimentation, which is particularly prominent during the low-turbidity influent period.

[0006] The main approaches to improving the flocculation effect of low-turbidity raw water include: First, increasing the dosage of coagulant, usually requiring the dosage of PAC to be increased to 1.5 to 3.0 times the conventional amount. Although this can improve floc formation to some extent, it increases operating costs and poses a risk of excessive aluminum residue. Second, adding coagulant aids (such as polyacrylamide PAM) can improve the floc structure to some extent, but it cannot fundamentally solve the problem of insufficient colloidal particles for extremely low-turbidity water. Third, extending the flocculation time, but this is usually difficult to significantly modify due to the scale of water plant structures and the area they occupy.

[0007] In terms of equipment modification, existing technologies have already included the practice of recirculating the sediment at the bottom of the sedimentation tank to increase the solid concentration in the reaction zone. For example, the integrated high-efficiency sedimentation device integrates the design of extracting sludge from the sludge thickening zone, stirring it in a dedicated coagulation zone, and then introducing it into the flocculation zone. However, such solutions have the following drawbacks: (1) The sludge extracted from the bottom of the sludge hopper is large-particle sludge (usually >100 μm in diameter and low in specific surface area) that has been fully settled and concentrated. Under the shearing of the swirling flow field at the front of the perforated swirling flocculation tank, there is a risk of particle breakage and redispersion, making it difficult to play the role of coagulation nuclei; (2) After the large-particle sludge is mixed with the low-turbidity raw water, due to the significant difference in particle size, it cannot effectively collide and combine with the nano- to micron-sized colloidal particles in the raw water; (3) Existing sludge recirculation solutions are usually designed for medium- to high-turbidity wastewater treatment scenarios and do not consider the differences in operating conditions of low-turbidity water purification projects and their matching relationship with the perforated swirling flow field.

[0008] In the design of perforated cyclone flocculation tanks, some existing engineering practices have considered gradually changing the pore size from small to large along the water flow direction to form a G-value distribution from strong to weak. However, existing gradual design usually optimizes parameters for medium-to-high turbidity water sources and does not combine "micro sludge coagulation nucleus supply" for targeted flow field matching design, thus having limited improvement effect on low turbidity water treatment.

[0009] During the operation of the inclined tube sedimentation tank, a layer of fine sludge will accumulate in the lower part of the inclined tube section near the inlet. This layer of fine sludge consists of particles with a diameter of d. 50 Composed of fine particles ranging from approximately 1 to 50 μm in size, rich in hydroxyl complexes from aluminum salt hydrolysis products and adsorbed natural organic matter, these particles possess a high specific surface area and abundant active adsorption sites, theoretically meeting the physicochemical requirements to act as contact flocculation nuclei. However, in existing water plant operations, this type of fine sludge is typically discharged into the wastewater system periodically without screening, and its potential value as a coagulation nucleus has not yet been identified and utilized.

[0010] In summary, the existing technology has the following core defects: when treating low-turbidity raw water, there is a lack of sufficient effective coagulation nuclei in the perforated cyclone flocculation tank; the conventional gradual pore design does not optimize the flow field for the introduction of coagulation nuclei; and the coagulation nuclei function of the fine sludge at the bottom of the inclined tube sedimentation tank is not effectively utilized. The separation of these three factors leads to unstable effluent quality, high coagulant consumption, and high water production costs during the low-turbidity period, which urgently need to be addressed. Summary of the Invention

[0011] The technical problem to be solved by this invention is that when low-turbidity raw water (turbidity 1~20 NTU) is treated in a perforated cyclone flocculation tank, the number of effective collision nuclei in the water body is insufficient, resulting in small floc size and unstable turbidity of the effluent after inclined tube sedimentation. At the same time, there are problems such as large amount of coagulant added and high water production cost.

[0012] The technical solution of this invention to the above-mentioned technical problem is as follows: A closed-loop channel of "selective extraction of micro-sludge—directional reflux" is constructed between the perforated cyclone flocculation tank and the inclined tube sedimentation tank. This channel allows for the selective extraction and directional reflux of micro-sludge (dm) from the bottom of the inclined tube section of the inclined tube sedimentation tank, which is rich in active adsorption sites. 50 (1~50 μm, SS concentration 800~3000 mg / L) is refluxed to the influent mixing zone of the flocculation tank at a volume ratio of 3%~8%, so that it forms a high G-value swirling flow field in the initial stage (35~70 s). - ¹) It fully collides and mixes with PAC and raw water colloids, using micro-sludge as coagulation nuclei to accelerate the destabilization and aggregation of colloids in low-turbidity raw water; in the later stage, the pore size gradually increases and the G value decreases (8~25 s). - ¹) In the swirling flow field, the flocs are protected and continue to grow, achieving a synergistic enhancement of flocculation effect through "directional supply of condensation nuclei and precise matching of flow field gradient".

[0013] "Directed supply of coagulation nuclei" is achieved through a selective micro-sludge extraction device at the bottom of the inclined tube section of the inclined tube sedimentation tank. This device employs an open-pore sludge collection pipe for low-speed extraction of fine particles (d) from the lower part of the inclined tube section. 50 The fine sludge (1~50 μm), with a large specific surface area and abundant active adsorption sites, is different from the large sludge particles extracted from the sludge hopper, ensuring that the particle size characteristics of the returned particles are suitable to act as coagulation nuclei.

[0014] "Precise matching of flow field gradient" is achieved through the segmented perforation design of the perforated vortex flocculation tank. The equivalent diameter of the perforations in the partition plates of each vertical compartment of the perforated vortex flocculation tank increases progressively in three stages from the inlet to the outlet (40~80 mm → 80~120 mm → 120~160 mm), resulting in a G-value ranging from 35 to 70 s⁻¹. - ¹Gradual transition to 8~25 s - ¹ The flow field: The higher G-value zone in the front section ensures that the micro-sludge, coagulant and raw water colloids fully collide and nucleate; the lower G-value zone in the back section protects the floc from being sheared and broken, and allows it to continue to grow to the size that meets the settling requirements.

[0015] The present invention has the following beneficial effects:

[0016] I. Improve the stability of effluent quality during low turbidity periods. By introducing micro sludge coagulation nuclei, the number of effective collision nuclei in the low turbidity raw water increases by more than an order of magnitude, significantly improving the flocculation nucleation rate. Within the same flocculation time, larger and denser settling flocs can be formed, ensuring that the effluent turbidity remains stable at ≤0.5 NTU.

[0017] Second, reduce the dosage of coagulant. Under the action of micro sludge coagulation nuclei, the dosage of PAC can be reduced from 5~15 mg / L in conventional low turbidity water treatment to 2~8 mg / L (calculated as Al2O3), reducing operating costs and reducing the risk of aluminum residue in the effluent.

[0018] Third, no new large-scale equipment is required, resulting in low modification costs. The micro sludge selective extraction device of this invention (perforated sludge collection pipe + variable frequency reflux pump) has a simple structure and can be upgraded from existing perforated cyclone flocculation tank and inclined tube sedimentation tank systems. The modification work is small and the water outage time is short, making it suitable for low-cost quality improvement and transformation of rural and small-to-medium-sized water plants.

[0019] IV. Strong adaptability, capable of handling extreme low turbidity conditions. This invention has a good treatment effect on influent turbidity of 1~20 NTU. When the influent turbidity is as low as 1~5 NTU or the water temperature is as low as 4~10℃, the effluent quality can be maintained stably by increasing the micro sludge return ratio (5%~8%), thus solving the problem of severe decline in flocculation efficiency of conventional processes under extreme low turbidity conditions.

[0020] Fifth, achieve the resource recycling of micro-sludge. The micro-sludge at the bottom of the inclined tube sedimentation tank is recycled and reused in the form of coagulation nuclei, reducing the amount of waste sludge discharge, which is in line with the development direction of energy-saving water treatment technology. Attached Figure Description

[0021] Figure 1 This is a flowchart of the method of the present invention.

[0022] Figure 2 This is a diagram showing the overall system structure of the present invention.

[0023] Figure 3 This is a structural diagram of the inclined tube sedimentation tank of the present invention.

[0024] The meanings of the labels in the attached diagram are as follows: 1—Raw water inlet pipe; 2—Inlet mixing zone; 3—Coagulant dosing pipe; 4—Micro sludge return pipe; 5—Perforated cyclone flocculation tank; 5a—Small-diameter vertical grid chamber at the front; 5b—Middle-diameter vertical grid chamber at the middle; 5c—Large-diameter vertical grid chamber at the rear; 6—Perforated baffle; 7—Transition water distribution channel; 8—Inclined tube sedimentation tank; 9—Perforated sludge collection pipe; 11—Control valve; 12—Variable frequency return pump; 15—PLC control cabinet; 16—Coagulant metering pump. Detailed Implementation

[0025] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The following embodiments and comparative examples all use the same influent water quality conditions for comparison. The engineering parameters involved are exemplary values; specific parameters should be determined through beaker tests and hydraulic calculations, taking into account the characteristics of the raw water quality and design specifications.

[0026] See Figure 1 The process flow of the treatment system of this invention is as follows: Low-turbidity raw water is introduced into the influent mixing zone (2) through the raw water inlet pipe (1). Polyaluminum chloride is injected into the mixing zone by the coagulant metering pump (16) through the dosing pipe (3). At the same time, the variable frequency return pump (12) returns the micro sludge from the inclined tube sedimentation tank (8) to the influent mixing zone (2) through the micro sludge return pipe (4). After the three are briefly intensified mixed in the mixing zone, they enter the perforated cyclone flocculation tank (5). The water flows through the front section (5a), middle section (5b), and rear section (5c) vertical grid chambers in sequence to complete multi-stage cyclone flocculation. The flocs continue to grow in an environment with progressively decreasing G values; the flocculated effluent is evenly distributed into the inclined tube sedimentation tank (8) through the transition distribution channel (7), and solid-liquid separation is completed in the inclined tube area (8a); the fine sludge is directionally extracted and returned to the inlet mixing zone (2) through the perforated sludge collection pipe (9) via the sludge collection pipe, control valve and variable frequency return pump (12), forming a closed loop utilization; the PLC control cabinet (15) automatically adjusts the speed of the variable frequency return pump (12) and the dosage of the coagulant metering pump (16) based on the measured signals of the online turbidity meter of the effluent channel and the online turbidity meter of the inlet.

[0027] See Figure 2 The perforated swirl flocculation tank (5) consists of 8 vertical cells connected in series, with each vertical cell separated by a perforated baffle (6). The perforations are alternately located at the top and bottom of the baffle along the vertical direction, creating an "upward-downward" reciprocating swirling flow pattern between adjacent vertical cells. In the front section (5a), the perforated baffle between cells 1-3 has a front perforation with an equivalent diameter of 40-60 mm; in the middle section (5b), the perforated baffle between cells 4-6 has a middle perforation with an equivalent diameter of 80-120 mm, increasing progressively between cells; and in the rear section (5c), the perforated baffle between cells 7-8 has a rear perforation with an equivalent diameter of 130-160 mm.

[0028] See Figure 3 The perforated sludge collection pipe (9) of the selective micro sludge extraction device is a UPVC pipe or a stainless steel round pipe, arranged horizontally along the full width of the inclined tube sedimentation tank (8), and fixed 10-15 cm above the bottom plate of the inclined tube by a fixed bracket. Circular sludge collection holes with a diameter of 8-15 mm are evenly distributed on the pipe body, with a total opening rate of 15%-30%. The perforated sludge collection pipe (9) is located at 1 / 4-1 / 3 of the inlet end along the water flow direction in the inclined tube area, above the boundary between the sludge hopper and the inclined tube area, and preferentially captures fine sludge particles (d) sliding down the inclined tube from a physical position.50 Approximately 1~50 μm), while coarse-grained sludge (d 50 Particles larger than 100 μm have already passed through the sludge collection pipe and fallen into the sludge hopper due to their rapid gravity deposition, thus achieving particle size selective extraction.

[0029] The automatic control logic of this invention is as follows: the online turbidity meter collects the effluent turbidity signal in real time and transmits it to the PLC control cabinet (15); the PLC control cabinet (15) adjusts the frequency of the variable frequency return pump (12) according to the measured values ​​of effluent turbidity and influent turbidity, and controls the proportion of micro sludge return flow to the raw water flow to be between 3% and 8%; at the same time, the PLC control cabinet (15) controls the coagulant metering pump (16) to add PAC according to the influent turbidity zone: the dosage is 4~8 mg / L when the influent turbidity is 1~5 NTU, and the dosage is 2~5 mg / L when the influent turbidity is 5~20 NTU (both are calculated as Al2O3).

[0030] Example 1 Enhanced treatment upgrade for a lake-type reservoir water supply project during the low turbidity period.

[0031] Water source quality characteristics: During the dry season, the influent turbidity is 3~12 NTU, the water temperature is 12~22℃, the pH is 7.0~7.8, the color is 10~20 degrees, and the ammonia nitrogen is 0.1~0.4 mg / L. The water treatment plant's capacity is Q = 5000 m³ / d (208 m³ / h, 0.0579 m³ / s).

[0032] Design parameters for the perforated cyclone flocculation tank: Eight vertical compartments are set along the water flow direction. The effective dimensions of each compartment (width × length × water depth) are 1.5 m × 1.5 m × 3.5 m, with an effective volume of approximately 7.9 m³ per compartment and a total effective volume of approximately 63 m³. The designed total flocculation residence time T = 63 / 208 × 60 ≈ 18 min, and the residence time per compartment t1 ≈ 136 s (2.3 min).

[0033] Perforated partition parameters: The equivalent diameter of the perforations in the front section (3 partitions between sections 1-3) is d1 = 50 mm. The perforations are located at the bottom of the partitions. The designed flow velocity through the perforations is v1 ≈ 0.49 m / s (calculated based on Q and approximately 60 perforations). The corresponding velocity gradient is: G1= √[v1² / (2Cd²νt1)] = √[(0.49)² / (2×0.4225×1.004×10 -6 [×136)] ≈ 46s - ¹; The equivalent diameters of the perforations in the middle section (three partitions between the 4th and 6th sections) are 90 mm, 100 mm, and 110 mm, respectively. The perforations are located on the upper part of the partitions, and the corresponding flow velocities are approximately 0.31 m / s, 0.24 m / s, and 0.19 m / s, respectively, with G values ​​of approximately 30 s⁻¹. - ¹、23 s - ¹、18 s - ¹; The equivalent diameter of the perforation in the latter section (two partitions between the 7th and 8th sections) is d3 = 140 mm, corresponding to a flow velocity of approximately 0.13 m / s, G3 ≈ 12 s - ¹. The G values ​​decrease sequentially from beginning to end, with a total GT value of approximately 3.1 × 10⁻⁶. 4 Satisfying 2×10 4 ~1×10 5 Design requirements.

[0034] Selective micro-sludge extraction device: An inclined tube sedimentation tank (width × length = 3.0 m × 6.0 m) is equipped with four UPVC perforated sludge collection pipes (outer diameter 80 mm, length 3.0 m). The pipes are installed 1.5 m from the inlet end of the inclined tube along the water flow direction, with the pipe axis 15 cm from the bottom plate and 300 mm above the top of the sludge hopper. Each pipe has approximately 80 evenly spaced 12 mm diameter circular sludge collection holes (hole spacing approximately 37 mm), totaling 320 holes across the four pipes, with a total sludge collection hole area of ​​approximately 362 cm². Under the design maximum return flow rate (8% × 208 = 16.6 m³ / h), the orifice flow velocity is approximately 0.13 m / s.

[0035] Coagulant Dosage and Recirculation Control: Under influent turbidity of 3-12 NTU, the PAC dosage (calculated as Al2O3) was set at 3-6 mg / L, which is approximately 40%-55% lower than the 8-15 mg / L required to maintain qualified effluent without micro-sludge recirculation; the micro-sludge recirculation flow rate was set at 4%-6% of Q (8-12 m³ / h), and the measured SS concentration of the recirculated micro-sludge was approximately 1200-2500 mg / L. 50 The measured value is approximately 8~25 μm. After implementing the process of this invention, the turbidity of the effluent during the low turbidity period is stably reduced to 0.2~0.4 NTU (100% compliance rate for ≤0.5 NTU), while the turbidity of the effluent before the modification was 0.8~3.5 NTU (approximately 0% compliance rate for ≤0.5 NTU and approximately 40%~55% compliance rate for ≤1.0 NTU).

[0036] In this embodiment, 0.10 mg / L of polyacrylamide coagulant is added before the effluent from the last section of the perforated cyclone flocculation tank (coagulant addition point 18) to further enhance the flocculant bridging effect; the surface load of the inclined tube sedimentation tank is controlled below 3.0 m³ / (m²·h), and the effluent turbidity is stably ≤0.3 NTU.

[0037] Example 2 Enhanced treatment during low temperature and low turbidity periods at a town-level water plant with a mountain stream-type water source.

[0038] Water source quality characteristics: During the dry season, the influent turbidity is 1~8 NTU, the water temperature is 4~15℃ (during the low temperature period in winter and spring), the pH is 6.8~7.5, and the total hardness is 80~120 mg / L (calculated as CaCO3). The low temperature and low turbidity superposition condition (water temperature 4~8℃, turbidity 1~5 NTU) is the key scenario to be addressed in the renovation. The water treatment capacity of the water plant is Q = 2000 m³ / d (83 m³ / h, 0.0231 m³ / s).

[0039] Parameters of the perforated cyclone flocculation tank: There are 6 vertical compartments, with effective dimensions (width × length × water depth) of 1.2 m × 1.2 m × 3.2 m per compartment and a volume of approximately 4.6 m³ per compartment. The total volume is approximately 27.6 m³. The designed total flocculation residence time T ≈ 20 min and the residence time per compartment t2 ≈ 199 s (3.3 min) are designed. The residence time is appropriately extended to compensate for the adverse effects of low temperature on flocculation kinetics.

[0040] Perforated baffle parameters: The equivalent diameter of the perforations in the front section (the two baffles between the 1st and 2nd compartments) is d1 = 45 mm. The perforations are located at the bottom of the baffles, and the flow velocity through the holes is approximately 0.52 m / s. Under a water temperature of 20°C, G1 = √[(0.52)² / (2×0.4225×1.004×10]]. -6 ×199)] ≈ 40 s - ¹, at a low temperature of 4°C (ν=1.567×10⁻¹⁰), -6 (m² / s) at G1|4°C ≈ 32 s - ¹; The equivalent diameters of the perforations in the middle section (the two partitions between the 3rd and 4th compartments) are 75 mm and 95 mm, respectively, corresponding to G values ​​of approximately 30 s. - ¹ and 19 s - ¹; The equivalent diameter of the perforation in the latter section (the two partitions between the 5th and 6th compartments) is d3 = 130 mm, the flow velocity in the holes is approximately 0.14 m / s, and G3 ≈ 11 s. - ¹. The overall GT value is approximately 2.8 × 10⁻⁶. 4 It meets the design requirements.

[0041] Low-temperature and low-turbidity operating conditions: When the raw water temperature is below 8°C and the turbidity is below 5 NTU, the system implements a low-temperature and low-turbidity enhanced mode: the micro-sludge recirculation ratio is increased to 6%~8% of Q (5.0~6.6 m³ / h); the flow velocity in the front-end orifice is maintained at 0.52 m / s (higher than the upper limit of the conventional flow velocity of 0.40~0.45 m / s at 4°C, to increase the number of collisions to compensate for the adverse effects of increased viscosity at low temperatures); the PAC dosage is increased to 6~8 mg / L (calculated as Al₂O₃); and the PAM dosage is increased to 0.15 mg / L. Under the above conditions, the effluent turbidity during the low-temperature and low-turbidity period is stably maintained below 0.3 NTU.

[0042] Selective micro-sludge extraction device: The net dimensions (width × length) of the inclined tube sedimentation tank are 2.4 m × 4.8 m. Three UPVC perforated sludge collection pipes (outer diameter of 75 mm, length of 2.4 m each) are arranged at the inlet end of the inclined tube 1.2 m along the water flow direction, with the tube axis 12 cm away from the bottom plate of the inclined tube. Each pipe has about 60 sludge collection holes with a diameter of 10 mm (hole spacing of about 40 mm), for a total of 180 sludge collection holes in the three pipes, with a total sludge collection hole area of ​​about 141 cm². The orifice velocity at the maximum designed return flow rate (8% × 83 = 6.6 m³ / h) is about 0.13 m / s, consistent with Example 1.

[0043] Comparative Example Using the same source water quality as in Example 1 (lake-type reservoir, influent turbidity 3~12 NTU, water temperature 12~22℃) and the same treatment scale (5000 m³ / d, 208 m³ / h), the existing conventional perforated cyclone flocculation + inclined tube sedimentation process was adopted, without the installation of a micro sludge return device. The partition plates of each compartment of the perforated cyclone flocculation tank adopted a uniform pore size throughout (without pore size gradual segmentation design). The effluent water quality and coagulant consumption during the low turbidity period were investigated.

[0044] Parameters of the perforated cyclone flocculation tank: The number of compartments and the effective size of the compartments are the same as in Example 1 (8 vertical compartments, each 1.5 m × 1.5 m × 3.5 m, residence time t ≈ 136 s). All 8 baffles use the same equivalent perforation diameter d = 90 mm. The perforations are alternately arranged vertically at the top and bottom of the baffles, forming a cyclone flow pattern in the same way but without a pore size gradient. Based on the design flow rate Q = 0.0579 m³ / s, each baffle has approximately 30 pores. The actual pore velocity v = 0.0579 / (30 × π × 0.045²) ≈ 0.304 m / s, and the overall velocity gradient G = √[(0.304)² / (2 × 0.4225 × 1.004 × 10⁻⁶)²]. -6 ×136)] ≈ 28 s - ¹, Uniform throughout, GT value approximately 3.1 × 10⁻⁶ 4(The total energy input is similar to that of Example 1). Total energy input: G = 28 s. - ¹Based on the value of the first segment of Example 1 (46 s) - ¹) and the latter segment value (12 s) - Between ¹), it is impossible to provide the strong mixing and collision effect of the high G value zone in the first part of Example 1 on the micro sludge coagulation nuclei and coagulant, nor can it provide the protective effect of the low G value zone in the second part of Example 1 on the already grown flocs.

[0045] Coagulant addition: No micro sludge return device is installed. In order to maintain the effluent turbidity as close to the standard as possible during the low turbidity period, the PAC dosage (calculated as Al2O3) is increased to 8~15 mg / L, and the PAM coagulant aid dosage is the same as in Example 1 (0.10 mg / L).

[0046] Treatment effect: Under low turbidity conditions of influent turbidity 3~12 NTU and water temperature 12~22℃, the effluent turbidity of the comparative example was 0.8~3.5 NTU, with an average of about 1.8 NTU. Among them, the proportion of ≤0.5 NTU was about 0%, and the proportion of ≤1.0 NTU was about 40%~55%. It was particularly obvious when the influent turbidity was below 5 NTU, and the effluent turbidity fluctuated more, frequently exceeding the control requirement of 1.0 NTU. The fundamental reason is: (1) The number of colloidal particles in the low turbidity raw water is insufficient, and there is a lack of sufficient coagulation nuclei. The whole process G = 28 s - ¹ The uniform swirling field cannot accumulate enough collisions within the effective flocculation time to allow the flocs to grow to a settling size; (2) Increasing the PAC dosage can increase the concentration of aluminum salt hydrolysis products, but in the absence of active nuclei, the initial coagulation efficiency of the aluminum salt hydrolysis products is limited; (3) The uniform pore size design ensures that the flocs are always in the same shear field during the growth process, with a higher shear force in the later stage (G = 28 s). - ¹Relative to the ideal later value of 8~15 s - ¹High levels of saturation cause continuous shearing of small alum flowers, further inhibiting their growth.

[0047] The comparative examples show that simply increasing the PAC dosage and using a uniform pore size design throughout the process cannot effectively solve the fundamental problem of insufficient flocculation nucleation in low-turbidity raw water. However, this invention, through the directional supply of micro sludge flocculation nuclei and gradient matching of the perforated swirling flow field, achieves significantly better effluent turbidity stability and compliance rate than the comparative examples, even with a PAC dosage reduction of approximately 40% to 55% (see table below).

[0048] Table 1 Comparison of main process parameters and treatment effects between the examples and comparative examples.

[0049] The system comparison between the above embodiments and the comparative examples shows that the present invention, through the synergistic mechanism of "directional supply of fine sludge coagulation nuclei" and "precise matching of gradient in the perforated swirling flow field," achieves a similar GT value (3.1 × 10⁻⁶) to the comparative examples. 4 Under these conditions, thanks to the nucleation effect of the refluxed micro-sludge and the enhanced collision nucleation in the higher G-value zone of the upstream section, the floc size and density are significantly improved. The turbidity of the effluent after inclined tube sedimentation is stably ≤0.5 NTU (Example 1: 0.2~0.4 NTU; Example 2: 0.25~0.45 NTU), while the turbidity of the comparative effluent is as high as 0.8~3.5 NTU, and the compliance rate of ≤1.0 NTU is only about 40%~55%, and the PAC consumption is 1.5~3 times that of this invention. This invention has significant technical advantages in improving the stability of effluent quality during low turbidity periods and reducing coagulant consumption. At the same time, it has low modification costs, wide applicability, and good engineering promotion value.

Claims

1. A method for treating low-turbidity raw water using perforated cyclone flocculation and micro-sludge recirculation, characterized in that, Includes the following steps: Step S1: Introduce low-turbidity raw water with an influent turbidity of 1~20 NTU into a multi-chamber perforated vortex flocculation tank, and simultaneously add polyaluminum chloride coagulant in the influent mixing zone of the flocculation tank at a dosage of 2~8 mg / L (calculated as Al2O3), with a mixing time of 15~30 s; Step S2: From the lower part of the inclined tube section of the inclined tube sedimentation tank connected in series downstream of the perforated cyclone flocculation tank, a selective micro sludge extraction device is used to continuously or intermittently extract suspended solids with a concentration of 800~3000 mg / L and a particle size d. 50 The micro sludge, which is 1~50 μm in size, is returned to the influent mixing zone at a volume ratio of 3%~8% of the original water flow. Step S3: In the front section of the perforated cyclone flocculation tank, the pore flow velocity is 0.35~0.60 m / s, and the velocity gradient G value is 35~70 s. - ¹, the reflux micro sludge collides fully with the hydrolysis products of coagulant and the colloidal particles of raw water in a high shear swirling field, and the micro sludge colloid ... Step S4: In the later section of the perforated cyclone flocculation tank, the pore size gradually increases along the water flow direction, the pore velocity decreases to 0.10~0.25 m / s, and the velocity gradient G value decreases to 8~25 s. - ¹, the initial flocs continue to collide and aggregate in a low-shear swirling flow field, and the total GT value throughout the flocculation process is controlled at 2×10. 4 ~1×10 5 ; Step S5: The flocculated effluent enters the inclined tube sedimentation tank for solid-liquid separation, and the effluent turbidity is ≤0.5 NTU; the selective micro sludge extraction device cyclically executes step S2; wherein, the total flocculation retention time is 15~25 min.

2. The method according to claim 1, characterized in that, The multi-chamber perforated vortex flocculation tank includes 6 to 10 vertical chambers, divided into three sections along the water flow direction: the front section consists of the 1st to 3rd vertical chambers, with the equivalent diameter of the perforations in the partitions between adjacent vertical chambers being 40 to 80 mm; the middle section consists of the 4th to 6th vertical chambers, with the equivalent diameter of the perforations increasing progressively to 80 to 120 mm; and the rear section consists of the 7th to the end vertical chambers, with the equivalent diameter of the perforations being 120 to 160 mm. The perforations in each partition are arranged alternately in the top 1 / 5 and bottom 1 / 5 regions of the partition, creating a reciprocating vortex flow pattern between adjacent vertical chambers.

3. The method according to claim 1, characterized in that, The selective micro-sludge extraction device includes a group of perforated sludge collection pipes arranged horizontally along the width of the inclined tube sedimentation tank. The net distance between the axis of each perforated sludge collection pipe and the bottom plate of the inclined tube is 5-20 cm, and the pipes are positioned at 1 / 4 to 1 / 3 of the length of the inclined tube inlet along the water flow direction. The flow velocity at the orifice of each sludge collection hole does not exceed 0.15 m / s. The particle size d of the extracted micro-sludge is... 50 The range is 1~50 μm.

4. The method according to claim 1, characterized in that, It also includes online monitoring and automatic feedback steps: an online turbidity detector is installed in the effluent channel of the inclined tube sedimentation tank to control the speed of the variable frequency return pump with the effluent turbidity signal; when the effluent turbidity exceeds 0.5 NTU, the micro sludge return flow rate is automatically increased to 5%~8% of the original water flow rate; when the influent turbidity is lower than 5 NTU, the dosage of polyaluminum chloride is simultaneously adjusted to 4~8 mg / L (calculated as Al2O3).

5. The method according to claim 1, characterized in that, Before the water effluent exits from the vertical grid chamber at the end of the perforated cyclone flocculation tank, add 0.05~0.20 mg / L of polyacrylamide coagulant to enhance floc bridging and improve floc particle size and settling properties.

6. The method according to claim 1, characterized in that, The effective water depth of each vertical cell in the perforated vortex flocculation tank is 3.0~4.5 m, and the hydraulic residence time of a single vertical cell is 2~3 min. After the water flows through the perforations, it forms a horizontal rotating flow in the vertical cell, and the average flow velocity in the cell is 0.05~0.15 m / s.

7. The method according to claim 1, characterized in that, When the raw water temperature is below 10℃, the micro sludge return flow rate is increased to 5%~8% of the raw water flow rate, and the flow velocity in the front section of the perforated vortex flocculation tank is increased to 0.50~0.60 m / s to compensate for the adverse effect of increased water viscosity on flocculation kinetic efficiency under low temperature conditions.

8. A perforated cyclone flocculation and micro-sludge recirculation treatment system for low-turbidity raw water implementing the method of any one of claims 1 to 7, characterized in that, include: The perforated cyclone flocculation tank consists of 6 to 10 vertical compartments connected in series. Perforated baffles are installed between adjacent vertical compartments. The equivalent diameter of the perforations increases gradually from the inlet end to the outlet end, and the perforations are alternately arranged on the upper and lower parts of the baffles. The inlet mixing zone is located upstream of the inlet end of the perforated cyclone flocculation tank and is connected to both the coagulant metering pump pipeline and the micro sludge return pipeline. The inclined tube sedimentation tank is connected to the outlet of the perforated cyclone flocculation tank via a transition water distribution channel, and a sludge discharge hopper is provided at the bottom. The selective micro sludge extraction device includes an open sludge collection pipe group arranged horizontally in the lower part of the inclined tube area of ​​the inclined tube sedimentation tank, a micro sludge return pipeline connected thereto, and a variable frequency return pump. The automatic control system includes an online turbidity meter installed in the effluent channel of the inclined tube sedimentation tank, a programmable logic controller (PLC) connected to the signal of the online turbidity meter, and a variable frequency reflux pump controller and a coagulant metering pump controller respectively connected to the PLC.

9. The processing system according to claim 9, characterized in that, The perforated sludge collection pipe assembly consists of 2 to 6 UPVC or stainless steel pipes with an outer diameter of 50 to 100 mm. Each pipe is horizontally arranged along the full width of the inclined tube sedimentation tank. The pipe body is evenly opened with circular sludge collection holes with a diameter of 8 to 15 mm, and the total area of ​​the holes is 15% to 30% of the outer surface area of ​​the pipe body. The pipe body is installed on the inclined tube support frame by a fixed bracket, and the net distance between the axis of the pipe body and the bottom plate of the inclined tube is 5 to 20 cm. Both ends of each pipe are connected to the main sludge collection pipe, which is connected to the variable frequency return pump via a control valve.

10. The processing system according to claim 9, characterized in that, The perforated baffle is made of reinforced concrete slab with a thickness of 100-200 mm or stainless steel plate with a thickness of 4-8 mm; the inclined tube of the inclined tube sedimentation tank has an inclination angle of 60°, a tube diameter of 35-80 mm, and an effective length of 800-1200 mm; the upward flow velocity in the clear water zone of the sedimentation tank does not exceed 4.0 m / h; a horizontal guide plate is installed at the bottom of the inclined tube zone above the sludge hopper to divide the inclined tube zone into a micro sludge extraction zone and a coarse sludge deposition zone; the equivalent diameter of the perforations in the front section baffle is 40-60 mm, and the perforations are located at the lower part of the baffle, 300-600 mm from the bottom plate; the equivalent diameter of the perforations in the middle section baffle is 80-120 mm, and the perforations are located at the upper part of the baffle, 300-600 mm from the top. At mm; the equivalent diameter of the perforations in the rear baffle is 130~160mm, and they are alternately arranged at the top and bottom of the baffle; the cross-sectional shape of the perforations is circular or rectangular, and the orifice area of ​​rectangular perforations is calculated based on the equivalent diameter; the automatic control system is also equipped with an online turbidity meter and an influent flow meter; the PLC stores the following control logic: when the influent turbidity is 1~5 NTU, the micro sludge return flow rate is set to 5%~8% of the raw water flow rate, and the coagulant dosage is 4~8 mg / L; when the influent turbidity is 5~20 NTU, the return flow rate is set to 3%~5%, and the coagulant dosage is 2~5 mg / L; all the above parameters are dynamically fine-tuned with the constraint that the effluent turbidity is ≤0.5 NTU.

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