An upflow filter backwash method and upflow filter

Abstract

The application discloses an upflow filter tank, a tank body (1) sequentially comprises a water and air distribution system (2), a filter bed (3) and a water collecting tank (4) from bottom to top; the water and air distribution system (2) comprises a water and air distribution pipe (222) arranged at the bottom of the filter bed (3), the water and air distribution pipe (222) is communicated with a filter water inlet pipe (211), a backwashing air inlet pipe (213), a backwashing water inlet pipe and a bottom sewage pipe (215) respectively; the water collecting tank (4) is communicated with a filter water outlet pipe (411) and a backwashing water outlet pipe (413) respectively; a plurality of cyclone holes (222a) are arranged on the water and air distribution pipe (222), and the inner wall of the cyclone hole (222a) is provided with a spiral water channel. The application further discloses a backwashing method of the upflow filter tank, and the steps comprise first bottom sewage, cyclone washing, second bottom sewage, single air washing and single water washing. The application can improve the cleaning effect, save backwashing water consumption and backwashing time.

Description

Technical Field

[0001] This invention belongs to the field of water treatment technology, specifically, it relates to a backwashing method for an upflow filter and an upflow filter. Background Technology

[0002] In existing technologies, upflow filters have water inlet at the bottom and outlet at the top, with the entire filter media serving as a contaminant-holding layer. Since the main contaminants are concentrated in the lower part of the filter media, the backwashing method is crucial for restoring operational efficiency. Currently, upflow filters often use a combination of lowering the water level and air-water backwashing. This involves first lowering the filter level to flush the filter media, followed by sequential air washing, combined air-water washing, water washing, and rinsing. However, the water flow direction in combined air-water washing is the same as the filtration direction, making flushing difficult, requiring a large amount of flushing water, and making it difficult to ensure the filter media is thoroughly cleaned. Over time, this can easily lead to filter media caking, resulting in long backwashing times and high backwashing water consumption. When applied to wastewater treatment, this also presents problems such as filter media caking. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a backwashing method for an upward flow filter and an upward flow filter, which can improve the cleaning effect and save backwash water consumption and backwashing time.

[0004] To solve the above-mentioned technical problems, the basic concept of the technical solution adopted by the present invention is as follows:

[0005] In a first aspect, the present invention provides an upward flow filter, the filter comprising a tank body 1, wherein a water distribution and air distribution system 2, a filter bed 3 and a water collection tank 4 are arranged sequentially from bottom to top in the tank body 1;

[0006] The filter bed 3 includes a filter media layer;

[0007] The water collection tank 4 is connected to the filter outlet pipe 411 and the backwash drain pipe 413 respectively.

[0008] The water and air distribution system 2 includes a water and air distribution pipe 222 installed at the bottom of the filter bed 3. The water and air distribution pipe 222 is connected to the filter water inlet pipe 211, the backwash air inlet pipe 213, the backwash water inlet pipe and the bottom drain pipe 215 respectively.

[0009] The water and air distribution pipe 222 is provided with a plurality of swirling holes 222a, and the inner wall of the swirling holes 222a has a spiral water channel.

[0010] Furthermore, the spiral channels of two adjacent swirling holes 222a rotate in opposite directions.

[0011] Furthermore, the spacing between adjacent swirling holes 222a near the inner wall of the pool body 1 is smaller than the spacing between adjacent swirling holes 222a near the center of the pool body 1.

[0012] Furthermore, the water outlet direction of the vortex hole 222a arranged near the inner wall of the pool body 1 is towards the inner wall of the pool body 1.

[0013] Furthermore, the backwash water inlet pipe includes a first backwash water inlet pipe 217 and a second backwash water inlet pipe 219. The first backwash water inlet pipe 217 is connected to the water and air distribution pipe 222 via a gas-liquid mixer 221. The gas-liquid mixer 221 quickly mixes water and air to form a gas-water mixture. The second backwash water inlet pipe 219 is connected to the water and air distribution pipe 222.

[0014] In another aspect, the present invention also provides a backwashing method for an upflow filter, the method comprising the following steps:

[0015] Step S1, First bottom drainage: Close the filter inlet pipe 211, and the bottom drain pipe 215 starts to drain. Drainage stops when the liquid level drops to the first set height above the filter bed 3.

[0016] Step S2, Cyclone Washing: After stopping the first bottom drainage, the air-water mixture is introduced into the water and air distribution pipe 222, and the water flow after passing through the cyclone hole 222a performs cyclone washing on the lower part of the filter media layer.

[0017] Step S3, Second bottom drainage: After the swirling flushing stops, the bottom drainage pipe 215 starts to drain sewage, and the drainage stops when the liquid level drops to the second set height above the filter bed 3;

[0018] Step S4, Single air wash: After stopping the second bottom drainage, backwash air is introduced through the backwash air inlet pipe 213 to loosen the filter material layer;

[0019] Step S5, Single water wash: After stopping the single air wash, backwash water is introduced through the backwash water inlet pipe to rinse the filter media layer. The impurities washed off are discharged through the backwash drain pipe 413.

[0020] Furthermore, it also includes:

[0021] Step S6, rinsing: Water is introduced through the filter inlet pipe 211 to rinse the filter bed 3, and the rinsed water is discharged through the backwash drain pipe 413.

[0022] Furthermore, if the turbidity of the effluent is in the first range at the end of step S5, then filtration is resumed;

[0023] If the turbidity of the wastewater is in the second range when step S5 ends, proceed to step S6 or extend step S5 so that the turbidity of the wastewater is in the first range and filtration is restored.

[0024] If the turbidity of the wastewater is in the third interval when step S5 ends, repeat steps S2 to S5 until the turbidity of the wastewater is in the first interval, and then resume filtration.

[0025] The first interval < the second interval < the third interval.

[0026] Furthermore, the first interval is ≤5 NTU, the second interval is >5 NTU and <10 NTU, and the third interval is ≥10 NTU.

[0027] Furthermore, the first set height is 200-600mm, and the second set height is 100-200mm.

[0028] Beneficial effects:

[0029] The upward flow filter provided by this invention utilizes a gas-liquid mixer to rapidly mix backwash water and air, dissolving the air into the water. The mixture is then released to the bottom of the filter bed through swirling holes in the water and air distribution pipes. By setting swirling holes with opposite rotation directions, the air-water mixture in adjacent holes moves in opposite directions under centrifugal force, creating a localized swirling scrubbing force in the lower filter media layer of the filter bed. This accelerates the lateral agitation of the lower filter media layer, rapidly cleaning pollutants adsorbed on the filter media surface. The gas-liquid mixture is released through the swirling holes, instantly forming nanobubbles that act on the filter media surface, significantly increasing the contact opportunity with the filter media surface. Compared to traditional large bubbles, this results in a superior cleaning effect.

[0030] The backwashing method for upflow filters provided by this invention, through the steps of first bottom discharge, cyclone rinsing, second bottom discharge, single air washing, and single water washing, can significantly save backwash water consumption and backwashing time, solve the problem of filter media caking caused by incomplete rinsing, and make backwashing more reasonable, more operable, and with better rinsing effect.

[0031] The specific embodiments of the present invention will now be described in further detail with reference to the accompanying drawings. Attached Figure Description

[0032] The accompanying drawings, as part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention, but do not constitute an undue limitation of the invention. Obviously, the drawings described below are merely some embodiments, and those skilled in the art can obtain other drawings based on these drawings without creative effort. In the drawings:

[0033] Figure 1 This is a schematic diagram of the equipment structure of the upward flow filter steel structure according to Embodiment 1 of the present invention;

[0034] Figure 2This is a schematic diagram of the reinforced concrete structure of the upward flow filter tank according to Embodiment 1 of the present invention;

[0035] Figure 3 yes Figure 2 A top-view structural diagram;

[0036] Figure 4 This is a schematic diagram of the water and air distribution pipe structure of Embodiment 1 of the present invention;

[0037] Figure 5 yes Figure 4 Enlarged view of the structure at point A;

[0038] Figure 6 This is a schematic diagram of the backwashing method for an upflow filter in Embodiment 2 of the present invention.

[0039] Figure 7 The present invention compares the turbidity effect of effluent from a traditional upflow filter after backwashing for 24 hours.

[0040] Figure 8 This invention compares the turbidity effect of the effluent after 72 hours of filtration in a traditional upflow filter after backwashing.

[0041] In the diagram: 1. Tank body; 2. Water and air distribution system; 3. Filter bed; 4. Water collection tank;

[0042] 211. Filter inlet pipe; 212. Filter inlet valve; 213. Backwash air inlet pipe; 214. Backwash air inlet valve; 215. Bottom drain pipe; 216. Bottom drain valve; 217. First backwash inlet pipe; 218. First backwash inlet valve; 219. Second backwash inlet pipe; 220. Second backwash inlet valve; 221. Gas-liquid mixer; 222. Water and air distribution pipe; 222a. Swirl orifice;

[0043] 411. Filter outlet pipe; 412. Filter outlet valve; 413. Backwash drain pipe; 414. Backwash drain valve; 415. Clear water channel; 416. Gate valve; 417. Drainage channel; 418. Flip valve.

[0044] It should be noted that these accompanying drawings and textual descriptions are not intended to limit the scope of the invention in any way, but rather to illustrate the concept of the invention to those skilled in the art by referring to specific embodiments. Detailed Implementation

[0045] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments will be clearly and completely described below with reference to the accompanying drawings. The following embodiments are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0046] In the description of this invention, it should be noted that the terms "upper", "lower", "front", "rear", "left", "right", "vertical", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention.

[0047] Terms such as “first” and “second” are used only to distinguish technical features and should not be interpreted as indicating or implying relative importance, or implicitly indicating the number of technical features indicated, or implicitly indicating the order of the technical features indicated.

[0048] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0049] When the filter reaches the set backwash cycle, set head loss, or effluent turbidity, backwashing is required. The following are specific embodiments of backwashing provided by this invention:

[0050] Example 1

[0051] This embodiment provides an upward flow filter, such as Figures 1-3 As shown, the upward flow filter includes a tank body 1, and the tank body 1 is provided with a water distribution and air distribution system 2, a filter bed 3, and a water collection tank 4 from bottom to top.

[0052] The water and air distribution system 2 is connected to the filter water inlet pipe 211, the backwash air inlet pipe 213, the bottom drain pipe 215, and the backwash water inlet pipe, respectively. The filter water inlet pipe 211 is equipped with a filter water inlet valve 212, the backwash air inlet pipe 213 is equipped with a backwash air inlet valve 214, the bottom drain pipe 215 is equipped with a bottom drain valve 216, and the backwash water inlet pipe is equipped with a backwash water inlet valve.

[0053] The filter bed has a pebble pad layer in three parts and a filter media layer on top. The filter media is heavy filter media.

[0054] The water collection tank 4 is connected to the clear water channel 415 through the gate valve 416. The clear water channel 415 is connected to the filter outlet pipe 411. The filter outlet pipe 411 is equipped with a filter outlet valve 412.

[0055] A clear water channel 415 is located on the outside of the filter tank body 1. A drainage channel 417 is located on the outside of the filter tank body 1 opposite to the clear water channel 415. The drainage channel 417 is connected to the inside of the filter tank body 1 through a flap valve 418. The bottom of the flap valve 418 is 200-300mm above the filter bed 3. The drainage channel 417 is connected to the backwash drain pipe 413. The backwash drain pipe 413 is equipped with a backwash drain valve 414.

[0056] The water collection trough 4 is located on three sides of the pool body 1, but not on the inner side of the drainage ditch 417.

[0057] Wastewater enters the tank 1 through the filter inlet pipe 211 and then enters the filter bed 3 through the water distribution and air distribution system 2. Particulate matter in the wastewater is intercepted. The purified water from the filter bed 3 is collected by the water collection tank 4 and finally flows out of the tank 1 through the clear water channel 415 and the filter outlet pipe, completing the filtration process.

[0058] The backwash inlet pipe includes a first backwash inlet pipe 217 and a second backwash inlet pipe 219. The first backwash inlet pipe 217 is connected to the water and air distribution pipe 222 via a gas-liquid mixer 221 and a first backwash inlet valve 218. The gas-liquid mixer 221 quickly mixes water and air to form a gas-water mixture. The second backwash inlet pipe 219 is connected to the water and air distribution pipe 222 via a second backwash inlet valve 220.

[0059] like Figure 4 As shown, multiple swirling holes 222a are arranged on the water and air distribution pipe 222. The inner wall of the swirling holes 222a has a spiral water channel to control the rotation direction of the outflowing water.

[0060] Preferably, the spiral channels of two adjacent vortex holes 222a rotate in opposite directions, see Figure 5 .

[0061] The backwash water and air are rapidly mixed in a gas-liquid mixer, with the air dissolving into the water. The mixture is then released to the bottom of the filter bed through swirling holes in the water and air distribution pipes. Since adjacent swirling holes rotate in opposite directions, the gas-water mixture in adjacent holes moves in opposite directions under centrifugal force, creating a localized swirling scrubbing force in the lower filter media layer of the filter bed. This accelerates the lateral agitation of the lower filter media layer and quickly cleans the pollutants adsorbed on the filter media surface. In addition, the gas-liquid mixture is released through the swirling holes, instantly forming nanobubbles that act on the filter media surface, significantly increasing the chance of contact with the filter media surface. Compared with traditional large bubbles, its cleaning effect is better.

[0062] As an optional implementation, to achieve better cleaning results, the spacing of the vortex holes 222a near the inner wall of the pool body 1 is greater than the spacing at the center of the pool body 1. This is because, in experiments, it was found that the rotation of the water exiting the vortex holes causes impurities to move towards the inner wall of the pool body 1 under the action of centrifugal force. Therefore, increasing the density of the vortex holes 222a near the inner wall of the pool body 1 can effectively remove impurities from the inner wall of the pool body 1.

[0063] Furthermore, swirl holes 222a are arranged near the inner wall of the pool body 1, with the water outlet direction facing the inner wall of the pool body 1. This can remove impurities attached to the inner wall. The effect is better when the angle between the water outlet direction and the inner wall of the pool body 1 is less than 30 degrees.

[0064] Example 2

[0065] like Figure 6 As shown, the present invention also provides a backwashing method for an upward flow filter, comprising the following steps:

[0066] Step S1, Bottom Sewage Discharge: Stop the filter water intake and start discharging sewage through the bottom sewage pipe 215. Stop discharging sewage when the pipe is lowered to the first set height above the filter media. This step is mainly to quickly remove the particles intercepted at the bottom of the filter bed 3. The first set height can be 200-600mm.

[0067] Step S2, cyclone washing: Dissolved air water is introduced into the water and air distribution system 2, and finally enters the filter media layer through the cyclone hole 222a, so that the filter media layer in the lower part of the filter bed 3 forms a local cyclone scrubbing.

[0068] This step utilizes the rapid mixing of backwash water and air in the gas-liquid mixer 221, where the air dissolves into the water and is finally released to the bottom of the filter bed 3 through the swirl holes 222a in the water and air distribution pipe 222. Since the adjacent swirl holes 222a rotate in opposite directions, the gas-water mixture in the adjacent holes moves in opposite directions under the action of centrifugal force, forming a local swirling scrubbing force in the lower filter media layer of the filter bed 3, accelerating the lateral agitation of the lower filter media layer, and quickly cleaning the pollutants adsorbed on the surface of the filter media.

[0069] Step S3, Bottom Sewage Discharge: Stop the cyclone flushing and start discharging sewage through the bottom sewage discharge pipe 215. Lower the pipe to the second set height above the filter media and stop discharging sewage. The pollutants washed down by the cyclone flushing are quickly discharged from the filter bed through the bottom sewage discharge pipe. The second set height can be 100-200mm.

[0070] Step S4, Single Air Wash: Stop bottom drainage and perform single air wash;

[0071] Step S5, Single Water Wash: Stop the single air wash and start backwashing. The backwash water further and rapidly rinses the filter media layer, and the impurities washed off are discharged from the top of the filter bed through the backwash drain pipe 413. This step is mainly to quickly discharge the pollutants washed off from the top of the filter bed 3, ensuring that the filter media is thoroughly rinsed.

[0072] As an optional implementation, rinsing can be performed after step S5.

[0073] Step S6, rinsing: Open the filter inlet valve 212, and the water flowing out of the filter inlet pipe 211 rinses the filter bed 3. The rinsing water is discharged from the backwash drain pipe 413 through the upper part of the filter tank.

[0074] The purpose of rinsing the filter bed 3 with filtered water is to further flush out the pollutants remaining in the upper filter media layer of the filter bed 3, so as to ensure the quality of the effluent from the filter bed.

[0075] Furthermore, the turbidity of the wastewater can be monitored to adjust the filtration process, as follows:

[0076] If the turbidity of the effluent is in the first range when step S5 ends, then filtration is resumed;

[0077] If the turbidity of the wastewater is in the second range when step S5 ends, proceed to step S6 or extend step S5 so that the turbidity of the wastewater is in the first range and filtration is restored.

[0078] If the turbidity of the wastewater is in the third interval when step S5 ends, repeat steps S2 to S5 until the turbidity of the wastewater is in the first interval, and then resume filtration.

[0079] Optionally, the first interval is ≤5 NTU, the second interval is >5 NTU and <10 NTU, and the third interval is ≥10 NTU.

[0080] In an optional implementation, it further includes:

[0081] Step S6, rinsing: Start rinsing the filter bed 3 with the incoming filtration water. The rinsing water is discharged from the backwash drain pipe 413 at the top of the filter tank.

[0082] The filter bed 3 is rinsed with filtered water to further clean the fine impurities and particles remaining between the filter media, ensuring the quality of the effluent.

[0083] Experimental data:

[0084] 1. A comparative experiment was conducted between the backwashing method of the present invention (filter 1#) and the traditional upflow filter backwashing method (filter 2#), with a treatment capacity of 350m³. 3 / d, treating wastewater from aquaculture, with a backwash cycle set at 24 hours for backwashing:

[0085] (1) The backwashing steps of this invention are: bottom drain → vortex rinsing → bottom drain → single air rinsing → single water rinsing. The entire backwashing time is 15 minutes, and the backwashing water consumption is 13 m³ / min. 3 .

[0086] (2) The traditional backwashing steps are: bottom drain → single air wash → combined air and water wash → single water wash → bottom drain → single air wash → combined air and water wash → single water wash. The entire backwashing time is 40 minutes, and the backwashing water consumption is 30 m³ / min. 3 .

[0087] The turbidity comparison effect after 24 hours of backwashing filtration is as follows: Figure 7 As shown.

[0088] As can be seen, under the same conditions, the filtration effect of the backwashing method of the present invention is comparable to that of the traditional backwashing method, but the backwashing time is reduced by 62.5% and the backwashing water consumption is reduced by 56.7%.

[0089] 2. A comparative experiment was conducted between the backwashing method of this invention and the traditional backwashing method of an upflow filter, with a treatment capacity of 350m³. 3 / d, treating wastewater from aquaculture, with a backwash cycle set at 72 hours for backwashing:

[0090] (1) The backwashing steps of this invention are as follows: bottom drain → vortex rinsing → bottom drain → single air rinsing → vortex rinsing → bottom drain → single air rinsing → single water rinsing. The entire backwashing time is 25 minutes, and the backwashing water consumption is 22 m³ / min. 3 .

[0091] (2) The traditional backwashing steps are: bottom drain → single air wash → combined air and water wash → single water wash → bottom drain → single air wash → combined air and water wash → single water wash. The entire backwashing time is 54 minutes, and the backwashing water consumption is 40 m³ / min. 3 .

[0092] The turbidity comparison effect after 72 hours of backwashing filtration is as follows: Figure 8 As shown.

[0093] Therefore, under the same conditions, the filtration effect of the backwashing method of the present invention is comparable to that of the traditional backwashing method, but the backwashing time is reduced by 53.7% and the backwashing water consumption is reduced by 45%.

[0094] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-described technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. An upward flow filter, characterized in that, The filter includes a tank body (1), and a water distribution and air distribution system (2), a filter bed (3) and a water collection tank (4) are arranged sequentially from bottom to top in the tank body (1). The filter bed (3) includes a filter media layer; The water collection tank (4) is connected to the filter outlet pipe (411) and the backwash drain pipe (413) respectively; The water and air distribution system (2) includes a water and air distribution pipe (222) installed at the bottom of the filter bed (3). The water and air distribution pipe (222) is connected to the filter water inlet pipe (211), the backwash air inlet pipe (213), the backwash water inlet pipe and the bottom drain pipe (215), respectively. The water and air distribution pipe (222) is provided with multiple swirling holes (222a), and the inner wall of the swirling holes (222a) has a spiral water channel; The backwash water inlet pipe includes a first backwash water inlet pipe (217) and a second backwash water inlet pipe (219). The first backwash water inlet pipe (217) is connected to the water and air distribution pipe (222) through a gas-liquid mixer (221). The gas-liquid mixer (221) quickly mixes water and air to form a gas-water mixture. After the gas and liquid are mixed, they are released in the vortex hole to form nanobubbles that act on the surface of the filter material. The second backwash water inlet pipe (219) is connected to the water and air distribution pipe (222).

2. The upflow filter according to claim 1, characterized in that, The spiral channels of two adjacent swirling holes (222a) rotate in opposite directions.

3. The upflow filter according to claim 2, characterized in that, The spacing between adjacent swirling holes (222a) near the inner wall of the pool body (1) is smaller than the spacing between adjacent swirling holes (222a) near the center of the pool body (1).

4. The upflow filter according to claim 3, characterized in that, The vortex hole (222a) arranged near the inner wall of the pool body (1) has its water outlet direction facing the inner wall of the pool body (1).

5. A backwashing method for an upflow filter as described in any one of claims 2-4, characterized in that, The method includes the following steps: Step S1, First bottom drainage: Close the filter inlet pipe (211), the bottom drain pipe (215) starts to drain, and the drain stops when the liquid level drops to the first set height above the filter bed (3); Step S2, Cyclone Washing: After stopping the first bottom drainage, the air-water mixture is introduced into the water and air distribution pipe (222), and the water flow after passing through the cyclone hole (222a) performs cyclone washing on the lower part of the filter material layer; Step S3, Second bottom drainage: After the swirling flushing stops, the bottom drain pipe (215) starts to drain, and the liquid level drops to the second set height above the filter bed (3) to stop the drainage; Step S4, Single air wash: After stopping the second bottom drainage, backwash air is introduced through the backwash air inlet pipe (213) to loosen the filter material layer; Step S5, single water wash: After stopping the single air wash, backwash water is introduced through the backwash water inlet pipe to rinse the filter media layer. The impurities washed off are discharged through the backwash drain pipe (413).

6. The backwashing method according to claim 5, characterized in that, Also includes: Step S6, rinsing: Water is introduced through the filter inlet pipe (211) to rinse the filter bed (3), and the rinsed water is discharged through the backwash drain pipe (413).

7. The backwashing method according to claim 6, characterized in that, If the turbidity of the effluent is in the first range when step S5 ends, then filtration is resumed; If the turbidity of the wastewater is in the second range when step S5 ends, proceed to step S6 or extend step S5 so that the turbidity of the wastewater is in the first range and filtration is restored. If the turbidity of the wastewater is in the third interval when step S5 ends, repeat steps S2 to S5 until the turbidity of the wastewater is in the first interval, and then resume filtration. The first interval < the second interval < the third interval.

8. The backwashing method according to claim 7, characterized in that, The first interval is ≤5 NTU, the second interval is >5 NTU and <10 NTU, and the third interval is ≥10 NTU.

9. The backwashing method according to claim 5, characterized in that, The first set height is 200~600mm, and the second set height is 100~200mm.

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