Operating method of a dewatering device and dewatering device

Abstract

To provide an operation method of a dewatering apparatus capable of reducing risks associated with changes in sludge concentration.SOLUTION: An operation method of a dewatering apparatus measures a height of sludge fed into an inlet 2 of the sludge and independently controls the rotation speed of at least a second screw 4 according to the measured height of the sludge.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a method of operating a dewatering apparatus and a dewatering apparatus. Background Art

[0002] Conventionally, a screw press (i.e., a dewatering apparatus) has been known as an apparatus that compresses sludge (liquid-containing material) discharged from liquid treatment facilities such as water and sewage treatment plants and night soil treatment plants, and separates water from the sludge.

[0003] This screw press includes a filtration cylinder formed of a screen (perforated plate) and a screw disposed inside the filtration cylinder. The screw has a screw shaft arranged concentrically with the filtration cylinder and screw blades fixed to an outer surface of the screw shaft. The screw blades are rotated by a rotation mechanism connected to the screw shaft, thereby compressing and dewatering the sludge introduced into the filtration cylinder.

[0004] A back pressure plate for damming sludge is disposed at the downstream open end of the filtration cylinder. The back pressure plate retains cake (dewatered sludge) fed by rotating screw blades to form a plug cake (plug). This plug cake applies back pressure to the cake fed in subsequently, and further compresses the cake, thereby reducing the moisture content of the sludge in the filtration cylinder. Prior Art Documents Patent Documents

[0005] Patent Document 1 Japanese Unexamined Patent Application Publication No. 2018-51582 Patent Document 2 Japanese Unexamined Patent Application Publication No. 2001-340900 Patent Document 3 Japanese Unexamined Patent Application Publication No. Sho 58-054999 Patent Document 4 Japanese Unexamined Patent Application Publication No. 2021-159878 Summary of the Invention [Problems that the invention aims to solve]

[0006] Patent Document 1 discloses a twin-screw press. This type of screw press comprises a first screw and a second screw arranged in series within a filter cylinder. These first and second screws are configured to rotate at different speeds by separate drive devices. Therefore, by independently controlling the rotational speeds of the first and second screws, a plug cake can be formed within the filter cylinder and the sludge inside the filter cylinder can be compressed without the need for a back pressure plate.

[0007] However, the twin-screw press disclosed in Patent Document 1 does not have a back pressure plate and is therefore highly susceptible to changes in sludge concentration. If the sludge supply rate decreases, voids may form in the second screw, potentially increasing the water content of the sludge. On the other hand, if the sludge supply rate increases, sludge leakage may increase or the hopper level may rise.

[0008] Patent Document 2 discloses a hopper level monitoring and control method for a screw press that does not have a back pressure function such as a tapered cone. This is because, in a screw press without a back pressure function, changes in sludge concentration, especially decreases in concentration, can easily create voids, and as a result, there is a risk that the water content of the sludge will increase.

[0009] However, Patent Document 2 only discloses an operating method for a single-axis screw press that does not have a back pressure adjustment mechanism such as a plug or back pressure plate, and it is difficult to simply apply such an operating method to a twin-axis screw press.

[0010] Patent Document 4 discloses a configuration in which the hopper level is adjusted by the rotation of the first screw alone. However, since the hopper level can also change depending on the state of the second screw, the range of hopper levels that can be adjusted by the rotation of the first screw may be limited. More specifically, under conditions in which the sludge is sufficiently dewatered, if the rotation speed of the first screw is accelerated to lower the hopper level at a certain rotation speed of the second screw, it has been observed that the hopper level does not decrease because the sludge rotates together with the first screw.

[0011] Therefore, in order to lower the hopper level, it is necessary to increase the rotational speed of the second screw, and based on this premise, it is necessary to adjust the screw press based on the results obtained from image analysis and machine learning. Patent document 4 does not disclose the order in which the first and second screws are changed, but in order to avoid the above-mentioned co-rotation, the order in which they are changed must be carefully considered.

[0012] The present invention aims to provide a method for operating a dewatering apparatus and a dewatering apparatus that can reduce risks caused by changes in sludge concentration (for example, an increase in the water content of the sludge, an increase in the hopper level, and the cake rotating together with the screw). [Means for solving the problem]

[0013] In one embodiment, a method for operating a coaxial dewatering apparatus comprising a first screw and a second screw is provided. The method for operating the dewatering apparatus measures the height of the sludge introduced into the sludge inlet and independently controls the rotational speed of at least the second screw according to the measured height of the sludge.

[0014] In one embodiment, when changing the rotational speed of the first screw and the rotational speed of the second screw to a higher rotational speed, the rotational speed of the second screw is changed first, and then the rotational speed of the first screw is changed. In one embodiment, when changing the rotational speed of the first screw and the rotational speed of the second screw to a lower rotational speed, the rotational speed of the first screw is changed, and simultaneously or afterward, the rotational speed of the second screw is changed. In one embodiment, the rotation speeds of the first screw and the second screw are controlled in a stepwise manner, where the rotation speeds of the first screw and the second screw are changed in steps according to the height of the sludge.

[0015] In one embodiment, the rotation speeds of the first screw and the second screw are controlled by a steep change method, in which the rotation speeds of the first screw and the second screw are changed rapidly according to the height of the sludge. In one embodiment, the control method for independently controlling the rotational speed of at least the second screw includes a stepwise change method in which the rotational speed of the first screw and the rotational speed of the second screw are changed in steps according to the height of the sludge, and a steep change method in which the rotational speed of the first screw and the rotational speed of the second screw are changed abruptly according to the height of the sludge.

[0016] In one embodiment, a dewatering apparatus is provided, comprising: a first screw and a second screw whose rotational speeds can be controlled independently of each other; a level sensor that detects a signal corresponding to the height of the sludge introduced into the sludge inlet; and a control device that measures the height of the sludge based on the signal detected by the level sensor and independently controls the rotational speed of at least the second screw according to the measured height of the sludge. [Effects of the Invention]

[0017] By independently controlling the rotation speeds of the first and second screws according to the measured sludge height, the risks associated with changes in sludge concentration can be reduced. [Brief explanation of the drawing]

[0018] [Figure 1] It is a diagram showing a dewatering system. [Figure 2] It is a diagram showing an embodiment of a dewatering device. [Figure 3] Figures 3(a) and 3(b) are diagrams showing a non-contact level sensor. [Figure 4] It is a graph showing an embodiment of the operating state of a screw press according to a step change method. [Figure 5] It is a graph showing another embodiment of the operating state of a screw press according to a step change method. [Figure 6] It is a diagram showing the control flow of a control device in the step change method. [Figure 7] It is a graph showing an embodiment of the operating state of a screw press according to an acceleration change method. [Figure 8] It is a graph showing another embodiment of the operating state of a screw press according to an acceleration change method. [Figure 9] It is a diagram showing the control flow of a control device in the acceleration change method. DETAILED DESCRIPTION OF EMBODIMENTS

[0019] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Figure 1 is a diagram showing a dewatering system. The dewatering system 200 is a system for performing dewatering treatment on sludge (organic sludge) as organic waste. As shown in Figure 1, the dewatering system 200 includes a sludge storage tank 201 that stores sludge, a flocculation tank 203 that prepares flocculated sludge, and a sludge supply pump 202 that supplies sludge in the sludge storage tank 201 to the flocculation tank 203.

[0020] The dewatering system 200 comprises a polymer dissolving tank 205 in which a polymer agent (i.e., a flocculant) is dissolved, and a polymer supply pump 206 that supplies the polymer agent in the polymer dissolving tank 205 to the flocculation tank 203. The flocculated sludge in the flocculation tank 203 is prepared by adding the polymer agent to the sludge. The dewatering system 200 comprises a thickener 204 that separates water from the flocculated sludge flocculated in the flocculation tank 203 to produce a concentrated sludge with low fluidity, and a dewatering device 100 that dewaters the concentrated sludge (liquid-containing material) concentrated in the thickener 204.

[0021] Figure 2 shows one embodiment of a dewatering device. As shown in Figure 2, the dewatering device 100 comprises a cylindrical screen casing (filter tube) 1, a coaxial (two-axis) first screw 3 and a second screw 4 arranged concentrically with the screen casing 1 within the screen casing 1 to transport concentrated sludge in a predetermined transport direction D, a first rotation mechanism 7 for rotating the first screw 3, and a second rotation mechanism 20 for rotating the second screw 4 independently of the first screw 3. Hereinafter, the dewatering device 100 may be referred to as a screw press 100.

[0022] The screen casing 1 is formed from a screen (perforated plate) such as perforated metal and has a double structure with an outer cylinder and an inner cylinder. A sludge inlet (in other words, a hopper) 2 is formed at the upstream end of the screen casing 1. Sludge introduced into the screen casing 1 from the inlet 2 is transported within the screen casing 1 in a predetermined transport direction D by rotating first screw 3 and second screw 4.

[0023] The dewatering system 200 includes a control device 6 that controls the operation of its constituent devices. As shown in Figure 1, the control device 6 is configured to control the operation of the sludge supply pump 202, the polymer supply pump 206, the coagulation tank 203, the thickener 204, and the screw press 100. More specifically, the control device 6 is configured to control the operation of the agitator 203a that agitates the mixture of polymer and sludge in the coagulation tank 203.

[0024] As shown in Figure 2, the control device 6 is configured to control the operation of the first rotation mechanism 7 and the second rotation mechanism 20. The second screw 4 is connected to the first screw 3 so that it can rotate independently of the first screw 3. The first screw 3 and the second screw 4 extend through the screen casing 1 and the discharge chamber 33, respectively. The discharge chamber 33 is connected to the screen casing 1. The plug cake, which will be described later, is discharged from the screen casing 1 into this discharge chamber 33. The axial length of the second screw 4 is shorter than the axial length of the first screw.

[0025] The first screw 3 has a first screw shaft 3A that is frustoconical (tapered) in shape, with its diameter gradually increasing towards the downstream side in the sludge transfer direction D, and a first screw blade 3B fixed to the outer surface of the first screw shaft 3A. The second screw 4 has a second screw shaft 4A that is cylindrical, and a second screw blade 4B fixed to the outer surface of the second screw shaft 4A. The second screw 4 is positioned downstream of the first screw 3 in the sludge transfer direction D.

[0026] The upstream end of the screen casing 1 is sealed by a sealing wall 8. One end of the first screw shaft 3A (the upstream end in the transport direction D) extends through this sealing wall 8. A water seal device 10 is installed in the sealing wall 8 to seal the gap between the sealing wall 8 and the first screw shaft 3A. The upstream end of the first screw shaft 3A, which extends through the sealing wall 8, is rotatably supported by bearings 11 and 12 installed on a base (not shown), while its axial movement is constrained. Note that one of the bearings 11 and 12 may be omitted.

[0027] The upstream end of the first screw shaft 3A is connected to a first rotation mechanism 7 for rotating the first screw 3. In this embodiment, the first rotation mechanism 7 includes a first drive unit (e.g., an electric motor) 14, a sprocket 15 fixed to the rotation shaft of the first drive unit 14, a sprocket 16 fixed to the first screw shaft 3A, and a chain 17 wrapped around these sprockets 15 and 16.

[0028] The sprocket 16 is located between the bearings 11 and 12. When the first drive unit 14 of the first rotation mechanism 7 is driven, the sprocket 15 fixed to the rotation axis of the first drive unit 14 rotates, causing the sprocket 16 fixed to the first screw shaft 3A via the chain 17 to rotate. As a result, the first screw 3 is rotated by the first rotation mechanism 7. The first drive unit 14 is connected to a control device 6, which is configured to control the operation of the first drive unit 14.

[0029] The second screw shaft 4A of the second screw 4 is positioned concentrically with the first screw shaft 3A. The outer diameter of the second screw shaft 4A is the same as the maximum diameter of the first screw shaft 3A. The second screw shaft 4A has a reduced diameter portion that extends through the inner wall 33A of the discharge chamber 33.

[0030] The upstream end of the second screw shaft 4A is rotatably supported on the first screw shaft 3A via a sliding bearing (not shown), and the downstream end of the second screw shaft 4A is rotatably supported while its axial movement is constrained by bearings 22 and 23 installed on a base (not shown). Note that bearing 23 may be omitted.

[0031] The downstream end of the second screw shaft 4A is connected to a second rotation mechanism 20 for rotating the second screw 4. In this embodiment, the second rotation mechanism 20 includes a second drive unit (e.g., an electric motor) 24, a sprocket 25 fixed to the rotation shaft of the second drive unit 24, a sprocket 26 fixed to the second screw shaft 4A, and a chain 27 wrapped around these sprockets 25 and 26.

[0032] The sprocket 26 is positioned between the bearings 22 and 23. When the second drive unit 24 of the second rotation mechanism 20 is driven, the sprocket 25 fixed to the rotation axis of the second drive unit 24 rotates, causing the sprocket 26 fixed to the second screw shaft 4A via the chain 27 to rotate. As a result, the second screw 4 is rotated by the second rotation mechanism 20.

[0033] The second drive unit 24 is connected to the control device 6. The second drive unit 24 has a built-in inverter (not shown), and the control device 6 is configured to control the operation of the second drive unit 24 via the inverter. That is, the control device 6 can control the rotational speed and rotational direction of the second drive unit 24 via the inverter. The second drive unit 24 can rotate the second screw 4 independently of the first screw 3. The first drive unit 14 also has a built-in inverter that can change the rotational speed and rotational direction of the first drive unit 14.

[0034] The first screw blade 3B extends spirally along the axial direction of the first screw shaft 3A, and the second screw blade 4B extends spirally along the axial direction of the second screw shaft 4A. The combined length of the portion of the first screw 3 to which the first screw blade 3B is fixed and the portion of the second screw 4 to which the second screw blade 4B is fixed is equal to or longer than the axial length of the screen casing 1.

[0035] A small gap is formed between the screen casing 1 (more specifically, the inner cylinder) and the first screw blade 3B, allowing the first screw blade 3B to rotate without contacting the screen casing 1. Similarly, a small gap is formed between the inner surface of the screen casing 1 and the second screw blade 4B, allowing the second screw blade 4B to rotate without contacting the screen casing 1.

[0036] The rotating first screw blades 3B and second screw blades 4B can transport the sludge introduced into the screen casing 1 from the inlet 2 formed at the upstream end of the screen casing 1 toward the discharge chamber 33 (i.e., in the transport direction D).

[0037] In this embodiment, the winding direction (i.e., the helical direction) of the second screw blade 4B is opposite to the winding direction of the first screw blade 3B. Therefore, when sending the sludge introduced from the inlet 2 to the discharge chamber 33, the second screw 4 is rotated in the opposite direction to the first screw 3, as shown in Figure 2.

[0038] In one embodiment, the winding direction of the second screw blade 4B may be the same as the winding direction of the first screw blade 3B. In this case, when the sludge introduced from the inlet 2 is sent to the discharge chamber 33, the second screw 4 will be rotated in the same direction as the first screw 3.

[0039] As shown in Figure 2, the screen casing 1 is divided into a dewatering region 1A where the first screw 3 is located and a plug-forming region 1B where the second screw 4 is located. The space in which sludge is transported in the dewatering region 1A is formed by the inner surface of the screen casing 1, the first screw blades 3B, and the first screw shaft 3A.

[0040] As shown in Figure 2, the cross-sectional area of ​​this transfer space gradually decreases along the sludge transfer direction D. Therefore, as the sludge introduced from the inlet 2 is transported through this transfer space by the first screw blade 3B, the sludge is compressed and dewatered. The filtrate that has passed through the screen (perforated plate) of the screen casing 1 is collected by a filtrate receiver 38 located below the screen casing 1. A drain 39 is connected to the filtrate receiver 38, and the filtrate collected by the filtrate receiver 38 is discharged from the screw press 100 via the drain 39.

[0041] The space through which sludge is transported in the plug formation region 1B is formed by the inner surface of the screen casing 1, the second screw blades 4B, and the second screw shaft 4A. As shown in Figure 2, the cross-sectional area of ​​this transport space is constant. In the plug formation region 1B, a plug cake is formed by the sludge (i.e., cake) dewatered in the dewatering region 1A.

[0042] The cake within the plug-forming region 1B is compressed by the second screw blades 4B, which obstruct its movement, resulting in a cake with a low water content. This low water content cake forms a plug cake that obstructs the movement of subsequent cakes. The plug cake formed around the second screw shaft 4A applies back pressure to the subsequent cakes, further compressing them. The liquid separated from the plug cake in the plug-forming region 1B is collected by the filtrate receiver 38 and discharged from the screw press 100 via the drain 39.

[0043] After the plug cake is formed, the control device 6 drives the second rotation mechanism 20 to rotate the second screw 4 (or operates the second rotation mechanism 20 to increase the rotational speed of the second screw 4). By rotating the second screw 4 in the opposite direction to the rotational direction of the first screw 3, the plug cake is gradually sent to the discharge chamber 33 (i.e., discharged). In this way, the formation and discharge of the plug cake are performed continuously, so the screw press 100 can be operated with a plug cake always present in the plug formation region 1B.

[0044] In conventional screw presses, if the cylindrically compressed plug cake hardens too much, this hardened plug cake can clog the screen casing, preventing it from being discharged. Furthermore, there is a risk that this hardened plug cake may rotate together with the screw. As a result, the screw press cannot continue to operate.

[0045] In addition to the plug cake rotating together with the screw, there are risks associated with changes in sludge concentration, such as an increase in the moisture content of the sludge introduced into the screen casing 1 and an increase in the hopper level (i.e., the height of the sludge introduced into the inlet 2). Therefore, the following describes a configuration that reduces these risks.

[0046] The screw press 100 is equipped with a level sensor 50 that detects a signal corresponding to the hopper level. The control device 6 is electrically connected to the level sensor 50 and is configured to measure the hopper level based on the signal detected by the level sensor 50, and to independently control the rotational speed of the first screw 3 and the rotational speed of the second screw 4 according to the measured hopper level.

[0047] In the embodiment shown in Figure 2, the level sensor 50 is a non-contact measuring instrument (e.g., a laser rangefinder). Such a level sensor 50 can detect the height of the sludge (i.e., the signal) at a position lower than the first screw blade 3B, and as a result, the control device 6 can measure a wide range of hopper levels. In one embodiment, the level sensor 50 may be a contact measuring instrument (e.g., a guide pulse, an electrode rod).

[0048] Figures 3(a) and 3(b) show a non-contact type level sensor. As shown in Figures 3(a) and 3(b), the level sensor 50 may not be able to accurately detect a signal corresponding to the hopper level due to the influence of sludge adhering to the inner wall 2a of the inlet 2. Therefore, it is desirable to position the level sensor 50 at a distance from the inner wall 2a of the inlet 2.

[0049] The control device 6 is configured to control the rotational speeds of the first screw 3 and the second screw 4 based on a stepwise change method that gradually changes the rotational speeds of the first screw 3 and the second screw 4 according to the hopper level, and a steep (accelerated) change method that rapidly (i.e., acceleratingly) changes the rotational speeds of the first screw 3 and the second screw 4 according to the hopper level.

[0050] In both the step-change method and the acceleration-change method, the control device 6 can set (divide) the hopper level into multiple levels. In this embodiment, the control device 6 sets the hopper level to four levels (H, MH, ML, L). Level H is the highest level. Level L is the lowest level. Levels MH and ML are levels between level H and level L, with level MH being higher than level ML. In one embodiment, the control device 6 may set level HH, which is higher than level H, and if the hopper level reaches level HH, it may determine that there is a malfunction in the screw press 100 and stop the operation of all equipment (i.e., the entire dewatering system 200).

[0051] When the hopper level reaches level H, the supply of sludge to the screw press 100 is stopped, and the sludge dewatering operation by the screw press 100 continues. With this configuration, the sludge in the inlet 2 is gradually discharged without any new sludge being supplied to the inlet 2. As a result, the hopper level gradually decreases. When the hopper level reaches a level below level H (for example, level MH), the sludge supply stoppage is released. Note that the level that triggers the release of the sludge supply stoppage is not limited to level MH, and a new dedicated level can be set.

[0052] Here, stopping the sludge supply means stopping the operation of the sludge supply pump 202, stopping the operation of the polymer supply pump 206, and stopping the operation of the agitator 203a of the coagulation tank 203. In this case, the operation of the thickener 204 may be stopped or continued.

[0053] If the sludge supply is stopped and the sludge is dewatered by the screw press 100, but the hopper level does not decrease, it is possible that the cake is rotating together with the first screw 3 (and / or the second screw 4).

[0054] Therefore, if the hopper level does not decrease for a predetermined time after the hopper level reaches level H, the control device 6 may determine that the above-mentioned co-rotation is occurring and temporarily increase the rotational speed of the second screw 4 (and / or the first screw 3) above the current rotational speed. With this configuration, the cake can be discharged quickly and the above-mentioned co-rotation can be eliminated.

[0055] In one embodiment, the control device 6 may temporarily increase the rotational speed of the second screw 4 to approximately the same as, or greater than, the rotational speed of the first screw 3 (rotational speed of the second screw 4 ≥ rotational speed of the first screw 3). For example, the rotational speed of the first screw 3 may be increased while the rotational speed of the second screw 4 is also increased, but preferably, the rotational speed of the first screw 3 is maintained while only the rotational speed of the second screw 4 is increased.

[0056] When the hopper level reaches level L, the sludge discharge operation by the screw press 100 is stopped, and the supply of sludge to the screw press 100 continues. With this configuration, new sludge is supplied to the inlet 2, and the sludge in the inlet 2 gradually accumulates. As a result, the hopper level gradually rises. When the hopper level reaches a level above level L (for example, level ML), the sludge discharge operation is resumed. Note that the level that triggers the resumption of the sludge discharge operation is not limited to level ML, and a new dedicated level can be set.

[0057] Figure 4 is a graph showing one embodiment of the operating state of a screw press using a step-change method. In Figure 4, the horizontal axis represents time, and the vertical axis represents the hopper level. The step-change method is an operating method in which the operation of the first screw 3 and the second screw 4 is switched after the hopper level reaches a predetermined level and a predetermined time has elapsed.

[0058] As shown in Figure 4, for example, if the hopper level between level MH and level ML drops to level ML, the control device 6 reduces the rotational speed of the first screw 3 from the high setting value R1-H (i.e., the first rotational speed) to the low setting value R1-L (i.e., the second rotational speed). Subsequently, the control device 6 reduces the rotational speed of the second screw 4 from the high setting value R2-H (i.e., the first rotational speed) to the low setting value R2-L (i.e., the second rotational speed). Note that the first rotational speed is a higher rotational speed than the second rotational speed.

[0059] When the control device 6 changes the rotational speed of the first screw 3 and the second screw 4 to lower rotational speeds, it changes the rotational speed of the first screw 3, and after a predetermined waiting time has elapsed, it changes the rotational speed of the second screw 4. This sequence makes it possible to reduce the speed difference between the first screw 3 and the second screw 4, thereby reducing the back pressure applied to the cake in the plug forming region 1B.

[0060] In one embodiment, the predetermined waiting time corresponds to the setting change time of the inverter of the second drive unit 24 (for example, about 5 seconds). In another embodiment, the waiting time may correspond to the time of one rotation of the second screw 4. By creating a waiting time, the cake in the switching portion between the first screw 3 and the second screw 4 is sufficiently replaced. As a result, the risk of co-rotation caused by the formation of an extremely hard cake can be reduced.

[0061] If the waiting time is too long, the water content of the sludge will increase. Therefore, in one embodiment, the waiting time may be the time equivalent to one rotation of the first screw 3, which has a rotational speed greater than that of the second screw 4. In another embodiment, the waiting time may be the time equivalent to 0.5 rotations to 1 rotation of the first screw 3.

[0062] As shown in Figure 4, if the rotational speed of the first screw 3 is changed to a low setting value R1-L, and the rotational speed of the second screw 4 is changed to a low setting value R2-L, and then the hopper level drops to level L, the control device 6 stops the operation of the screw press 100 (more specifically, screws 3 and 4). Stopping the operation of the screw press 100 corresponds to the deceleration of the first screw 3 and the second screw 4. Therefore, in this case, it is desirable for the control device 6 to stop the operation of the first screw 3, and after a predetermined waiting time has elapsed, to stop the operation of the second screw 4. In one embodiment, the waiting time may include 0 seconds. In this case, the control device 6 stops the operation of the first screw 3 and the second screw 4 simultaneously.

[0063] When the screw press 100 is stopped, sludge accumulates in the inlet 2, causing the hopper level to rise. When the hopper level reaches level ML, the control device 6 sets the rotation speed of screws 3 and 4 to a low set value (R1-L, R2-L) and restarts the operation of the screw press 100.

[0064] In this case, the control device 6 starts operating the second screw 4, and after a predetermined waiting time has elapsed, starts operating the first screw 3. This sequence makes it possible to reduce the speed difference between the first screw 3 and the second screw 4, and to reduce the back pressure applied to the cake in the plug-forming region 1B. The waiting time may be the same as the waiting time described above.

[0065] If the hopper level rises to level MH after the screw press 100 resumes operation, the rotation speed of the second screw 4 is changed from the low setting value R2-L to the high setting value R2-H. After a predetermined waiting time has elapsed, the rotation speed of the first screw 3 is changed from the low setting value R1-L to the high setting value R1-H. If the hopper level still reaches level H after this change, the control device 6 decides to stop the sludge supply.

[0066] The control device 6 continues to operate the screw press 100 while stopping the sludge supply, causing the hopper level to decrease. When the hopper level drops to level MH, the control device 6 releases the sludge supply stop. More specifically, the control device 6 restarts the operation of the sludge supply pump 202, the polymer supply pump 206, and the agitator 203a of the coagulation tank 203. In one embodiment, the hopper level at which the sludge supply stop is released is not limited to level MH, but may be any level.

[0067] In the embodiment shown in Figure 4, the screw press 100 is operated between a low setting and a stop. In this embodiment, the control device 6 is configured to change the rotational speed of the first screw 3 and the rotational speed of the second screw 4 in two setting values ​​(i.e., a low setting and a high setting), but it may also be changed in three or more setting values. The setting value at the start of operation of the screw press 100 may be set to any value.

[0068] Figure 5 is a graph showing another embodiment of the operating state of a screw press using a step-change method. In Figure 5, the horizontal axis represents time, and the vertical axis represents the hopper level. In the embodiment shown in Figure 5, when the hopper level drops to level ML, the control device 6 reduces the rotational speed of the first screw 3 from a high setting value R1-H to a low setting value R1-L, and then reduces the rotational speed of the second screw 4 from a high setting value R2-H to a low setting value R2-L.

[0069] Subsequently, when the hopper level rises to level MH, the control device 6 increases the rotational speed of the second screw 4 from the low setting value R2-L to the high setting value R2-H, and then increases the rotational speed of the first screw 3 from the low setting value R1-L to the high setting value R1-H. Thus, in the embodiment shown in Figure 5, the screw press 100 is operated between the high setting value and the low setting value.

[0070] Figure 6 shows the control flow of the control device in the step-change method. As shown in Figure 6, the control device 6 starts control using the step-change method when the hopper level is between level MH and level ML. First, the control device 6 sets the rotational speed of screws 3 and 4 to a low set value (see step S101).

[0071] The control device 6 determines whether the hopper level is below level L (see step S102), and if the hopper level is below level L (see "Yes" in step S102), it stops the operation of screws 3 and 4 (see step S103). Subsequently, sludge accumulates in the inlet 2, and the control device 6 determines whether the hopper level is below level MH (see step S104). If the hopper level is below level MH (see "Yes" in step S104), the control device 6 executes step S101 again. If the hopper level is not below level MH (see "No" in step S104), the control device 6 continues to stop the operation of screws 3 and 4.

[0072] If the hopper level is not below level L (see "No" in step S102), the control device 6 determines whether the hopper level is above level MH (see step S105). If the hopper level is not above level MH (see "No" in step S105), the control device 6 continues to step S101.

[0073] If the hopper level is MH or higher (see "Yes" in step S105), the control device 6 changes the rotation speed of screws 3 and 4 from a low setting to a high setting (see step S106). Then, the control device 6 determines whether the hopper level is H or higher (see step S107), and if the hopper level is H or higher (see "Yes" in step S107), it stops the sludge supply (see step S108). At this time, the control device 6 continues the operation of screws 3 and 4.

[0074] The control device 6 continues to operate the screws 3 and 4 and determines for a predetermined time whether the hopper level is at or above level H (see step S109). If the hopper level remains at or above level H after the predetermined time has elapsed (see "Yes" in step S109), the control device 6 determines that co-rotation is occurring and executes a sludge discharge operation (see step S110). More specifically, the control device 6 temporarily increases the rotational speed of the second screw 4 (and / or the first screw 3) above its current rotational speed. In this embodiment, the control device 6 changes the rotational speed of the second screw 4 (and / or the first screw 3) to a setting higher than the high setting value and quickly discharges the sludge that is causing the co-rotation.

[0075] If the hopper level is not level H or higher (see "No" in step S109), the control device 6 determines whether the hopper level is level MH or lower (see step S111). If the hopper level is level MH or lower (see "Yes" in step S111), the control device 6 executes step S106. If the hopper level is not level MH or lower (see "No" in step S111), the control device 6 executes step S108.

[0076] In step S107, if the hopper level is not level H or higher (see "No" in step S107), the control device 6 determines whether the hopper level is level ML or lower (see step S112). If the hopper level is level ML or lower (see "Yes" in step S112), the control device 6 executes step S101. If the hopper level is not level ML or lower (see "No" in step S112), the control device 6 executes step S106.

[0077] In this embodiment, the control device 6 is configured to control the rotational speed of the screws 3 and 4 according to the hopper level. In one embodiment, the control device 6 may set high and low set values ​​for the sludge flow rate, polymer flow rate, rotational speed of the agitator 203a of the coagulation tank 203, and rotational speed of the concentrator 204 according to the hopper level.

[0078] In this embodiment, the control device 6 has a high setting value, a low setting value, and a setting value corresponding to stopping the screws 3 and 4, but it is not necessarily required to have all of the setting values. In one embodiment, the control device 6 may have some of the setting values ​​among the high setting value, the low setting value, and the setting value corresponding to stopping the screws 3 and 4.

[0079] Figure 7 is a graph showing one embodiment of the operating state of a screw press using an accelerated change method. Figure 8 is a graph showing another embodiment of the operating state of a screw press using an accelerated change method. In both Figure 7 and Figure 8, the horizontal axis represents time, and the vertical axis represents the hopper level. Figure 7 shows the change in hopper level when the change in sludge concentration is extremely large, and Figure 8 shows the change in hopper level when the change in sludge concentration is relatively small.

[0080] The acceleration change method is an operating method in which, when the hopper level is within a predetermined range, the operation of the first screw 3 and the second screw 4 is changed after a predetermined time has elapsed. When the hopper level is located between levels MH and ML, the set values ​​of the rotational speeds of screws 3 and 4 are not changed, and the operation of screws 3 and 4 continues as is. When the hopper level reaches a level between levels ML and L, the control device 6 reduces the rotational speed of screws 3 and 4 based on a predetermined change unit after a predetermined time has elapsed. In one embodiment, the change unit may be a predetermined numerical value, or it may be a relational expression (e.g., calibration curve, change table) created based on past operating results of the screw press 100.

[0081] The following describes the case where the unit of change is a numerical value. In the acceleration change method as well, in order to reduce the back pressure applied to the cake in the plug formation region 1B, the sequence of changes in the rotational speed of screws 3 and 4 is the same as in the embodiment described above (step change method).

[0082] After the hopper level reaches a level between level ML and level L, and a predetermined time has elapsed, the control device 6 rotates the first screw 3 at a target setting value ((R1-0)-(R1-D)) obtained by subtracting the reduction setting value R1-D from the current setting value R1-0, and then rotates the second screw 4 at a target setting value ((R2-0)-(R2-D)) obtained by subtracting the reduction setting value R2-D from the current setting value R2-0.

[0083] After a predetermined time has elapsed, the control device 6 rotates the first screw 3 at a target setting value ((R1-0)-(R1-D)×2), which is a further modification of the current setting value ((R1-0)-(R1-D)) for the rotational speed of the first screw 3, and then rotates the second screw 4 at a target setting value ((R2-0)-(R2-D)×2), which is a further modification of the current setting value ((R2-0)-(R2-D)) for the rotational speed of the second screw 4.

[0084] In this manner, the control device 6 rapidly (i.e., accelerates) changes (declines) the rotational speeds of screws 3 and 4 over time. The control device 6 repeats this accelerated change until the hopper level falls outside the range between level ML and level L. In this embodiment, when the rotational speeds of screws 3 and 4 are accelerated, the control device 6 subtracts a multiple (n times) of the reduction setting value from the current set value over time, but the method for accelerating the reduction of the rotational speeds of screws 3 and 4 is not particularly limited.

[0085] If the hopper level drops to level L, the control device 6 stops the operation of screws 3 and 4. As a result, sludge accumulates in the inlet 2, causing the hopper level to rise. When the hopper level reaches level ML, the control device 6 restarts the operation of screws 3 and 4 at the rotational speed just before they stopped. Even in this case, the control device 6 first starts the operation of the second screw 4, and then starts the operation of the first screw 3. In one embodiment, the hopper level that acts as a trigger to release the stop of the operation of screws 3 and 4 is not limited to level ML, but may be any level above level L, and a new set value may be set for restarting operation.

[0086] If the hopper level is located between level H and level MH, the control device 6 changes the rotation speed set values ​​of screws 3 and 4 to higher set values ​​based on a predetermined change unit after a predetermined time has elapsed. For example, the control device 6 rotates the second screw 4 at a target set value ((R2-0)+(R2-U)) obtained by adding the acceleration set value R2-U to the current set value R2-0, and then rotates the first screw 3 at a target set value ((R1-0)+(R1-U)) obtained by adding the acceleration set value R1-U to the current set value R1-0.

[0087] After a predetermined time has elapsed, the control device 6 rotates the second screw 4 at a target setting value ((R2-0)+(R2-U)×2), which is a further modification of the current setting value ((R2-0)+(R2-U)) for the rotational speed of the second screw 4, and then rotates the first screw 3 at a target setting value ((R1-0)+(R1-U)×2), which is a further modification of the target setting value ((R1-0)+(R1-U)) for the rotational speed of the first screw 3.

[0088] In this manner, the control device 6 accelerates (increases) the rotational speed of screws 3 and 4 over time. The control device 6 repeats this accelerated change until the hopper level falls outside the range between level H and level MH. In this embodiment, when accelerating the rotational speed of screws 3 and 4, the control device 6 adds a multiple (n times) of the acceleration setting value to the current setting value over time, but the method for accelerating the rotational speed of screws 3 and 4 is not particularly limited.

[0089] When the hopper level rises to level H, the control device 6 decides to stop the sludge supply. The control device 6 continues to operate the screw press 100 while stopping the sludge supply, so the hopper level decreases. When the hopper level drops to level MH, the control device 6 releases the sludge supply stop and drives the sludge supply pump 202 and polymer supply pump 206 to resume the supply of sludge and polymer. In one embodiment, the hopper level that acts as a trigger for releasing the sludge supply stop is not limited to level MH, but may be any level below level MH.

[0090] In this embodiment, the control device 6 is configured to control the rotational speed of the screws 3 and 4 according to the hopper level. However, in one embodiment, the control device 6 may set set values ​​(i.e., deceleration set value, acceleration set value) related to the sludge flow rate, polymer flow rate, rotational speed of the agitator 203a of the coagulation tank 203, and rotational speed of the concentrator 204.

[0091] In one embodiment, the control device 6 may have some of the following set values: a set value corresponding to the continuous operation of screws 3 and 4, an acceleration set value, a deceleration set value, and a set value corresponding to the stopping of screws 3 and 4.

[0092] Figure 9 shows the control flow of the control device in the accelerated change method. As shown in Figure 9, the control device 6 starts control using the accelerated change method when the hopper level is between levels MH and ML. First, the control device 6 continues to receive signals from the level sensor 50 while the screws 3 and 4 continue to operate, that is, while doing nothing (see step S201). Then, the control device 6 determines whether the hopper level has reached level L (see step S202), and if the hopper level has reached level L (see "Yes" in step S202), it stops the operation of screws 3 and 4 (see step S203).

[0093] Subsequently, the control device 6 determines whether the hopper level has reached level ML (see step S204). If the hopper level has reached level ML (see "Yes" in step S204), it releases the shutdown of screws 3 and 4 (see step S205) and executes step S201. If the hopper level has not reached level ML (see "No" in step S204), it continues in step S204.

[0094] If the hopper level has not reached level L (see "No" in step S202), the control device 6 determines whether the hopper level is between level ML and level L (see step S206). If the hopper level is between level ML and level L (see "Yes" in step S206), the control device 6 accelerates the deceleration of screws 3 and 4 (see step S207) and continues operation (see step S201).

[0095] If the hopper level is not between levels ML and L (see "No" in step S206), the control device 6 determines whether the hopper level is at or above level H (see step S208). If the hopper level is at or above level H (see "Yes" in step S208), the control device 6 stops the sludge supply (see step S209). After that, the control device 6 determines whether the hopper level is at or above level H even after a predetermined time has elapsed (see step S210).

[0096] If the hopper level remains at or above level H after a predetermined time has elapsed (see "Yes" in step S210), the control device 6 determines that co-rotation is occurring and executes a sludge discharge operation (see step S211). If the hopper level is not at or above level H after a predetermined time has elapsed (see "No" in step S210), the control device 6 determines whether or not the hopper level has reached level MH (see step S212). If the hopper level has not reached level MH (see "No" in step S212), the control device 6 executes step S209.

[0097] If the hopper level reaches level MH (see "Yes" in step S212), the control device 6 releases the sludge supply stop (see step S213) and continues the operation of screws 3 and 4 (see step S201).

[0098] If the hopper level is not at or above level H (see "No" in step S208), the control device 6 determines whether the hopper level is between level MH and level H (see step S214). If the hopper level is between level MH and level H (see "Yes" in step S214), the control device 6 accelerates (increases speed) the screws 3 and 4 (see step S215) and continues operation (see step S201). If the hopper level is not between level MH and level H (see "No" in step S214), the control device 6 executes step S202.

[0099] In the embodiments described above, the control device 6 is configured to change the rotational speed of the screws 3 and 4 according to the hopper level in both a step-change method and an acceleration-change method. However, in one embodiment, the control device 6 may be configured to change the sludge supply amount according to the hopper level. More specifically, the control device 6 may operate the devices included in the components of the dewatering system 200 (sludge supply pump 202, polymer supply pump 206, rotational speed of the agitator 203a of the coagulation tank 203, and rotational speed of the concentrator 204) according to the hopper level.

[0100] When reducing the sludge supply, the control device 6 operates the sludge supply pump 202 and the polymer supply pump 206 to reduce the sludge and polymer supply, and then reduces the rotational speed of the agitator 203a of the coagulation tank 203 and the rotational speed of the thickener 204. On the other hand, when increasing the sludge supply, the control device 6 increases the rotational speed of the thickener 204, then increases the rotational speed of the agitator 203a of the coagulation tank 203, and then operates the sludge supply pump 202 and the polymer supply pump 206 to reduce the sludge and polymer supply. This operating method prevents a temporary increase in the sludge supply to the thickener 204 and enables stable operation of the thickener 204. The waiting time may be the same as in the embodiment described above.

[0101] In one embodiment, the control device 6 may employ a combination of a step-change method and an acceleration-change method. For example, the control device 6 may alternately execute the step-change method and the acceleration-change method according to the hopper level to change the rotational speed of the screws 3 and 4.

[0102] The embodiments described above are intended to enable persons with ordinary skill in the art to implement the present invention. Various modifications of the above embodiments can be made naturally by those skilled in the art, and the technical idea of ​​the present invention can be applied to other embodiments as well. Therefore, the present invention is not limited to the embodiments described, but is to be interpreted in the broadest sense according to the technical idea defined by the claims. [Explanation of symbols]

[0103] 1. Screen casing 2 Inlet 3. First Screw 3A First screw shaft 3B First screw blade 4. Second Screw 4A Second screw shaft 4B Second screw blade 6. Control device 7. First Rotation Mechanism 8 Blocking wall 10 Water sealing device 11,12 Bearings 14. First drive unit 15, 16 sprocket 17 chain 20. Second Rotation Mechanism 22,23 Bearings 24. Second drive unit 25, 26 sprocket 27 chain 33 Discharge Chamber 38 Filtrate receiver 39 Drain 50-level sensor 100 Dehydration equipment (screw press) 200 Dehydration System 201 Sludge Storage Tank 202 Sludge supply pump 203 Coagulation tank 203a Stirrer 204 Concentrator 205 Polymer dissolution tank 206 Polymer supply pump

Claims

1. A method for operating a coaxial dewatering apparatus comprising a first screw arranged in a dewatering region and a second screw arranged in a plug forming region, The height of the sludge introduced into the sludge inlet is measured. The rotational speed of the first screw and the rotational speed of the second screw are controlled independently of each other according to the measured sludge height. When changing the rotational speed of the first screw and the rotational speed of the second screw to a higher rotational speed, the rotational speed of the second screw is changed first, and then the rotational speed of the first screw is changed. A method for operating a dewatering apparatus, wherein when changing the rotational speed of the first screw and the rotational speed of the second screw to a lower rotational speed, the rotational speed of the first screw is changed, and simultaneously or afterward, the rotational speed of the second screw is changed.

2. A method for operating a dewatering apparatus according to claim 1, wherein the rotation speed of the first screw and the rotation speed of the second screw are controlled in a stepwise manner, in which the rotation speed of the first screw and the rotation speed of the second screw are changed in steps according to the height of the sludge.

3. A method for operating a dewatering apparatus according to claim 1, wherein the rotation speed of the first screw and the rotation speed of the second screw are controlled in a steep change manner, in which the rotation speed of the first screw and the rotation speed of the second screw are changed rapidly according to the height of the sludge.

4. A control method for independently controlling the rotational speed of the first screw and the rotational speed of the second screw is: A step-by-step change method is used, in which the rotation speed of the first screw and the rotation speed of the second screw are changed in steps according to the height of the sludge. A method for operating a dewatering apparatus according to claim 1, comprising a steep change method for rapidly changing the rotational speed of the first screw and the rotational speed of the second screw according to the height of the sludge.

5. A first screw located in the dewatering region and a second screw located in the plug forming region, whose rotational speeds can be controlled independently of each other, A level sensor that detects a signal corresponding to the height of the sludge introduced into the sludge inlet, The system includes a control device that measures the height of the sludge based on a signal detected by the level sensor and independently controls the rotational speed of the first screw and the rotational speed of the second screw according to the measured height of the sludge. The control device is When changing the rotational speed of the first screw and the rotational speed of the second screw to a higher rotational speed, the rotational speed of the second screw is changed first, and then the rotational speed of the first screw is changed. A dewatering apparatus that, when changing the rotational speed of the first screw and the rotational speed of the second screw to a lower rotational speed, changes the rotational speed of the first screw, and simultaneously or afterward changes the rotational speed of the second screw.

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