JP2008221060A - Control method and control apparatus for constant cake water content in screw press - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a control method and a control apparatus for a constant cake water content in a screw press where the water content in a dehydrated cake is made constant. <P>SOLUTION: The standard water content (WO) of a dehydrated cake, the frequency (N) of a screw shaft (7), the chemical feed rate (α) of a flocculant or the like are beforehand set, and, in the case the water content (W) of the dehydrated cake measured with a water content gauge (16) arranged at the cake exhaust part (12) of a screw press (1) is deviated from the standard water content (WO), the reduction of the chemical feed rate (α) is preferentially considered, and the frequency (N) of the screw shaft (7) and the chemical feed rate (α) are controlled, thus the cake water content can be constantly controlled while reducing the amount of the chemical to be used corresponding to the variation in sludge properties on real time, and further, the management of a treatment process after the dehydration is made easy. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an operation control method for a screw press, and in particular, to control the water content of a cake discharged from the terminal side of an outer cylinder screen at a constant level, the rotation of a screw shaft is controlled and polymer agglomeration that agglomerates sludge. The present invention relates to an operation control method and an operation control device for a screw press that adjusts a drug injection rate of the agent.

  Conventionally, dewatering treatment of sewage sludge at many treatment plants uses a dehydrator such as a belt press, a centrifugal dehydrator, a screw press, etc., that dehydrates sludge by conditioning with a polymer flocculant. When operating these dehydrators, the amount of sludge treated, the polymer flocculant injection rate, and the moisture content of the cake are the main control items. The cake moisture content depends on the sludge properties and the tempering of the polymer flocculant. Affected by the condition. In addition, the dehydrated cake is processed as incineration or compost, but it is desirable to keep the water content of the dehydrated cake constant for the operation management of these facilities. Incineration disposal affects fuel consumption, and landfill disposal affects transportation costs and landfill capacity. Therefore, it is desired that the dehydrated cake is discharged at a uniform predetermined moisture content with a moisture content as low as possible.

As an operation control method for this type of screw press, for example, as shown in Patent Document 1, a part of the discharged dewatered cake is pumped to a flat plate-shaped measuring die, and dewatered through the measuring die. The moisture content of the cake is measured with a microwave transmission moisture meter, and compared with the preset data, and the cake moisture content is adjusted to adjust the sludge flocculant addition rate and to control the rotation of the screw shaft. A method for controlling the operation of the dehydrator is described.
JP 2003-177105 A

  As a method for measuring the moisture content of a dehydrated cake using a microwave described in Patent Document 1, the microwave freely travels straight through space, but reflects on a metal surface and propagates while being attenuated in the dielectric. It uses the characteristic of doing. And even among dielectrics, water is a substance that exhibits a particularly high dielectric constant, and utilizes the principle that the phase difference between microwave transmission and reception is clearly indicated by the amount of moisture in the dehydrated cake, and dehydrated in real time. The moisture content of the cake can be measured.

  However, in the configuration described in Patent Document 1, it is necessary to sample a part of the dewatered cake discharged from the dehydrator and transfer it to the measuring device with a pressure pump. For this reason, a separate pump is required, and extra running costs are required. Moreover, in a measuring apparatus with a small opening area, there exists a possibility that a dewatering cake may be clogged when it stops for a long time, and there exists a subject that the removal work of a dewatering cake is required.

  The cake moisture content constant control method according to the present invention includes a screw shaft in which screw blades are wound around an outer cylinder screen having a filtration surface on the periphery, and the sludge supplied to the start end portion of the outer cylinder screen is supplied to the screw shaft. The screw that separates the filtrate from the outer cylinder screen while rotating and removes the dehydrated cake from the terminal part of the outer cylinder screen, and controls the pressure of sludge supplied to the starting end of the outer cylinder screen to be constant. In the press, measure the moisture content of the dehydrated cake, and if the measured moisture content of the dehydrated cake falls outside the preset standard moisture content range, increase or decrease the rotational speed of the screw shaft or the chemical injection rate of the flocculant. Thus, the moisture content of the dehydrated cake is controlled to return to the range of the standard moisture content. Specifically, when the moisture content of the dehydrated cake is larger than the reference moisture content, the screw shaft rotation number is decreased, and when the moisture content of the dehydrated cake is smaller than the reference moisture content, the chemical injection rate of the flocculant is decreased. The moisture content of the dehydrated cake is controlled to return to the standard moisture content range, so that the moisture content of the dehydrated cake should be kept constant with priority given to reducing the amount of chemicals used in response to changes in sludge properties in real time. Can do.

  Also, when the lower limit value of the screw shaft rotation speed is set in advance and the cake moisture content does not recover to the reference moisture content by controlling the screw shaft rotation speed, the flocculant drug injection rate is increased. Thus, the moisture content of the dehydrated cake is controlled so as to return to the range of the reference moisture content, so that the operation is not performed at the rotational speed of the screw shaft that hinders normal operation. In addition, when the upper limit value of the chemical injection rate is set in advance and the cake moisture content does not recover to the reference moisture content and reaches the upper limit value by controlling the chemical injection rate, an alarm is given and the chemical injection rate of the flocculant is set. Resetting and controlling the moisture content of the dehydrated cake to return to the range of the standard moisture content, so even if the sludge properties fluctuate significantly, abnormalities can be detected, and the moisture content of the dehydrated cake is kept constant with an appropriate amount of chemical used Can be maintained.

  In addition, when the lower limit value of the flocculant chemical injection rate is set in advance and the cake moisture content does not recover to the reference moisture content by controlling the flocculant chemical injection rate, the rotational speed of the screw shaft is reduced. Since the moisture content of the dehydrated cake is controlled so that it returns to the standard moisture content range, the moisture content of the dehydrated cake can be kept constant by controlling the combination of the two parameters with priority given to reducing the amount of chemicals used. it can. In addition, when the upper limit value of the rotational speed of the screw shaft is set in advance and the cake moisture content does not recover to the reference moisture content by controlling the rotational speed of the screw shaft, an alarm is given and the screw shaft Since the rotation speed is reset and the moisture content of the dehydrated cake is controlled to return to the standard moisture content range, abnormalities can be detected even if the sludge properties fluctuate significantly, and the dehydrated cake can be The driving | running which keeps a rate constant can be performed.

  The apparatus for carrying out constant control of the moisture content of the cake is provided with a screw shaft in which screw blades are wound around an outer cylinder screen having a filtration surface on the periphery, and the sludge supplied to the starting end of the outer cylinder screen, When the screw shaft is rotated, the filtrate is separated from the outer cylinder screen, and when the dewatered cake is taken out from the terminal part of the outer cylinder screen, the pressure of the sludge supplied to the starting end of the outer cylinder screen is controlled to be constant. In the screw press, the moisture content meter that measures the moisture content of the dehydrated cake at the cake discharge part of the screw press, and the data detected by the moisture content meter are received and compared with the preset reference moisture content of the dehydrated cake. A discriminator for discriminating, a rotation indicator that receives a command signal from the discriminator and increases or decreases the number of rotations of the screw shaft, and a command signal from the discriminator to increase or decrease the drug injection rate of the coagulant The chemical liquid injection controller and the alarm indicator that receives the command signal of the discriminator and issues an alarm are provided, and the cake moisture content is always within the range of the reference moisture content. Therefore, it becomes easy to manage the treatment process after dehydration.

  A microwave moisture meter is arranged in the cake discharge part of the screw press, and a part of the compacted cake in the cake discharge part is continuously connected between a pair of transmitting and receiving antennas provided facing the microwave moisture meter. If the cake moisture content is directly and continuously measured, piping, equipment, power, etc. for extracting and measuring the dehydrated cake separately become unnecessary, and the cake moisture content according to the dehydration situation in real time Can be controlled.

  The present invention is configured as described above, and with respect to the fluctuation of the moisture content of the dehydrated cake, for the two parameters of the screw shaft rotation speed and the drug injection rate, it is preferentially controlled to reduce the drug injection rate. The moisture content of the dehydrated cake can be kept constant, and the amount of chemicals used can be reduced in response to fluctuations in sludge properties. Then, when processing as incineration or composting after dehydration, the amount of incinerated fuel used can be reduced and managed, or composting time and product quality control can be facilitated. In addition, a microwave moisture meter is installed in the discharge part of the screw press, and the moisture content of the dehydrated cake transferred to the discharge side in the screw press is measured directly and continuously. In addition to eliminating the need for piping, equipment, power, etc. for extracting and measuring the cake, it is possible to control the moisture content of the cake according to the dewatering situation in real time.

  The embodiment of the present invention will be described in detail with reference to the drawings. First, in FIG. 1, reference numeral 1 denotes a screw press, which is installed on frames 3 and 4 before and after a gantry 2. The screw press 1 is provided with a screw shaft 7 in which a screw blade 6 is wound around an outer cylinder screen 5 having a filtration surface on the periphery. The screw shaft 7 disposed inside the outer cylinder screen 5 increases in diameter in a tapered manner from the start end side to the end end side, and the outer cylinder screen 5 and the screw shaft 7 are spaced relative to each other in the extending direction. It is trying to decrease. A sludge supply pipe 8 is connected to the front end portion of the screw shaft 7, and the supply pipe 8 communicates with a supply hole 9 of the screw shaft 7 opened on the start end side of the outer cylinder screen 5. A screw drive shaft 10 is connected to the rear end of the screw shaft 7, and a drive sprocket 11 is fitted to the screw drive shaft 10. The sprocket 11 is driven by a screw driver 38 (not shown) to rotate the screw shaft 7. The sludge supplied from the supply hole 9 is transferred from the start end side to the end end side by the screw blade 6 and is concentrated and dehydrated while separating the filtrate from the outer cylinder screen 5. The outer cylinder screen 5 may be rotatable as required.

  The dehydrated cake discharge section 12 for discharging sludge (dehydrated cake) immediately after the dehydration process in the screw press 1 to the outside is a tapered cone-shaped presser for applying a back pressure to the discharged dehydrated cake. (Pressing plate) 13 is provided. The presser 13 is provided so as to reciprocate in the axial direction (left and right direction in FIG. 1) by a fluid pressure cylinder 14 such as an air cylinder or a hydraulic cylinder. Therefore, by pressing the presser 13 by the fluid pressure cylinder 14 and adjusting the back pressure acting on the dehydrated cake discharged from the dehydrated cake discharge section 12, the pressure is adjusted according to the dewaterability of the dehydrated cake. Can be adjusted.

  In the screw press 1, the dewatered cake discharge unit 12 is provided with a connecting pipe 15 that can be attached to and detached from the end of the outer cylinder screen 5. The connecting pipe 15 is provided with a microwave moisture meter 16 as a moisture content measuring means for measuring the moisture content of the dehydrated cake discharged from the dehydrated cake discharge unit 12. More specifically, the connection pipe 15 is detachably connected to the outer cylinder screen 5 via a flange, and the attachment portion of the microwave moisture meter 16 with respect to the connection pipe 15 is connected to FIG. In this way, a cylindrical mounting tube 17 that is long in the radial direction perpendicular to the axis of the connection tube 15 is integrally mounted.

  When the microwave transmitted from the transmitter is received by the receiver, the microwave moisture content meter 16 is between the transmission and reception of the microwave if moisture exists between the transmitter and the receiver. A phase difference is generated in the sample, and the moisture content of the dehydrated cake is measured by utilizing the fact that the phase difference and the moisture content are correlated.

  As shown in FIG. 2, the microwave moisture meter 16 includes a cylindrical main body 18 that can be inserted into a mounting tube 17, and a pair of transmission / reception antennas 19 </ b> A and 19 </ b> B is provided at the tip of the main body 18. Antenna support portions 20A and 20B provided so as to face each other are provided so as to protrude. The antenna support portions 20 </ b> A and 20 </ b> B are provided so as to protrude into the connecting tube 15 when the main body 18 is mounted in the mounting tube 17. The opposing directions of the transmitting and receiving antennas 19A and 19B are provided so as to be substantially orthogonal to the moving direction of the dehydrated cake that moves (flows) in the connecting pipe 15. In other words, the dewatering cake is provided so as to move between the pair of transmitting / receiving antennas 19A and 19B facing each other in a direction along the facing surface of the pair of transmitting / receiving antennas 19A and 19B.

  An upper portion of the main body 18 in the microwave moisture meter 16 is integrally provided with a flange 22 that can be joined to a mounting flange 21 provided at the distal end portion of the mounting tube 17. A measurement unit 23 is provided. The mounting flange 21 and the flange 22 can be integrally fixed by a plurality of mounting fixtures 24 such as bolts and nuts.

  FIG. 3 is a schematic configuration diagram of a cake press constant moisture content control device of a screw press, and a stock solution supply pipe 26 for extracting sludge from the sludge storage tank 25 is supplied with a constant flow stock solution supply pump SP, a concentration meter D, and a stock solution flow meter Fs. Is arranged to measure the sludge concentration and the raw sludge flow rate Qs. A chemical solution supply pipe 28 is connected to a polymer dissolution tank 27 provided in the sludge storage tank 25, and a flow rate adjustable polymer supply pump PP for supplying the flocculant to the sludge and addition of the flocculant are added to the chemical solution supply pipe 28. A chemical flow meter Fp for measuring the amount is provided. The chemical liquid supply pipe 28 is connected to the rear of the raw liquid supply pipe 26 provided with the raw liquid flow meter Fs. A concentration meter D and a raw solution flow meter Fs arranged in the raw solution supply pipe 26 and a chemical flow meter Fp arranged in the chemical solution supply pipe 28 are connected to the chemical solution injection controller 29, and the detected concentration of raw sludge and raw solution are detected. The detection signal of the sludge flow rate Qs is transmitted to the chemical solution injection controller 29, the amount of solid matter of the raw sludge flow rate Qs withdrawn from the sludge storage tank 25 at a constant flow rate is calculated, and the detection signal of the addition amount of the polymer flocculant is calculated. The polymer supply pump is sent to the injection controller 29 and compared with the supply amount of the polymer flocculant to be added according to the solid amount of sludge set in advance, so that the addition rate of the predetermined flocculant is obtained. The number of rotations of PP is controlled to supply a polymer flow rate Qp having an addition rate α suitable for the raw sludge flow rate Qs of the stock solution supply pipe 26.

  As shown in FIG. 3, the rear end of the stock solution supply pipe 26 is connected to the tank bottom of the sealed coagulation mixing tank 30, and the sludge to which the polymer flocculant is added is pressed into the coagulation mixing tank 30 from the bottom, Agitation slurry is produced by stirring and mixing with the agitator 31. A slurry supply pipe 32 for extracting the agglomerated slurry to the agglomeration mixing tank 30 with tank pressure is connected to the screw press 1, and the agglomerated slurry is supplied to the supply pipe 8 of the screw press 1 shown in FIG.

  A pressure gauge PSo is provided in the slurry supply pipe 32 for extracting the aggregated slurry from the aggregation mixing tank 30, and a pressure controller 35 for transmitting a supply pressure detection signal is connected to the pressure gauge PSo. Comparing the supply pressure PS of the agglomerated slurry received by the pressure controller 35 with a preset reference pressure PO and transmitting a command signal for the number of revolutions to be corrected to the stock solution supply pump SP to cope with changes in the sludge properties Then, the number of revolutions of the stock solution supply pump SP is controlled so that the stock solution flow rate Q is adjusted so that the supply pressure PS that fluctuates becomes the reference pressure PO.

  The electrical signal of the moisture content W measured by the microwave moisture meter 16 provided in the connecting pipe 15 disposed in the cake discharge section 12 of the screw press 1 is transmitted to the discriminator 36, and the discriminator 36 receives the data and It is determined by calculating a preset reference moisture content WO. The reference moisture content WO is a value (WO = WO ′ ± δ) obtained by adding an allowable value δ to the set reference value WO ′, and has an upper limit value and a lower limit value to give a width. When the measured moisture content W deviates from the reference moisture content WO, when the moisture content W is larger than the reference moisture content WO, the command signal of the discrimination result of the discriminator 36 is transmitted to the rotation indicator 37, and the screw driver The rotational speed N of the screw shaft 7 is decelerated and controlled via 38 so that the cake moisture content W falls within the range of the reference moisture content WO. Further, when the water content W is smaller than the reference water content WO, a command signal of the discrimination result of the discriminator 36 is transmitted to the chemical liquid injection controller 29, and the polymer supply pump is used so as to reduce the chemical injection rate α of the flocculant. The rotational speed of PP is controlled to adjust the cake moisture content W to be within the range of the reference moisture content WO.

  When the cake water content W does not reach the target value only by the deceleration control of the rotational speed N of the screw shaft 7, a signal is transmitted from the discriminator 36 to the chemical liquid injection controller 29 that controls the chemical injection rate α of the flocculant. Then, the rotational speed of the polymer supply pump PP is controlled so as to increase the chemical injection rate α, and the cake moisture content W is adjusted so as to be within the range of the reference moisture content WO. If the cake moisture content W does not reach the target value only by the above-described reduction control of the chemical injection rate α, a signal is transmitted from the discriminator 36 to the rotation indicator 37 that controls the rotational speed N of the screw shaft 7. Then, the rotational speed of the screw driver 38 is controlled so as to increase the rotational speed N of the screw shaft 7 so as to adjust the cake moisture content W to be within the range of the reference moisture content WO.

FIG. 4 is a flowchart of a screw press cake moisture content constant control method,
1: The operation of the screw press 1 is started.
2: Set the reference moisture content WO, the upper limit value Nmax and the lower limit value Nmin of the rotational speed N of the screw shaft 7 and the upper limit value αmax and the lower limit value αmin of the chemical injection rate α of the flocculant. A predetermined polymer flow rate Qp is added (Q = Qs + Qp), and the mixture is stirred and mixed in the agglomeration mixing tank 30, and the agglomerated slurry is supplied to the screw press 1 at the reference pressure PO.
3: The moisture content W of the dehydrated cake of the cake discharger 12 is measured by the microwave moisture meter 16 at each set time, and the detection signal is transmitted to the discriminator 36.
4: The discriminator 36 performs a comparison operation with the moisture content W of the dewatered cake and the preset reference moisture content WO.
5: When the moisture content W is larger than the upper limit value {WO ′ + δ} of the reference moisture content WO (W−WO ′> δ), the rotational speed N of the screw shaft 7 is compared with a preset lower limit value Nmin. .
6: When the rotational speed N of the screw shaft 7 is larger than the lower limit value Nmin (N> Nmin), a command signal for decreasing the rotational speed N of the screw shaft 7 is transmitted to the screw driver 38, and at each set time, the micro A water content meter 16 measures the water content W of the dehydrated cake in the cake discharger 12.
7: When the rotational speed N of the screw shaft 7 is equal to or lower than the lower limit value Nmin (N ≦ Nmin), the drug injection rate α is compared with the preset upper limit value αmax.
8: When the drug injection rate α is smaller than the upper limit αmax (α <αmax), a command signal for increasing the drug injection rate α is transmitted to the polymer supply pump PP, and the microwave moisture content meter 16 is set every set time. Then, the moisture content W of the dehydrated cake in the cake discharger 12 is measured.
9: When the moisture content W is smaller than the lower limit value {WO′−δ} of the reference moisture content WO (WO′−W> δ), the chemical injection rate α is compared with the preset lower limit value αmin.
10: When the chemical injection rate α is larger than the lower limit αmin (α> αmin), a command signal for decreasing the chemical injection rate α is transmitted to the polymer supply pump PP, and the microwave moisture content meter 16 is set every set time. Then, the moisture content W of the dehydrated cake in the cake discharger 12 is measured.
11: When the drug injection rate α is equal to or lower than the lower limit value αmin (α ≦ αmin), the rotational speed N of the screw shaft 7 is compared with the preset upper limit value Nmax.
12: When the rotational speed N of the screw shaft 7 is smaller than the upper limit value Nmax (N <Nmax), a command signal for increasing the rotational speed N of the screw shaft 7 is transmitted to the screw driver 38, and at each set time, micro A water content meter 16 measures the water content W of the dehydrated cake in the cake discharger 12.
The rotational speed N of the screw shaft 7 of the screw press 1 and the rotational speed of the polymer supply pump PP are controlled so that they are always within the set reference value (−δ ≦ W−WO ′ ≦ δ). If the moisture content W of the dehydrated cake does not return to the range of the reference moisture content WO even when the chemical injection rate α reaches the upper limit value αmax or the rotational speed N of the screw shaft 7 reaches the upper limit value Nmax, a command signal is sent to the alarm indicator 39. Is transmitted to generate an alarm that can be confirmed by sound or light (indicator on the lamp or panel), and the operation manager can detect a large variation in sludge properties. Then, review each parameter value of the initial setting that was set in advance and reset it either manually or automatically by the command signal from the alarm indicator, and keep the dehydrated cake moisture content constant at an appropriate chemical usage. It can be carried out.

  The screw press constant moisture content control method and the cake moisture content constant control device according to the present invention are controlled by combining two parameters of the screw shaft rotation speed and the chemical injection rate in response to fluctuations in the moisture content of the dehydrated cake. Therefore, the water content of the dewatering cake of the screw press can be made uniform. That is, it is possible to prevent the moisture content of the dehydrated cake from being changed due to changes in the properties of the sludge, and to keep the moisture content of the dehydrated cake constant in response to fluctuations in the sludge properties of the continuously supplied stock solution. The amount of chemicals used can also be reduced. Since the dehydrated cake is stabilized, the amount of incinerated fuel used can be reduced and managed, or the compost time and product quality can be easily controlled when incineration or composting after dehydration.

It is a vertical side view of the screw press used for the cake moisture content constant control apparatus which concerns on this invention. Similarly, it is sectional explanatory drawing which shows the attachment state of a microwave moisture meter. It is a schematic block diagram of the cake moisture content constant control apparatus of a screw press. Similarly, it is a flowchart of the cake moisture content constant control method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Screw press 5 Outer cylinder screen 6 Screw blade 7 Screw shaft 12 Cake discharge part 16 Microwave moisture content meter 19A Transmission antenna 19B Reception antenna 29 Chemical solution injection controller 36 Discriminator 37 Rotation indicator 39 Alarm indicator SP Stock solution supply pump PP Polymer feed pump W Cake moisture content WO Standard moisture content N Screw shaft rotation speed Nmax Screw shaft rotation speed upper limit value Nmin Screw shaft rotation speed lower limit value α Injection rate αmax Injection rate upper limit value αmin Lower limit of rate

Claims (8)

  A screw shaft (7) in which a screw blade (6) is wound around an outer cylinder screen (5) having a filtration surface on the periphery is provided, and sludge supplied to the starting end of the outer cylinder screen (5) When the filtrate is separated from the outer cylinder screen (5) while rotating (7) and the dehydrated cake is taken out from the terminal end of the outer cylinder screen (5), the filtrate is supplied to the starting end of the outer cylinder screen (5). In the screw press (1) for controlling the pressure of the sludge to be constant, the moisture content (W) of the dewatered cake is measured, and the measured moisture content (W) of the dewatered cake is set to a reference moisture content ( When it is out of the range of (WO), the water content (W) of the dehydrated cake is determined as the reference water content (WO) by increasing or decreasing the rotational speed (N) of the screw shaft (7) or the chemical injection rate (α) of the flocculant. It is controlled to return to the range of Cake moisture content constant control how to.   When the moisture content (W) of the dehydrated cake is larger than the standard moisture content (WO), the rotational speed (N) of the screw shaft (7) is decreased, and the moisture content (W) of the dehydrated cake is the standard moisture content (WO). 2. The method according to claim 1, wherein if smaller, the chemical injection rate (α) of the flocculant is decreased and the water content (W) of the dehydrated cake is controlled to return to the range of the standard water content (WO). Cake moisture content constant control method.   The lower limit (Nmin) of the rotational speed (N) of the screw shaft (7) is set in advance, and the cake water content (W) is set to the reference water content (WO) by controlling the rotational speed (N) of the screw shaft (7). When the lower limit value (Nmin) is reached without recovering until it reaches the lower limit value (Nmin), control is performed to increase the chemical injection rate (α) of the flocculant and return the moisture content (W) of the dehydrated cake to the range of the reference moisture content (WO). The cake water content constant control method according to claim 2, wherein   The lower limit (αmin) of the chemical injection rate (α) of the flocculant is set in advance, and the cake water content (W) does not recover to the reference water content (WO) by controlling the chemical injection rate (α) of the flocculant. When the lower limit (αmin) is reached, the rotational speed (N) of the screw shaft (7) is increased to control the water content (W) of the dehydrated cake to return to the range of the standard water content (WO). The cake moisture content constant control method according to claim 2, wherein the cake moisture content is constant.   The upper limit (αmax) of the chemical injection rate (α) is set in advance, and the cake moisture content (W) does not recover to the reference moisture content (WO) by the control of the chemical injection rate (α), and reaches the upper limit value (αmax). If reached, an alarm is issued and the chemical injection rate (α) of the flocculant is reset, and the moisture content (W) of the dehydrated cake is controlled to return to the range of the standard moisture content (WO). The cake water content constant control method according to claim 3.   An upper limit (Nmax) of the rotational speed (N) of the screw shaft (7) is set in advance, and the cake water content (W) is set to a reference water content (WO) by controlling the rotational speed (N) of the screw shaft (7). When the upper limit value (Nmax) is reached without recovery, the alarm is issued and the rotation speed (N) of the screw shaft (7) is reset, and the moisture content (W) of the dewatered cake is determined as the reference moisture content (WO The cake water content constant control method according to claim 4, wherein control is performed so as to return to a range of   A screw shaft (7) in which a screw blade (6) is wound around an outer cylinder screen (5) having a filtration surface on the periphery is provided, and sludge supplied to the starting end of the outer cylinder screen (5) When the filtrate is separated from the outer cylinder screen (5) while rotating (7) and the dehydrated cake is taken out from the terminal end of the outer cylinder screen (5), the filtrate is supplied to the starting end of the outer cylinder screen (5). In the screw press (1) for controlling the pressure of the sludge to be made constant, a moisture content meter (16) for measuring the moisture content (W) of the dehydrated cake in the cake discharge section (12) of the screw press (1), A discriminator (36) that receives data detected by the moisture content meter (16) and compares it with a preset reference moisture content (WO) of the dehydrated cake and receives a command signal from the discriminator (36). The rotation of the screw shaft (7) A rotation indicator (37) for increasing / decreasing (N), a chemical liquid injection controller (29) for receiving a command signal from the discriminator (36) and increasing / decreasing the drug injection rate (α) of the coagulant, and a discriminator ( 36) A cake moisture content constant control device comprising an alarm indicator (39) which receives the command signal of 36) and issues an alarm.   A microwave moisture meter (16) is disposed in the cake discharge section (12) of the screw press (1), and a pair of transmission / reception antennas (19A, 19B) provided facing the microwave moisture meter (16) The cake moisture content (W) is directly and continuously measured by passing a part of the compacted cake in the cake discharge section (12) continuously between the cakes. The cake moisture content constant control device described.

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