GB2069858A

GB2069858A - Belt pressure filter

Info

Publication number: GB2069858A
Application number: GB8102586A
Authority: GB
Original assignee: Kubota Corp
Current assignee: Kubota Corp
Priority date: 1980-01-30
Filing date: 1981-01-28
Publication date: 1981-09-03
Also published as: CA1157390A; DE3102417C2; GB2069858B; DE3102417A1

Abstract

A belt pressure filter comprises an endless filter belt moving through a gravity dewatering zone where sludge is supplied to the belt followed by a forcible dewatering zone where the sludge is pressed between two belts passing round rollers. Coagulating agent is mixed with the sludge in mixer 22 upstream of the gravity dewatering zone. The thickness of the sludge deposited on the belt in the gravity dewatering zone is continuously monitored and used as a measure of the solids concentration therein, and this figure is processed to control the belt speed and/or the dosage of coagulating agent, in order to produce a filter cake of uniform water content and to avoid wasting the coagulating agent. <IMAGE>

Description

SPECIFICATION Belt pressure filter The present invention relates to a belt pressure filter suited for treatment of a sludge orthe like pro d uced in various types of water treatment facilities.

More specifically, the present invention relates to a belt pressure filter which is capable of an automatic treatment in such a manner that in spite of variation of the sludge characteristics, such as concentration and the amount of organic substance, the water content of a cake as dewatered may be maintained relatively low and constant while dosage of a coagulat ing agent being dosed in a raw solution of a sludge may be maintained optimum and an abnormal con dition, such as poor coagulation can be readily found.

A belt pressure filter is a kind of a dewatering machine often used in a dewatering process and may be classified as a filtration type dewatering machine using a filter belt. Since a belt pressure filter employs a belt press type sludge dewatering system using mesh like filter belts and rolls, a driving power may be small and an increase of the amount of solid matter through injection of an agent is small, while a cake of a small water content can be obtained and therefore attention has been attracted to this system in that the same fully meets the requirement of energy saving.Generally, a belt pressure filter comprises a gravity dewatering zone for dewatering a sludge through the gravity, a roll press dewatering zone for dewatering the sludge by means of a roll press, a compressive dewatering zone for dewatering the sludge by compressive force, and a shearing stress dewatering zone for dewatering the sludge by a shearing stress and is basically constituted of two mesh like filter belts and rolls. This type of belt pressure filter need take into consideration, as the factors being controlled, four factors, i.e. a filter belt traveling speed, a sludge concentration as a typical factor representing sludge characteristics dosage of an agent being dosed or dosage of a coagulating agent, and a supply amount of the sludge.It is desired that these factors are properly controlled so that the water content of a cake as dewatered is maintained low and constant and in addition the dosage of a coagulating agent is maintained as low as possible.

The present invention achieves such purposes.

More specifically, since a material being supplied to a dewatering machineforthe purpose of processing, i.e. sludge characteristics such as a solid amount of a sludge or a concentration of a sludge, the amount of organic substance and particles of a sludge is changeable and, therefore, even if an amount of supply of a material being processed is maintained constant, a difference in the filtration amount is caused in the gravity dewatering zone.

Accordingly, the thickness of the material being processed supplied from a source material supply tank onto a filter belt is changeable. Usually, it is desired to make uniform the thickness throughout the width of the material being processed supplied from gravity dewatering zone onto a forcible dewatering zone comprising a roll press dewatering zone and there fore rolls are provided at the enterance of the forc ible dewatering zone for the purpose of adjusting the thickness of the material being processed or the other means for adjusting such thickness may be provided.Accordingly, if and when the concentra tion of the solid in the material being processed or the sludge becomes high and/or poor coagulation is caused and thus the amount of the material being processed increases, the material being processed becomes too congested at the roll for adjusting the thickness, which could cause leakage of the material.

On the contraty, if and when the amount of the material being processed is decreased, i.e. the concent ration of the solid material becomes low and/or poor coagulation is caused, it could happen that a desired water content cannot be attained. Thus, a problem was encountered that a dewatering performance is lowered due to a change of the sludge characteristics mainly a change of the concentration of a solid material. On the other hand, in the case where the solid concentration of a material being processed is changed, it is necessary to determine the dosage of a coagulating agent in association with the solid concentration. If the dosage of a coagulating agent is maintained constant, it could happen that a dewatering performance is lowered or too much dosage of a coagulating agent exceeding a required amount causes uneconimical waste of agent.

The inventive belt pressure filter comprises a gravity dewatering zone for dewatering a material being processed through the gravity exerted upon the material being processed or dewatered, and a forcible dewatering zone for forcibly dewatering the material being processed through external pressure.

An endless filter belt is disposed to travel from the above described gravity dewatering zone to the above described forcibly dewatering zone. A con stantflow rate of the material being processed is supplied to the above described gravity dewatering zone. Information concerning the sludge characteristics, mainly the concentration of the material being processed as supplied is obtained, wherein information concerning various changes of the sludge, such as coagulation condition, the amount of organic substance and the like may be included as the concentration associated information. The traveling speed of the above described filter belt is controlled to be in substantial proportion to the concentration responsive to the thus obtained concentration associated information, whereby the water content of the material being processed after dewatering is made uniform.

In a preferred embodiment of the present invention, means is provided in the above described gravity dewatering zone for detecting the thickness of the material being processed deposited on the above described filter belt. The information representing the thickness of the material being processed is used as the above described concentration associated information. Preferably, the thickness detecting means at least comprises a first level sensor for detecting the thickness of the material being processed deposited on the filter belt exceeding a predetermined lower limit, and a second level sensor detecting the thickness of the material being proces sed deposited on the filter belt exceeding a pre determined upper limit.The information represent ing the thickness of the deposited material being processed is determined as a combination of the log ical outputs of the first level sensor and the second level sensor. The traveling speed of the filter belt is controlled so that the traveling speed is increased responsive to the information representing the thickness exceeding the above described upper limit thickness obtained from the thickness detecting means until the thickness of the material being processed deposited on the filter belt becomes a thickness between the above described lower limit thickness and the above described upper limit thickness and the traveling speed of the filter belt is decreased responsive to the information representing the thickness smaller than the above described lower limit thickness obtained from the thickness detecting means until the thickness of the material being processed deposited on the filter belt becomes a thickness between the lower limit thickness and the upper limit thickness.

In a more preferred embodiment of the present invention, a coagulating agentforcoagulating the material being processed supplied to the above described gravity dewatering zone is mixed into the material being processed. It has been observed that a predetermined functional relation exists between the concentration of the material being processed and the optimum dosage of a coagulating agent. The above described functional relation is stored in advance in storage means. The optimum amount of a coagulating agent being dosed is evaluated responsive to the above described concentration associated information and based on the function stored in the above described function storage means.

In another more preferred embodiment of the present invention, an endless filter belt is adapted such that the same may travel at a constant speed.

Accordingly, a principal object of the present invention is to provide a belt pressure filter which is automatically controlled to make constant the water content of a cake obtained by a dewatering process in spite of a change of a solid concentration of a material being processed.

Another object of the present invention is to provide a belt pressure filter which is capable of auto maticaily dosing an optimum of a coagulating agent in accordance with a change of the solid concentration of a material being processed.

A further object of the present invention is to provide a belt pressure filter which is capable of automatically controling the dosage of a coagulating agent to be optimised, while the water content of a cake obtained by a dewatering process is maintained low and constant. In spite of a change of the solid concentration of a material being processed.

These objects and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

Fig. 1 shows a structural aspect of a belt pressure filter which constitutes the background of the invention; Fig. 2 is a graph showing a relation between the water content of a cake and the dosage of a coagulat ing agent; Fig. 3 is a graph showing a relation between the dosage of a coagulating agent and the solid concentration of a sludge; Fig. 4 is a view showing a whole structure of a belt pressure filter in accordance with one embodiment of the present invention, including a belt pressure filter main body and an automatic control circuit; Fig. 5 is a block diagram showing an outline of a computer portion shown in Fig. 4; Fig. 6A is a flow diagram for depicting a control operation of the belt pressure filter; Fig. 6B is a flow diagram showing in more detail the operation steps of the filter belt traveling speed in Fig. 6A;; Figs. 7 and 9 are time charts each showing a control state for depicting another embodiment for operating the filter belt traveling speed; Fig. 10 is a view showing an arrangement of means for detecting the material being processed coming outside from the filter belt for the purpose of making sure an operation state of the inventive belt pressure filter; and Fig. 11 is a perspective view showing an outline of the Fig. 10 detecting means.

Fig. 1 is a view showing a mechanical structure of a belt pressure filter which constitutes the background of the invention. The belt pressure filter shown basically comprises a first endless filter belt 2, and a second endless filter belt 4 disposed in partial contact with the first filter belt 2. The first and second filter belts 2 and 4 are adapted to be turned by means of suitable rollers.

The belt pressure filter also comprises a source material supply means 12 for supplying a source material being processed such as a sludge, a coagulating agent supply means 20 for supplying a coagulating agent for coagulating the material being processed, and a rotary mixer 22 for mixing the material being processed and the coagulating agent supplied from the source material supplying means 12 and the coagulating agent supplying means 20, respectively. The material supplying means 12 comprises a reservoir 6 for reserving and supplying the material being processed such as a sludge, and a pipe line 10 coupled to the above described reservoir 6 through a capacity variable pump 8. The coagulating agent supplying means 20 similarly comprises a coagulating agent reservoir 14, and a pipe line 18 coupled to the coagulating agent reservoir 14 through a capacity variable type pump 16. The rotary mixer 22 serves to mix the supplied material being processed and the coagulating agent to supply the mixture onto the above described first filter belt at a constant flow rate.

A gravity dewatering portion 26 is formed between the first filter belt 2 and the case side walls and partition wall 24 of the filter for filtering through its own weight to remove the water of the material being processed supplied from the mixer 22. Adjusting means such as a thickness adjusting roll 28 for adjusting the thickness of the material being proces sed fed upon being placed on the first filter belt 2 is provided downstream of the gravity dewatering portion 26 in terms of the traveling direction of the first filter belt. Furthermore, a second filter belt 4 is provided downstream of the first filter belt in terms of the traveling direction so that the first and second filter belts sandwiches the sludge having the thickness made constant by the thickness adjusting roll 28.In an area from this portion up to the point where the cake as dewatered is finally discharged, the first and second filter belts 2 and 4 are adapted to be urged toward each other. For simplicity of description, the portion where the first and second filter belts 2 and 4 are urged toward each other is referred to as a forcible dewatering portion 30. The forcible dewatering portion 30 consists of a roll press dewatering zone A in a linear path, a compressive dewatering zone B in a large diameter arcuate path passing through a roll Sofa large diameter, and a shear dewatering zone C disposed to pass through a number of rolls in a zigzag manner. The diameters of a number of rolls disposed in the shear dewatering zone C are selected to become smaller from the start of the zone toward the end of the zone.The dewatering principles at the respective zones A, B and C will be described subsequently in more detail.

Basically the sludge supplied from the rotary mixer 22 to the first filter belt 2 is dewatered at the gravity dewatering portion 26 through its own weight. Furthermore, the sludge as adjusted to the predetermined thickness by the thickness adjusting roll 28 is fed to the forcible dewatering portion 30 as the first filter belt 2 is traveled, whereby the material being processed is forcibly dewatered. The cake thus forcibly dewatered is finally fed to the discharging portion 32, where the first and second filter belts 2 and 4 are separated and as a result the dewatered cake is discharged.

Now the dewatering principle at the above described respective zones will be briefly described, First at the gravity dewatering zone 26 the first filter belt 2 of a mesh structure functions as a strainer and the sludge flock remains on the inclined filter belts, while the free water is removed as a filtrate as a function of the gravity. The amount of water produced through a dewatering process is largely influenced by the amount of a coagulating agent which is mixed into a sludge. For example, generally the water content of sewage sludge that has passed therethrough is approximately 90%. Then the sludge is adjusted in the roll press dewatering zone A by the cake thickness adjusting roll 28 to a cake of a specified uniform thickness which is different depending on the nature of the sludge.At that time it follows that the sludge is compressed by itself and furthermore a large gap between the flocks is reduced. Since the sludge is fed downstream of the roll while the same is rotated, a dewatering effect is accordingly increased and in addition the effects of stabilizing the travel of the filter belts and of preventing wrinkles from occurring are achieved. Then the sludge is further pressed from upward and downward to be dewatered by a relatively weak force by means of pressed rolls disposed so that the gap between the first and second filter belts is gradually decreased. In the case of a sewage sludge, the water content of the cake at the end of this zone is approximately 80 to 86%.Since the sludge has increased own plasticity to be a real cake form by the time when the same reaches a pressing process in the pressing zone B, a compressive force is applied to a cake by a tension of the filter belts and by the roll 5 of a large diameter, whereby a dewatering operation is expedited. In the case of a sewage sludge, the water content of a case at the end of this process is approx ornately 80 to 83%. At the following shear dewatering zone C a dewatering operation is performed by the maximum compressing force and an auxiliary shear force.More specifically, since the inner and outer filter belts are fed in the same traveling speed, a displacement is caused between the inner and outer filter belts due to the thickness of the cake when the roll is rotated and the above described shearing stress is caused in association with such displacement, whereby a dewatering operation of the compressed cake is further expedited. In the case of a sewage sludge, the water content of the finally obtained cake is approximately 68 to 80%.

A level meter 34 is provided at the start of the above described gravity dewatering portion 26 for the purpose of detecting the thickness of the material being processed deposited on the filter belt 2. The level meter 15 comprises a long sensor S1 for detecing a small thickness and a short sensor S2 for detecting a large thickness and these sensors S1 and S2 are selected to be of such lengths that only the long sensor S1 becomes operable when the thickness of the material being processed is in a normal range and the long sensor S1 does not become operable or both the long and short sensors S1 and S2 become operable when the thickness of the material being processed is in an abnormal state. Preferably such level meter may comprise an electrode type level switch.To this end, the longest sensor S3 which serves as a common electrode is provided.

The purpose of employing the level meter 34 is to detect the concentration of the material being processed supplied from the rotary mixer 22 onto the filter belt 2. The input weight of the sludge supplied from the mixer 22 is maintained constant in the embodiment shown. Therefore, assuming that the traveling speed of the filter belt is also constant, then the higher the concentration of the sludge being supplied the larger or higher the thickness or level of the sludge deposited on the filter belt 2 and vice versa. More specifically, the concentration of the sludge is proportional to the level of the sludge deposited on the filter belt. Accordingly, measurement of the level leads to measurement of the concentration of the sludge. According to the embodiment of the present invention, the traveling speed of the filter belt is controlled responsive to detection of the level of the sludge at the gravity dewatering portion and thus responsive to detection of the concentration of the sludge supplied from the rotary mixer 22, so that the water content in the cake may be maintained constant. The manner of such control will be more apparent with reference to a depiction in conjunction with Fig. 2 et seq.

As described previously, there are four factors to be taken into account in the inventive belt pressure filter, i.e. the filter belt traveling speed, the sludge concentration, the coagulating agent dosage, and the sludge supply amount The last mentioned factor of the sludge supply amount is set to be constant.

Accordingly, it is important to first grasp a correlation of the first mentioned three factors. As described previously, in consideration of su bse- quent process, it is desired to maintain the water content in a cake as dewatered constant and as low as possible. It has been observed that the water content of the cake is a function of the coagulating agent dosage, the coagulating agent amount and the filter belt speed. Fig. 2 is a graph showing a relation between the water content in a cake and the dosage of a coagulating agent. As seen from the graph, the curve of water content- dosage is different depending on the kinds of sludge and the optimum dosage is accordingly different depending on the kinds of sludge. By development of the relation shown in Fig.

2, a relation between the coagulating agent dosage and the sludge concentration as shown in Fig. 3 is obtained. As is clear from Fig.3, it would be appreciated that the coagulating agent dosage is reverse proportional to the sludge concentration. With the above described relation in mind, the present invention will be more specifically described in the following.

According to the present invention, it is assumed that the sludge supply amount is maintained constant. On the other hand, the sludge concentration is changeable. Accordingly, the solid quantity contained in the sludge is changeable in proportion to the concentration of the sludge. Assuming that the traveling speed of the filter belt is maintained constant, then the water content in a cake as dewatered is reverse proportional to the concentration of the sludge. More specifically, the higher the concentration the lower the water content. Therefore, if the concentration of the sludge is increased, the traveling speed of the filter belt is to be increased, while if the concentration of the sludge is decreased, the traveling speed of the filter belt is to be slowed down, in order to maintain the water content constant.More specifically, in order to maintain constant the water content in a cake, it is necessary to make the filter belt traveling speed proportional to the concentration of the sludge. In other words, the traveling speed of the filter belt is to be determined in proportion to the concentration of the sludge being supplied, i.e. the solid quantity. Conversely, if the traveling speed of the filter belt is determined, then accordingly the concentration of the sludge being supplied is determined. Therefore, if the concentration of the sludge is determined, then the optimum dosage of a coagulating agent is determined from the relation shown in Fig. 3.Since the flow rate of the sludge being supplied is kept con stant and the concentration of the sludge as deter mined is substantially the solid quantity, the dosage of a coagulating agent is deterjnined by determining the optimum dosage.

According to the present invention, attention is paid to the above described correlation and at the outset information associated with the concentration of the sludge being supplied to the dewatering apparatus, i.e. information concerning the thickness of the material being processed deposited on the first traveling filter belt 2 (Fig. 1) in the embodiment shown, is obtained and then the traveling speed of the filter belt is controlled based on the above described information (specifically, so as to be in proportion to the concentration of the sludge), whereupon the optimum dosage of a coagulating agent is determined from the thus determined traveling speed of the filter belt, whereby the optimum amount of the coagulating agent is added or dosed into the rotary mixer 22, whereby the water content in a cake as dewatered is low and constant while the optimum dosage of the coagulating agent is dosed based on the concentration of the sludge in such situation.

Fig. 4 is a view showing the whole structure of the belt pressure filter in accordance with one embodiment of the present invention and comprises a belt pressure filter main portion and an automatic control circuit. It is pointed out that like portions have been denoted by the same reference characters used in Fig. 1. Basically, the Fig. 4 embodiment comprises a belt pressure filter main body, and a computer portion for automatically controlling the belt pressure main body. The digital outputs obtained from the sensors S1 and S2 of the level meter 34 are applied to a filter belt traveling speed operating circuit 40.

More specifically, the detected output of the sensor S1 for detecting the low level is applied to the filter belttraveling speed operating circuit 40 through a data line D1 and the detected output obtained from the sensor for detecting the high level is applied to the filter belt traveling speed operating circuit 40 through a data line D2. The thickness or level of the sludge deposited on the filter belt is determined by a combination of the logical outputs obtained through these data lines D1 and D2 from the sensors S1 and S2, respectively.For example, if and when the outputs from the data line D1 and D2 are both the logic zero, then this means that the level is lowerthanthe predetermined lower limit level and thus the concentration is too small; if and when the output from the line D1 is the logic one and the output from the line D2 is the logic zero, then the level is in the predetermined normal range and accordingly the concentration is in a proper state; and if and when the outputs from the lines D1 and D2 are both the logic one, the level is higher than the predetermined upper limit level and accordingly the concentration of the sludge is too high. An additional sensor, not shown, may be provided forthe purpose of detecting an abnormally high level that makes the control inoperable and, if such abnormal high level sensor is provided, then the abnormal high level detected output is applied to a coagulating agent dosage operating circuit 50 through the data line D3. Detection of this abnormal high level is a kind of an abnormal state detection. The embodiment is adapted such that in the case of such abnormal state the operation deviates from the original sequence so that the dosage of the coagulating agent is exceptionally increased, deviating from the predetermined functional relation.In addition, apart from the above described abnormal high level sensor, a further additional abnormal highest level sensor, not shown, may be provided for the purpose of instantaneously detecting the extremely abnormally highest level and stopping the run of the machine, whereby overrunning of the sludge from the side wall can be avoided.

The above described filter belt traveling speed operating circuit 40 is responsive to the digital signals supplied through the data lines D1 and D2 to make an arithmetic operation to evaluate the traveling speed of the filter belt in accordance with the predetermined program. The filter belt traveling speed as evaluated by the filter belt traveling speed operating circuit 40 is withdrawn in the form of an analog output. The analog output is applied to an eddy current coupled control motor M, for example, so that the rotation speed is controlled. Thus the traveling speed of the filter belt is controlled, as desired. The filter belt traveling speed thus evaluated by the filter belt traveling speed operating circuit 40 is further applied to the coagulating agant dosage operating circuit 50.The coagulating agent dosage operating circuit 50 is responsive to the traveling speed of the filter belt to make an arithmetic operation to evaluate the dosage of a coagulating agent in accordance with the function which has been in advance stored. The dosage thus evaluated is used to control an agent supply pump 16, such as an eddy current coupled control pump through a control plate. Meanwhile, when the belt pressure filter is to be started, the sludge supply amount, the filter belt traveling speed and the coagulating agent dosage can be manually entered through a typewriter 70, for example. The sludge supply amount thus manually entered through the typewriter 70 is applied through a sludge amount operating circuit 60 to a sludge supply pump 8, so that the operation of the sludge supply pump is controlled.Since the embodiment of the present invention has been adapted such that the sludge supply amount may be constant, inherently the sludge supply amount operating circuit 60 can be dispensed with; however, preferably the circuit 60 is provided in preparation for an occurrence of an abnormal situation. To that end, the output from the coagulating agent dosage operating circuit 50 is applied to the sludge supply amount operating circuit 60. The reason is that such an abnormal situation could occur in which only dosage of the coagulating agent exceeding a predetermined amount is not sufficient to eliminate poor coagulation, when the sludge supply amount need be decreased.

Fig. 5 is a block diagram showing an outline of a computer portion shown in Fig. 4. Basically, the computer comprises a central processing unit 110, a first read only memory 120 for storing a predetermined program, a second read only memory 130 for storing predetermined functions for operating the dosage of a coagulating agent, a random access memory 140 for storing data, and an input/output port 150.Digital input signals being obtained from the belt pressure filter, i.e. the filter running signal, the automatic/manual signal of the filter, the abnormal high level signal, the high level detected signal, and the low level detected signal; analog outputs supplied to the belt pressure filter, i.e. the digital outputs indicating the filter belt running speed operating amount, the coagulating agent flow rate operating amount, and the sludge flow rate operating amount, and a stop command of the filter due to abnormality, are transferred through the input/output interface 160 and the data bus 170 for communication with the central processing unit 110, the read only memories 120 and 130, the random access memory 140 and the input/output port 150.A control bus 180 and an address bus 190 are provided among the central processing unit 110, the read only memories 120 and 130, the random access memory 140 and the input/output port 150. More specifically, the above described first read only memory 120 is used to store the program shown in Figs. 6A and 6B to be described subsequently and the second read only memory 130 is used to store predetermined functions as shown in Fig. 3 for operating the dosage of a coagulating agent. On the other hand, the random access memory 140 is used as a storage for data being transferred. The central processing unit 110 performs a processing operation in accordance with the program stored in the read only memory 120.

Fig. 6A is a flow diagram for explaining the controlling operation of the belt pressure filter. When the program starts, at the step S1 the digital signals as entered are read out and stored in the random access memory 140 (Fig.5). These digital signals comprise the filter running signal indicating whether the filter is in operation, and three level signals being detected by the level meter 34 shown in Fig. 1, i.e.

the abnormal high level detected signal, the high level detected signal, and the low level detected signal. The last mentioned level detected signals are each represented as the logic one signal obtained when each of the corresponding levels is detected. In the light of the thickness of an actual sludge deposited, any one of the above described three levels is detected in a normally controlled range. More specifically, these three levels comprise (1) the level lower than the low level, (2) the level between the low level and the high level, and (3) the level higher than the high level. In the case of the first mentioned level, the outputs of the low level sensor S1 and the high level sensor S2 are both the logic zero.In the case of the second mentioned level, i.e. the intermediate level, the output of the low level sensor S1 is the logic one and the output of the high level sensor S2 is the logic zero. In the case of the third mentioned level, i.e., in the case of the level higher than the high level, the outputs of the low level sensor S1 and the high level sensor S2 are both the logic one. At the above described step S1 a combination of such logical signals is obtained and stored in the random access memory until the following cycle. Then at the step S2 it is determined whether the running signal of the belt pressure filter is ON. In the case of the running state of the filter, the running signal is ON and therefore the program proceeds to the step S3.

At the step S3 it is determined whether the initial values of the sludge supply flow rate, the filter belt running speed and the dosage of the coagulating agent have been set. It is a common practice that the initial values are set by manual operation after the running signal becomes ON at the beginning.

Accordingly, since the initial values has not been set at the cycle at the start, the program proceeds to the step S4. At the step S4 information necessary for setting of the initial values is entered manually by means of the typewriter 70 shown in Fig. 4. Usually, only the information concerning the sludge flow rate is manually set by means of a typewriter and the like for the purpose of setting the initial values. The set value of the sludge flow rate is applied to the filter belt running speed operating circuit 40 and the coagulating agent dosage operating circuit 50 in Fig.

4. The initial value of the filter belt running speed and the dosage of the coagulating agent are proportional to the sludge flow rate as manually set. The respective proportion constants a and P may be stored in a memory, for example in the read only memory 130 or alternatively such may be stored in the random access memory using a typewriter and the like. The necessary initial values are thus set at the step S4. After the initial values are set, the program proceeds to the step S15 and at the step S15 the analog outputs based on the set initial values are obtained.

In the cycles after the initial values are set, the program proceeds from the step S3 to the step S5. At the step S5 it is determined whether the thickness of the sludge deposited on the filter belt 2 shown in Fig.

1 is an abnormal high level, i.e. the concentration of the sludge is abnormally high. The step S5 is aimed to detect an abnormality and usually the level of the sludge as deposited is within any one of the above described three level ranges. Accordingly, in a normal case, the program proceeds from the step S5 to the step S6. At the step S6 it is determined whether the operation is in a cycle time for controlling the filter belt traveling speed. Usually, this time cycle has been set to an arbitrary time period of 30 to 300 seconds. For example, assuming that the cycle time has been set to 30 second, then a control operation of the filter belt traveling speed is made once per every 30 second. At the step S6 it is determined whether the operation has reached such control cycle time.If the operation has reached such control cycle time, then the program proceeds to the step S7. At the step 57 it is determined whether the flow rate of the sludge being supplied has fluctuated.

Since usually the sludge supply amount has been set to a constant value, no fluctuation occurs in the sludge supply amount, insofar as a normal operation continues. In this context, it would be appreciated that the step S7 is aimed to detect an abnormality of the filter. Following the step S7, the program proceeds to the step S8. At the step S8 an arithmetic operation is performed to evaluate the traveling speed of the filter belt based on the thickness of the sludge as read and stored in the previous cycle and the thickness of the sludge currently read out. The detail of the step S8 for evaluating the traveling speed of the filter belt will be described subse quently in more detail with reference to Fig. 6B.

When the traveling speed of the filter belt is thus evaluated, then the program proceeds to the step S9, where it is determined whether the traveling speed of the filter belt has increased three times consecutively. The number of three times is by way of an example and the number may be largerthan that. In the case of the embodiment shown, there are three levels being detected by the level meter and the speed has been controlled responsive to a fluctuation among these three levels. When the level increases the speed is accordingly increased and vice versa, according to the embodiment shown, and therefore the fact that the speed is increased three times consecutively means that the concentration of the sludge is too high for an accelerating control of the filter belt traveling speed to follow.In this context, the step S9 may also be said to detect an abnormality of the filter. In a normal case, the number of consecutive increases of the speed would be two at the most, as described above, and therefore the program then proceeds to the step S10. At the step S10 it is determined whether the traveling speed of the filter belt is lower than a predetermined abnormal value VH. In other words, in a normal control, the traveling speed of the filter belt has been restricted to be smaller than the predetermined abnormal value VH. Accordingly, the step S10 is also aimed to detect an abnormality of the filter.Considering a normal case, therefore, the program proceeds to the following step S1 1. At the step S11 it is determined whether an operation is in a control cycle time for injection of a coagulating agent. This control cycle time has been usually set to an arbit .rary time period of 10 to 120 minutes. Assuming that the cycle time has been set to a time period of 10 minutes, an injection or dosing control of the coagulating agent is made per every ten minutes. If and when the operation has reached the control cycle time, the program then proceeds from the step S11 to the step S12. At the step S12 an arithmetic operation is performed to evaluate the dosage of the coagulating agent.The agent to be evaluated, i.e. the injection amount or dosage Fp of the coagulating agent, can be calculated by the following equation:

where V is the traveling speed of the filter belt in the current cycle time as calculated at the step S8, f(x) is a function of the sludge concentration and the optimum dosage, which is determined in advance through experimentation and is shown in Fig. 3. The information concerning this function has been stored in advance in the read only memory 130 shown in Fig. 5. The values V and Fs are stored in the random access memory 140 in Fig. 5. The information concerning the dosage of the coagulating agent evaluated at the step S20 is withdrawn at the following step S13 as an analog output. The information concerning the filter belt traveling speed calculated at the previously described step S8 is also withdrawn at the step S13 as an analog output. These analog outputs thus obtained are applied to the belt pressure filter, as described previously.

Thus the program proceeds through the steps for a normal operation as described in the foregoing.

Now description will be made of a case where an abnormal situation is determined at the above described abnormality determining steps. If and when it is determined at the step S7 that there is a fluctuation in the sludge flow rate, the program proceeds from the step S7 to step S14. At the step S14 the filter belt traveling speed Vn and the coagulating agent dosage Fpn are calculated in accordance with the following equations:

where n is a suffix denoting that the value is a current value and (n-1) is a suffix denoting that the value is a value of the previous cycle. If and when the filter belt traveling speed has become largerthan the predetermined abnormal speed VH at the step S10, then the program proceeds from the step S10 to step 515.

At the step S15 the coagulating agent dosage is increased. The purpose of this dosage is to increase a coagulation ratio of the sludge by temporarily increasing the coagulation agent dosage, thereby to decrease the concentration of the sludge, if and when the concentration of the sludge is too high for only a control of the filter belt traveling speed is to follow. Since an unlimited increase of the dosage of the coagulating agent is uneconimical, at the step S15 a timer is started concurrenly with the start of the increase of the coagulating agent dosage, so that a period of the increase of the coagulating agent dosage is limited to only a predetermined time period. Following the step S15, atthe step S16 it is determined whether the time period set by the above described timer has timed up.Since at the beginning the above described time period has not been timed up, the program proceeds to the step S13. If and when an increase of the coagulating agent dosage is still continuing even after the above described preset time period, this means that only the increase of the coagulating agent dosage cannot follow and therefore at the following step S17 the flow rate of the sludge being supplied is decreased.

At the following step S18 it is determined whether the flow rate of the sludge being supplied has become smaller than a predetermined minimum supply amount If and when the flow rate of the sludge has decreased to be smaller than the predetermined minimum value, then the automatic control cannot follow and therefore at the following step S19 the operation is brought to a stop by way of an abnormality stop.

Fig. 6B is a flow diagram showing the detail of the operation for evaluating the traveling speed of the filter belt at the step S8 shown in Fig. 6A. The flow diagram shown in Fig. 6B is adapted to determine whetherthe level is lowerthanthe low level, higher than the high level, or in the level between the low and high levels (hereinafter referred to as an intermediate level) with respect to these three levels determined based on a logical combination of the outputs obtained from the low level sensor S1 and the high level sensor S2, and then to determine what is the level in the previous cycle, thereby to evaluate the traveling speed of the filter belt based on the current level and the previous level.Therefore, before entering into a description in conjunction with Fig. 6B, various operation symbols used in the flow will be described. VN is a target value of the traveling speed of the filter belt in the case where t N. Vh is the latest value of the traveling speed of the filter belt when a change of the state occurs from the high level to the low level or from the intermediate level to the lew level, i.e. a change of level down occurs. V1 is the latest value of the traveling speed of the filter belt when a change of the state occurs from the low level to the high level or from the intermediate level to the high level, i.e. when a change of level up occurs. These latest values are stored in the random access memory. AV is a speed modification constant or a speed adjustment constant and is a predetermined relatively small value.Meanwhile, the constant AV is stored in advance in the read only memory. A point to be noted is that a change of the level downward means too large a value of the traveling speed of the filter belt, while a change of the level upward means too slow a value of the traveling speed of the filter belt. With the foregoing description in mind, the flow diagram shown in Fig.

6B will be described.

(1) In the case where the detected level of the sludge as deposited is smaller than the predetermined low level: In such situation, the program proceeds from the step S31 for determining whether the level is smaller than the low level to the step S32. At the step S32 it is determined what level was in the previous cycle. The level in the previous cycle has been stored in the random access memory 140 at the step S1 in Fig. 6A.

If and when the previous level is smaller than the low level, then the program proceeds to the step S33. At the step S33, the target value VN of the traveling speed of the filter belt at the current cycle is determined based on the condition that both the current level and the previous level are smaller than the low level. More specifically, the fact that the program proceeds to the step S33 means that the traveling speed of the filter belt in the previous cycle is too large. Therefore, at the step S33 the following arithmetic operation is performed: VN =VN#l-AV More specifically, since the traveling speed VN-1 of the filter belt in the previous cycle is too large, the speed is decreased by the speed modification constant AV.The speed VN thus determined is stored in the random access memory 140 shown in Fig. 5. If and when the level detected and stored in the previous cycle is the intermediate level, then the program proceeds from the step S32 to the step S34. Then at the step S34 the following arithmetic operation is performed: Vh = VN-1

The fact that the program proceeds from the step S36 to the step S34 means that the level has decreased from the intermediate level in the previous cycle to the low level in the current cycle.

Accordingly, it is necessary to increase the target value of the traveling speed of the filter belt in the current cycle for the purpose of controlling the traveling speed of the filter belt. To that end, the value obtained by subtracting a half of the speed modification constant AV from the average value of the latest value of the traveling speed of the filter belt when the level is decreased and the latest value of the traveling speed of the filter belt when the level is increased, is determined as the current traveling speed of the filter belt. Furthermore, the latest traveling speed of the filter belt when the level is decreased is the traveling speed, Van~1, of the filter belt determined in the previous cycle.The reason is that the level has been decreased from the previous intermediate level to the level lower than the current low level. V1 is the latest traveling speed of the filter belt when the level is increased. Thus, the traveling speed of the filter belt is adjusted by adopting the average value of the traveling speeds of the filter belt when the level is increased to the latest value and the level is decreased to the latest value. Meanwhile, the reason why AY is subtracted is that the 2 decrease of the level was one step, i.e. from a level down from the previous intermediate level to the current low level.If and when the previous level is the high level, an abrupt level down of two steps must have occurred from the high level to the low level and in this case the following arithmetic operation is performed at the step S35:

Since the level down from the previous level to the current level is abrupt atthattime, in other words since the previous traveling speed of the filter belt is too fast, the speed modification constant AV is subtracted for the purpose of adjustment of the speed.

Thus, if there occurs a change of the level, basically the average value of the latest traveling speeds of the filter belt when the level down and the level up occur and in addition the speed modification component is halved in accordance with the extent of the level down.

(2) In the case where the level of the sludge as deposited is higherthanthe high level: In this case the program proceeds from the step S31 through the step S36 to the step S37. Atthe step S37 it is determined what is the level detected at the previous cycle in the same manner as that in the previously described step 532. If and when the level detected atthe previous cycle is lower than the low level, then the program proceeds to the step S38.

The fact that the program proceeds from the step S37 to the step 538 means that there occurred a change of level up of two steps from the low level in the previous cycle to the level higher than the high level. In other words, this means that the traveling speed of the filter belt in the previous cycle was too slow. Accordingly, the target value of the traveling speed of the filter belt in the current cycle is determined at the step S38 in accordance with the following equation:

The fundamental idea is the same as that in the case of the previously described mode (1) and the average value of the latest traveling speeds of the filter belt on the occasion of the level up and the level down is evaluated, whereupon the speed modification constant is added thereto, because there occurred a level up of two steps.If and when the detected level in the previous cycle is the intermediate level, then the program proceeds from the step S37 to the step S39. This means that there occurred a gradual level up from the intermediate level in the previous cycle to the high level of the current cycle. Therefore, at the step S39 the target value of the traveling speed of the filter belt in the current cycle is obtained by adding a half of the speed modification constant to the average value of the respective latest traveling speeds of the filter belt on the occasion of the previously described level down and the level up.Meanwhile, it would be appreciated thatV1 would become a value corresponding to the traveling speed of the filter belt in the previous cycle, i.e. VN#l, because a level up occurs in each of the steps S38 and 539. If the level detected in the previous cycle is higher that the high level, then both levels in the previous and current cycles are the high level. Accordingly, this means that the traveling speed of the filter belt in the previous cycle was not high enough to decrease the level. Therefore, at the step S40 a target value of the traveling speed of the filter belt in the current cycle is obtained by adding the speed modification factor AV to the traveling speed of the filter belt in the previous cycle.

(3) In the case where the level of the sludge as deposited is the intermediate level: In this case the program proceeds through the steps S31 to S36 to the step S41. At the step S41 it is determined what is the level in the previous cycle in the same manner as described in conjunction with the previous steps S32 and S37. If the level in the previous cycle is lower than the low level, the program proceeds to the step S42. The fact that the program proceeds from the step S41 to the step S42 means that there occurred a change of level up from the level lower than the low level in the previous cycle to the intermediate level in the current cycle.

Accordingly, V1 is the traveling speed of the filter belt in the previous cycle, i.e. VN-l. Since the level in the current cycle is the intermediate level, it is not necessary to make speed modification and simply a target value of the traveling speed of the filter belt in the current cycle is obtained by simply adopting the average value of the respective latest traveling speeds of the filter belt on the occasion of the level up and the level down. If and when the level in the previous cycle is the intermediate level, then there is no level change between the previous and the current cycles and therefore the traveling speed of the filter belt in the current cycle may be the same as the traveling speed of the filter belt in the previous cycle as at the step S43.If and when the level in the previous cycle is higher than the high level, the program then proceeds to the step S44. The fact that the program proceeds from the step S41 to the step S44 means that there occurred a change of the level down from the high level in the previous cycle to the intermediate level in the current cycle. Accordingly, Vh is the traveling speed of the filter belt in the previous cycle, i.e. VN-1- The traveling speed of the filter belt in the current cycle is, as in the case of the step S42, the average value of the respective latest traveling speeds of the filter belt on the occasion of the level up and the level down.

As described in the foregoing, it would be appreciated that the target value of the traveling speed of the filter belt in the current cycle is determined based on the level of the deposited sludge detected in the current cycle and the level of the sludge detected and stored in the previous cycle and in consideration of the degree of a level change and the direction of a level change between the levels in the previous and current cycles. In particular, according to the Fig. 6B embodiment, the average value of the respective latest traveling speeds of the filter belt on the occasion of the level down and the level up is used as a reference without causing an abrupt change of the traveling speed and therefore a smooth speed control can be achieved.

Figs. 7 to 9 are time charts for depicting a control state for describing another embodiment for evaluating the traveling speed of the filter belt. In the case of the previously described Fig. 6B embodiment, the traveling speed of the filter belt in the subsequent cycle was determined by using the average value of the latest traveling speeds of the belt on the occasion of the level up and the level down as a reference and by adding or subtracting a predetermined speed modification constant. By contrast, according to the Fig. 7 embodiment, if and when the detected level has become the high level, i.e. the detected output of the high level sensor S2 becomes the logic one, the traveling speed of the filter belt is increased stepwise by adding the speed modification constant AV to the traveling speed of the filter belt in the previous cycle.

On the other hand, if and when the level is decreased from the high level to the intermediate level due to the increase of the traveling speed of the filter belt, i.e. when the digital output of the high level sensor S2 becomes the logic zero, the speed is decreased to the previously described traveling speed of the filter belt when the high level was reached, i.e. to the intermediate set speed (V + AV) which is the sum of V plus the speed adjustment constant AV. On the other hand, when the detected level becomes lower than the low level, i.e. the digital output of the low level sensor S1 becomes the logic zero, the speed is stepwise decreased by the speed modification constant AV from the traveling speed of the filter belt at that time.When the level is increased to reach the intermediate level due to the decrease of the traveling speed, i.e. when the output of the low sensor S1 becomes the high level, the speed is increased to the traveling speed at the time point of the level down to the latest value this time, i.e. to the intermediate set speed (V' - AV) which is V' minus the speed modification constant AV.

The previously described Fig. 7 embodiment was adapted such that the speed is increased or decreased abruptly to the intermediate set speed. On the other hand, the Fig. 8 embodiment is adapted to change the speed stepwise by the speed adjustment constant AV.

The Fig. 9 embodiment is adapted such that an allowable maximum speed (Vmax) and an allowable minimum speed (Vmin) are stored in advance in the read only memory or the random access memory and when the output of the high level sensor S2 becomes the logic one the traveling speed is at once changed to the maximum speed, whereupon when the detected level becomes the intermediate level, i.e. to the set range, the speed is changed from the maximum speed to the intermediate set speed and conversely when the detected level becomes lower than the low level the traveling speed of the filter belt is at once decreased to the minimum speed, whereupon the speed is changed to the intermediate set speed at the time point of the intermediate level, i.e.

when the output from the low level sensor S1 becomes the logic one. According to the control manners of the traveling speed of the filter belt as shown in Figs. 6B to 9, the traveling speed of the filter belt is adjusted such that if and when the thickness of the material deposited on the filter belt to be detected, i.e. the detected level, becomes other than the intermediate level (the set range), the above described detected level may be changed to the intermediate level and if and when the level is to be changed in the intermediate level (the set range) the traveling speed of the filter belt is always adjusted to the intermediate set speed.Accordingly, if and when the detected level becomes higherthan the high level, for example, even if the traveling speed of the filter belt is increased so that the detected level may be changed to the intermediate level, if such state is maintained, then conversely the detected level would come to exceed the intermediate level to be lower than the low level. By contrast, according to the present invention, when the detected level becomes the intermediate level, the level is automatically controlled to be prevented from exceeding the intermediate level by decreasing the speed to the speed smaller than the speed at that time point and larger than that before the adjustment. As a result, as a whole adjustment of the traveling speed of the filter belt due to a change of such state can be achieved in such a state that a new level change after the adjustment is suppressed as much as possible, in spite of a-change of such state in the material being processed as supplied.

Figs. 10 and 11 are views showing an arrangement and an outline of an apparatus for detecting a material being processed leaking from between the filter belts for making sure the operation of the inventive belt pressure filter. As described in conjunction with Fig. 1, the shear dewatering zone C comprises a plurality of rollers disposed in parallel and in a zigzag fashion such that the diameter of each roller is decreased from the upstream to the downstream in terms of the traveling direction of the filter belts 2 and 4. A rotating shaft 210 is provided in the vicinity of a roller 200 at the downstream end in a swinging manner about the axis P1 in parallel with the rotation axis P of the roller 200. Material receiving members 220 are provided integrally of the above described rotation shaft 210.The material receiving members 210 are in situ positioned downward of both side ends of the filter belts 2 and 4. A limit switch 230 is provided at one end of the above described shaft 210, so that the material being processed leaking during a dewatering process from both sides of the filter belts 2 and 4 is received by the members 220.

The limit switch 230 becomes operable as a function of a change of the weight of the material being processed as received by the receiving members 220.

Referring to Fig.11, the structure of the detecting means 240 will be described in more detail. The receiving member 220 comprises a cup 280 having a water leaking aperture 270 at the bottom thereof.

The cup 230 is integrally coupled to the above described shaft 210 through a rod 290. The material being processed as leaked from between the filter belts 2 and 4 is received by the above described cup 280. If and when the material being processed as processed exceeds a predetermined weight, the above described shaft 210 is rotated counterclockwise about the axis thereof. As the shaft 210 is rotated, the limit switch 230 provided in association with the above described shaft 210 is brought to a conductive state. The purpose of providing the water leaking aperture 270 in the above described cup 280 is to prevent a leaked liquid coming in the cup 280 on the occasion of a normal operation from remaining in the cup 280 to make the limit switch 230 undesirably operated.

The above described limit switch 230 is connected in series with an alarm apparatus 250 through a power supply 260. As a result, when the limit switch is turned on responsive to detection of the leaked material being processed, the alarm apparatus 250 is automatically operated, thereby to notify an operator of leakage of the material being processed.

Upon learning the alarm, the operator can manually adjust a supply amount of the coagulating agent and a supply amount of the material being processed and can take any necessary steps to adjust the traveling speed of the filter belts 2 and 4.

Although a cup-like vessel was shown as an example of the receiving members 220 in the embodiment shown in Figs. 10 and 11, it is to be pointed out that the geometry of the cup is not limited to such structure. Furthermore, although in the above described embodiment the detecting means 240 was disposed at the end of the shear dewatering zone C where a leaking phenomenon is necessarily caused due to the maximum pressure between the roll belts at that zone, the detecting means may be disposed at any other place, such as the start portion or the intermediate portion of the shear dewatering zone C.

As described in detail in the.foregoing, according to the present invention the traveling speed of the filter belts in controlled following a change of the concentration of the sludge being supplied and optimum dose of a coagulating agent can be determined, while a supply amount of a material being processed or the sludge is maintained constant, and therefore the water content of a cake as finally obtained as a result of a dewatering process can be maintained low and constant, without necessity of dosing the coagulating agent unnecessarily.

It is to be pointed out that the embodiments described in the foregoing were shown only by way of example and various changes and modifications can be made by those skilled in the art without departing from the scope and spirits of the prevent invention. For example, although a level meterwas employed for the purpose of detecting the concentration of the sludge in the above described embodiments, any other types of concentration meter, such as that for measuring the concentration using an ultrasonic wave in terms of attenuation thereof, that for measuring the concentration using a scattered light beam, that for measuring the concentration using gamma rays in terms of attenuation, and the like may be employed for the purpose of the present invention.

Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended

Claims

claims. CLAIMS

1. A belt pressure filter including a gravity dewatering zone for dewatering a material being dewatered through the gravity, and a forcible dewatering zone for forcibly dewatering the material being processed through external pressure, comprising: an endless filter belt provided to travel through said gravity dewatering zone and said forcible dewatering zone, material supply means for supplying said material being processed to said gravity dewatering zone, concentration associated information providing means operatively coupled to said material supply means for providing information associated with the concentration of said material being processed supplied from said material supply means, and filter belt traveling speed control means responsive to said concentration associated information provided from said concentration associated information providing means for controlling the traveling speed of said filter belt so as to be in substantial proportion to said concentration.

2. A belt pressure filter in accordance with claim 1, wherein said traveling speed control means is adapted to control the traveling speed of the filter belt at every predetermined cycle.

3. A belt pressure filter in accordance with claim 1, wherein said concentration associated information provid ing means comprise thickness detecting means for detecting the thickness of said material being processed deposited on said filter belt in said gravity dewatering zone, and said concentration associated information comprises information representing the thickness of said material being processed.

4. A belt pressure filter in accordance with claim 3, wherein said thickness detecting means comprises a first level sensor for detecting that the thickness is smaller than a predetermined lower limit thickness of said material being processed deposited on said filter belt, a second level sensor for detecting that the thickness is largerthan a predetermined upper limit thickness of said material being processed deposited on said filter belt, and said information representing the thickness of said material being processed as deposited is determined by a combination of the logical outputs of said first and second level sensors.

5. A belt pressure filter in accordance with claim 4, wherein said filter belt traveling speed control means is adapted to be responsive to the information representing the thickness being largerthan said upper limit thickness obtained from said thickness detecting means for increasing the traveling speed of said filter belt so that the thickness of said material being processed deposited on said filter belt may become an intermediate thickness between said lower limit thickness and said upper limit thickness and to be responsive to said information representing the thickness being smaller than said lower limit thickness obtained from said thickness detecting means for decreasing the traveling speed of said filter belt so that the thickness of said material being processed deposited on said filter belt may become an intermediate thickness between said lower limit thickness and said upper limitthickness.

6. A belt pressure filter in accordance with claim 5, wherein said filter belt traveling speed control means comprises first storage means for storing information representing the thickness of said material being processed detected by said thickness detecting means at every control cycle, thickness change direction determining means for comparing the information representing the thickness in the previous control cycle stored in said first storage means and the information representing the thickness detected in the current control cycle for determining the change direction of the thickness of said material being processed between said previous control cycle and said current control cycle, second storage means responsive to the thickness change direction determining output from said thickness change direction determining means for storing the filter belt traveling speed in said previous control cycle, arithmetic operation means for evaluating information representing an intermediate speed based on the latest filter belt traveling speed on the occasion of a change in the thickness increasing direction and the filter belt traveling speed on the occasion of a change in the thickness decreasing direction as stored in said second storage means, and intermediate speed control means responsive to said intermediate speed information obtained from said arithmetic operation means for controlling the traveling speed of said filter belt to become said intermediate speed.

7. A belt pressure filter in accordance with claim 6, which further comprises third storage means for storing information representing a predetermined relatively small speed value, and wherein said filter belt traveling speed control means comprises addition means responsive to the thickness increasing change determined output from said thickness change direction determining means for adding said predetermined speed value stored in said third storage means to said intermediate speed and responsive to the thickness decreasing change determined output from said thickness change direction determining means for subtracting said predetermined speed value stored in said third storage means from said intermediate speed.

8. A belt pressure filter in accordance with any one of the preceding claims, which further comprises coagulating agent dosing means for dosing a coagulating agent to said material being processed for the purpose of coagulating said material being processed supplied to said gravity dewatering zone, function storing means for storing a predetermined function between the concentration of said material being processed and the optimum dosage of said coagulating agent, coagulating agent dosage operating means responsive to said concentration associated information obtained from said concentration associated information providing means for evaluating the amount of said coagulating agent being dosed based on said predetermined function stored in said function storing means, and coagulating agent supply control means responsive to the information concerning dosage of said coagulating agent determined by said coagulating agent dosage operating means for controlling the supply amount of said coagulating agent supply means to become said determined coagulating agent amount.

9. A belt pressure filter in accordance with any one of claims 1 to 7, which further comprises detecting means for detecting the level where said material being processed deposited on said filter belt overruns from the filter.

10. A belt pressure filter including a gravity dewatering zone for dewatering a material being dewatered through the gravity, and a forcible dewatering zone for forcibly dewatering the material being processed through external pressure comprising: an endless filter belt provided to travel at a constant speed through said gravity dewatering zone and said forcible dewatering zone, material supply means for supplying said material being processed to said gravity dewatering zone, coagulating agent dosing means for dosing a coagulating agent to said material being processed for the purpose of coagulating said material being processed supplied to said gravity dewatering zone, concentration associated information providing means operatively coupled to said material supply means for providing information associated with the concentration of said material being processed supplied from said material supply means, and function storing means for storing a predetermined function between the concentration of said material being processed and the optimum dosage of said coagulating agent, coagulating agent dosage operating means responsive to said concentration associated information obtained from said concentration associated information providing means for evaluating the amount of said coagulating agent being dosed based on said predetermined function stored in said function storing means, and coagulating agent supply control means responsive to the information concerning dosage of said coagulating agent determined by said coagulating agent dosage operating means for controlling the supply amount of said coagulating agent supply means to become said determined coagulating agent amount.

11. A belt pressure filter in accordance with claim 10, wherein said concentration associated information providing means comprises thickness detecting means for detecting the thickness of said material being processed deposited on said filter belt in said gravity dewatering zone, and said concentration associated information comprises information representing the thickness of said material being processed.

12. A belt pressure filter in accordance with claim 11, wherein said thickness detecting means comprises a first level sensor for detecting that the thickness is smaller than a predetermined lower limit thickness of said material being processed deposited on said filter belt, a second level sensor for detecting that the thickness is largerthan a predetermined upper limit thickness of said material being processed deposited on said filter belt, and said information representing the thickness of said material being processed as deposited is determined by a combination of the logical outputs of said first and second level sensors.

13. A belt pressure filter in accordance with any one of claims 10 to 12, which further comprises detecting means for detecting the level where said material being processed deposited on said filter belt overruns from the filter.

14. A belt pressure filter substantially as herein described with reference to the accompanying draw ings.