Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a belt pressure
filter suited for treatment of a sludge or the like produced
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 variations in the sludge characterist-
ics, such as the concentration of organic substances, the
water content of a dewatered cake may be maintained
relatively low and constant while the dosage of a coagulating
agent that is added to a raw sludge solution may be maintained
at an optimum and an abnormal condition, such as poor coagulat-
ion can be readily discovered.
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, the driving power may be small and any
increase in the amount of solid matter through injection of
an agent, is small, so that a cake having a small water
content can be obtained. Therefore, attention has been
attracted ~o such a system since it fully meets the require-
ment of saving energy. Generally, a belt pressure filter
comprises a gravity dewatering zone for dewatering a sludge
by gravi~y, a roller press dewatering zone for dewatering the
sludge ~y means of a roller press, a compressive dewatering
zone fqr dewatering the sludge by compressive force, and a
shearing stress dewatering zone for dewatering the sludge by
a shearing stress. A belt pressure filter comprises basically
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two mesh-like filter belts and rollers. This type of belt
pressure filter needs to take into consideration, as the
factors being controlled, four factors, i.e. the filter belt
traveling speed, the sludge concentration as a typical factor
representing a sludge characteristic dosage of an agent being
dosed or a dosage of a coagulating agent, and the amount or
quantity of sludge being supplied. It is desired that these
factors are properly controlled so that the water content of
a dewatered cake 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, the characteristics of a material
being supplied to a dewaterilg machine for the purpose of
processing may vary. The amount or concentration of solids in
a sludge or the amount of organic substance and the number of
particles in a sludge may vary. Therefore, even if the amount
of supplied material to be processed is maintained constant, a
difference in the filtration amount may be caused in the
gravity dewatering zone. Accordingly, the thickness of the
material supplied from a source material supply tank onto a
filter belt may be changeable. Usually, it is desired to make
uniform the layer thickness throughout the width of the
material as it is supplied from the gravity dewatering zone
onto a forced dewatering zone comprising a roller press de-
watering zone. Therefore, rollers are provided at the entrance
of the forced dewatering zone for the purpose of adjusting the
layer thickness of the material being processed. However,
other means for adjusting such layer thickness may be provided.
Accordingly, if and when the concentration of the solids in the
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sludge material becomes too high and/or a poor coagulation is
present or when the amount of sludge being processed
increases, the material being processed may cause a congestion
at the roller for adjusting the layer thickness, which could
cause a leakage of the material. On the other hand, if and
when the amount of the material being processed is decreased,
i.e. when the concentration of the solid material is low and/
or a poor coagulation is present it could happen that a
desired water content cannot be attained. Thus, there is a
10 problem that the dewatering performance or efficiency is
lowered due to a change in the sludge characteristics mainly
due to a change in the concentration of the solid material
components of the sludge. If the concentration of solids
changes, it is necessary to determine the dosage of a co-
agulating agent in association with or as a function of the
solids concentration. If the dosage of a coagulating agent
is maintained constant, it could happen that the dewatering
performance or efficiency is lowered or too large a dosage of
a coagulating agent exceeding a required amount would cause
an uneconomical waste of agent.
According to the invention is provided a belt
pressure filter including a gravity dewatering zone for de-
watering a material being dewatered by gravity, and in a
forced dewatering zone for forced dewatering the material
being processed through external pressure, comprising endless
filter belt means arranged to travel through the gravity
dewatering zone and through the for forced dewatering zone,
material supply means supplying the material being processed
to the filter belt means in the gravity dewatering zone, layer
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thickness information providing means operatively located
for sensing the thickness of a layer of material deposited
onto the filter belt means by the material supply means for
providing information representing the layer thickness of the
material being supplied by the material supply means for
processing, and filter belt traveling speed control means
operatively connected to the layer thickness information
providing means for controlling the traveling speed of the
filter belt means so that the filter belt speed is sub-
stantially proportional to the layer thickness.
Preferably, the thickness detecting means comprises
at least a first level sensor for detecting a layer thickness
which exceeds a predetermined lower limit, and a second level
sensor for detecting a layer thickness exceeding a pre-
determined upper limit. The information representing the
layer thickness is determined as a combination of the logical
outputs of the first level sensor and of the second level
sensor. The traveling speed of the filter belt is controlled
so that the traveling speed is increased when the information
representing the thickness is exceeding said upper limit until
the layer thickness of the filter belt is between the above
lower and upper limits. The traveling speed of the filter
belt is decreased when the information representing the
thickness is smaller than the lower limit until the layer
thickness on the filter be~t is again between the lower and
upper limits.
One aspect of the invention comprises a belt pressure
filter including a gravity dewatering zone for dewatering a
material by gravity, and a forced dewatering zone for a
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for a forced dewatering of the material by external pressure,
comprising an endless fil~er belt arranged to travel at a
constant speed through the gravity dewatering zone and the
forced dewatering zone, material supply means for supplying
the material to the gravity dewatering zone, coagulating -
agent dosing means for adding a coagulating agent to the
material for the purpose of coagulating the material, the
coagulating agent being supplied to the gravity dewatering
zone, means for providing an information representing a
solids concentration in the material, the solids concentration
information providing means being operatively coupled to
the material supply means, function information storing
means for storing a predetermined function between the solids
concentration of the material and the optimum dosage of the
coagulating agent, coagulating agent dosage operating means
responsive to the solids concentration information for
evaluating the amount of the coagulating agent based on the
predetermined function stored in the function information
storing means to provide a coagulating agent dosage
information, and coagulating agent supply control means
responsive to the coagulating agent dosage information for
controlling the coagulating agent supply means so that the
supplied amount of coagulating agent corresponds to a
determined quantity of coagulating agent. It has been
observed that a predetermined functional relationship exists
between the solids concentration of the material being
processed and the optimum dosage of a coagulating agent. Such
functional relationship is stored in advance in a memory means.
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The optimum amount of a coagulating agent being dosed is
evaluated in response to the solids concentration associated
information and based on the function stored in said memory
means.
In another preferred embodiment of the present
invention, an endless filter belt is adapted such that the
same may travel at a constant speed.
Accordingly, the present invention may provide a belt
pressure filter which is automatically controlled to assure
a constant water content of a cake obtained by a dewatering
process in spite of any change in the solids concentration
of the material being processed.
The present invention may also provide a belt
pressure filter which is capable of automatically dosing an
optimum amount of a coagulating agent in accordance with a
change of the solids concentration of a material being
processed.
The present invention may also provide a belt
pressure filter which is capable of automatically controlling
the dosage of a coagulating agent to make the dosage optimal,
while the water content of a cake obtained by the dewatering
process is maintained low and constant, in spite of any
change in the solids concentration of the 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
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present invention given by way of example when taken in
conjunction with the accompanying drawings.
Fig. 1 shows a structural side view of a belt
pressure filter which constitutes the background of the
invention;
Fig. 2 is a graph showing the relationship between
the water content of a cake and the dosage of a coagulating
agent;
Fig. 3 is a graph showing the relationship between
the dosage of a coagulating agent and the solid concentration
of a sludge;
Fig. 4 is a view showing the 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 for controlling the filter belt
traveling speed in Fig. 6A;
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Figs. 7 to 9 are time charts each showing a control
state for depicting another embodiment for controlling the
filter belt traveling speed;
Fig. 10 is a view showing an arrangement of means for
detecting the material being processed coming from the filter
belt for the purpose of assuring a certain operational state
of the present belt pressure filter; and
Fig. 11 is a perspective view showing an outline of
the detecting of Fig. 10.
DESCRIPTION OE THE PREFERRED EMBODIMENTS
Fig. l 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 advanced by means
of suitable drive and guide rollers.
The belt pressure filter also comprises a source
material supply means 12 for supplying a source material to be
processed such as a sludge, a coagulating agent supply means 20
for supplying a coagulating agent for coagulating the material
and a rotary mixer 22 for mixing the material and the coagulat-
ing agent. The material supplying means 12 comprises a
reservoir 6 for storing and supplying the material and a pipe
line l0 coupled to the 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 pump 16. The rotary mixer 22 mixes the
supplied material and the coagulating agent and supplies the
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mixture onto the above described first filter belt 2 at a
constant flow rate.
A gravity dewatering portion 26 is formed between the
first filter belt 2, the case side walls, and the partition
wall 24 of the filter for removing by gravity the water from
the mixed material. An adjusting means such as a layer thick-
ness adjusting roller 28 for adjusting the thickness of the
material on the fi~st filter belt 2 is provided downstream of
the gravity dewatering portion 26 as viewed in the traveling
direction of the first filter belt. Furthermore, the second
filter belt 4 is located downstream of the first filter belt in
the traveling direction so that the first and second filter
belts 2 and 4 sandwich the sludge having the layer thickness
made constant by the thickness adjusting roll 28. Between the
roller 28 and the point where the dewatered cake is finally
discharged, the first and second filter belts 2 and 4 are 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 forced dewatering portion 30
~ which comprises a roller press dewatering zone A in a linear
path, a compressive dewatering zone B in a large diameter
arcuate path passing through a roller S of large diameter, and
a shear dewatering zone C disposed to pass through a number of
rollers in a zigzag manner. The diameters of a number of
rollers disposed in the shear dewatering zone C are selected to
become smaller from the input end of the zone toward the output
end of the zone. The dewatering principles at the respective
zones A, B and C will be described below in more detail.
Basically the sludge supplied from the rotary mixer
22 to the first filter belt 2 is dewatered in the gravity
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portion 26 through its own weight. Furthermore, the sludge
adjusted to a predetermined layer thickness by the thickness
adjusting roller 28 is fed to the forced dewatering portion 30
by the movement of the first filter belt 2 whereby the sludge
material being processed is dewatered by compression exerted
by the rollers. The pressure dewatered cake is finally fed to
the discharging portion 32, where the first and second filter
belts 2 and 4 are separated whereby the dewatered cake is dis-
charged.
Now the dewatering principle at the above described
respective zones will be briefly described. First, in 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 by gravity. The amount of water produced by such
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 after passing
through the gravity dewatering zone 26, is approximately 90%.
Then the sludge is adjusted in the roller press dewatering
zone A by the layer thickness adjusting roller 28 to a cake
layer of a specified uniform thickness which may differ
depending on the nature of the sludge. The compression of the
sludge reduces its volume since large gaps between the sludge
flocks or flakes are reduced. Since the sludge is fed down-
stream of the roller 28 while the same is rotated, the dewater-
ing effect is increased. In addition, the travel of the filter
belts is stabilized by preventing wrinkles from occurring in
the belts. Then the sludge is further pressed from above and
below for dewatering by a relatively weak forced exerted by
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pressure rollers disposed so that the gap between the first
and second filter belts 2, 4 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 its own plasticity to assume a real cake form by
the time when the same reaches the compression stage in the
pressing zone B, a compressive force is applied to the cake by
tensioning the filter belts and by the rollers 5 having a
large diameter, whereby the dewatering operation is expedited.
In the case of a sewage sludge, the water content of the cake
at the end of this process is approximately 80 to 83%. In the
following shear dewatering zone C the dewatering operation is
performed by the maximum compressing force and by an auxiliary
shearing force. More specifically, since the inner and outer
filter belts 2, 4 are advanced at the same traveling speed, a
displacement is caused between the inner and outer filter
belts due to the layer thickness of the cake when the roller is
rotated and the above described shearing stress is applied to
the cake between the belts to said displacement, whereby the
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 or layer thickness is provided at
the start of the above described gravity dewatering portion 26
for the purpose of detecting the layer thickness of the
material deposited on the filter belt 2. The level meter 34
comprises a long sensor Sl' for detecting a small layer thick-
ness and a short sensor S2' for detecting a large layer thick-
ness. These sensors Sl' and S2' are selected to be of such
lengths that only the long sensor Sl' becomes operable when the
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thickness of the material is in a normal range and the long
sensor Sl does not become operable or both the long and short
sensors Sl' and S2' become operable when the layer thickness of
the material on the belt 2 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 de~ect the solids concentration of the material
on 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, the higher the concentration of
the solids in the sludge the larger or higher is the layer
thickness of the sludge deposited on the filter belt 2 and
vice versa. More specifically, the solids concentration of the
sludge is proportional to the level of the sludge deposited on
the filter belt 2. Accordingly, measuring of the level or
layer thickness provides in effect the solids concentration of
the sludge. According to the present invention, the traveling
speed of the filter belt is controlled in response to the
detection of the layer thickness of the sludge in the gravity
dewatering portion and thus responsive to the solids concentrat-
ion of the sludge supplied by 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 from the following
description in conjunction with Fig. 2 and the following
figures.
As described above, the invention takes the following
four factors into account for controlling the belt pressure
filter, i.e. the filter belt traveling speed, the sludge solids
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concentration, the dosage of the coagulating agent, and the
amount of the sludge supply. The last mentioned factor of the
sludge supply amount is set to be constant. Accordingly, it is
important to first explain the correlation of the first three
factors. As described previously, in consideration of sub-
sequent process steps it is desired to maintain the water
content of a dewatered cake as 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, of the effectiveness
of the coagulating agent and of the filter belt speed. Fig. 2
is the 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 as a function of dosage
differs depending on the kinds of sludge. The optimum dosage
accordingly also differs 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. It is clear from Fig. 3 that the
coagulating agent dosage is inversely proportional to the
sludge concentration. With the just described relationship in
mind, the present invention will be more specifically described
in the following.
According to the present invention, it is assumed
that quantity of supplied sludge is maintained constant. On
the other hand, the sludge concentration is changeable or
variable. Accordingly, the quantity solids 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 dewatered
cake is inversely proportional to the concentration of the
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sludge. More speclfically, 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, 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.
For this purpose it is necessary to make the filter belt
traveling speed proportional to the solids concentration of the
sludge. 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 constant and the concentration of the sludge as determined
is substantially the quantity of solids in the sludge, the
required dosage of a coagulating agent is determined by determin-
ing the optimum dosage.
According to the present invention, attention is paid
to the above described correlation. At the outset information
associated with the solids concentration of the sludge is
obtained in the form of the layer thickness of the material
deposited on the first traveling filter belt 2 (Fig. 1) in the
embodiment shown. Then the traveling speed of the filter belt
is controlled based on that information so that the belt speed
is proportional to the concentration of the sludge Then the
optimum dosage of a coagulating agent is determined from said
traveling speed of the filter belt, and added or dosed into the
rotary mixer 22, whereby the water content in a dewatered cake
is low and constant while the optimum dosage of the coagulating
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agent is based on the solids concentration of the sludge.
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. The same portions
have been denoted by the same reference characters as used in
Fig. l. sasically~ the Fig. 4 embodiment comprises a belt
pressure filter, and a computer portion for automatically
controlling the belt pressure. The digital outputs obtained
from the sensors Sl and S2 of the level meter 34 are applied to
a filter belt traveling speed operating circuit 40 through a
data line Dl 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 layer 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 Dl and D2 from the
sensors Sl' and S2', respectively. For example, if and when
the outputs from the data line Dl and D2 are both the logic
zero, then this means that the level is lower than a pre-
determined lower limit level and thus the concentration is toosmall. If and when the output from the line Dl 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 proper. If and when the outputs from the
lines Dl 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 for the purpose of detecting an
abnormally high level that makes the control. If such an
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additional level sensor is provided, then the abnormally highlevel detected output is applied to a coagulating agent dosage
operating circuit 50 through the data line D3. Detection of
such an abnormally high level indicates an abnormal condition.
The embodiment is adapted such that in the case of such abnormal
condition the operation deviates from the original sequence .so
that the dosage of the coagulating agent is exceptionally
increased, deviating from the predetermined functional relation.
A further abnormal, highest level sensor, not shown, may be
provided for the purpose of instantaneously detecting the
extremely abnormally highest level for stopping 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 Dl 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 provided in the form of an analog
output. The analog output is applied to an eddy current
coupled control motor M, for example, for controlling the
travel speed of the belts. The filter belt traveling speed
thus evaluated by the filter belt traveling speed operating
circuit 40 is further applied to the coagulating agent dosage
operating circuit 50 for arithmetically evaluating the dosage
of a coagulating agent in accordance with a function which has
been stored in advance. The dosage thus determined is used to
control an agent supply pump 16, such as an eddy current coupled
0 control pump including a control plate. When the belt pressure
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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 supplv amount is applied through a sludge amount
operating circuit 60 to a sludge supply pump 8, for controlling
the operation of the pump 8. 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. Such an abnormal
situation could occur in which a coagulating agent dosage
merely exceeding a predetermined amount is not sufficient to
eliminate poor coagulation, when the amount of supplied sludge
must be decreased.
Fig. 5 is a block diagram showing an outline of the
computer portion of 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 dosing
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 be~t 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
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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 below and the second read only memory 130 is used to
store predetermined functions as shown in Fig. 3 for dosing 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 controll-
ing operation of the belt pressure filter. When the program
starts, at the step Sl 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 for
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. Based of the thickness of an actual sludge deposit
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or layer on th~ belt, 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, t2) 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 Sl' and the high level sensor S2' are both the
logic zero. In the case of the second mentioned leveli.e. the
intermediate level, the output of the low level sensor Sl'
Sl' 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 Sl' and the high
level sensor S2' are both the logic one. In the above
described step Sl a combination of such logical signals is
obtained and stored in the random access memory until the
following cycle. Then in step S2 it is determined whether the
running signal of the belt pressure filter is ON. If the
filter runs, the running signal is ON and therefore the program
proceeds to the step S3. In 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. In practice that the initial values are set manually
after the running signal becomes ON at the beginning. Accord-
ingly, since the initial values have not been set in the cycle
at the start, the program proceeds to the step S4. In 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 sludgP 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
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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 ~ and ~ may be stored in a
memory, for example in the read only memory 130 or alternatively
they may be stored in the random access memory using a type- -
writer or the like. The necessary initial values are thus set
in step S4. After the initial values are set, the program
proceeds to step S15 whereby 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. In step S5
it is determined whether the thickness of the sludge layer
deposited on the filter belt 2 shown in Fig. 1 is at an
abnormally 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 which determines 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 seconds, then a control operation of the filter
belt traveling speed is made once every 30 seconds. In step
S6 it is determined whethe~ the operation has reached such ~ -
control cycle time, if so the program proceeds to step S7. In
step S7 it is determined whether the flow rate of the sludge
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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, as long as a normal operation
continues. In this context, step S7 is aimed to detect an ab-
normality of the filter. Following the step S7, the program
proceeds to the step S8. In step S8 an arithmetic operation is
performed to evaluate the traveling speed of the filter belt
based on the thick~ness of the sludge as read and stored in the
previous cycle and the thickness of the sludge layer is
currently read out. The detail of the step S8 for evaluating
the traveling speed of the filter belt will be described below
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, wherein 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 larger than that. In the embodi-
ment shown, there are three levels that may be detected by the
level meter 34 and the speed has been controlled in response 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 SlO.
In step SlO it is determined whether the traveling speed of the
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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 pre-
determined abnormal value VH. Accordingly, the step S10 is
also aimed at detecting an abnormality of the filter. Consider-
ing a normal case, therefore, the program proceeds to the
following step Sll which determines 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 arbitrary
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 every ten
minutes. If and when the operation has reached the control
cycle time, the program then proceeds from the step Sll to the
step S12. In 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 calculat~ed by the following equation:
Fp = V x f( F )
wherein V is the traveling speed of the filter belt in the
current cycle time as calculated in step S8, f(x) = f(F ) is a
function of the sludge concentration and the optimum dosage,
which is determined in advan-ce through experimentation and
is shown in Fig. 3. Fs is the sludge flow rate as shown in
step 4 in Fig. 6A-(l). 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 in step S20 is withdrawn during
the following step S13 as an analog output. The information
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concerning the filter belt traveling speed calculated during
the previously described step S8 is also withdrawn in step S13as an analog output. These analog outputs thus obtained are
applied to the belt pressure filter control, as described above.
Thus the program proceeds through the steps for a
normal operation as described in the foregoing. An abnormal
situation as determined by the above described abnormality
determining steps will now be described. If and when it is
determined in step S7 that there is a fluctuation in the sludge
flow rate, the program proceeds from step S7 to step S14. In
step S14 the filter belt traveling speed Vn and the coagulating
agent dosage Fpn are calculated in accordance with the following
equations:
Vn = Vn-l x F n
n-l
Fs
FPn Pn-l Fs
n-l
where n is a suffix denoting that the value is a current value
and (n-l) is a suffix denoting that the value is a value of the
previous cycle. If and when the filter belt traveling speed
has become larger than the predetermined abnormal speed VH in -
step S10, then the program proceeds from the step S10 to step
S15. In step S15 the coagulating agent dosage is increased
temporarily. The purpose of this dosage increase is to
increase the coagulation ratio of the sludge, thereby to
decrease the concentration of the sludge, if and when the
concentration of the sludge is too high to be handled only by
a following control of the filter belt traveling speed. Since
an unlimited increase of the dosage of the coagulating agent
is uneconomical, a timer is started in step 15 concurrently
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with the start of the increase of the coagulating agent dosage,
so that a period during which the coagulating agent dosage is
increased is limited to a predetermined time period only.
Following thP step S15, in step S16 it is determined whether
the time period set by the above described timer has run out.
Since at the beginning the above described time period has not
run out, the program proceeds to 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 the increase of the coagulating agent dosage alone cannot
correct the situation and therefore in the following step S17
the flow rate of the sludge being supplied is decreased. In
following step S18 it is determined whether the flow rate of
the sludge being supplied has become smaller than a predetermin-
ed 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 in 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 in step S8 shown in Fig. 6A. The flow diagram shown in
Fig. 6B is adapted to determine whether the level is lower than
the low level, higher than the high level, or in the level
between the low and high levels, hereinafter referred to as an
intermediate level. These three levels are determined by a
logical combination of the outputs obtained from the low level
sensor Sl' and the high level sensor S2', and then to
determine what was the level in the previous cycle, thereby to
evaluate the traveling speed of the filter belt based on the
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, ~ .
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current level and the previous level. Therefore, beforedescribing in conjunction with Fig. 6B, various operation
symbols used in the flow diagram of Fig. 6B 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 occurs from
the high level to the low level or from the intermediate level
to the low level, i.e. a change of level decrease occurs. Vl
is the latest value of the traveling speed of the filter belt
when a change occurs from the low level to the high level or
from the intermediate level to the high level, i.e. when a
level increase occurs. These latest values are stored in the
random access memory. ~V is a speed modification constant or
a speed adjustment constant and is a predetermined relatively
small value. The constant AV is stored in advance in the read
only memory. A downward change of the level means that the
traveling speed of the filter belt is too high, while an upward
change of the level means of the traveling speed of the filter
belt is too small. With the foregoing description in mind, the
flow diagram shown in Fig. 6B will now be described.
First the case will be considered when the detected
level of the sludge layer as deposited is smaller than the
predetermined low level:
In such a situation, the program proceeds from the
step S31 for determining whether the level is smaller than the
low level to the step S32 which determines what the level was
in the previous cycle. The level in the previous cycle has
been stored in the random access memory 140 in step Sl in Fig.
6A. If and when the previous level is smaller than the low
level, then the program proceeds to step S33. In step S33,
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the target value VN of the traveling speed of the filter belt
at the current cycle is determined on the basis of 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, in step S33 the following arithmetic
operation is performed:
N N-l I
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 ~V. 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 step S32 to step S34. Then in step
S34 the following arithmetic operation is performed:
Vh = VN-l
V = Vh + Vl - ~V
N 2
The fact that the program proceeds from step S36
to step S34 means that the level has decreased from the inter-
mediate 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
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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 VN 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. Vl 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. The reason why ~ is subtracted is that the
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
decrease of two steps must have occurred from the high level to
the low level and in this case the following arithmetic operat-
ion is performed in step S35:
Vh + Vl
VN 2 ~ ~V.
Since the level decrease from the previous level to
the current level is abrupt at that time, in other words since
the previous traveling speed of the filter belt is too fast,
the speed modification constant ~V 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 decreases
and increases occur and in addition the speed modification
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, ~
.
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component is halved in accordance with the extent of the level
decrease.
Second, the case will be considered when the level of
the sludge as deposited is higher than the high level:
In this case the program proceeds from the step S31
through the step S36 to the step S37. In step S37 it is
determined what is the level detected at the previous cycle in
the same manner as~described above for step S32. If and when
the level detected for the previous cycle is lower than the
low level, then the program proceeds to the step S38 which means
that there occurred a level increase by 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. According-
ly, the target value of the traveling speed of the filter belt
in the current cycle is determined in step S38 in accordance ~ :
with the following equation:
Vh + Vl
V = 2 + ~V
The fundamental idea is the same as that in the case
of the previously described first case or mode and the average
value of the latest traveling speeds of the filter belt on the
occasion of the level increase and the level decrease is
evaluated, whereupon the speed modification constant is added
thereto, because there occurred a level increase 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
increase from the intermediate level in the previous cycle to
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the high level in the current cycle. Therefore, in 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 as in the
previously described level increase and decrease. It will
be appreciated that Vl would become a value corresponding to
the traveling speed of the filter belt in the previous cycle,
i.e. vN 1' because a level increase occurs in each of the steps
S38 and S39. If the level detected in the previous cycle is
higher than the high level, then both levels in the previous
and current cycles are at the high level. This means that the
traveling speed of the filter belt in the previous cycle was
not high enough to decrease the level. Therefore, in 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 ~V to the traveling speed of the filter belt in the
previous cycle.
Third the case will be considered when the level of
the sludge as deposited is at the intermediate level:
In this case the program proceeds through the steps
S31 to S36 to the step S41. In step S41 it is determined what
was 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 step S42 which means that there
occurred a level increase from the level lower than the low
level in the previous cycle to the intermediate level in the
current cycle. Accordingly, Vl is the traveling speed of the
filter belt in the previous cycle, i.e. VN 1 Since the level
A` -29-
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in the current cycle is the intermediate level, it is not
necessary to make any speed modification and simply a target
value of the traveling speed of the filter belt in the current
cycle is obtained by adopting the average value of the respect-
ive latest traveling speeds of the filter belt when there was
a level increase or decrease. If and when the level in the
previous cycle is at 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 inlstep S43. If and when the level
in the previous cycle is higher than the high level, the
program then proceeds to the step S44 which means that there
occurred a level decrease from the high level in the previous
cycle to the intermediate level in the current cycle. Accord-
ingly, 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 step S42, the
average value of the respective latest traveling speeds of
the filter belt when there was a level increase or decrease.
In view of the foregoing, 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 when there
was a level increase or decrease is used as a reference without
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causing an abrupt change of the traveling speed and therefore
a smooth speed control can be achieved.
Figs. 7 to 9 are time charts showing a control state
for describing another embodiment for evaluating the traveling
speed of the filter belt. Contrary to the embodiment of Fig.
6B described above, in the embodiment of Fig. 7, 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 ~V 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 + ~V). On the other hand, when the
detected level becomes lower than the low level, i.e. the
digital output of the low level sensor Sl' becomes the logic
zero, the speed is stepwise decreased by the speed modification
constant ~V 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 Sl' becomes the high level, the
speed is increased to the traveling speed at the time when the
level decreased to the latest value at the present time, i.e.
to the intermediate set speed (V' - V).
The above described embodiment of Fig. 7 was adapted
0 such that the speed is increased or decreased abruptly to
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the intermediate set speed. On the other hand, the embodimentof Fig. 8 is adapted to change the speed stepwise by the speed
adjustment constant ~V.
The embodiment of Fig. 9 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 when the intermediate
level, i.e. when the output from the low level sensor Sl'
becomes the logic one. According to the control function 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 the thickness of the material deposited on the filter belt
to be detected, i.e. the detected level, assumes a value other
than the intermediate level or a set range the above described
detected level may be changed to the intermediate level. If
the level is to be changed to the intermediate level or set
range the traveling speed of the filter belt is always adjusted
to the intermediate set speed. Accordingly, if the detected
level becomes higher than 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, the detected level would pass through
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the intermediate level to be lower than the low level. However,
according to the present invention, when the detected level
becomes the intermediate level, the level is automatically
controlled so as to prevent that the intermediate level is
passed through by decreasing the speed to a speed smaller than
the speed at that time and larger than that before the adjust-
ment.
Figs. 10 and 11 are views showing an arrangement of
an apparatus for detecting a material being processed leaking
from between the filter belts for assuring the operation of
the present 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 as viewed in the traveling direction of the
filter belts 2 and 4. A ro,tating shaft 210 is provided in the
vicinity of a roller 200 at the downstream end for tilting about
the axis Pl in parallel with the rotation axis P of the roller
200. Material receiving members 220 are secured to the
rotation shaft 210. The material receiving members 220 are
positioned below and adjacent to both side edges of the filter
belts 2 and 4. A limit switch 230 is provided at one end of
the above described shaft 210, so that any material leaking from
both s~des of the filter belts 2 and 4 is received in the
members 220. The limit switch 230 is operated by the weight of
the material received in 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 there-
3 of. The cup 280 is connected to the above described shaft 210.
1 15739~
When the material received in the cup 280 exceeds a predetermin-
ed weight, the above described shaft 210 is rotated counter-
clockwise about the axis thereof. As the shaft 210 is rotated,
the limit switch 230 closes a circuit with a power supply 260
and an alarm device 250. The purpose of the water leaking
aperture 270 in the cup 280 is to prevent any leaked liquid
flowing into the cup 280 during normal operation from remaining
in the cup 280 thereby to prevent an undesirable closing of the
limit switch 230.
When more material is received in the cup 280 than can
flow out the limit switch 230 closes the circuit for the alarm
device 250 through the power supply 260, whereby an alarm is
given to notify an operator of an excessive leakage. The
operator can then manually adjust the supplied amount of the
coagulating agent and the amount of supplied material, he can
also 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 a 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 along the belts, such
as the start portion or the intermediate portion of the shear
dewatering zone C. -
As described in detail in the foregoing, according to
0 the present invention the traveling speed of the filter belts
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is controlled following a change of the concentration of the
sludge being supplied and the optimum dose of a coagulating
agent can be determined, while the amount of the supplied
material to be processed or the sludge is maintained constant.
Therefore the water content of a cake as finally obtained as
a result of a dewatering process can be maintained low and
constant, without any need for supplying excessive quantities
of coagulating agen~t.
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 present
invention. For example, although a level meter was employed for
the purpose of detecting the concentration of the sludge in the
above described embodiments, any other types of concentration
meter may be used. The concentration may be measured by
using an ultrasonic wave or gamma rays and ascertaining an
attenuation thereof. The concentration could also be measured
by using a scattered light beam.
Although the present invention has been described and
illustrated in detail, it is to be 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.
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