Note: Descriptions are shown in the official language in which they were submitted.
CA 02306405 2000-04-20
SEWAGE SCREENING APPARATUS
Tho invention relates to a sewage screening apparatus for removing solid
materials from a sewage flow.
It is recognised that the effluent flow entering a sewage treatment plant
contains solid materials, such as rags, paper, polythene and other plastic
sheeting, and the like, which cannot be processed by the treatment plant.
Solids
can be removed from the flow by screens or sieves which capture the solids.
The screens or sieve arc then periodically or continuously operated for
cl~anning
to remove the captured solids, commonly referred to as screenings, for de-
watering and compaction and subsequent disposal. Qne such screening
apparatus known in the art is the "continuous-belt screen" which includes a
continuous belt screen, driven by means of a drive mechanism, which presents
a continuous, moving screening area to the input sewage flow to effect removal
of screenings within the sewage flow.
Tt is inevitable that faecal materials from the efrluent flow entering the
sewage
treatment plant will become entrapped with the screenings. In order to improve
the working environment of personnel handling the extracted screenings and to
minimise potential health hazards, it is desirable that screenings removed
from
the flow arc as free as possible from any faecal material. Usually, screenings
removed from the sewage flow by the continuous-belt screen are passed to a
secondary washing stage, remote from the screening apparatus, to wash the
screenings prior to compaction and removal from the site. Thus, additional
conveying means are required for conveying the screenings to the secondary
washing stage and subsequently onwards to the de-watering and compaction
CA 02306405 2000-04-20
-2~
stage. The use of a secondary washing stage is disadvantageous in terms of
complexity and cost.
EP 0 592 508 describes an alternative apparatus for cleaning screenings to be
removed from a sewage flow. In this apparatus, removal of the screenings is
effected by means of a screw conveyor having a perforated trough through
which sewage in the liquid phase passes. A.n agitator, in the form of an
impeller
device, is used to create turbulence in the sewage flow causing faecal
contaminants to be washed from the screenings prior to their entry to the
screw
conveyor and their subsequent removal from the flow. The screenings cannot
pass through the perforated trough and are compacted by the screw conveyor
for subsequent disposal from the apparatus.
A disadvantage of the apparatus is that it has a relatively low sewage flow
capacity and, typically, the apparatus can only be used with sewage flow rates
of less than 300-400 litres per second. Thus, the apparatus is not well suited
for
use as a primary sewage screening system, In particular, the apparatus cannot
be employed in a sewage treatment plant serving areas where it is necessary to
process a large sewage flow, such as areas of high population. Furthermore,
the
washing action is effected by repeatedly removing and reintroducing
screenings into the region of turbulence by periodically reversing the
direction
of the screw conveyor during operation. Repeatedly reversing the direction of
the screw conveyor in this way is mechanically inconvenient.
It is an object of the present invention to provide an apparatus for removing
washed screenings from a sewage flow which alleviates the problems of the
prior art.
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According to the present invention there is provided a sewage screening
apparatus comprising;
a screen arrangement having a continuous belt screen for capturing screenings
contained within the sewage flow to enable their subsequent removal
therefrom, the screen arrangement having a head space above the level of the
sewage flow when in use; and
a washing apparatus, housed within the head space of the screen arrangement,
into which screenings captured by the screen arrangement and an aqueous
liquid are introduced, wherein the washing apparatus includes impeller means
for generating turbulence within the aqueous liquid to erect washing of the
screenings prior to their removal from the sewage screening apparatus.
The apparatus has a large flow capacity by virtue of the large screening area
presented by the continuous belt screen. The apparatus therefore provides the
advantage that it can be used for sczeening a high volume sewage flow and is
therefore particularly suitable for use in a sewage treatment plant serving
highly populated areas. Furthermore, owing to its increased capacity, the
apparatus is suitable for use as a primary sewage screening apparatus.
rn addition, as washing of the screenings removed from the sewage flow is
effected within the washing apparatus housed within the screen arrangement,
the screenings removed from the screen arrangement need not be passed
through a second, remote washing stage prior to their removal from the sewage
treatment plant.
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The washing apparatus includes a washing tank having impeller means located
therein, the washing tank having a back plate and a front plate. The impeller
means may be one or more rotary impeller. At Ieast one rotary impeller may be
mounted in the back plate of the washing tank. In addition, or alternatively,
at
least one rotary impeller may be mounted in the front plate of the washing
tank.
The back plate of the washing tank preferably forms part of a back plate of
the
screen arrangement
The apparatus also includes a drive mechanism for driving the belt screen, the
drive mechanism being mounted externally of the scxeen arrangement.
It is usual in conventional continuous belt screen arrangements for the drive
mechanism to take the form of a driving wheel located within the head space of
the screen arrangement. Mounting the driving mechanism externally of the
screen arrangement enables the washing apparatus to be accommodated in the
head space of the screen arrangement. Furthermore, contamination of the drive
mechanism by the sewage flow is avoided.
The drive mechanism comprises a driven chain carrying a plurality of bracket
members for cooperation with a plurality of link members mounted on the belt
screen, each of the link members having a drive pin member projecting
therefrom, the bracket members cooperating with the projecting pin member to
lift the link members and thereby drive the belt screen.
More particularly, each of the link members has two drive pin members, one
projecting from each side of the lirxlc member, and each bracket member is
bifurcated, the two fingers of the bracket member extending one on either side
of the link member to engage with a corresponding drive pin member.
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The drive mechanism is not limited to use in the screening apparatus of the
present invention and may be employed in any sewage screening apparatus
having a driven screening belt.
The screening apparatus further includes brushing means, such as a rotary
brush, housed within the head space of the screen arrangement, to effect
removal of screenings captured by the continuous belt screen into the washing
apparatus. The washing tank also includes a weir over which an outlet flow
from the washing tank, including the washed screenings, flows into
conveyance means for conveying the outlet flow to a subsequent dc-watering
and compaction stage. The conveyance means may be a launder or a screw
conveyor. The washing tank also includes covering means to prevent
screenings removed from the continuous belt screen from passing directly into
the conveyance means without passing through the washing tank. The washing
tank also includes scraping means for scraping materials, including
screenings,
from the rotary brush into the washing tank. The scraping means may be
provided by an upper edge of a side wall of the washing tank.
The washing tank also includes a drainage point within its base to enable
drainage of the washing tank Preferably, the base of the washing tank is of
substantially calved or rounded form or the base is formed from separate
plates
having mitred corners. ~ base of this shape serves to enhance the mixing and
turbulation of the tank contents.
The invention will now be described, by way of example only, with reference
to the fohowing drawings in which;
Figure 1 is a perspective view of a conventional screen arrangement;
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Figure 2 is a plan view of the apparatus shown in Figure I;
Figure 3 is a front view of an embodiment of the present invention;
Figure 4 is a side view of a part of the apparatus of Figure 3;
Figure 5 is a perspective view of the apparatus of Figure 3 housed within the
main sewage flow channel;
Figure 6 is a perspective view of a part of the drive mechanism for the
apparatus shown in Figures 3-5;
Figure 7 is a side view of the drive mechanism shown in Figure 6;
Figure 8 is an enlarged side view of a part of the driving connection of the
drive mechanism shown in Figures 6 and 7;
Figure 9 is a perspective vices of an apparatus in accordance with an
alternative
embodiment of the invention;
Figure 10 is a front view of a part of the apparatus is Figure 9; and
Figure 11 is a side view of a part of the apparatus in Figure 10.
Referring to Figure 1, a conventional screening apparatus, referred to
generally
as I0, for use in a sewage treatment plant includes a number of screen panels
12, each panel 12 being formed from a perforated metal sheet. The screen
CA 02306405 2000-04-20
panels 12 are connected so as to form a continuous loop providing a belt
screen
1 l, each screen panel 12 being connected to the adjacent screen panels, one
on
each side, by hinging means 14. Conveniently, the hinging means 14 may be in
the form of a series of interdigitated lugs (not shown in detail), arranged
along
the edge of each panel 12, in which a retaining pin is received, the retaining
pin
passing through the lugs on adjacent panels 12 to secure adjacent panels
together.
The continuous belt screen 11 is driven by a drive mechanism (not shown in
Figure 1) such that the screen panels 12 are conveyed upwardly on one side of
the arrangement 10 and dowawardly on the other side in a direction, as
indicated by arrows 18. Such screen arrangements are well lmown in the art
and may be referred to as "continuous belt screens", as the screen panels 12
form a continuous belt, driven by the drive mechanism, which presents a
substantially continuous, moving screening area to the input sewage flow.
At the input stage, the sewage flow 16 contains solid materials (not shown),
such as rags, paper, plastics materials and stones, which cannot be processed
by
the treatment plant. The screen panels 12 are formed from sheets of perforated
metal, the perforations of which 22 enable the liquid phase of the sewage to
pass through the panels 12 onward to the subsequent processing stages of the
treatment plant. However, the perforations 22 are such that solid materials in
the sewage flow I6, having a greater dimension than the perforations, are
captured. The liquid phase of the sewage is therefore passed out from the
screen aaangement 10 through the continuous belt screen 11 on each side and
through that part at the base, whilst the screenings are retained.
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.$.
The flow path of the liquid phase of the sewage through the screen arrangement
can be seen more dearly in Figure 2, which shows the screen arrangement
10 housed within the main sewage chaanel 24 of the processing plant. Scat
points 26 between the belt screen and the main channel 24 separate the input
sewage flow 16 from the screened output flow 28.
Hooks or prongs 30 are arranged internally on the belt screen 11 at spaced
locations around the loop. The hooks 30 are usually located between adjoining
screen panels 12 and serve to capture the screenings contained within the
sewage flow 16 as the belt screen passes continuously therethrough. The
screenings may be removed from the belt screen 11 by means of a rotary brush
32 housed within the head space 20 of the screen arrangement 10 as they are
conveyed through the head space 20 past the rotary brush 32.
The apparatus also includes a hopper 34, housed within the head space 20, into
which captured screenings, contaminated by faecal materials, are dislodged
from the belt screen 11 by the rotary brush 32. At the upper part of the
screen
amdngement 10, aqueous liquid 36 is passed through a sparge washing system
40 onto the belt screen I 1 to flush any materials which become entrapped in
the
perforations 22 of the screen panels 12. The scrceniags removed by the rotary
brush 32, along with any materials flushed from the belt screen 11 by the
liquid
36, are then discharged from the hopper 34 into a launder 42 which carries the
screenings to a subsequent washing stage. The subsequent washing stage is
remote from the screen arrangement 10 and therefore additional conveying
means are required for conveying the screenings to the secondary washing
stage and subsequently onwards to the de-watering and compaction stage. The
complexity and cost of the apparatus is therefore increased.
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.g_
It is common for the continuous belt screen to be driven by means of a
rotating
wheel mechanism located within the head space 20 at the top of the screen
arrangrmcnt 10. However, if the head space 20 accommodates this rotating
wheel mechanism, the rotary brush 32 cannot then easily be accommodated
therein. The present invention overcomes this problem, as will be described
hereinafter.
Referring to Figure 3, the screening apparatus of the present invention
includes
a screen arrangement 10, as shown in Figure 1, and a washing apparatus,
referred to generally as 43. The washing apparatus 43 is housed within the
head
space 20 of the screen arrangement 10, the head space 20 being that region
within the belt screen 11 above the level of liquid sewage. The washing
apparatus may be housed anywhere above the sewage level within the screen
arrangement 10 but is preferably housed towards the upper portion of the head
space 20.
The washing apparatus is generally of the kind described in EP 0 557 030, and
includes a washing tank 46 and an impeller device 48, such as a rotary
impeller, housed within the washing tank 46. A washing liquid 50 is introduced
into the washing tank 46 by means of a spurge washing system (not shown),
fiDing the washing tank 46 to level 52. The washing liquid 50 also serves to
flush any materials from the belt screen 11 which have become entrapped in
the perforations 22 of the screen panels 12. The impeller 48 is driven by a
motor (not Shown in Figure 3) and serves to create a turbulence effect within
the contents of the washing teak 46.
As the belt screen 11 is moved continuously by means of the drive mechanism
the hooks 30, arranged internally on the belt screen 11, capture the
screenings
CA 02306405 2000-04-20
44 within the flow such thax they are conveyed upwardly on the hooks 30
towards the head space 20. As the screenings are conveyed passed the rotary
brush 32 on the belt screen 1 I, the brush 32 removes the screenings from the
belt screen 11 into the washing tank 46. Preferably, the hooks 30 are grouped
together to form rows of hooks spaced internally around the loop. The hooks
may be located between adjoining screen panels 12, although they need not be
included between each of the adjoining panels 12.
The screenings introduced into the washing tank 46 arc therefore subjected to
a
rigorous swirling motion, as indicated by arrows 54, produced by the driven
rotary impeller 48. The swirling motion serves to wash the screenings and
thereby removes any faecal contaminants which are entrapped with the
screenings.
Having been subjected to the intense washing action within the tank 46, the
screenings pass over a weir 56 within the washing tank 46 into a launder 5 8.
A
suffciently large volume of washing liquid also passes over fine weir 56 into
the launder 58, thereby ensuring the washed screenings are carried to the
subsequent de-watering and compaction stage by the flow of liquid.
The washing tank 46 includes a baffle plate 60 arranged underneath the launder
58. Along with the profile of the launder 58, the baffle plate 60 serves to
separate the washing tank 46 into two parts; a primary washing part, in which
the region of turbulence washes the screenings introduced thereto (as
indicated
by arrows 54) and a secondary outlet part fronn which washed screenings are
discharged to the launder 58 in the outlet flow.
The washing tank 46 also includes a cover 62, arranged above the launder 58,
to prevent screenings brushed from the belt screen 11 by-passing the washing
CA 02306405 2000-04-20
.11.
process within the tank 46 and falling directly into the launder 58.
Preferably,
the side wall 64 of the washing tank 46 extends upwardly towards the rotary
brush 32, thereby providing a scraping edge to aid removal of screenings from
the brush 32 into the washing tank 46.
Figure 4 shows a side view of the washing tank 46 housed within the screen
arrangement 10. The impeller 48 is driven by a motor 70 mounted in a back
plate 72 of the washing tank 46 such that the motor is external to the washing
tank 46. Preferably, the back plate 72 also forms part of the back plate of
the
screen arrangement 10. In this way, the motor 70 is also mounted externally to
the screen arrangement 10, thereby avoiding contamination of the motor 70 by
the effluent flow within the screen arrangement 10. Removal of the back plate
72 enables easy access to the internal parts of the washing tank 46, for
example
for cleaning purposes.
Preferably, the direction of rotation of the impeller 48 is towards the
inconung
screenings (i.e. anti-clockwise as viewed in Figure 3). In this way, the
swirling
motion imparted to the contents of the washing tank 46 acts on the screenings
so as to force them into the primary washing part of the washing tank 46.
Thus,
the screenings are maintained within the region of turbulence for as long as
possible, thereby optimising the washing effect.
It is advantageous if the bast 78 of the washing tank 46 is of substantially
curved or rounded form or is formed from separate plates having mitred
comers. A base having this shape serves to enhance the mixing and turbulation
of the tank contents. Periodically it may be necessary to remove solid
materials,
such as stones, which collect in the base region 78 of the washing tank 46.
For
this purpose a drainage point 80 may be provided in the base region 78 of the
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-~a-
washing tank 46. The drainage point 80 also ambles the contents of the tank 46
to be emptied, for example if access to the impeller is required if any damage
has occurred thereto. As described previously, access to the impeller 48 can
be
achieved by removing the back plate 72.
As well as washing faecal contaminants from the screenings, the swirling
motion generated by the impeller assists in breaking the faecal materials into
finely comrainuted form. As a further aid to commi.nution of the faecal
materials, the inner surface of the washing tank 46 may be provided with an
abrasive lining so that, as the faecal materials are thrown against the wall
of the
tank by the turbulent flow within the washing tank, mechanical attrition of
the
faecal materials occurs. For example, the abrasive lining on the inner wall of
the washing tank 46 may be a metallic or ceramic particle based abrasive
coating.
The comminution of the faecal materials in this way is advantageous as any
liquefied faecal materials passing over the weir with the screenings will be
separated from screenings in the subsequent de-watering and compaction stage
and can then be re-supplied to the sewage flow channel for further processing
in the sewage treatment plant. Maintaining the faecal materials within the
sewage flow ensures the biological loading of the sewage treatment plant is
maintained.
The outlet flow from the weir 56 therefore includes washed screenings,
liquefied faecal material and washing liquid. The outlet flow discharged over
the weir 56 into the launder 58 is conveyed to the subsequent de-watering and
eompaction stage of the apparatus. The de-watering and compaction stage may
include a screw compactor ofthe kind described in EP 0 557 030. Such a screw
CA 02306405 2000-04-20
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compactor includes a perforated trough region and a rotatable screw. The
washing liquid and the liquefied faecal materials in the outlet flow can pass
through the perforated trough, whilst the washed screenings are compacted by
the rotating screw. The compacted screenings can then be safely disposed of in
an incinerator or into a removal vehicle and the washing liquid and the
liquefied faecal materials, being free from screenings, are returned to the
sewage flow for further treatment.
It will be appreciated that the launder used for conveying screenings from the
weir 56 to the subsequent de-watering and compaction stage may be replaced
with another suitable conveyance means, such as a screw conveyor.
The screen arrangement 10 is housed in the main sewage channel 24, as shown
in Figure 5. For clarity, the washing apparatus shown in Figure 3 is not shown
in Figure 5 but, as described hereinbefore, is located above the sewage level,
illustrated generally by the dotted line $1. The continuous belt screen 11
presents a large screening area to the input sewage flow 16, the screening
area
being provided by the belt screen 11 on two vertical sides of the screen
arrangement 10 and at the bast of the screen ancangement. The screening
apparatus therefore has a large sewage flow capacity and, typically, is
capable
of screening an input sewage flow at a rate of up to 3000 litres per second.
The
apparatus is therefore particularly suitable for use in areas having a high
population. Furthermore, owing to the large sewage flow capacity, the
apparatus is suitable for use as a primary sewage screening apparatus.
The dimensions of the washing tank 46 housed within the screen arrangement
10 will be determined by the volume of sewage to be passed through the
sewage screening apparatus. For washing tanks of larger dimension it is
CA 02306405 2000-04-20
_14_
preferable to include two or more impeller devices 48 within the tank 46. Each
of the impellers may be mounted on the back plate 72 or, alternatively, one or
more impellcs may be mounted on a front plate of the washing tank 46
oppositcly facing the back plate 72. The washing liquid introduced into the
washing tank 46 is an aqueous liquid and may include a washing compound to
aid the washing process.
In use, the impeller device 48 is driven by the motor 70 during the period of
operation of the continuous belt screen 11. However, it is preferable if the
impeller is driven for a longer period than the belt screea so that the
washing
action continues after operation of the belt screen has ceased. Thus, all the
screenings removed from the belt screen are washed fully. It may also be
preferable to pause operation of the impeller during operation of the belt
screen
11 as re-starting of impeller rotation can cause any materials which have
become attached to the impeller to be dislodged therefrom. In addition,
aeration
within the tank contents which results from the impeller action causes the
screenings to float to the top of the task 46 (i.c. towards the weir 56) when
impeller motion stops.
In the present invention, the drive mechanism for the belt screen is mounted
externally to the screen arrangement 10 on a sidewall of the screen
arrangement
10, the drive mechanism engaging with both sides of the belt screen 11 as it
passes through the head space 20 to effect the driving connection. Referring
to
Figures 6-8, only the drive connection for one side of the belt screen 11 is
shown for clarity. The drive mechanism for this side includes a roller chain
90,
formed from a plurality of chain links 92, and driven by means of a chain
wheel 94 mounted on a drive shaft 96. The drive shaft 96 is driven by means of
a motor 97 in the direction of arrow 102, the roller chain 90 therefore being
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driven by the chain wheel 94 in the direction of arrows 104. Idler wheels
98,100 hold the roller chain 90 under tension, the roller chain tension being
adjustable by means of an adjustable tensioning plate 106. The face of the
adjustable tensioning plate 108 is formed from a high density plastic
material.
Mounted at each end of the screen panels 12 forming the belt screen 11 is a
link
member 110. As described previously, the screen panels 12 are connected
together by means of hinge pins 14, passing through a plurality of
interdigitated
lugs (not shown) arranged along the edge of each screen panel 12. The hinge
pins 14 also pass through apertures foamed in the link members 110, thereby
securing the link members 110 to their respective screen panel 12. Each of the
link members 110 also includes a drive pin 114, or peg, projecting laterally
therefrom.
The roller chain 90 carries bifurcated brackets 112, referred to as "lifting"
brackets, by means of a connection with the chain links 92, the brackets being
spaced on the roller chain 90 at locations corresponding to the positioning of
the link members 110 of the belt screen 11. As a lifting bracket 112 is
carried
by the driven roller chain 90 around the base 91 of the roller chain 90 and
subsequently upwards, the upper surface of the each of the fingers of the
bracket 112 comes into contact with a corresponding drive pin 114 on a link
member 110. The bracket 112 thereby serves to "lift" the drive pins 114, and
hence the link member 110, in an upward direction, thus driving the belt
screen
11. The tensioning plate 106 serves to ensure that the brackets 112 embrace
the
link members 110 at the correct depth so that the bracket provides the
required
lifting force for the drive pins 114.
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The Link members 110 may be formed from a high density plastic material and
are shaped to cooperate with vertically extending guide tracks 116 for guiding
the link members 110 as they are driven in an upward direction by the driven
roller chain 90. The guide tracks 116 may be formed from a high density
plastic material and are supported by a steel angle track 118 which may form
part of the outer housing of the screen artangemcnt 10. Equivalent guide
tracks
for the link members mounted on the other side of the belt screen 11 are also
provided (not shown in Figure 6).
External mounting of the drive mechanism leaves sufficient room in the head
space 20 of the screen arrangement for accommodation of the washing tank 46
and the rotary brush 32. Furthermore, as the drive mechanism is mounted
externally, contamination of the drive mechanism by the efiQuent is avoided.
In
addition, the driving motion effected by the lifting mechanism of the brackets
112 and the link members 110 is particularly effective and the mechanism has a
longer operating life than conventional drive mechanisms.
The drive mechanism also provides an advantage over conventional drive
mechanisms which are mounted on the top curve of the screen arrangement. By
mounting the drive mechanism on a vertical side of the screen arrangement, the
bearing load on the top curve is minimised and the wear on the apparatus is
reduced. The operating life of the apparatus is therefore improved.
With reference to Figures 9, 10 and I 1, there is shown an alternative
embodiment of the invention in which the washing tank 46 is provided with a
valve arrangement 130 which is operable between open and closed positions to
control the discharge of liquid and washed screenings from the washing tank
46. When the valve arrangement 13 0 is in the open position, liquid within the
CA 02306405 2000-04-20
_ 17.
washing tank 46 and washed screenings are able to flow through an outlet port
(not shown) provided in the washing tank 46, into the launder 58 to be
discharged from the screening apparatus. When the valve arrangement 130 is
in the closed position, the outlet port is closed such that liquid and the
screenings are unable to flow from the washing tank 46 into the launder 58.
The valve arrangement 130 is operable in response to as output signal from a
sensor 132, as shown in Figure 10, for sensing the level of liquid within the
washing tank 46. In use, when the level of liquid within the washing tank 46
reaches an upper level 134, the output signal generated by the liquid Ievel
sensor 132 is used to initiate opening of the valve arrangement 130. The valve
arrangement 130 remains open for a period of time which is sufficient to
permit
the washing tank 46 to drain to a liquid level no less than a lower level 136.
When the level of liquid in the washing tank 46 reaches the lower level 136,
the
output signal generated by the sensor 132 is used to initiate closure of the
valve
arrangement 130. Under such circumstances, liquid within the washing tank 46
and washed screenings are unable to flow through the outlet port into the
launder 5 8.
The output signal from the sensor 132 is also used to control operation of the
motor 70 for the impeller 48 such that, when the level of liquid within the
washing tank 46 reaches the lower level 13 6, operation of the motor 70 is
ceased. Operation of the impeller 3 8 therefore ceases when the level of
liquid
within the washing tank 46 reaches the lower liquid level 136.
?ypically, the output signals generated by the sensor 132 may be supplied to
an
electronic control unit (not shown), or a computer processor, for controlling
operation of the valve arrangement 130 and the drive mechanism for the screen
CA 02306405 2000-04-20
_ 18.
arrangement. When the output signal from the sensor 132 indicates that the
Ievel of liquid in the washing tank 46 has reached the upper level 134 and the
valve arrangement 130 is opened, a signal is provided to the drive mechanism
for the screen arrangement to halt operation of the belt screen 11. This
prevents any unwashed screeniags delivered to the washing tank 46 whilst the
valve arrangement 130 is open from being discharged into the launder 58. It
will be appreciated that the apparatus shown in Figures 9, 10 and 11 enables
washed screenings to be discharged from the apparatus in batches, rather than
continuously.
The inset in Figure 6 shows the direction of travd of the belt screen 11 (as
indicated by the arrows 18) when the drive mechanism is mounted on the right
hand, vertical side of the screen arrangement and is driven in the direction
of
arrow 104. However, it will be appreciated that the direction of travel 18 may
be reversed by mounting the drive mechanism on the left hand vertical side of
the screen arrangement.
The drive mechanism described hereinbefore is not limited to use in the sewage
screening apparatus of the present invention and may be employed in any
sewage screening apparatus having a driven screening belt.
