Note: Descriptions are shown in the official language in which they were submitted.
2050~~~
METHOD AND APPARATUS FOR REMOVING GRIT FROM SEWAGE
Field of the Invention
The invention relates to an improved method of removing
grit from sewage and apparatus for removing the grit
incorporating this method.
Sewage entering a treatment works can be described as a
mixture of four pollution forms, which are rags and paper,
grit from soil and roads, organic sludge and dissolved
material. The rags and paper suspended in the flow are
usually removed by passing the flow through mechanically
raked screens. The organic sludge is removed by settlement
in large sedimentation tanks in which the flow conditions
are quiescent. This material has almost the same specific
gravity as water and settles slowly. In turbulent flow
conditions, it remains effectively in suspension in the
water. The dissolvable material in the water has to be
treated. The grit from soil and roads has a specific
gravity considerably greater than water. There are a number
of methods available for removal of the grit and it is this
field of treatment of sewage to which this invention is
directed.
Sewage therefore has to be treated in four phases which
are usually carried out in the following sequence. Firstly,
screenings removal, then grit settlement/removal, then
2~~a~~~
2.
primary sedimentation and then bio-oxidation. Final
settlement or other final processes may be added.
Occasionally, a further fine screening stage is introduced
after the grit removal. The screenings would clog up pipes
and plant if allowed to remain in the flow until the
settlement stage. Grit could be left in the flow to the
primary settlement stage, but may settle out of the
connecting pipes and channels causing a maintenance
problem. The main reason for removing as much of the grit
as possible is to ease the removal and treatment of the
settled sludge. The heavier material of the grit renders
the sludge more solid and can lead to blockage. During
treatment of the sludge, the grit often causes an action
similar to concrete setting and has to be dug out of the
plant. This binding and setting of grit is known as
accretion.
The grit is transported in water by means of several
mechanisms. If the flow is fast and turbulent, the majority
of the grit will be transported in suspension. At more
normal levels of flow, speed and turbulence, the grit
particles bounce along the bottom of the channel often
colliding with each other in an action know as saltation.
At lower flow speeds, the grit forms dunes which are moved
slowly along by detaching from their downstream faces in
regions of turbulence and moving by saltation to the front
of the next dune downstream. If the flow speed is even
lower than this, then the dunes build up and the saltation
transport between the dunes stops. In correctly designed
sewers and channels in treatment works, a velocity of 0.8
to 1.2 metres per second is aimed at so that grit transport
is either in suspension or by saltation.
Description of the Prior Art
2~~~~~~
3.
A first example of apparatus for removing grit from
sewage that is well known is shown in figures 1 and 2 of
the drawings. This is known as a detritor. The sewage is
passed from a channel through a set of flow straighteners
S in a much widened out channel. The widening out of the flow
reduces the velocity of the water considerably. If the
velocity is reduced to 0.3 metres per second, then grit
falling out onto the bottom of the chamber is not moved
forward. The diameter of the settling basin is between 4
and 12 metres and therefore takes up a large plan area. A
scraper member incorporating three scraper
arms is rotatably mounted on a member which is suspended
from a bridge across the chamber. The scraper is submerged
in the flow, but there are no bearings within the chamber
which would be damaged by the grit. These rotate and move
the grit that has fallen onto the base of the channel
towards a hopper mounted at the side of the channel. In the
example shown, the chamber is in the form of a circular
dish.
Not all the grit is removed by this technique, as
particles of less than 0.5 mm diameter can be transported
out of the basin. Further reducing the velocity of flow
through the settling basin would ensure that even grit
particles of this size would be retained. However, a
compromise has to be adopted, because some organic
particles have the same settling mechanics as these fine
grits. If flow conditions in the settlement basins were
more quiescent, the large proportions of organic sludge
would be removed with the grit, posing a washing and
disposal problem. The problem with this design is that
contrary to the basic premise of the design, many of these
basins do not have uniform velocity distribution and
streams of faster flowing water often occur sweeping larger
grits through the basin. This results in many basins
retaining as little as 30$ of the incoming grit. To allow
for any unevenness on the basin surface, the scrapers
cannot be in direct contact with the base but have to be
4.
spaced therefrom. This means that there is always a small
amount of grit which is going to be left on the base of the
chamber. Moreover, if for any reason, there is a mechanical
failure, grit will settle on the base of the chamber in
dunes, and the scrapers will be then set in position. The
only way that the detritor can then be used is for
operators to manually come in and remove the grit from the
base of the chamber before it can be reused.
A second method for removal is illustrated in figures 3
and 4 of the drawings in an apparatus which is a form of
Pista grit trap. This apparatus relies on the flow being
caused to rotate in a large circular hopper with a conical
bottom. The rotation is caused by paddles which are
rotatably mounted and driven by an electric motor mounted
on a bridge across the tank. The rotation of the flow
causes particles heavier than water, i.e. the grit, to move
to the centre and downwards. The grit collects at the
bottom of the cone and is withdrawn. An inverted baffle is
mounted within the hopper to separate the grit from the
rotating flow. This unit can allow grit to be washed out of
the basin and if air is used to agitate the grit in the
bottom, some grit is washed out along with settled organic
matter.
A new grit removal apparatus which has recently been
introduced is the type shown schematically in figures 5 and
6 of the drawings. The unit comprises typically a drum of
0.5 to 6 metres diamater where the flow enters the
cylindrical drum in a tangential direction. This tends to
set up a rotating flow which causes the heavier than water
particles to descend in the drum and the lighter than water
particles to ascend. The action of the unit is similar to
vortex separators and hydrocyclones used in industry.
These three methods and apparatus are the three
proprietary products which are available on the market for
separating grit from sewage.
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5.
There are two other methods of removing grit from sewage
which are used.
A first method is illustrated in the drawings and
consists of a constant velocity tank which uses an elongate
S rectangular tank which is long enough to straighten the
flow and therefore allow the grit to settle on the base of
the channel. The grit is then sucked out of the base of the
channel by the suction of a pump mounted on a travelling
bridge over the channel. The second method is a spiral flow
grit channel which sets up a spiral flow of the water
within the tank. Fine bubbles of air are fed into the tank
along one of its walls which induces a flow upwards to
maintain the spiral flow. It exaggerates the difference
between the specific gravity of the grit and the specific
gravity of the water, since the water becomes entrained
with air and its specific gravity therefore reduces. This
means that the grit falls out of suspension very quickly.
Summary of the Invention
According to the invention, there is provided a method of
removing grit from sewage comprising passing sewage
containing grit along a channel, the sewage passing
initially through a straight elongate portion of length
sufficient to stabilise the flow and subsequently passing
around a bend having an angle of at least 10 degrees from
the straight portion and, downstream of the bend, removing
grit from the flow via a port situated at the inside of the
bend at the base of the channel.
The method depends on the mechanism which in nature
causes rivers to take a meandering path across plains. When
water flows through a straight channel, if the channel is
long enough for flow conditions to reach a stable regime
such that upstream disturbances have no longer an effect,
2~~~~~5
6.
then the velocity near the walls and bottom will be low
with a central core of higher velocity in the centre of the
channel and near the free water surface. Grit will be
transported by saltation and in suspension near the bottom
in the region where the rate of increase of velocity is
large.
As the channel curves in a bend, a rotation of flow is
set up where the strength of the rotation is a function of
the curvature radius as compared to the width of the
channel, the velocity and the angle through which the flow
is turned. A rotation so set up forms a higher velocity
core in the form of a helix which has the result that
material heavier than water, i.e. the grit, will be moved
to the bottom of the channel on the inside of the curve
whilst lighter material will be moved to the top near the
water surface on the outside of the curve. Thus, downstream
of the bend, grit travels along the bed close to the wall
of the inner curve edge of the channel and against the
lower part of the wall and if a port is located on that
inside edge at the bottom of the channel, the heavier than
water grit will be removed from the flow. The water and
lighter material will be transported forward.
Sewage grit comprises a range of particle sizes with a
specific gravity considerably greater than water. The
proportion of any specific particle size in a sample of
sewage grits can vary. The total quantity and rate at which
grit arrives at a sewage treatment works is variable from
site to site. It depends, among other things, on the type
of catchment area, the hydraulic characteristics of the
sewerage system and the flow rate at the time. The
operation of the apparatus is independent of the grit
loading and thus has a wide range of applications.
Some organic particles have the same settling mechanics
as fine grits, and so in prior art removal systems
utilising the settlement process, the grits often become
cantaminated with organic material by further processing
prior to disposal of the grits to tip. In the apparatus in
accordance with the invention, since only a relatively
small proportion of the total flow is passed into a grit
ra:movc1 hopper for settlement, there will only be limited
contamination. The apparatus will typically be located
dawnstream of fine sewage screens. Therefore, providing
that 'the screens are designed in accordance with~good
practice, it is unlikely that gross solids will penetrate
to the apparatus in any significant quantities. However, it
is a feature of the apparatus that gross solids will be
diverted to the outside of the channel limiting the
potential for gross solids capture.
Apparatus in accordance with the invention will comprise
a channel including a straight elongate portion of constant
width having a length sufficient to stabilise the flow and
a bend portion being at an angle of at least 10 degrees to
the axis of the straight portion of the channel; at least
one port situated at the base of the channel downstream of
the bend on the inside of the bend; and a grit hopper
coupled to the or each port, coupled to a draw off point to
remove the grit for disposal.
The length of the straight portion has to be long enough
to allow stable flow patterns to be established. If the
channel upstream of the straight portion is curved, then
the length of the straight portion would have to be longer
if the curvature is opposite to that chosen in the
apparatus, or shorter if the upstream bend is in the same
direction as that on the apparatus. The straight portion of
the channel will typically have a length of the order of at
least 10 times the width of the channel. It should also be
at least 10 times the maximum depth of flow to ensure that
grits approaching the bend are transported in the lower
part of the flow near to the bed of the channel. Typically,
the cross-section of the channel is constant, especially
8 .
throughout the length of the straight portion. However, it
is not essential that the width of the channel is constant.
If the cross-section is to change, it has to be changed
very gradually to maintain stability of flow.
The channel downstream of the bend should be straight and
have the same cross-section as the straight portion of the
channel and bend. In order to avoid interference with the
operation of the rest of the apparatus, the length of the
downstream channel, measured from the end of the bend,
should not be less than twice the channel width. In
practice, the length of the downstream channel will be much
longer, especially if a standing wave flow measurement
flume is installed downstream.
Although the channel may have a sloping or curved base,
it is preferred that the channel has a rectangular cross-
section.
The bend can be a right hand or left hand bend and the
radius of the bend is chosen for the degree of effect that
is required.
The angle of the bend and radius of the bend are
interrelated. A sharp turn without a bend or any
combination of angle and bend radius will, to a degree,
concentrate grits against the inside face of the turn.
Irrespective of the bend radius, there is only a slight
effect at bend angles of up to 10 degrees. However, as the
angular deflection is increased, the radius of the bend
must also be increased if flow is not to separate and
spread the grits across the width of the channel. For
instance, if the angle of the bend is greater than 10
degrees and the radius is less than 2.5 to 3 times the
width of the channel, the flow will exhibit a tendency to
separate. The angular change of direction depends on the
average velocity, the radius and the channel width. The
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9.
angle will usually be less than 45 degrees but can be up to
130 degrees. There is a slight effect with any small angle,
but the effect is only pronounced with angles of more than
degrees.
S Preferably) the angle of the bend is between 20 and 40
degrees and the radius of the bend is between 5 to 15 times
the channel width. More preferably, the angle described by
the bend is 30 degrees and the radius of the bend is 12
times the channel width.
10 Typically, the width of the channel will be between 600
mm and 1200 mm, although it is possible for the apparatus
to be made with different sizes of channels.
The relationship between the depth of the approach flow
of sewage approaching the bend and the width of the
straight portion is an important factor in the correct
operation of the apparatus. In a preliminary treatment
plant, it is normal practice to locate a standing wave
flume downstream of the grit removal plant. The main
purpose of the flume is to measure the flow. In doing so,
the flume also sets the depth in the channel to give that
required for the grit and screenings removal plants.
Preferably, the apparatus will be installed with an
appropriately sized downstream standing wave flume to set
the required water depth/flow parameters.
Preferably, the preliminary treatment channels are
designed so that, as far as possible, grits are transported
through screens to the grit removal plant over the full
range of expected inflows. This would require a minimum
channel velocity of approximately 0.70 m/s. Excessive
velocities must be avoided if efficient screenings removal
is to be achieved. This can be achieved if maximum mean
flow velocities do not exceed approximately 1.00 m/s.
10.
The apparatus will remove grits over the range of
approach velocities likely to be encountered in a
preliminary treatment works designed in accordance with
good practice. The lower limit of approach velocity is that
which will transport grit in the straight portion. The
apparatus will remove grits from sewage flowing at
velocities well in excess of those likely to be encountered
in practice but at velocities in excess of 1.30 m/s, the
efficiency of capture begins to reduce. The optimum maximum
approach velocity is 1.00 m/s.
The width of the channel to give the above range of
approach velocities is dependent on the downstream
hydraulic controls. The operation of the apparatus is
independent of the channel width but it is preferred that
the depth of the approach flow should be approximately 1.0
to 1.2 times the channel width at maximum flow.
If the apparatus is installed in accordance with the
above criteria, head losses due to the apparatus will be
low, of the order of 0.030 m.
The location of the port through which the grit passes
into the collection hopper is important to the efficient
operation of the apparatus. If the port is positioned
upstream of the end of the bend, some of the approaching
grits will not be concentrated against the inner wall of
the bend. If the port is positioned too far downstream of
the end of the bend, the grits will naturally begin to
spread across the width of the channel. In either case, the
efficiency of the apparatus will be reduced. Therefore, for
optimum performance, preferably the leading edge of the
port commences at the end of the bend.
The port is typically rectangular in shape but there are
a number of possible optional port arrangements which will
work satisfactorily. The main port area should be
concentrated on the floor of the channel but could, for
2~~~~~
example, extend up the inside wall. However, a simple
rectangular floor port is preferred.
The port may simply comprise a hole through which the
grit falls to the collection hopper. However, it is
preferred that the port consists of a grit removal device
including an inlet port, a flow deflector and an exhaust
port, arranged such that the sewage containing the grit
flows through the inlet port, deposits the grit and flows
out of the exhaust port into the channel.
The length and width of the inlet port is important to
the efficient operation of the apparatus. Any width and
length of inlet port will collect grits. The minimum width
of the inlet port is dictated by the practicalities of
construction and the need to prevent partial blockage in
the unlikely event that gross solids penetrate past the
upstream fine screens and attempt to enter the grit
collection hopper. Too wide an inlet port will increase the
proportion of organic solids collected. The preferred width
of the port is half the width of the channel at the bend.
If the inlet port is too short, grit capture will not be
totally effective and if it is too long, secondary flow
effects will become dominant and the effectiveness of the
flow deflector will be reduced. The preferred length of the
inlet port is equal to the width of the channel.
The flow deflector is positioned at the downstream end of
the inlet port. The main purpose of the deflector is to aid
the capture of grits thrown up into the flow owing to the
interaction of particles as they saltate down the channel
and bend. Additionally, the deflector diverts flow into the
inlet port which improves particle capture and directs the
grits down into the grit collection hopper towards the
point of removal.
12.
The flow deflector is in the form of a plate which
e:Ktends into the flow between the inlet port and the
e:Khaust port .
The inclination of the flow deflector is not crucial to
the operation of the apparatus. However, if the inclination
to the vertical is excessive, the effectiveness of the
deflector will be reduced. Preferably, the flow deflector
is vertical.
Preferably, the width of the flow deflector is at least
one quarter of the channel width. Flow deflector widths
approaching half the channel width will lead to excessive
head losses and increase the potential for the capture of
organic particles. The preferred width of the flow
deflector is one third of the channel width.
Preferably, the flow deflector extends to a height above
the channel bed of not less than the maximum water level in
the approach channel. To improve the penetration of grits
into the collection hopper and avoid flow short circuiting
through the exhaust port, the flow deflector should extend
below the base of the channel to a depth of not less than
0.3m or half the approach channel width, whichever is the
greater.
The thickness of the flow deflector is on the one hand
limited by practical miminum material thicknesses, and on
the other by the need to avoid the formation of a hydraulic
control. The flow deflector is constructed from metal or
plastic plate or in the case of wider units from either
pre-cast or insitu farmed concrete. In any event, the
thickness of the flow deflector should preferably not
exceed O.100m. It is possible to use a flow deflector with
an inlet port without an exhaust port, but preferably an
exhaust port is included.
5
13.
The cross-sectional area of the exhaust port controls the
total quantity of flow passing through the grit colection
hopper. If the exhaust port cross-sectional area is too
small, grit capture will not be as positive. If, however,
the exhaust port area is too large, excessive quantities of
water will pass through the grit collection hopper and in
so doing carry a proportion of the finer grit particles
into the downstream channel. It is a feature of the
apparatus that by adjusting the cross-sectional area of the
exhaust part, a varying range of grit particle sizes can be
retained in the grit collection hopper.
The exhaust port is conveniently placed in the floor of
the channel and is of the same width as the inlet port.
Other arrangements which allow flow to pass through the
grit collection hopper are equally acceptable. The length
of the exhaust port is typically less than the inlet port
and is preferably one quarter of the channel width.
The positioning of the exhaust port or ports is not
crucial to the operation of the apparatus. However, the
quantities of finer particles exhausted will be reduced if
it is located immediately downstream of the deflector.
Typically, the grit hopper will follow established
practice of having steeply sloping benching to a draw off
point. Alternatively, it will be a basin with a flat or
more gently inclined base and be equipped with mechanical
rakes and scrapers to move the grit for disposal.
Since all that this apparatus requires is a straight
channel which then has a bend in it with a grit removal
port, much less space is required than for a settlement
basin. No mechanical scraping mechanisms are required which
means that there is less possibility of mechanical
breakdown. Construction costs are much lower than for
previous methods. The head loss across the device is made
out of the normal hydraulic losses of the channel. This
2fl~~~~~
14.
could lead to saving in energy loss of up to approximately
300 mm head of water which is typically about 1/10 of the
whole loss through a treatment works. The action of the
apparatus depends only upon the geometry and therefore its
action is automatic and will not vary with time. Since
there is no requirement for the velocity of flow to be
reduced, the apparatus will be effective over a wide range
of flows and the efficiency will not be flow sensitive.
Thus, there will not be so much necessity for carefully
controlling the flow through the apparatus. By varying the
size of the exhaust port and the angle of the deflector,
the size of particles to be removed can be varied. Thus,
the apparatus can be engineered to remove any particular
size of grit required.
Less organic matter can be removed with the grit due to
the turbulence on the main flow helix having a washing
action.
The grit can be removed from the hopper by continuous
gentle draw off to the washing and disposal arrangements or
grit can be drawn off intermittantly using the storage
capacity of the hopper which is in line with established
practice. If the grit is stored in the hopper, it is usual
to inject a small flow of compressed air near the bottom,
so that the rising bubbles break up accreted grit and
release organic particles. With this device, grit lifted
near to the surface would not be reintroduced to the main
flow, as can be the case for many prior art apparatus
available at present.
Preferably, a sewage treatment works will include several
sets of apparatus in accordance with the invention. If one
or more of the units need to be taken out of commission,
when flow in the channel ceases and it is drained down,
there will be no residual grit and other matter, so that
the maintenance and odour problems which arise in the
detritor will be eliminated.
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15.
It will be appreciated by the skilled addressee of the
specification that the parameters of channel width, flow
velocity, bend angle, bend radius and position of port are
all interdependent, but the choice of suitable parameters
will be readily apparent to the skilled addressee.
Brief Description of the Drawings
Examples of apparatus in accordance with the invention
will now be described and contrasted with the prior art,
with reference to the accompanying drawings in which:-
Figure 1 is a plan view of a first example of prior art;
Figure 2 is a cross section in direction I-I of figure l;
Figure 3 is a glan view of a second example of prior art;
Figure 4. is a section through the apparatus shown in
Figure 3;
Figure 5 is a schematic section through a third example of
apparatus of the prior art;
Figure 6 is a plan view of the apparatus shown in Figure 5;
Figure 7 is a plan view of a fourth example of apparatus of
the prior art;
Figure 8 is a longitudinal section through apparatus shown
in Figure 7;
Figure 9 is a lateral section through apparatus shown in
Figure 7;
Figure 10 illustrates the typical path followed by grit at
a bend in a channel;
Figure 11 is a schematic plan view of a first example
apparatus in accordance with the invention;
Figure 12 is a section in direction II-II of Figure 11;
Figure 13 is a section in direction III-III of Figure 11;
Figure 14 is a schematic plan view of a second example of
apparatus in accordance with the invention;
Figure 15 is an enlarged view of part of Figure 14;
Figure 16 is a sectional elevation along A-A of Figure 15;
Figure 17 is a sectional elevation along B-B of Figure 15;
Figure 18 is a sectional elevation along C-C of Figure 15;
16.
Figure 19 is a detailed view of the hopper used in the
apparatus of Figure 14;
Figure 20 is a section along D-D of Figure 19;
Figure 21 is a section along E-E of Figure 19;
Figure 22 is a schematic view of a variety of possible plan
views of modifications to the apparatus in accordance with
the invention;
Figures 23A to 23D are sections through the channel;
Figure 24 is a section through the channel showing a
modified inlet port;
Figure 25 illustrates modifications to the deflector; and
Figure 26 is a schematic plan view of a third example of
apparatus in accordance with the invention.
Description of the Preferred Embodiments
A detritor 1 is shown in Figures 1 and 2 which relies on
the widening and substantial decrease in flow speed of the
sewage. The sewage is passed from a straight channel 1 into
a much wider channel 5 including a circular basin 7 for
settlement purposes. The flow passes through a set of flow
straighteners 9 which are intended to straighten the flow
and even out the velocity of the sewage as it flows through
the tank 5. A scraper member 11 is suspended from a bridge
across the basin 7 (not shown) and is mounted to rotate. It
has two, three or four arms 13 under which are mounted
blades to scrape along the base of the basin 7. The
reduction in the velocity of the sewage through the basin 7
causes the grit to form dunes on the base of the basin. The
scraper blades 13 push the grit around until they pass into
exit port 15 to pass out into a grit hopper. The detritor
has a number of disadvantages which have been outlined
above, but in normal flow, the main disadvantage is that
the velocity is not constant throughout the chamber and
therefore, in the areas where the flow is much faster keeps
the grit iii suspension and therefore allows the grit to be
passed out of the outlet channel 17. The movement of the
scraper member 11 itself disturbs some grit as it passes
17.
the outlet weir 17 and causes grit to be reentrained in the
flow, allowing it to be passed to the outlet channel. Other
disadvantages lie in that if the scraper lI is stopped by
operatives or, say, by power failure, grit builds up in the
basin 7 and can cause the mechanism to be overloaded with
the result that the scraper cannot rotate or mechanical
damage could occur.
The Pista grit trap 19 illustrated in Figures 3 and 4
depends on the rotation of the flow around a conical tank
21. Paddles 22 are rotated by an electric motor 24 mounted
on a bridge across the tank. The grit tends to fall to the
base of the tank to an outlet 23. An inverted v-shaped
baffle 25 tends to separate the grit from the rest of the
flow.
In the third example of apparatus of the prior art
illustrated in figures 5 and 6, the removal of grit depends
on a vortex separation effect where a tangential inlet to a
cylindrical drum introduces a rotating flow which causes
the grit heavier than water to descend and particles
lighter 'than water to ascend.
A fourth example of apparatus is illustrated in figures 7
to 9 which consists of an elongate tank which is long
enough for the flow to straighten and the grit to settle on
the base of the tank. The grit is then removed by suction
of a pump mounted on a bridge to traverse the length of the
tank.
In an apparatus in accordance with the invention, sewage
is passed along a straight channel of constant cross
section 27 of length at least 10 times the width of the
channel. The channel then bends in plan view with an angle
of 45 degrees to the straight channel and the radius of the
bend is three times the width of the channel. Positioned at
the downstream end of the bend or a short distance
thereafter is the first of two ports 29 on the inside edge
18.
of the curve. These are straight-edged ports which have
vertical sides as shown in Figure 12. They are positioned
at 15 degrees to the channel. Grit is caused to fall
through the ports 29 to grit hopper 31 shown in Figure 13.
Trais is asymmetric with a sloping bottom. Figure 10
illustrates a typical grit path described at a bend which
shows the desired position for the ports 29. By using two
ports ensures that all the grit is removed.
In a second example of apparatus in accordance with the
invention, sewage containing grit is passed along a channel
33 of constant width 35 and cross-section throughout. The
cross-section is rectangular.
The channel is arranged such that the sewage passes
initially along a straight elongate portion 37 having
length 39 equal to 10 times the width 35 of the channel.
The sewage then passes through bend portion 41 which
describes an angle 43 to the axis of the straight portion
37 of 30 degrees. The radius 45 of the bend is equal to 12
times the width 35 of the channel.
Downstream of the bend 41 is a port 47 situated at the
inside of the bend 41 and at the base of the channel 33.
The port 47 is shown in detail in figures 15 to 18 of the
drawings. It is situated in a downstream channel 49 which
is straight and of the same cross-section as straight
portion 37. It has a length of at least twice the channel
width 35.
The port 47 consists of a grit removal device including
an inlet port 51, an exhaust port 53 and a flow deflector -
55. The inlet port 51 comprises a hole in the base of the
channel 33. The length 57 of the hole is equal to the width
of the channel. The width 59 of the inlet port 51 is
equal to half the width 35 of the channel so that the
~~':~~~~~
19.
proportion of organic solids collected is kept to a minimum
whilst ensuring that the grit in the sewage can be
collected.
The inlet port 51 is arranged so that it lies at the end
61 of the bend portion 41.
Downstream of the inlet port 51 is exhaust port 53 which
has the same width 59 as the inlet port 51, for
convenience. The length 63 of the exhaust port is equal to
one quarter of the channel width 35.
Between the inlet port 51 and the exhaust port 53 is flow~
deflector 55. This comprises a plate which extends into the
sewage flow and down under the base of the channel 33.
The flow deflector 55 is vertical and has a width 65
equal tv one third of the channel width 35. The height of
the flow deflector is such that it extends to not less than
the maximum level in the approach channel 37.
It extends below the base of the channel to a depth of
one half of the channel width 35.
The deflector 55 is here a metal plate having a thickness
of less than O.lm
The inlet port 51 and the exhaust port 53 both feed to
grit collection hopper 67, shown in detail in Figures 19 to
21. The hopper 67 has steeply sloping benching 69 to draw-
off point 71.
The apparatus is used by passing grit containing sewage
through the channel with a depth of 1 to 1.2 times channel
width 33. The velocity of the flow is typically 1.00 m/s.
The sewage is passed through the straight portion 37
whose length is greater than 10 times its depth to allow a
20.
stable flow pattern to be established. The grits in the
sewage are transported in the lower part of the flow near
the bed of the channel. The sewage then passes through bend
portion 41 where the grits are concentrated against the
inside face of the turn. The flow develops a core of high,.
velocity rotation in the form of a helix so that the grit
moves to the bottom of the~channel at the inside of the
curve whilst the lighter material is moved to the top near
the water surface on the outside of the curve.
1G At the end edge 61 of the bend 41 is inlet port 51
through which sewage containing a large quantity of grit
flows. The flow deflector 55 aids the capture of the grits
thrown up into the flow owing to the interaction of
particles as they saltate down the channel and bend. The
flow deflector 55 helps to direct the flow into the inlet
port 51 and directs the grit downwards into hopper 67.
The flow deflector 55 directs the flow deep into the grit
hopper 67. The grit is deposited and the flow then flows
upwards out of the hopper 67 and out through exhaust port
53. The sewage returning to the channel is substantially
without grit and the sewage may then be further processed.
Figure 22 illustrates a variety of plan views of
variations to the apparatus, specifically the different
radii and angles of bends which can be utilised. The bend
may be a left hand bend (A-C, G-I or M-O) or right hand
bend (D-F, J-L, P-R). It can be very sharp (A-F) or can be
gradual with a large radius (M-R).
The angle described by the bend can be as small as 10
degrees (C,I,O,D,J,P) or large, of 45 degrees
(A,G,M,F,L,R). Four examples of cross-sections of the
~~'..?~~J~
21.
channel 33 are illustrated in Figures 23A to 23D. The
apparatus shown in Figure 14 has a rectangular cross-
sesction as shown in Figure 23A. In some cases, the cross-
ss;ction may be arranged to slope along one edge (23B) or
across the base (23C). Alternatively, one edge may be
rounded (23D).
Although the inlet port may comprise a hole in the base
of the channel, it may also be arranged as shown in Figure
24. Here the port 73 extends up the side of the channel on
the inside edge of the bend.
The flow deflector 55 plate shown in Figure 14 is
vertical. It is possible for the plate to be arranged at an
angle to the vertical and the limits of the angle are shown
in Figure 25 in dotted lines.
Although the channel width has to be constant throughout
the length of the straight portion, the channel width does
not have to be constant throughout its whole length. Figure
26 illustrates a third example of apparatus, in accordance
with the invention, where the channel widens after the bend
in the region of the grit collection device. The width of
the inlet port is half the width of the channel in the
straight portion.
