Note: Descriptions are shown in the official language in which they were submitted.
~.76~8~
The present invention relates to filtration systems and
methods of filtration for removing impurities from water.
One conventional filter system consists of a bed of
granular material such as sand or crushed coal supported on a
bed of gravel. When water passes through the filter, suspended
particles and flocculant material come in contact with the
granular material and adhere to it. This reduces the effective
size of the water passages through the filter and produces a
straining action within the filter. With time, more and more
material becomes trapped in the filter media, the pores clog,
and the hydraulic headloss through the bed becomes excessive.
When the headloss becomes excessive, the granular media
is cleaned by backwashing. During backwashing, previously
filtered water is passed upwards through the filter at a
sufficient velocity to force a fluidization of the granular
material which results in an expansion of bed depth by about 30
to 50 percent. Material which has been filtered out and adheres
to the media is dislodged by the shearing action of this
backwash water and is carried off to waste.
It is generally considered that about 5 percent of the
filtered water is required for backwashing the dirty filter
material. However, one survey of water filtration systems has
shown that the volume of backwash water has been reported to be
as much as 27 percent of the filtered water. The turbidity of
this backwash water requires that it be treated prior to
disposal to receiving waters.
During backwashing, filter medias become hydraulically
graded to leave the finest grain sizes at the top and the
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1~76~
coarsest grain sizes at the bottom. This grain size
distribution through the filter depth is the reverse of what is
desired if the filtered solids are to penetrate deeply into the
filter bed. Poor filter bed utilization occurs if the fine
grains of the upper surface of the filter clog. ThiS problem
can be partially avoided by using a multimedia filter.
Multimedia filters use different ~ranular materials of different
specific gravities so that che hydraulic grading places these
lower mass, larger diameter particles on the upper surface of
the filter. Obvlously, a properly, hydraulically graded filter
can only be achieved by using a large variety of media.
An extensive investigation of the effect of various
filtration parameters on the backwash characteristics of single
and multimedia filters has been comp]eted and published ("Filter
Backwash Water Economy", J.D. ~ur~e, M.Enq. thesis, Dept. of
Civil Engineering, Royal Military College, Kingston, Ontario.~
In that investigation, marginal reductions in the backwash water
required to clean filters were achieved~ Some of these
reductions can be attributed to the careful control exercised
during the test program. Nevertheless, a dramatic deviation
from conventional filtration techniques will be required if
water filtration wastes are to be minimized.
It is accordingly an object of the present invention to
provide a novel and improved filtration system and method of
filtration which reduce filter backwash waste disposal.
According to the present invention, there is provided a
method of filtration, which comprises the steps of:
passing an influent liquid throu~h a single cylindrical
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117~i18~
filter bed of annular cross-section~ said filter bed having
particles of substantially uniform grain size of between ~50
and 400 ~m in the direction of thickness of the filter bed;
subesquently passing a backwash of filtered liquid through
the filter bed in a second direction transverse to the first
direction at a sufficlent velocity to fluidi.ze the filter bed and
dislodye the filtered solids from the filter bed to form a suspen-
sion of solids in the backwash;
~, repeatedly recycling the backwash through the filter bed;
and
finally purging the filter bed with clean water.
Also according to the present invention, there is pro-
vided a filtration system, comprising:
a single cylindrical fllter bed of annular cross-section
having particles of substantially uniform grain size of betweer
250 and 400 ~m;
means for passing an influent liquid through said filter
bed in the direction of the thickness of said filter bed;
means for passing a backwash of fi.ltered liquid through
said filter bed in a direction perpendicular to the thickness of
the filter bed at a sufficient velocity to fluidize said filter
bed and dislodge filtered solids from the filter bed to form a
suspension of solids in the backwash; and
means for passing fresh water through said filter bed in
said perpendicular direction.
This invention will be more readily understood from the
following description of a conventional filtration system and a
filtration system embodying the present invention, taken in
conjunction with the accompanying diagrammatic drawings, in which:
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6~81
Figure 1 shows a perspective view of a conventional
filter bed;
Figure 2 shows a perspective view of a filter bed
embodying the present invention;
Figure 3 shows a graph i3.1ustrating the penetration of
i solids into a filter bed; and
Figure ~ shows a filtration system.
A soLid cylindrical core from a conventional filter bed
indicated generally by reference numeral 9, is shown in Iligure 1
cotnprising a bed of coal 10 on a bed of sand 11, whic~ in turn
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~L~76~8~
overlies a bed of gravel 12. Influent is passed downwardly
through the filter bed 9 as indicated by arrow 14, the r~sultant
effluent leaving the bottom of the filter bed 9 a5 indicated by
.arrow 15, and a backwash is subsequently passed upwardly through
the bed as indicated by arrow 16. According to Darcy's law, the
flow through this filter bed i~ given as:
Qc = KCAc ~h
Lc (Equation 1.0)
where Qc = flow rate (m3/s~
Ac = filter area (m )
Xc = coefficient of permeability (m/s)
~h = head loss through filter (m)
~c = depth of filter bed (m)
In operation, the filter bed is backwashed by applying
a reverse flow of water up through the filter bed at a
sufficient velocity to fluidize the granular media. This
reverse flow is maintained for a fixed period of time (usually
five minutes) to allow the shearing action of the fluid to
dislodge the filtered out solids. An expression for the
backwash volume of water required for the conventional filter
bed is given as:
Vc ~ VcActb (Equation 2.0)
where Vc = volume of backwash water (m3)
Vc = backwash velocity (m/s)
tb = required backwash time (s~
L ..
6~
Now consider a filter section which occupies the same
volume as that shown in Figure 1, but constructed as a hollow
cylinder as shown in Figure 2, in which there is illustrated a
hollow cylindrical filter bed 20 of annular cross section,
referred to below as "the annular filter bed 20~, the influent,
effluent and backwash being indicated by reference numerals 14a,
15a and 16a. The flow is now permitted to pass through the
cylindrical wall, i.e. in the direction of the thickness of the
annular filter bed 20, and, according to Darcy's law, is given
as:
QA = 2 x Lc KA dy (Equation 3.0)
dx
where QA = flow through annular section (m3/s)
x = any radius limited between ri and rO (m)
Lc = length of annular section (m)
~ = slope of hydraulic surface from rO to ri
x
quation 3.0 is of the orm
dx = 2~L KA d
X C _, y
hich when integrated between the limits of rO and ri gives
2~L. K
Ln (ro/ri) = ~A- ~- (YO Yi
But (yO - Yi) equals the head loss, ~h, across the filter.
Thus QA = 2 Lc KA ~h
Ln (rO/ri) (Equation 4.0)
When this annular filter bed is backwashed vertically,
i.e. axially, in the same manner as the conventional filter,
then the annular area through which the backwash water must flow
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l ~
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1~L76~81
is obviously smaller than ~he cylindrical cross-sectional area
of the conventional filter cylinder. The volume of backwash
water required is
VA vA1~rO ri) tB
(Equation 5.0)
where VA = volume of backwash water (m3)
VA = backwash velocity (m/s)
Since backwashing is an erosion process which results
from the viscous shear of the washing fluid, the time requi/red
for backwashing is reasonably consistent between filters. The
required velocity of the backwash water, however, will vary
directly as the square of the diameters of the fil~er media
particles. In comparison to the larger particles, the lower
viscous shear experienced by the smaller particles is balanced
by the thinner deposits found on their surfaces, thus requiring
similar backwashing t~mes.
It is generally believed that the deposition of solids
with regard to penetration depth into the approach face of a
filter, follows a logarithmic function. This implies that the
greatest mass of filtered solids will be deposited on the
approach face of the filter, and will logarithmicaily diminish
with depth into the filter. ~n practice, it is recognized that
excessive headloss and plugging of the approach face of filters
~B - 7
6~81
signals the requirement for filter cleaning. Thus the mass of
filtered sol~ds assumed to be trapped by a f$1ter media would be
approximately estimated by the $ntegral of the concentration and
depth logarithmic relationship, multiplied by the area of the
filter approach face. Much of the filter volume is unused
unless solids penetrate deeply into the filter bed.
Tw~ ~n~ of sand were tested bo deb~ne a suitable uniform
~n size for an ~ ular filber and n~m~l grain si~s of 250 bo 400 m
gave satisfacbory results. No~l sizes have an a¢ ~ 1 bole ~ oe of -0
+5% and so the ach~l size of gr ~ lies in the range 250 to 420~m.
various fil ~ bed dep ~ ranging from 5 cm to 40 cm were ~nd bo be
satisfactory for solids penetration resulting from the f~tratl'on of
solutions of bentonite and alu~. Bentonite dosage ranged from
12 to 24 mg/L producing a turb~dity up to 5.0 FTU and alum
dosage was reduced from lS to 5 mg/1. The headloss across the
filter bed was maintained constant, allowing the flow rate to
diminish with t$me. When the flow rate was reduced to
approximtely one half of the original flow rate, filtration was
terminated and the f~lter bed was analysed for solids
concentration at various depths. Filter beds which were too
thick showed no util$zation of void space for solids depos~tion
in the lower strata. These thicker beds also had a lower
in$tial iltration rate than the shallower filter beds. Good
filter bed utilization for solids depos$t$on ~as found for beds
approx$mately 10 cm deep. This depth of bed also gave effluent
turbidities of less than O.S FTU, and initial f$1trat$on rates
of up to 3Q0 ~ m2 min w$th a head loss of 30 cm. Typical
illustrations of the solids deposition w~th depth in the 10 cm
deep filter bed are shown in Figure 3 for 300~m sand. These
B tests have shown that a good quallty effluent can
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~1761~31
be produced from a uniform sized! fine grain, filter media which
does not surface plug.
To backwash this filter media, a closed circuit
recycling reservoir system is used as illustrated in Figure 4~
The system shown in Figure 4 comprises a filter bed 20 contained
in a housing 21 having at its bottom an outlet 22, which
communicates through pipe 23 with a backwash water recycle
reservoir 24, through pipe 25 with a clean water reservoir 2~,
and through pipe 27 with tank 28, the pipes 23, 25 and 27 being
provided with respective flow control valves 30, 31 and 32. The
tank 28 is connected by pipe 34 to a turbidimeter 35, which is
connected in turn by pipe 36 to the clean water reservoir 26.
Dirty water is supplied to the filter bed 20 through
pipe 38 from a holding tank 39 by a pump 40 and a further pump
41 is provided between the outlet 22 and the pipes 23 and 25~
Pipe 42 connects the backwash water recycle reservoir
24 to the filter bed 20 in the housing 21 and is provided with a
pump 43. The backwash reservoir has a bottom outlet 44 provided
with a flow control valve 45 ànd communicating with a sludge
waste tank 46.
In operation, dirty water is fed by the pump 40 to the
filter bed 20 and the effluent from the latter passes through
the turbidimeter 35 to the clean water. When the filter bed 20
re~uires cleaning, water from the backwash reservoir 24 is
pumped sufficient velocity to fluidize the filter bed 20 and
dislodge the filtered solids from the filter bed 20. The
filtered solids, once dislodged, stay in suspensio~ in the
recirculating backwash water, even during the reverse flow
11~7611~
through the expanded filter bed, and are fed by the pump 43 to
the backwash reservoir. To purge this murky water from the
filter bed prior to recommencing a filtration cycle, a plug of
clean, filtered water is drawn from the filtered water reservoir
by the pump 41, the valve 31 being opened and the valves 30 and
32 being closed, and is pumped at a fluidizativn velocity
through the filter bed 20. When the interface between the clean
water and the murky water emerges from the top of the filter bed
20, the backwash cycle is stopped. ThUS, the volume of clean
water reguired for filter cleansing is equal to the pore space
of the expanded filter bed. An expression for this volume of
water for a 30 pércent bed expansion is given as:
V 1 3 n V (~quation 62
where VF = volume of filtered water required (m3)
n - filter media porosity
Vm ~ volume of filter media bed (m3)
The uniform grain size of the filter bed 20 ensures
that the coefficient of permeability measured horizontally
through a wall section of filter remains constant through the
vertical depth at the annular section. Non-uniform grain sizes
would be hydraulically redistributed during backwashing and
produce a horizontal coefficient of permeability gradient which
would increase with depth. Therefore, sand is used for the
filter bed which has a high sphericity, giving more consistent
sieved grain sizes than angular beach sand. The no~inal 300 ~um
sand used has a size range between 300 and 315 ~m or
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~76181
300,+5%,-0% r~ grain size, and a sphericity of about 0.7 as
measured by Stokes terminal settlin9 velocityO
Lc ~ 2.0 m
r9 = 0.5 m
ri ~ 0.4 m
KA = 0~3 x 10 3 m/s
Rc ~ 4.0 x 10 3 m/s
The ratio of flows through the annular filter bed 20
and a conventional filter contained in the same 5pace can be
found from the ratio of e~uation 4.0 to equation 1.0 as follows:
2~LC XA ~h
./Qc ~ ~ (Equation 7.0)
c
Applying the same head loss across both filter beds and
substituting appropriate di~ension and permeability values gives
QA/QC = 10.75
Thus the rate of flow through the present annular filter bed is
10.75 times greater than that through the conventional filter
bed occupying the same volume of space.
The ratio of the approach face area of the annular
filter bed to that of the conventional filter bed contained in
the same space is:
AA 2~ rOLc (Equation 8.0)
AC ~rO
= 8 for ~le measur~ts ~iven.
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~17618~
Since the solids deposition into the approach face controls the
frequency of backwash cycles, it can, for the present, be
considered that the conventional filter bed requires
bac~washing, due to headloss, approximately 8 times more
frequently than the annular filter bed. Thus, the volume of
water filtered by the annular filter bed is 8 times greater than
that produced by the conventional filter bed between respective
backwash cycles.
Under similar headloss conditions, it is found that a
10 cm thick test section of the filter bed 20 has the same
filtration ability to assimilate solids as the thicker section
found in the conventional filter bed. During backwashing,
however, the required velocity of backwash water varies as the
square of the diameter of the filter media particle size. For
the annular filter, a preferred particle size of 300t'm is used.
Similarly, average particle size for conventional filter sand
beds is about 500r~m with a uniformity coeeficient of about 105.
The ratio of the required backwash water velocities for the
conventional and annular filters is:-
v 500 = 2.77
300
Thus, to filter an equivalent amount of water, the ratio of thebackwash volume for the conventional filter bed in comparison to
an annular filter bed occupying the same volume is:-
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c = VC ~ rO tb AA (Equation 9.0)
VA vA ~ (rO ri ) b Ac
2.77 rO
rO ~ ri
= 61.5
Without considering the recirculation feature to be
used with the backwash water in the annular filter bed, it can
be shown analytically that the conventional filter bed will
require approximately 60 times more water to clean it, than an
annular filter bed producing the same quantity of clean wster.
For the conventional filter, a backwash velocity of
about 1.0 m/min is required during a backwash cycle lasting 5
minutes and the conventional filter bed requires backwashing 8
time to produce the same volume of filtered water as the annular
filter. Thus the conventional filter requires a backwash volume
of approximately
Vc ~ 1.0 x rO 2 x 8 x 5 2 31.5 m3
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~1761~3~
Cons~der~ng the recycling feature for the backwash
water in the annular filter, the volume of backwash water
required as given by equation 6.0 for n equal 0.42 is:
VF = 1.3 x 0.42 x ~rO 2 _ ri 2) Lc
_ .31 m3
The present system may b,e used to produce filtered
water for cities, towns etc. by replacing the conventional sand
filters in ~se, and can also be used for treating other liquid
aqueous wastes once further developed.
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Thus, when recirculation is considered, the ratio of
the backwash water required for the conventional filter in
comparison to the present annular filter is:
V = ~i-- = 100 ~quation 10.0)
It has been found that, with the above-described
filtration system:-
a. the filter media size is prefereably of 250 to400~m particle size
b. the filter media is preferably uniform in grain size
c. bed depth (wall thickness) is preferably greater
than 5 cm and less than 40 cm
d. scouring water ùsed to clean the filter may be very
dirty yet still function appropriately
e. traces of dirty scouring water may be flushed from
the media with a plug flow of clean water
f. the cleaning of the filter is water velocity
dependent, and the present filter permits reduction
of the cross section through which this water
velocity must pass, thus reducing the volume
required.
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