Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
L BACKGROU~ID AND COMMERCIAL PRIOR ART
The conventional method of backwashing or cleaning
granular media filters, as commonly used for removing sus-
pended solids from waters or wastewaters, is to pass water
upward through the bed at sufficiently high velocities that
the media are first fluidized, that is, the media grains are
suspended or floated in the upward flowing backwash water,
and secondly, expanded by as much as 50% over the settled bed
height so tha~ particles of entrapped solids can be removed
lo from between the media grains. After a backwash operation of
this type, a single-medium filter bed is left stratified so
that the smaller grains are in the upper part of the bed and
the larger grains are in the lower part of the bed. When
multiple-media filters - usually consisting of coal over
sand, or coal over sand over garnet - are used, the sizes of
the media are selected so that the lower density and usually
larger grain size media overlie the higher density and usually
smaller grain size media.
When-using multiple-media filter beds, a zone of
~20 intermixed media may occur in which the small sizes:of. the
.:: higher density media are interspersed.with the:larger sîzed
. . ~ , , .................... .. , . . . . ;.. . .
grains o~ the lower density media~; The.exten~ of~intermixing
depends on the relative size and density of tke media~wit~in
each layer and may be controlled to some extent b~ appropriate
selection of size and gradation of each medium and by the rate
and duration of back~ash~
. Conventional filter designs range from those having
- .,.. -.:, ., ...- ~ ,
~ essentially no intermixing to those`having intërm xing
". . ;
~7~3~4
1 throu~h most of the bed depth. Filters having as many as
four distinct layers distinguished ~y media type and grain
size or both have been used in full-scale water and waste-
water filtration ~lants. The amount of intermixing is, to
a large extent, a filter design option.
. :;
Conventional fluidization and expansion backwash
methods are not always as effective as desired. The ineffec-
tiveness of scouring of solids with water alone is well
known, and is the reason for development of auxiliary media
lo scouring tschniques such as air wash preceding water wash
and media scouring with high velocity jets of water prior to
and during water wash. Even these methods are not as effec-
tive as desirable and examples of dirty media and ~Imudballs~
remaining after backwashing by such techniques also are well
known.
An alternate backwash method is to use air and water
simultaneously throughout much of the backwash cycle. Both
research tests and field experience have shown this method
to be much more effective in cleaning filter medla than
20 conventional backwash methods. Prior to the method described
- , ~
by this invention, the simultaneous use of air and water at the
same time water is flowing from the backwash colIector has been
.. ~ . .
restricted in practice to single-medium beds that did not rèquire
stratirication or separation into two or~more layers of dlfferent
sizes or tyEesof media. The simultaneous use of air and water
for durations longer than required to fill the volume between
the lowest water level prevailing at the end o:the filtration
sequence and the overflow weir of the backwash collector will re-
. - ~ ... . . .- -
sult in loss of significant amounts of media unless some ~ositive nYans`
~117~13~4
-
1 is used to control media loss.
When simultaneous air-water wash has been used with
multiple-media filters, its use has been restricted to a
very short period during which water rises from a level
slightly above the surface of the media to the edge of the
waste backwash water collector. The air flow must then be
terminated to prevent media loss, and the water wash is
continued, usually at an increased rate, to wash entrapped
solids from the bed and to separate the media into respective
lo layers of different size or density. Using conventional
media grain-size and density combinations requires that a
relatively high rate water-only step follow the air wash
or hydraulic scouring to fluidize and expand t-hese media so
that solids washout and stratification occur. For example,
a dual media mixture of 1.0 mm effective size (E.S.) coal
over 0.5 mm E.S. silica sand can be backwashed using water
at a rate as low as 8 gallons per minute per square foot
(gpm/sq.ft.) simultaneously with air at rates as low as 2
standard cu.ft. per minute per square foot (scfm/sq/ft.),
whereas washou~ of solids from these two media and subsequent
restratification using~ water alone, as used in conventional
backwash methods, require water rates as high as 20 gpm/sq.t.;
-:-3 and when using larger grain size combinations, even higher
rates of water become necessary. However, it is desirable
to be able to use a simultaneous air-water wash method with
multiple-media filters that can be extended for a longer
backwash duration than the rise time ~rom slightly above the
surface of the media to the edge o the waste backwash water
collector without the loss of media and subsequently will
.
.7~13~
1 restratify the filter bed in a manner suitable for filtration_
PATENT PRIOR ART
A number of multiple-media ilters and systems of
filtration and backwashing have been patented. Rice et al
patent 3,343,680 discloses a three-media filter wherein
particles of all three media are intermixed. The filter is
said to be designed so that in filtration use the number of
particles per unit area continually increase in the direction
of water flow through the bed. In preparing the filter, it
is disclosed that ~he initially stratified layers of the
beds are placed in the filter, and then "backwashing the bed
until the particle distribution has reached a substantially
constant orientation" (col. 2, lines 37-40). -Only backwashing
with water alone is described. It is stated that thereafter
both filtration and backwashing can be carried out without
substantially changing the particle distribution.
Hsiung et al in patent 3,876,546 disclose an extension
four-media hed of the Rice et aI filter-bed. As with the
- ~ Rice et al bed, the particles of the differen~ media of the
bed are said to be intermixed so that there is~a continually
increasing ~umber of particles per unit area in the dlrection
of water flow through the bed during filtration~ Theibed is
backwashed with water at a rate su~ficient to adequately
fluidize the bed, the backwashing being with water only
except that air scourin~ or hydra~Ii~ scouring may precede
water wash.
Multiple-media beds have been proposed which are
designed to avoid or minimize intermixing of the stratified
:, - `
, ~17~364
1 layers of the~different.media~ Hirsch 3,497,068 discloses a
multiple-media bed in which the different media possess equal
hydraulic uplift properties when subjected to backwashing.
Presumably, therefore, each layer of the bed expands to
permit removal of the entrapped solids without intermixing
.:.3
of the layers, thereby permitting the layers to settle back
to their original stratified condition after backwashing
is concluded. The backwashing is by water alone.
Hirs 3,925,202 discloses a dual-media filter bed
lo where the layers are partially intermixed. The recommended
method OL backwashing is to first scour the bed with air,
and then to use a water wash without air. An alternate
method .is mentioned in which air is introduced simultaneously
with the backwash water (col. 4, lines 28-36), but this method
was not related to media size, combination, preferred backwash
rate, or desired media separation~
The problem~of reverse stratification during backwashing
is discussed in Hirs patent 4,048,068. Bed designs nd a
multiple-tank filter system are disclosed for avoiding such
- 20 reverse stratification, which may require regr.ading of the
beds to a more stratiied condition before filtration can be
continued. Another system for avoiding the necessity of
regrading layers of a multiple-media bed is described in Hirs
~ patent 3,814,247.
Simultaneous air-water backwashing of single-media
filters has been proposed including the.desirablility of
equipping such filters with baffled backwash troughs to reduce
media loss. See Scholten et aI patent 4,076,625 and Row et al
. - 6 - .
3~4
1 patent 2,453,345. Neither of these patents sugsests how such
filters can be used with multiple-media beds.
SUMMARY OF THE INVENTION
The backwash method of this invention is applicable
to the use of two or more layers of media differentiated in
size or density or both and backwashed in such a manner that
the bed is substantially mixed during backwash by simul-
taneously passing air and water through the bed. The duration
of this backwash operation is extended sufficiently that the
air and water flow simultaneously while waste backwash water
passes over the weir of the backwash collector. After the
air application is discontinued, at a time determined by
cleanliness of the backwash water o~ by the duration of back-
wash or by other appropriate means, the flow of water is con-
tinued to separate the media into respective layers distin
guished hy ~ize and density or both. By properly selecting
the backwash rate and the grain size and density of each
individual mediu~, various types and sizes of media can be
substantially separated without lncreasing the water rate
20 above that used during simultaneous air-water wash. For
.. . ..
example, a filter bed consis~ing of 1.0 mm E.S.~anthracite
coai.ha~ing a S~eCifiG .gravi~.~E.aoout:1.7 (~-~ght relativ.e.to equal volume of
water) over 0.5 mm E.S, garnet sand or other granular m~terial
having a specific gravity great~r than 2~60 can be washed .
effectively when using water at 10 to 12 gpm~sq~ft~ sîmul-
taneously with air at 2 to 3 scfm/sq~ft~ and then can be
stratified effectively without ha~ing to increase the ~ack-
wash water rate. Th~s same filter media combination, wh.en
backwashed by conventional methods~ typ.ically requires about
36~
1 20 ypm/sq.ft. to expand the bed during backwash sufficiently
to remove entrapped solids and to separate the media into
its respective layers. Although -the backwash water rate
used in the method of the present invention can be held
substantially constant throughout the entire backwash
.... :1 ~ .
operation, and this is a preferable procedure, the method
of backwashing cf this invention can also be used with a
low rate of water during simultaneous air-water-wash, which
is followed by a higher rate of water alone to separate
10 the media after the air flow is terminated~
The method also can be usPd for backwashing filter
bed combinations having media sizes and densities other than
those referred to above, providing the backwas-h air and water
rates are adjusted to those appropriate for the different
combination.
Features of the present invention include:
(1~ providing a method of back~Jashing granular-media
filtars comprised of two or more media sizes or densities or
both by using air and water simultaneously w~île waste back-
.., :.
20 wash water is passing into a bac~wash water collector,
(2) controlling media loss during simultaneous air-
water wash of multiple-media filters by using a system of
baffles surrounding the backwash collector or other suitable
means to separate the air from the was~e backwash water
preceding its exit over the backwash water collector; ànd
~ 3~ providing a method of washing multiple-media
filters with the simultaneous use of air and water but without
- 8 -
1 requiring an increase in the backwash water rate to separate
the media into size or density layers after the air flow is
terminated.
The improvements in backwashing achieved by the
method of this invention provide an increased abllity to scour
entrapped solids from the surface of the media grains of
multiple-media filters and from the interstitial spaces
between grains. Further, these solids can be removed
effectively at lower backwash rates than used in conventional
lo backwash techniques. The method also permits effective
cleaning of multiple-media filter beds containing larger
grain sizes than are used in conventional filter designs.
This feature permits significant increase in run time between
backwashing and produces savings in filter operating costs and
costs for treatment of waste backwash water. Conventional
multiple-media filters usually are restricted to the use of
media that will expand 20 to 50% at backwash rates of 15 to
20 gpm/sq.ft. This means that the use of anthracite coal
media having a specific gravity of 1.6 to 1.7 is limited to
effective sizes of about 1.0 mm; silica sand (S.G. - 2.6 to
- 2.7) effective sizes are limited to about 0.6 mm; and garnet
~S.G. about 4.1~ or ilmenite (S.G. abou~ 4.6) sizes are limited
to about 0.3 mm E.S. Larger sizes of these media are commer-
cially available but when used in multiple-media ilters,
require such high backwash rates as to be impractical. For
example, the use of 1.5 mm coal over Q~8 mm sand~re~u~res
about 30 gpm/sq.ft. backwash rate to expand the bed ~0 to
30%, and although the use of such rates is possible, the size
of physical appurtenances such as v~lves, piping, pumps, etc,
~17~3~4
j
1 cause filter operating costs to increase appreciably.
BRIEF DESCRIPTION OF THE DRAWINGS
,
The accompanying drawings and diagrams are illustrative
of preferred embodiments of the method of this invention.
~ IG. 1 is an elevational sectional view of a water
filtration apparatus, including a baffle and wash trough
assembly in the upper portion thereof, and a multiple-media
filter bed in the lower position;
FIG. 2A is a fr~ment~ry enla~ged view of a portion
oof the multiple-media bed of FIG~ l illustrating the appearance
of the bed during normal filtration; -
FIG. 2B is an enlarged fragmentary view of a cross-
section of the multiple-media bed simllar to FIG. 2A but
illustrating the appearance of the bed during combined air-
water backwashing;
FIG~ 3 is a diagram illustra~ing t~e preferred size
relation between the coal medium and the silica sand medium
for use as adjacent layers in a multiple~media bed~
FIG. 4 is a diagram showing the preferred size relation
-~ 20 between a coal medium and garnet sand or ilmenite medium for
use as adjacent layers of multiple-media beds; and
- FIG. 5 is a diagram illustrating the preferred size
relation between a silica sand medium and garnet sand or
ilmenite medium for use as adjacent lâyers in multiple-media
beds.
.
~17~364
DESCRIPTION OF PREFERRED EMBODIMENTS
Looking first at FIG. l thereis shown a vertically
extending tank 10, which as shown is formed of metal tviZ.
steel), but it can be constructed of other ma~erials, such
as concrete. It will also be understood that although the
tank, as shown, is circular in horizontal cross-section,
other shapes may be used.
The tank 10 has an open top, circular side walls 11,
and a closed bottom 12. It also will be understood that
lo enclosed tanks for filtration under pressure may be used.
Spaced upwardly ~rom bottom 12 there is provided an under-
drain plate 13, supported by bracing 14 and connected to the
tank sides 11 by means to form a water-tight seal therebetween.
Extending through plate 13 are a plurality of nozzle tubes 15
having strainer caps 16 on the top thereof above plate 13.
During downflow filtration the water passes downwardly
through the strainers 16 and the nozzles 15 into the underdrain
chamber 17 and is removed through pipe connection 18 to the
filtered water outlet pipe 19. The same arrangement can be
used for upflow filtration the water to be filtered entering
chamber 17, which now serves as a water introduction chamber,
rather than as an underdrain chamber, the water then passing
~ upwardly through the nozzle tubes 15 and the stainer cap 16.
During washing, water is supplied under pressure through
pipe 20 to pipe 18 and chamber 17 for passing upwardly through
nozzle tubes 15 and strainers 15~ Al~o during washing r air
is supplied under pressure through pipe 21 which connects with
underdrain chamber 17. ~ir enters the nozzles through holes
.:. ` ;!
~7~364
1 in the upper portions thereof, while water enters through the
lower ends of the nozzle tubes. Further details of this
filtration apparatus are described in Scholten and Young
patent 4,076,625. Similar apparatus is available commercially
from General Filter Company, Ames, Iowa. Alternate means of
....~
` ` adding the air to the filter can be used, for example, as
through a distribution grid placed within or immediately
below the filter media bed. The pipes 19, 20 and 21 are usually
provided with separate shut-off valves as is drainpipe 22.
lo A multiple-media filter bed is provided above plate 13.
Where the fine granular material, such as sand, extends to
the drain plate 13, the nozzles 15 may be equipped with
strainer caps 16 having a series of narrow annular slots
through which the water flows while retaining the granular
medium above plate 13. (See U.S~ patent 4,076,525) The
filter medium comprising the multiple-media bed will contain
two or more different filtering materials, such as granular
materials of different average size, different denisty, etc.
.
- For example, the multiple media bed may comprise a
20 two-media, three-media, or four-media bed, the media being
selected in accordance with media previously used for such
multiple-media beds. Two or more layers of media will be
~3
~ provided which are differentiated in size or density or both.
Commonly,- there will be a dist~nct upper layer of a medium of
lower densi~y and larger ef~ective size and at least one
distinct layer of a medium of higher density and smaller
effecti~e size. For example, in a two-media bed, the upper
layer may be comprised of coal particles and the lower layer
particles of silica sand. AIternatively, in a two-media bed,
`the upp~r layer may be o~p~ised;o ooal particles, and the lower layer of
.;
- 12 -
1 either garnet or ilmenite sand. As a further example, in a
three-media bed, the uppermost layer mayib~ o~rised of ~x~ pa~ticles,
the intermediate layer particles of silica sand, and the lower
layer particles of garnet or ilmenite sand. For other useable
multiple-media beds, reference may be had to the media dis-
closed in the United States patents 3,343,680 and 3,876,546.
Returning to FIG. 1, the wash trough and baffle
assembly used for preventing media loss during simultaneous
air and water backwash will now be described. A wash water
lo collection trough 27 is supported to extend horizontally
across the upper portion of tank 10. Trough 27 has an upper
edge on at least one side thereof functioning as an overflow
weir. As shown in FIG. 1, trough 27 provides overflow weirs
on both sides thereof, being respectively designated by the
numbers 28 and 29. Trough 27 as shown in FIG. 1 can vary in
design and construction detail without affecting its function.
Baffles 33 and 35 are supported adjacent to both sides
of the trough in spaced relation to the trough and extending
horizontally along the weir edge 28. Baffle 33, referred to
as the long baffle lies adjacent to side 27a of trough 27
and includes a portion 33a extendlng below the trough bottom
- 30 and a portion 33b extending to a level above weir edge 28,
.~
thereby defining a restricted flow channel 35 ~or the passage
of backwash water into trough 27. Baffle 35, referred to
herein as the short baffle, is supported adjacent the other
side of the trough (side 27b3 and extends horizontally along
weir edge 29. Baffle 35 also incIudes a portion 35b
extending to a level above weir edge 2g, thereby defining a
second restricted flow channel 36 for the passage of backwash
- 13 -
1~7~ ;4
1 water into the trough 27. The lower end of portion 35a of
baffle 35 terminates above and is spaced from the lower end
portion 33a of the long baffle 33. For further details,
reference may be had to patent 4,076,625.
It will be understood that the above detailed physical
description of a granular medium filter and its appurtenances
such as the backwash water collection trough and air-water
separator baffles are given here to aid in~.the description
of the backwash method of the invention and that tank size
lo and configuration and trough and baffle assembly designs
can vary without affecting the purpose and execution of
the subject backwash method.
With the foregoing background, the multiple-media
washing method of the present invention can ~e understcod.
It is carried out in a filter including a tank ha~ing a
~ratifi~d`nL~tip~.~media bed therein including a distinct
upper layer of a medium of lower density ~and preferably
also a larger ef~ective particle size) and at least one
distinct lower Iayer of a medium of higher densi~y (and
- 20~.~preferably also sm~ller effective particle siæej.~ An upflow
air-water wash means- is provided which permits simultaneous
washing of the bed with a mixture of air and water followed
.. ~ .
: by washing with water alone, and there is provided water
withdrawal means in the uppe~ portion of the tank above the
bed including means for diverting media suspended in the
wash water from exiting therewith~ As the first step of the
washing method, there is passed a wash mixture of air and
water upwardly through the bed at comblned air and water flow
rates sufficient to cause partial intermixing o the distinct
, .
- 14 ~
~.~.713~4
l layers. The combined air-water backwash flow is continued
while simultaneously removing water from the tank through
the wash water withdrawal means in the upper portion of the
tank. Advantageously, from 25 to 75% or more of the water
used for the wash c~cle can be passed through the bed and
exit through the water withdrawal means during the simultane-
our air-water wash. The combined air-water wash is then
interrupted. For example, the air flow is discontinued
while the water flow is continued to complete the washing
lo and to obtain restratification of the layers. The water
flow may be continued at the same rate as in the combined
air-water washing, or at a higher rate if re~uired to
obtain the desiréd regrading of the layers. In either case,
the water rate should be sufficient to cause the intermixed
portions of the media to at least properly separate, and
the water wash flow should be continued until the distinct
layers of the bed have reformed to substantially the sa~e
extent of separation as before the start of the wash cycle.
In a specific embodiment, the filter apparatus is
20 similar to the one described with respect to FIG. l. It may
be used as a downflow filter with the washing performed in an
upflow direction, that is, the multiple-media bed will be
subjected to "backwashing." Furthex, baffle means of the
kind described with respect to FIG. l may be interposed in
the path of the backwash water flow from the tank into the
- 15 -
.
~17~4
,
1 backwash water removal trough for diverting media suspended
in the backwash water from exiting therewith.
It will be understood that similar procedures can be
employed in connection with upflow filters in which the
filtering and washing 10ws are in the same direction, and
that other means besides baffles may be employed to divert
media suspended in the wash water, such as occurs particularly
in the simultaneous air-water wash, from exiting with the
waste backwash water.
EXAM2LES
Returning to FIG. 1, a backwash procedure in
accordance with the method of this invention. After a number
of backwashings, the multiple-media filter bed will achieve
a xelatively fixed orientation with the higher density, and
usually smaller grain size medium, such as garnet or silica
sand, on the bottom, and the lower density and usually larger
diameter, medium such as anthracite coal on top. As illustrated
more clearly in FIG. 2A, an intermediate zone of intermixed
media may occur in which the smaller sizes of the higher
20 density media are interspersed with the larger size grains
of the lower density media. The extent of intermixing
depends on the relative size and density of the media layers
and may be ~ontrolled by appropriate selection and gradation
of each medium and by the rate and duration of bac~wash.
.
As the backwash operation is initiated, water is
passed from pipe ~0 through pipe 18, into plenum 17, through
media retaining nozzle assemblies 15 and 16 and upward throùgh
~~- the filter bed. Air is supplied within a few seconds of
- 16 -
~7~3~
1 turning on the backwash water--and in fact can precede the
turning on of the water--through pipe 21 and is metered into
the filter bed 22 through properly-sized orifices located
in the tailpipes 15 of the media-retaining nozzle assemblies.
-' The simultaneous passage of air and water through
.:~
the multiple-media bed causes substantial mixing of the
media layers except that with some combinations of media
sizes and backwash rates, a predominance of the high density
medium may remain in the lower levels of the bed and a pre-
lo dominance of the lower density medium may remain in the
upper reaches of the bed. Substantially complete mixing is
the preferred effect, but the method hereof is also applicable
to partial intermixing of the adjacent layers, that is, the
intermixing during washing, as shown in FIG. 2B, is sub-
stantially greater than during filtration, as illustra~ed in
FIG. 2A.
As water and air flow continue simultaneously, the
water begins to flow through restricted flow channels 34
and 3~ located between air-water separator baffles 33 and 35
and trough 27 and over the edges~ 28 and 2~ of t~e backwash
water collection trough 27 carryin~ with it the sol~ds
washed from the filter media~ This simultaneous flow o~ air
and water is pe~nitted to continue until the filter bed IS
substantially clean as determined by clean}~nes$.~f t~e
backwash water or by timed duration or by other suitable
means. In the method of th.is inVention, the backwash water
(or wash water) must flow into the was~ water collector while
air and water are applied simultaneously.
, _ .
3~
1 After the media bed is cleaned of entrapped solids,
the air flow is turned off by closing valve 21a and the
water flow is continued until the media bed is restratified
sufficiently for subsequent filter operation. In the
preferred application, the same water rate is used for re-
stratification as is used during simultaneous air-water wash,
but a higher water rate can be used to restratify the bed if
required. The exact rates of air and water to be used in the
method of the invention will necessarily vary for different
lO combinations of media size and density.
While the method of this invention as described --
above: is nat limited to speciic media combinations, media
sizes, or media densities, there are certain embodiments
which are presently believed to be the mostidesirable fox
commercial use. Further, with respect to these media com-
binations, there are preferred size ratios between the adja-
cent media layers. These combinations and the size ratios
are illustrated in FIGS. 3, 4, and 5. ~ith reference to these
figures and as used herein, the term "effective size" is
20defined as the media grain dLameter at which 10%~of the
total grains by weight are smaller and 90~- are larger.
- (ASTM Committee E-ll, 1969.)
...... ~
~ FIG~ 3 illustrates the wa~ ~n which useable or
preferred size ratios may b~ determined ~etween ~ coal medium
us~d as the next stratified layer ov~r a silica sand layer,
that is, the coal is the larger particle size, lower density
medium, while the silica sand is a smalier particle size,
higher density medium. Such a com~ination may be used as
- the two-media of a dual-media filter~ or as the upper and
.. . .
~ ~8
,, , ~.
1 intermediate media la~ers of a three-media filter, using garnet
or ilmenite sand as the lowermost layer, which has the
smallest size particles of highest density.
As will be noted with reference to FIG. 3, the
effective size of the coal medium in millimeters (mm) is
plotted on the vertical axis, while the effective size of
the silica sand medium in millimeters (mm) is plotted on
the horizontal axis. In this lllustration the coal has
a specific gravity of from 1.5 to 1.7. The shaded region en-
lo closed by the tetrahedran A-B-C-D including the boundary line
therearound represents the desirable size ratios of the coal
particles to the silica sand particles. In the interest of
precise illustration, the coordinates for the points A, B,
C, and D are shown on the diagram. For example/ point A
represents a ratio of 0.67 mm coal to 0.30 mm silica sand, etc.
The size ratios defined by the diagram of FIG. 3 are pre-
~erably used in a coal and silica sand dual-media bed.
FIG. 4 illustrates preferred size ratios for a dual-
media bed using coal particles as the larger lower density
20 medium over garnet or ilmenite sand as the smaller higher
densi~y medium. In this illustration, the coal has a specific
gra~ity of from 1.5 to 1.7. The useable or preferred ratios of
:~ the coal ~o the garnet or ilmenit~ sand are defined by the
region enclosed by the ~etrahedran E-F-&-H including the
boundary line therearound. The coordinates for the points
E, F, G, and H are shown on the diagram. For ~xample, point
E represents a ratio of 1.2 mm coal to 0.3 mm garnet or
ilmeni'ce sand.
-- 19 --
1~7~64
1 I FIG. 5 illustrates Ihe preferred ratios for a
silica sand medium used over a garnet or ilmenite sand
medium. These ratios may be used in a dual-media:bed
employing silica sand as the upper media and garnet or
ilmenite sand as the lower media, or in a three-media
bed using coal as the upper media, silica sand as the
intermediate media, and garnet or ilmenite sand as the
lowermost media. In the latter case, the ef~ective size
ratios of the uppermost coal layer to the intermediate
lo silica sand layer will be defined by the ratios of FIG. 3.
With reference to the foregoing examples as
illustrative of the method of this invention, the combined
air-water backwash flow is continued for at least two
minutes or more and preferably for.at least five minutes.
During the combined backwash., usually 25% or more of.the
water used for the wash cycle passes through the mixed
media bed and into the water removal means. For example,
from 40 to 75% of the water used for the wash cycle can be
used in this manner.
While the foregoing disclosure assumes.that the
invention will be applied primarily to multiple-media beds
where a low-density, large grain size medium overlies a high-
density, small grain size medium,~it will be apparent to
those skilled in the filtration arts that the invention may
.~
be applied in an equivalent embodiment wherein the multiple-
media bed comprises a low-density, small grain size medium
over a high-density, large grain size medium.
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