U.S. patent application number 09/860871 was filed with the patent office on 2001-10-25 for fluid treatment media support system.
Invention is credited to Savage, E. Stuart.
Application Number | 20010032813 09/860871 |
Document ID | / |
Family ID | 21780444 |
Filed Date | 2001-10-25 |
United States Patent
Application |
20010032813 |
Kind Code |
A1 |
Savage, E. Stuart |
October 25, 2001 |
Fluid treatment media support system
Abstract
A filter media support system that reduces media clogging and
head loss in granular filtration systems by providing a layered
porous plate. The porous plate can have multiple layers of fine
sized and coarse sized pores. The porous plate is positioned
between the media and the filter bottom. The filter media support
system is securely anchored to the infrastructure of the underdrain
system thereby inhibiting media penetration of the filter bottom
and avoiding seal failures. The infrastructure can be air lateral
piping fitted beneath the underdrain blocks of the support system.
The anchors can be secured to pipe clamps circumscribing the air
laterals.
Inventors: |
Savage, E. Stuart;
(Brunswick, ME) |
Correspondence
Address: |
Jo Katherine D'Ambrosio
Payne & D'Ambrosio, L.L.P.
800 Wilcrest, Suite 160
Houston
TX
77042
US
|
Family ID: |
21780444 |
Appl. No.: |
09/860871 |
Filed: |
May 18, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09860871 |
May 18, 2001 |
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PCT/US97/06800 |
Apr 24, 1997 |
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09860871 |
May 18, 2001 |
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09176147 |
Oct 21, 1998 |
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6261453 |
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60017052 |
Apr 26, 1996 |
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Current U.S.
Class: |
210/274 ;
210/293 |
Current CPC
Class: |
B01D 24/24 20130101;
B01D 24/4631 20130101 |
Class at
Publication: |
210/274 ;
210/293 |
International
Class: |
B01D 024/22; B01D
024/46 |
Claims
1. A fluid treatment media support system for a fluid treatment
filter having a filter bottom, underdrain blocks, air laterals, and
filter media, the system comprising: a rigid, porous plate adapted
to support the filter media over the underdrain blocks, the porous
plate comprising integral layers of different pore size.; anchors
extending from the porous plate and adapted to attach to air
laterals positioned below the the porous plate.
2. The filter media support system of claim 1 wherein the porous
plate includes a relatively coarse pore size layer adjacent the
filter bottom and a relatively fine pore size layer above the
coarse pore size layer.
3. The filter media support system of claim 2 wherein the porous
plate further includes a relatively coarse pore size layer above
the fine pore size layer.
4. The filter media support system of claim 2 wherein the coarse
layer has a pore size of from 500 to 5000 microns and the fine
layer has a pore size of from 150 to 1500 microns.
5. The filter media support system of claim 3 wherein the coarse
layers have a pore size of from 500 to 5000 microns and the fine
layer has a pore size of from 150 to 1500 microns.
6. The filter media support system of claim 1 wherein the porous
plate comprises sintered polyethylene.
7. The filter media support system of claim 1 wherein the porous
plate is made of a material selected from the group consisting of
ceramics, metals and polymers.
8. A fluid treatment media support system for a fluid treatment
filter having a filter bottom, underdrain blocks, an
infrastructure, and filter media, the system comprising: a rigid,
porous plate adapted to support the filter media over the
underdrain blocks, the porous plate comprising integral layers of
different pore size; anchors extending from the porous plate and
adapted to attach to the infrastucture positioned below the the
porous plate.
9. The filter media support system of claim 8 wherein the
infrastructure includes a plurality of air laterals running under
the underdrain blocks and the anchors are adapted to be secured to
the air laterals.
10. The filter media support system of claim 8 wherein the
underdrain blocks are arranged end-to-end in rows over the air
laterals, the porous plate has a larger horizontal dimension than
the individual underdrain blocks so that the porous plate is
adapted to cover a plurality of the underdrain blocks, and the
anchors are adapted to extend between adjacent ends of the
blocks.
11. The filter media support system of claim 10 wherein upper ends
of the anchors are secured to bars positioned over the porous plate
running transversely to the rows of the underdrain blocks.
12. The filter media support system of claim 10 wherein the porous
plate includes lap joints adapted to be parallel to the rows of
underdrain blocks.
13. The filter media support system of claim 12 wherein the anchors
pass through a bore formed through an overlap of the joint between
adjacent porous plate sections.
14. The filter media support system of claim 8 wherein the
underdrain blocks comprise lateral sides and the anchors are
adapted to extend from the porous plate, pass the lateral sides of
the underdrain blocks to the infrastructure.
15. A fluid treatment media support system for a fluid treatment
filter having a filter bottom, underdrain blocks, air laterals, and
filter media, the system comprising: a rigid, porous plate adapted
to support the filter media over the underdrain blocks, the porous
plate comprising integral layers of different pore size.; anchors
extending from the porous plate and adapted to attach to the
underdrain blocks positioned below the the porous plate.
Description
CROSS REFERENCES TO RELATED CASES
[0001] This is a continuation of U.S. Provisional Patent
Application, Ser. No. 60/017,052 filed Apr. 26, 1996, now
abandoned, International Application No. PCT/US97/06800, filed Apr.
24, 1997 and U.S. patent application Ser. No. 09/176,147, now U.S.
Pat. No. ______.
FIELD OF THE INVENTION
[0002] The present invention relates to a fluid treatment media
support system for granular filters. More specifically the
invention relates to a fluid treatment media support system using a
porous plate, a layered porosity pattern in the porous plate, and
an anchoring system for the porous plate. The fluid treatment media
supported by the system of this invention can be a filtration media
or other media such as an ion exchange resin.
BACKGROUND OF THE INVENTION
[0003] Water, wastewater and industrial liquid granular filtration
units typically have a filter media support system that separates
the filter media from the underdrain system and filter bottom. The
underdrain system is the primary support for the filter media, and
also serves to collect the filtrate and provide for the uniform
distribution of air and water during the backwash of the filter
system.
[0004] Underdrain systems are often made of concrete blocks having
spaces to allow for piping, such as air laterals, that are part of
the backwash air distribution system. A precast concrete,
plastic-jacketed underdrain block is disclosed in U.S. Pat. No.
4,923,606. Nozzle-less type underdrain systems with large openings
for the passage of the filtrate and the backwash water are
preferred because they do not plug as easily as nozzle type
underdrains. Because the openings in nozzle-less underdrains are
larger than the size of the individual grains of the media,
however, it is necessary to use a media support system between the
underdrains and the media.
[0005] A media support system serves several purposes that are
conflicting. For example, very fine media, such as 0.1 to 0.5 mm
sand, may be used in potable water type filters. Consequently, a
very fine media support is needed to separate this media from the
underdrain system and filter bottom and prevent plugging and loss
of filter media. Plugging of the underdrain system filter bottom
causes a loss of the filtering capacities of the bed and downtime
of the filter system. However, large or coarse-pore media support
is necessary to promote the formation of larger air bubbles which
are desired because they wash a filter better than fine bubbles of
air. Jung & Savage, Deep Bed Filtration, Journal American
WaterWorks Association, February, 1974, pp. 73-78.
[0006] Two types of media support systems have been in common use:
(1) support gravel beds comprised of graded gravel placed between
the filter media and the filter bottom (or underdrain system) and
(2) uniformly porous plates that are anchored to the side walls of
the filter or to the underdrain blocks.
[0007] When layered gravel beds are used for media support systems,
the bed of gravel is usually 12 to 18 inches in height with several
layers of varying size gravel. The layers of gravel adjacent to the
media and filter bottom are usually coarse and the intermediate
layer or layers smaller or finer in size. The finer intermediate
gravel layer inhibits the penetration of the media to the
underdrain blocks. The coarser gravel in the top or cap layer,
however, inhibits plugging of the fine gravel layer. If the finer
media penetrates the gravel layers during filtration, it
accumulates in the cap layer and is then washed out during the
backwash cycle of the filtration process.
[0008] U.S. Pat. No. 1,787,689 to Montgomery and U.S. Pat. No.
1,891,061 to Friend et al., for example, disclose a water treating
tank containing zeolite water softeners. The gravel beds of the
tanks are arranged in an hourglass configuration with layers of
coarser and finer gravels.
[0009] Gravel layers have several disadvantages including
difficulty in installation, the need for deeper filter boxes to
allow for the depth of the gravel and higher costs. Also, the
gradation of the gravel layers tends to be disturbed during the
filtration and backwashing processes and downtime may be required
to restore the desired gradation.
[0010] Porous plates have been used to replace gravel layers.
Porous plates are typically manufactured from sintered plastics.
Plastic porous plates, however, are usually buoyant and need to be
secured in some way to prevent lifting, especially during the
backwash cycle. Prior art methods of securing the porous plate
include a combination of screwing and caulking or grouting the
plate to the underdrain blocks as disclosed in U.S. Pat. No.
5,149,427 to Brown, or bolting the plate to the underdrain
blocks.
[0011] U.S. Pat. No. 4,882,053 to Ferri discloses a porous plate
used in a filter system without underdrain blocks; the porous plate
is attached by a retaining angle secured to each wall of the filter
box. The retaining angle holds the plate in place and a seal is
made by a sealant bead applied between the side walls and the
porous plates.
[0012] Problems arise with the above-referenced methods of
anchoring the porous plates. Small irregularities in the floor of
the filter, the underdrain blocks and the plates can cause seal
failures between the plates. Seal failure allows media to penetrate
the media support system, causes a progressive failure of the
filter underdrain and then of the filter system itself. The
underdrains, effluent piping, and clearwell may become plugged with
media and the filter bottom may collapse due to excessive pressures
which develop during backwash.
[0013] U.S. Pat. Nos. 5,149,427 and 5,232,592 to Brown disclose a
cap for filter underdrain blocks comprising a porous, planar body.
The body of the cap is said to be adapted to support a fine grain
filter media without the media penetrating therethrough. The pores
in the cap body are approximately 700-800 microns in size.
[0014] U.S. Pat. No. 4,882,053 to Ferri, mentioned above, discloses
a support or drain plate for filter media comprising porous
heat-fusible polyethylene in a traveling bridge filter. The porous
drain plates have narrow heat fused, non-porous bands extending
vertically through the plates. These bands provide rigidity to the
plates said to decrease bowing and subsequent channeling of water
during backwash experienced with lap joints. However, the
non-porous bands would tend to reduce permeability during
filtration and increase head loss.
[0015] U.S. Pat. No. 667,005 to Davis discloses a filter bottom for
a granular bed that includes three sheets or layers of wire cloth.
The upper layer and lower layer are coarse with the intermediate
layer being a fine mesh. U.S. Pat. 2,267,918 to Hildabolt discloses
a porous article formed from metal powders and having plural layers
of different porosity. U.S. Pat. No. 5,468,273 to Pevzner et al.
discloses a nickel-based filter material having three strata of
different porosity used for removing contaminants from air.
SUMMARY OF THE INVENTION
[0016] The filter media support system of the present invention is
a barrier between the media of a filter and its underdrain system.
The filter media support system reduces media clogging and head
loss by providing a layered porous plate having multiple layers of
fine sized and coarse sized pores to restrain media grains and
waste solids from entering and damaging the underdrain system. The
filter media support system further provides an anchor for securely
anchoring the porous plate to the infrastructure of the filter
bottom, thereby inhibiting media penetration to the filter bottom
and avoiding seal failures.
[0017] In one aspect, the present invention provides a system for
supporting granular filter media above a filter bottom. The system
has a porous plate which is placed over the filter bottom to
support the filter media. The porous plate includes adjacent layers
of different porosity. Preferably, the porous plate includes a
relatively coarse pore size layer adjacent to the filter bottom,
and a relatively fine pore size layer above the coarse pore size
layer. If desired, the porous plate can also include a relatively
coarse pore size layer above the fine pore size layer. The coarse
layer preferably has a pore size of from 500 to 5000 microns, and
the fine layer preferably from 150 to 1500 microns.
[0018] The porous plate is preferably supported on a layer of
underdrain blocks on the filter bottom. The porous plate preferably
has a larger horizontal dimension than that of the individual
underdrain blocks. In this manner, a plurality of underdrain blocks
support the porous plate. The porous plate can be anchored to air
laterals beneath the underdrain blocks, or other infrastructure.
The porous plate preferably comprises sintered polyethylene,
although it could also be made from ceramics, metals, polymers and
the like. The porous plate preferably includes lap joints between
adjacent sections.
[0019] In another aspect, the present invention provides a filter
which has upright walls defining at least one compartment housing
granular filter media supported above a filter bottom on the porous
plate with the layers of different porosity just described.
[0020] In a further aspect, the present invention provides a filter
system for supporting granular filter media above a filter bottom
which has a layer of underdrain blocks placed over infrastructure
of the filter bottom. A porous plate is placed over the underdrain
blocks to support the filter media. Anchors extend from the porous
plate through the layer of underdrain blocks to secure the porous
plate to the infrastructure. The infrastructure can include a
plurality of air laterals running beneath the underdrain blocks,
and the anchors are preferably secured to the air laterals. The
underdrain blocks are preferably arranged end-to-end in rows over
the air laterals and the porous plate preferably has a larger
horizontal dimension than the individual underdrain blocks. In this
manner, the porous plate covers a plurality of the underdrain
blocks, and the anchors can extend between adjacent ends of the
blocks.
[0021] The upper ends of the anchors are preferably secured to bars
positioned over the porous plate which run transversely to the rows
of the underdrain blocks. The porous plate can include lap joints
parallel to the rows of underdrain blocks. The anchors preferably
pass through a bore formed through an overlap of the joint between
adjacent porous plate sections. The sides of adjacent underdrain
blocks are preferably interconnected by lugs.
[0022] Yet another aspect of the invention is a filter having
upright walls defining at least one compartment housing granular
filter media supported above a filter bottom which includes a
porous plate anchored to the infrastructure of the filter bottom as
just described.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a perspective view, partially cut away, of a
section of the filtration system illustrating the filter media
support system according to one embodiment of this invention.
[0024] FIG. 2 is a perspective view of a section of the filtration
system illustrating the backwash flow through the filter media
support system of FIG. 1.
[0025] FIG. 3 is a cross-section of the filter media support system
of FIG. 1 taken along lines 3-3.
[0026] FIG. 4 is an enlarged view of a section of FIG. 3.
[0027] FIG. 5 is a perspective view, partially cut away, of the
layered porosity plate according to one embodiment of this
invention.
[0028] FIG. 6 is a cross-section of the filter media support system
of FIG. 1 taken along lines 6-6.
[0029] FIG. 7 is an enlarged view of a section of FIG. 6.
[0030] FIG. 8 is a plan view of the filter media support system of
FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
[0031] The filter media support system of this invention is
directed to a porous plate, preferably of graded porosity, and a
anchor system for securely anchoring a porous plate to the
infrastructure of the underdrain system. The media support system
of this invention is not a filter in itself and does not perform
filter functions. Fitration occurs within the filter media. The
media support system of this invention serves two functions: 1) it
supports the filtering media and 2) restrains media grains and
waste solids from entering the underdrain system where they can
cause extensive damage, clogging and headloss. FIG. 1 illustrates a
section of a filtration system 10 and a porous plate 20 securely
anchored within that system 10. A filtration system 10 is usually
used to filter water, including potable water and wastewater and
can also be used for ion exchange or other absorption processes.
The filtration system 10 has a filter box 100 containing granular
media 90, such as sand, anthracite, activated carbon, ion exchange
resin, or the like, or a combination thereof. Filter influent flows
into the filter box 100, through the media 90 for the removal of
suspended solids. During the water treatment process, the filter
influent is filtered by the media 90 and than drains through the
underdrain system 50 to the bottom of the filter box 102 where it
collects in a sump 104. The porous plate 20 of this invention is a
barrier between the filter media and the underdrain to restrain the
media and any suspended solids from passing through and damaging
the underdrain 50. Backwash sends any media grains and waste solids
that are restained by pores of the porous plate 20 back up into the
filtration media 90.
[0032] During the backwash phase of the filtration cycle, normal
downward filtration stops and an upflow of liquid, usually water,
and gas, usually compressed air, cleanse the filter system. As seen
in FIG. 2, backwash water from backwash pumps (not shown) is pumped
into the sump 104 and through the filter system 10. Backwash air is
supplied via headers 110 located on either side of the filter box
100, and through air laterals 60 into the filter system 10.
[0033] The porous plate 20 is positioned between the media 90 and
the underdrain blocks 40, thereby supporting and separating the
filter media 90 from the underdrain system 50. As illustrated in
FIG. 5, the porous plate 20 has a reverse gradation of coarse and
fine pore layers. In a preferred embodiment of the invention, a
relatively coarse pore layer 20c is adjacent the underdrain blocks
40 and another relatively coarse pore layer 20a is adjacent the
filter media 90. A relatively fine pore layer 20b lies between the
two coarse pored layers 20a, 20c. Varying size pores are beneficial
in media support systems. A fine pore layer 20b is necessary to
separate fine media 90, 0.1 to 0.5 mm sand for example, from the
underdrain system. The fine pore layer 20b prevents clogging of the
underdrain system 50 and loss of filter media 90. The coarse pore
layer 20c of the porous plate 20 promotes the formation of large
air bubbles which wash the filter system better than fine air
bubbles. Also, if any media penetrates the porous plate 20 during
the filtration cycle, it will accumulate in the top coarse pore
layer 20a and is readily washed out during the backwash cycle.
[0034] In a preferred embodiment, the pore size of the coarse
layers 20a, 20c range from 500 to 5000 microns. The pores in the
fine pore layers range from 150 to 1500 microns.
[0035] The porous plate 20 of this invention may be manufactured
from ceramics; metals, particularly sintered metals such as nickel,
titanium, stainless steel and the like; and polymers, such as
polyethylene, polypropylene or polystyrene; or any suitable
material. In a preferred embodiment, the material is a sintered
polyethylene. The porous plate 20 can be formed by sintering
heat-fusible particles to the desired shape. Other heat-fusible
materials may be used such as polypropylene or the above referenced
group of materials. The porous plate 20 can include different
adjacent layers of different porosity fused integrally together, or
the layers can be formed by stacking sheets of different porosity
together where each sheet corresponds to a specific porosity
layer.
[0036] The length and width of the porous plates 20 may vary
according to the size of the underdrain blocks 40 or bottom of
filter box 102. In a preferred embodiment, the porous plate 20 has
a larger horizontal area or dimension than the individual
underdrain blocks 40 so that the porous plate 20 covers a plurality
of underdrain blocks 40. In another preferred embodiment, the
porous plates have widths in multiples of the width of the
underdrain blocks 40. The preferred thickness of the porous plate
20 varies from 1 inch or less to 2 inches or more, depending on the
particular application.
[0037] A porous plate 20 manufactured from sintered polymers tends
to be buoyant and float. FIGS. 4 and 7 illustrate the improved
anchoring of the porous plate 20 of one embodiment of this
invention. The porous plate 20 is secured to the infrastructure 60
of the bottom of filter box 102 rather than the side walls 106 of
the filter box 100 as done in the prior art media support systems.
In one alternative, the porous plate 20 can be anchored to the
underdrain blocks 40. Anchoring the porous plate 20 to the
infrastructure 60 improves the seal to prevent lifting and bowing,
especially during the backwash cycle. Infrastructure 60 includes
but is not limited to air laterals, air headers, floor of filter,
sump cover plates, air lateral anchors, piping and support.
[0038] In a preferred embodiment of this invention, the porous
plate 20 is anchored to the air lateral piping 60 which supplies
the backwash air. The air laterals 60 are run in spaces 42 between
block legs 44 of the underdrain blocks 40. An air lateral 60 can be
placed between the legs 44 of every other row of blocks 40. A
preferred underdrain block 40 is described in U.S. Pat. No.
4,923,606 the disclosure of which is hereby incorporated by
reference in its entirety. Briefly, as best seen in FIGS. 6 and 7,
the underdrain blocks 40 are arranged end-to-end in rows over the
air laterals 60, and the sides of adjacent underdrain blocks 40 are
interconnected by lugs 48. Preferably, the porous plate 20 has a
larger horizontal area than the individual blocks 40 so that the
porous plate 20 covers a plurality of the underdrain blocks 40.
Anchors 26 extend from the porous plate 20 between adjacent ends of
the blocks 40 to the air laterals 60. An indentation (not shown) is
preferably formed in the opposing ends of the adjacent blocks 40 to
accommodate the cross-section of the anchors 26. Alternatively, the
anchors 26 could extend directly through an aperture formed in the
blocks 40 to an attachment point on the bottom of filter box 102.
In still another alternative, anchors 26 can extend between the
lateral sides of the blocks 47 down to the infrastructure.
[0039] Preferably, the upper ends of the anchors 26 are secured to
bars 30 positioned over the porous plate 20. The bars 30 preferably
run transversely to the underdrain blocks 40 and help to hold the
porous plates securely in place. This inhibits bowing or lifting of
the porous plate 20. Suitable bars 30 are manufactured of a
corrosion-resistant metal such as stainless steel and are
approximately 2 inches in width and 1/4 inch in depth. The
preferred anchor 26 is a threaded rod manufactured from a
corrosion-resistant metal such as stainless steel. The anchor 26 is
secured to the porous plate 20 by a fastener, preferably a nut 27a
and an oversized washer 27b. Additional sealants may be used to
prevent leakage in the bore through the plate 20 around the rod
26.
[0040] FIG. 6 illustrates sections of the porous plate 20 joined
together by overlapping the ends of adjacent sections of the porous
plate 20 at lap joints 24. The lap joints 24 run parallel to the
rows of underdrain blocks 40. The anchors 26 pass through the bar
30, through the porous plate 20 by means of a bore in the lap
joints 24 and between the underdrain blocks 40, and are secured to
the air laterals 60. Preferably, the anchors 26 are secured to the
air laterals 60 by pipe clamps 62 circumscribing the air laterals
60 as illustrated in FIGS. 4 and 7. Lateral support angles 76
grouted into the bottom of filter box 102 can provide additional
support for the air laterals 60. As depicted in FIG. 3 support
brackets 36 can also be used, if desired, to secure the porous
plate 20 to the walls of the filter box 100.
[0041] The porous plate 20 of the present invention may be
installed in new filtration systems or retrofitted into existing
systems. A filter box 100 having side walls 106 and a bottom 102 is
constructed conventionally with an infrastructure 50 of air lateral
piping 60 across the bottom of filter box 102 and a sump 104 and
sump cover plate 105 for collection of filtrate during the
filtration process and for the supply of backwash water during
backwashing operations. Pipe clamps 62 are placed around the air
laterals 60 and anchors 26 secured to the pipe clamps 62. The
underdrain blocks 40 are arranged in rows over the air laterals 60
so that the air laterals 60 lie in spaces 42 between the block legs
44 with an air lateral 60 under every other row of blocks 40. The
blocks 40 are spaced apart to create a gap 45 which provides for
air and water flow. The anchors 26 extend upward between the blocks
40. The beveled configuration of the top of the blocks 40 creates a
channel into the gap 45. The blocks 40 can be interconnected with
lugs 48 sized to provide the desired size of gap 45. Additional
sealing can be provided by grouting the perimeter blocks 40 to the
filter box 100. The blocks 40 should be of a weight to resist
lifting and shifting, especially during the backwash phase but not
so heavy as to prohibit easy handling.
[0042] After the underdrain system is in place, the sections of the
porous plate 20 are placed over the rows of blocks 40 and joined by
lap joints 24 which run parallel to the blocks 40. Bores,
preferably pre-formed, pass through the upper lips 24a and lower
lips 24b of the adjacent sections of the porous plate 20 for
receiving anchors 26 extending upwards from the rows of blocks 40,
thereby improving the seal of the lap joints 24. A stainless steel
bar 30, running transversely to the blocks 40, is placed over the
lap joints 24. The anchors are then secured by nuts 27a and washers
27b. Larger sheets of porous plate 20 can be made by further
sealing the lap joints 24 by means of mastic, epoxy glues or
thermal welding; however, this should be avoided as much as
possible to minimize decreasing the permeability of the porous
plate 20. The anchors 26 thus extend through the bar 30, through
the bores in the lap joints 24, between the underdrain blocks 40
and are secured to pipe clamps 62 circumscribing the air laterals
60.
[0043] After the filtration media support system is in place,
filter media 90 may be installed and operation of the filtration
cycle initiated as the filter influent flows into the filter box
100. Periodically, the filtration process may be stopped so that
the filtration system may be backwashed.
[0044] The anchors 26 of the present invention securely hold the
porous plate 20 to the air laterals 60, thereby reducing lifting
and bowing that is induced especially by the pressures exerted
during the backwash cycle. The graded porosity layers of the plate
20 create larger air bubbles during the backwash cycle which wash
the filter system better than fine bubbles, and yet provide fine
pores for inhibiting media particles 90 from entering the
underdrain system 50 during the filtration cycle.
EXAMPLE
[0045] Air spreading tests are performed to observe and record the
impact of the reverse-gradient porous plate of this invention on
backwash air distribution. During the first test, a 600-700 micron
3/4-inch thick porous plate is put in place. Underdrain blocks,
specifically 8-inch wide T-blocks are installed in the test column,
the column is filled with water up to the overflow weir and
backwash air added at a rate of 2.0 CFM/ft.sup.2. The test is
repeated at air rates of 4.0 CFM/ft.sup.2 and 6.0 CFM/ft.sup.2. A
standard is used to measure the size of the air bubbles. The
results are photographed and data recorded. An uneven air pattern
occurs during the backwash and the air bubbles are relatively
small.
[0046] The tests are repeated with the layered porosity porous
plate in place at the same three air rates. The porous plate has
coarse-pore layers of about 3/8-inch thickness having a pore size
of approximately 600 microns and an intermediate fine-pore layer of
about 3/8-inch thickness having a pore size of approximately 350
microns. The thickness of the entire plate is about 11/8 inches.
The porous plate produces a more even pattern of air distribution,
relatively larger air bubbles, and the pressure drop is comparable
to the uniform-porosity plate.
[0047] The foregoing description is illustrative and explanatory of
preferred embodiments of the invention, and variations in the size,
shape, materials and other details will become apparent to those
skilled in the art. It is intended that all such variations and
modifications which fall within the scope or spirit of the appended
claims be embraced thereby.
* * * * *