U.S. patent application number 10/674694 was filed with the patent office on 2004-05-06 for scouring method.
This patent application is currently assigned to U.S. Filter Wastewater Group, Inc.. Invention is credited to Beck, Thomas W., Johnson, Warren T., Kopp, Clinton, McMahon, Robert, Zha, Fufang.
Application Number | 20040084369 10/674694 |
Document ID | / |
Family ID | 25645333 |
Filed Date | 2004-05-06 |
United States Patent
Application |
20040084369 |
Kind Code |
A1 |
Zha, Fufang ; et
al. |
May 6, 2004 |
Scouring method
Abstract
A method and apparatus for removing fouling materials from the
surface of a plurality of porous membranes (9) arranged in a
membrane module (4) by providing, from within the module, by means
(10) other than gas passing through the pores of said membranes,
gas bubbles in a uniform distribution relative to the porous
membrane array such that the bubbles move past the surfaces of the
membranes (9) to dislodge fouling materials therefrom. The
membranes (9) are arranged in close proximity to one another and
mounted to prevent excessive movement therebetween. The bubbles
also produce vibration and rubbing together of the membranes to
further assist removal of fouling materials.
Inventors: |
Zha, Fufang; (Hurlstone
Park, AU) ; Kopp, Clinton; (Bismarck, ND) ;
McMahon, Robert; (Concord, AU) ; Johnson, Warren
T.; (Bligh Park, AU) ; Beck, Thomas W.; (North
Richmond, AU) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
2040 MAIN STREET
FOURTEENTH FLOOR
IRVINE
CA
92614
US
|
Assignee: |
U.S. Filter Wastewater Group,
Inc.
Warrendale
PA
|
Family ID: |
25645333 |
Appl. No.: |
10/674694 |
Filed: |
September 30, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10674694 |
Sep 30, 2003 |
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10369813 |
Feb 18, 2003 |
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10369813 |
Feb 18, 2003 |
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09336059 |
Jun 18, 1999 |
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6555005 |
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09336059 |
Jun 18, 1999 |
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PCT/AU97/00855 |
Dec 18, 1997 |
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Current U.S.
Class: |
210/636 ;
210/321.89; 210/500.23; 210/639; 210/650 |
Current CPC
Class: |
B01D 2321/168 20130101;
B01D 2321/185 20130101; B01D 63/022 20130101; B01D 61/18 20130101;
B01D 2321/04 20130101; B01D 63/16 20130101; B01D 65/02 20130101;
B01D 2321/16 20130101; B01D 63/024 20130101; B01D 2321/2058
20130101; B01D 65/08 20130101; B01D 2315/06 20130101 |
Class at
Publication: |
210/636 ;
210/639; 210/650; 210/321.89; 210/500.23 |
International
Class: |
B01D 061/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 20, 1996 |
AU |
PO 4312 |
Sep 1, 1997 |
AU |
PO 8918 |
Claims
What is claimed is:
1. A method for filtering a feed liquid, the method comprising:
providing a vessel; providing a membrane module, the membrane
module comprising a plurality of porous hollow fiber membranes, the
membranes comprising a plurality of pores and an outer surface,
wherein the membranes are mounted in a header in close proximity to
one another so as to prevent excessive movement therebetween,
wherein the membranes form an array, wherein gas bubbles may be
introduced into the membrane module, and wherein the membrane
module is contained within the vessel; providing a feed liquid to
the vessel, the feed liquid comprising a fouling material, wherein
the feed liquid is provided to the vessel at a rate sufficient to
cause an overflow; applying a transmembrane pressure to the
membranes in the module, whereby a filtrate passes through pores in
the membranes, thereby producing a concentrated feed comprising the
fouling material in the vessel; and removing the fouling material
from the vessel, wherein the fouling material is carried out of the
vessel in the overflow therefrom.
2. The method according to claim 1, further comprising: connecting
the header to a source of a pressurized gas; and providing, through
the header but not through the pores of the membranes, gas bubbles
in a uniform distribution relative to the porous membrane array
such that the gas bubbles move past the outer surfaces of the
membranes and vibrate the membranes to dislodge the fouling
material therefrom.
3. The method according to claim 1, further comprising: mounting
the membranes relative to one another so as to produce a rubbing
effect between the membranes when vibrated.
4. The method according to claim 3, wherein the hollow fiber
membranes are arranged in at least one bundle.
5. The method according to claim 4, wherein the bundle is
surrounded by a perforated cage, whereby excessive movement between
the hollow fiber membranes is prevented.
6. The method according to claim 4, comprising the additional step
of: providing gas bubbles from within the module through gas
distribution holes or openings in the header.
7. The method according to claim 1, further comprising: providing
gas bubbles from within the module through at least one tube
situated within the module.
8. The method according to claim 7, wherein the tube comprises a
plurality of holes.
9. The method according to claim 7, wherein the tube comprises a
comb of tubes.
10. The method according to claim 1, further comprising: draining
down a liquid within the vessel to remove accumulated solids
dislodged from the membranes.
11. The method according to claim 10, wherein the draining down
comprises periodically draining down.
12. The method according to claim 10, wherein the draining down
comprises continuously draining down.
13. The method according to claim 1, further comprising: scouring
the membranes.
14. The method according to claim 13, wherein the step of scouring
comprises liquid backwashing.
15. The method according to claim 13, wherein the step of scouring
comprises pressurized gas backwashing.
16. The method according to claim 13, wherein the step of scouring
comprises chemically cleaning.
17. The method according to claim 13, wherein the step of scouring
comprises chemically dosing.
18. The method according to claim 13, wherein the scouring is
continuous.
19. The method according to claim 13, wherein the scouring is
intermittent.
20. A filtration system comprising: a membrane module comprising a
plurality of porous hollow membrane fibers, each of the fibers
having an upper end and a lower end, the fibers extending
longitudinally between and mounted at the upper end to an upper
potting head and at the lower end to a lower potting head, wherein
the fibers are sealed at the lower end and open at the upper end to
allow removal of a filtrate, the fibers being arranged in close
proximity to one another and mounted in a bundle in a substantially
taut manner between the upper potting head and the lower potting
head to prevent excessive movement therebetween, wherein the fibers
are surrounded by a perforated cage to further prevent excessive
movement of the fibers, the fibers being substantially uniformly
mounted in the lower potting head relative to a distributed array
of aeration holes in the lower potting head, wherein the aeration
holes are sized and located such that bubbles, formed by a
pressurized gas passing therethrough when the module is immersed in
a liquid, pass substantially uniformly between the fibers, wherein
the lower potting head is connected to a source of the pressurized
gas, and wherein the fibers are arranged to be vibrated by the gas
bubbles, the fibers being mounted relative to one another so as to
produce a rubbing effect between the fibers when vibrated by the
gas bubbles; and a vessel, wherein the membrane module is situated
in the vessel, the vessel comprising a feed inlet whereby a feed
liquid is provided to the vessel at a rate sufficient to cause an
overflow, such that at least one fouling material is carried out of
the vessel in the overflow.
21. The filtration system according to claim 20, further comprising
a porous sheet through which a pressurized gas is supplied, whereby
gas bubbles are provided from within the module.
22. The filtration system according to claim 20, further comprising
at least one porous tube through which a pressurized gas is
supplied, whereby gas bubbles are provided from within the
module.
23. The filtration system according to claim 22, wherein the porous
tube comprises a comb of porous tubes.
24. A method of removing accumulated solids from an outer surface
of a plurality of porous hollow fiber membranes, the method
comprising: providing a plurality of porous hollow fiber membranes,
the porous hollow fiber membranes extending longitudinally in an
array to form a membrane module, wherein the membranes are arranged
in close proximity to one another and mounted to prevent excessive
movement therebetween, wherein the module is contained within a
vessel; providing, from within the array, by means other than gas
passing through the pores of the membranes, uniformly distributed
gas bubbles, the distribution being such that the bubbles pass
substantially uniformly between each membrane in the array to scour
the surface of the membranes, vibrate the membranes, and remove
accumulated solids from within the membrane module; and removing
accumulated solids from the vessel, wherein the accumulated solids
are carried out of the vessel in an overflow of a concentrated feed
therefrom.
25. The method according to claim 24, wherein the membranes are
mounted vertically to form the array and the bubbles pass generally
parallel to a longitudinal extent of the fibers.
26. The method according to claim 25, wherein the uniformly
distributed gas bubbles are provided at a lower end of the array.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of application Ser. No.
10/369,813, filed Feb. 18, 2003, which is a continuation of
application Ser. No. 09/336,059, filed Jun. 18, 1999, which is a
continuation, under 35 U.S.C. .sctn. 120, of International Patent
Application No. PCT/AU97/00855, filed on Dec. 18, 1997 under the
Patent Cooperation Treaty (PCT), which was published by the
International Bureau in English on Jul. 2, 1998, which designates
the U.S. and claims the benefit of Australian Provisional Patent
Application No. PO 4312, filed Dec. 20, 1996 and Australian
Provisional Patent Application No. PO 8918, filed Sep. 1, 1997.
FIELD OF THE INVENTION
[0002] The present invention relates to the use of a gas bubble
system to remove fouling materials from the surface of membranes
used in filtration systems and the like.
BACKGROUND OF THE INVENTION
[0003] A variety of membrane filtration systems are known and many
of these use pressurised systems operating at high transmembrane
pressures (TMP) to produce effective filtering and high filtrate
flux. These systems are highly effective but are also expensive to
produce, operate and maintain. Simpler systems using membrane
arrays freely mounted vertically in a tank and using suction
applied to the fibre lumens to produce TMP have also been
developed, however, these systems have been found in the past to be
less effective than the pressurised systems.
[0004] Examples of such known systems are illustrated in U.S. Pat.
No. 5,192,456 to Ishida et al, U.S. Pat. No. 5,248,424 to Cote et
al and WO 97/06880 to Zenon Environmental Inc.
[0005] The Ishida et al patent describes an activated sludge
treating apparatus where air flow is used to clean the outer
surface of the filter membrane. In this arrangement the air blower
used for biological treatment of the waste water is also used as a
secondary agitation source to clean the surface of the membranes.
The membrane modules are of the plate type. The membranes also have
a low packing density and thus do not have the problems associated
with cleaning tightly packed fibre bundles. Air is bubbled from
beneath the modules and is supplied externally from the membrane
array.
[0006] The Cote et al patent again describes a system of cleaning
arrays of fibres. In this case the fibres are mounted in a skein to
form an inverted U-shaped or parabolic array and the air is
introduced below the array to produce bubbles which contact the
fibres with such force they keep the surfaces relatively free of
attached microorganisms and deposits of inanimate particles. The
fibres are freely swayable as they are only attached at either end
and this assists removal of deposits on their outer surface. The
bubbles of gas/air flow are provided from a source external of the
fibre bundle and move generally transverse to the lengths of fibre.
This limits the depth of fibre bundle which can be effectively
cleaned.
[0007] The invention disclosed in the Zenon Environmental, Inc. PCT
Application No. WO 97/06880 is closely related to the Cote et al
patent. In this document the fibres are unconfined, vertically
arranged and dimensioned to be slightly longer than the distance
between the opposed faces of the headers into which the fibre ends
are mounted to allow for swaying and independent movement of the
individual fibres. The skein is aerated with a gas distribution
means which produces a mass of bubbles which serve to scrub the
outer surface of the vertically arranged fibres as they rise
upwardly through the skein.
[0008] Our own International Patent Application WO96/07470
describes an earlier method of cleaning membranes using a gas
backwash to dislodge material from the membrane walls by applying a
gas pressure to the filtrate side of the membranes and then rapidly
decompressing the shell surrounding the feed side of the membranes.
Feed is supplied to the shell while this gas backwash is taking
place to cause turbulence and frothing around the membrane walls
resulting in further dislodgment of accumulated solids.
SUMMARY OF THE INVENTION
[0009] The present invention relates particularly to a plurality of
porous membranes arranged to form a membrane module arranged in a
relatively tightly packed bundle. These porous membranes may be in
the form of fibres or plate type membranes as described in the
above prior art.
[0010] The present invention seeks to overcome or at least
ameliorate the problems of the prior art by providing a simple
effective system and method for removing fouling materials from the
surface of the porous membranes by use of gas bubbles.
[0011] According to one aspect, the present invention provides a
method of removing fouling materials from the surface of a
plurality of porous membranes arranged in a membrane module, the
porous membranes forming an array, the module having a header used
to mount the membranes, the header connected to a source of
pressurized gas, the method comprising providing, through the
header, gas bubbles in a uniform distribution relative to the
porous membrane array such that said bubbles move past the surfaces
of said membranes to dislodge fouling materials therefrom, said
membranes being arranged in close proximity to one another and
mounted to prevent excessive movement therebetween. The porous
membranes may comprise hollow fibre membranes. Preferably, the
fibre membranes are arranged in bundles surrounded by a perforated
cage which serves to prevent said excessive movement
therebetween.
[0012] According to a second aspect, the present invention provides
a membrane module comprising a plurality of porous membranes, said
membranes being arranged in close proximity to one another and
mounted to prevent excessive movement therebetween, the membranes
forming an array, the module having a header used to mount the
membranes, the header connected to a source of pressurized gas so
as to permit formation of gas bubbles such that, in use, said gas
moves through said header, and said bubbles move past the surfaces
of said membranes to dislodge fouling materials therefrom.
[0013] The gas bubbles may be provided from within the module by a
variety of methods including gas distribution holes or openings in
the header, a porous tube located within the module or a tube or
tubes positioned to output gas within the module, the tubes may be
in the form of a comb of tubes containing holes which sit within
the module. Another method of providing gas bubbles includes
creating gas in-situ by means of spark type ozone generators or the
like. Further types of gas provision are detailed below and in the
preferred embodiments of the invention.
[0014] According to one preferred form, the present invention
provides a method of removing fouling materials from the surface of
a plurality of porous hollow fibre membranes mounted and extending
longitudinally in an array to form a membrane module, said
membranes being arranged in close proximity to one another and
mounted to prevent excessive movement therebetween, the method
comprising the steps of providing, from within said array, via the
header connected to a source of pressurized gas, uniformly
distributed gas bubbles, said distribution being such that said
bubbles pass substantially uniformly between each membrane in said
array to scour the surface of said membranes and remove accumulated
solids from within the membrane module.
[0015] For preference, said membranes are mounted vertically to
form said array and said bubbles pass generally parallel to the
longitudinal extent of said fibres. Preferably, said uniformly
distributed gas bubbles are provided at the lower end of the array.
Optionally, a backwash may be used in conjunction with the removal
process to assist solids removal from the membrane pores and outer
surface of the membranes.
[0016] For preference, the membranes comprise porous hollow fibres,
the fibres being fixed at each end in a header, the lower header
having a plurality of holes formed therein through which gas is
introduced to provide the gas bubbles. The fibres are normally
sealed at the lower end and open at their upper end to allow
removal of filtrate. Some of the fibres may also be used to provide
bubbles of scouring gas to the array by feeding gas through
selected ones of the fibres in the array. The fibres are preferably
arranged in cylindrical arrays or bundles.
[0017] Filtrate is normally withdrawn from the fibres by
application of suction applied thereto, however, it will be
appreciated that any suitable means of providing TMP may be used. A
porous sheet may be used in conjunction with the holes or
separately to provide a more uniform distribution of gas bubbles.
The porous sheet also provides the added advantage of preventing
solids ingressing into the air supply plenum chamber.
[0018] According to a further preferred aspect, the present
invention provides a membrane module comprising a plurality of
porous hollow membrane fibres extending longitudinally between and
mounted at each end to a respective potting head, said membrane
fibres being arranged in close proximity to one another and mounted
to prevent excessive movement therebetween, one of said potting
heads having a uniform distributed array of aeration holes formed
therein and said fibres being substantially uniformly mounted in
said one potting head relative to said aeration holes.
[0019] According to a preferred further aspect, the present
invention provides a filtration system including a membrane module
according to said second aspect wherein said filter module is
positioned vertically in a tank containing feed liquid to be
filtered, means to apply a transmembrane pressure to said fibres in
said array to cause filtrate to pass through pores in said fibres
and means to supply continually or intermittently a supply of
pressurized gas to said aeration holes so as to produce gas bubbles
which move upwardly and uniformly between said fibres to scour the
outer surfaces thereof.
[0020] Optionally, when the module is contained in a separate
vessel, periodic draindown of the vessel is carried out after the
scouring step to remove solids accumulated during the scouring
process. Apart from draindown, other methods can be used for
accumulated solids removal. These include continual bleed off of
concentrated feed during the filtration cycle or overflow at the
top of the tank by pumping feed into the base of the tank at
regular intervals at a rate sufficient to cause overflow and
removal of accumulated solids. This would be typically done at the
end of a backwash cycle.
[0021] It should be understood that the term "gas" used herein
includes any gas, including air and mixtures of gases as well as
ozone and the like.
[0022] It will be appreciated that the above described invention
may be readily applied to our own modular microporous filter
cartridges as used in our continuous microfiltration systems and
described in our earlier U.S. Pat. No. 5,405,528. These cartridges
may be modified by providing gas distribution holes in the lower
plug and providing a manifold for supplying gas to said holes such
that, in use, the gas passes through the holes and forms scouring
bubbles which pass upward through the filter medium. In a preferred
arrangement, the filter medium would be sealed at the lower end and
filtrate withdrawn under a vacuum from the upper end while the
cartridge or cartridges were positioned in a tank containing the
feed.
[0023] The embodiments of the invention will be described in
relation to microporous fibre membranes, however, it will be
appreciated that the invention is equally applicable to any form of
membrane module.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] Preferred embodiments of the present invention will now be
described, by way of example only, with reference to the
accompanying drawings in which:
[0025] FIG. 1 shows a simplified cross-sectional view of one
embodiment of a membrane module in accordance with the present
invention;
[0026] FIG. 2 shows a simplified two part representation of the
potting arrangement of the membrane module according to one
preferred form of the invention;
[0027] FIG. 3 shows an enlarged view of the potting base of FIG.
2;
[0028] FIGS. 4A and 4B show the pin formations in the annular
portion of the potting base and the plunger portion of the potting
base, respectively;
[0029] FIG. 5 shows schematic diagram of a filtration system using
the membrane module of FIG. 1;
[0030] FIG. 6 shows a simplified cross-sectional view of an
alternate embodiment of the membrane module according to the
present invention;
[0031] FIG. 7 shows a simplified cross-sectional view of an
alternate embodiment in terms of feeding of air to the membrane
module of the present invention;
[0032] FIGS. 8A and 8B shows two graphs illustrating the suction
performance of the module under different conditions;
[0033] FIG. 9 shows a graph of resistance increase over time with
30 minute suction stage;
[0034] FIG. 10 shows a graph of resistance increase over time
between backwashes without a porous sheet;
[0035] FIG. 11 shows a graph of resistance increase over time
between backwashes with the porous sheet;
[0036] FIG. 12 shows a graph of resistance changes over time with
gas bubble scouring at regular intervals but no liquid backwash of
the fibre membranes;
[0037] FIG. 13 shows a similar graph to FIG. 12 illustrating the
effect of no bubble scouring on backwash efficiency; and
[0038] FIG. 14 shows a similar graph to FIG. 12 illustrating the
effect of applying gas bubble scouring to the outer side of the
fibre bundle only.
[0039] FIGS. 15a-c show a comb of tubes containing holes, the tube
sitting within a module and providing pressurized gas bubbles.
[0040] FIG. 16 shows a module incorporating a porous sheet through
which pressurized gas is supplied to provide gas bubbles.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0041] Referring to FIG. 1, the membrane module 4, according to
this embodiment, comprises a cylindrical array or bundle of hollow
fibre membranes 5 extending longitudinally between upper and lower
potting heads 6, 7. Optionally, a screen or cage 8 surrounds the
array 5 and serves to hold the fibres 9 in close proximity to each
other and prevent excessive movement. The fibres 9 are open at the
upper potting head 6 to allow for filtrate removal from their
lumens and sealed at the lower potting head 7. The lower potting
head 7 has a number of holes 10 uniformly distributed therein to
enable gas/air to be supplied therethrough. The fibres are fixed
uniformly within the potting heads 6 and 7 and the holes 10 are
formed uniformly relative to each fibre 9 so as to provide, in use,
a uniform distribution of gas bubbles between the fibres.
[0042] The holes are formed as part of the potting process as
described below. The arrangement of the holes relative to one
another as well as the arrangement of fibres relative to the holes
and each other has been found to effect the scouring efficiency of
the gas bubbles.
[0043] The maldistribution of gas within the fibre bundle can be
overcome by appropriate distribution and sizing of holes to ensure
that bubble flow around the fibres is uniform across the bundle. In
a cylindrical bundle of closely packed fibres it has been found
that the distance traveled through the bundle by bubbles introduced
towards the centre of the bundle is larger than those introduced
towards the outer extremity of the bundle, resulting in a higher
resistance to bubble flow at the centre of the bundle than at its
border or periphery.
[0044] As outlined above, one method of addressing the
maldistribution of gas bubbles is to provide a porous sheet (not
shown) across the holes to provide an even pore distribution and
thus a uniform gas flow. Another method is to provide a
distribution of hole size relative to the distribution of
resistance. Since the gas flowrate (Q) per unit area (A) is
inversely proportional to the resistance (R),
[0045] Q/A.about.1/R the relationship between the hole diameter (d)
and the resistance becomes
[0046] d.about.(R).sup.1/2 using the above relationship it is
possible to design a hole size and position configuration which
compensates for resistance differences within the bundle. For
example, if the resistance at the centre of the bundle is 50%
higher than that at its periphery, the hole size at the centre
(d.sub.c) and on the periphery (d.sub.p)would be the following for
a uniform distribution of gas:
[0047] d.sub.c/d.sub.p=1.5.sup.0.5=1.22
[0048] Known methods of forming holes require the drilling of holes
or other forms of post-potting formation. Such methods have the
disadvantage of requiring avoidance of the fibres/membranes when
drilling or the like to avoid damage. This imposes limitations on
the fibre packing density and hole size as, where fibres are
tightly packed, it very difficult to drill holes without
interfering with or damaging the fibres. Further, it is difficult
to accurately locate holes relative to the fibres/membranes.
[0049] The process used in one aspect of the present invention
seeks to overcome or at least the ameliorate the problems and
disadvantages outlined above.
[0050] According to this aspect, the present invention provides a
method of forming openings in a membrane pot for use in gas
distribution comprising the steps of: providing a mould for potting
membrane ends, said mould having provided therein formations for
forming said openings during the potting process; positioning said
membrane ends in said mould which is filled with a curable potting
material; allowing said potting material to at least partially cure
and, demoulding said membranes.
[0051] Preferably, said membranes ends are uniformly distributed in
relation to said formations. In another aspect, the invention
includes a membrane assembly including at least one membrane pot
formed according to the above method.
[0052] Referring to FIGS. 2-4, the preferred method of forming the
gas distribution holes will be described. As shown in the right
side part of FIG. 2, the potting apparatus (shown empty) comprises
a potting mould 20 mounted on a vertically movable platform 21
which is raised and lowered by means of hydraulic cylinder 22. The
centre of each mould 20 is provided with a vertically movable
ejector plunger 23 operated by and hydraulic ejector cylinder 24. A
fibre guide or collar 25 fits around the periphery of the mould to
guide and hold the fibre ends during the potting process as well as
retaining the potting mixture, typically polyurethane, within the
mould. The fibres are held within a sleeve 26 when inserted into
the guide 25. The base 20' of the mould 20 has a plurality of
upstanding pins 27 which serve the dual purpose of assisting even
distribution of the fibre ends and forming the gas distribution
holes in the pot. The pins are sized and distributed as required
for correct gas bubble distribution. One form of pin distribution
is shown in FIG. 4.
[0053] In use, the guide 25 is placed about the mould 20 and the
mould 20 filled to the required level with potting material. The
platform 21 is then raised to lower the fibre ends into the mould
20. The fibre ends are normally fanned before insertion to ensure
even distribution and also trimmed to ensure a uniform length.
[0054] Once the potting material has partially cured, the pot is
ejected from the mould by raising the central ejector portion 23 of
the mould. The mould 20 is normally heated to assist curing. If
desired, the mould 20 may be centrifuged during the potting process
to assist the penetration of the potting material into the fibre
walls.
[0055] This process normally results in the ends of the fibres in
this pot being sealed, however, it will be appreciated that, by
appropriate transverse cutting of the pot, the fibre ends may be
opened for withdrawal of filtrate from the lumens.
[0056] A trial module 4 of this type was packed with 11,000 fibres
(o.d./i.d. 650/380 .mu.m). The fibre lumens at the lower end were
blocked with polyurethane and 60 holes of 4.5 mm in diameter
distributed within the fibre bundle. The lower end was connected to
an air line sealed from the feed.
[0057] FIG. 5 illustrates the setup of the trial unit. The module 4
was arranged vertically in the cylinder tank 15 and the filtrate
withdrawn from the top potting head 6 through suction. Air was
introduced into the bottom of the module 4, producing air bubbles
between fibres to scrub solids accumulated on membrane surfaces. To
remove solids clogged within membrane pores, a small quantity of
permeate was pumped through fibre lumens (permeate backwash). One
method of operation was to run suction for 15 minutes, then
aeration for 2 minutes 15 seconds. After a first minute of
aeration, a permeate backwash is introduced for 15 seconds. The
cycle returns to suction. After several cycles, the solids in the
cylinder tank 15 were concentrated and the water in the tank 15 was
drained down to remove concentrated backwash.
[0058] In the preferred embodiment shown in FIG. 1, gas/air should
be uniformly distributed and flow through the small holes 10 at the
lower end of the module 4 so that air bubbles can be produced
between fibres 9. Air bubbles then flow upwards producing shear
force to scour solids accumulated on the membrane surfaces. If the
resistance around the holes 10 is variable due to varying
resistance provided by different regions of the fibre bundle,
gas/air will tend to flow through those holes associated with a
lower resistance, resulting in by-pass flow through these
holes.
[0059] In the manufacture of membrane modules 4, it is desirable to
pot the fibres 9 in a uniform distribution relative to the holes
10. Moreover, smaller and more holes will help distribution of
gas/air, but holes that are too small will reduce bubble size and
thus the shear force applied to the outer surface of the fibres. It
is preferable that size of holes should be within the range of 0.01
to 5 mm, however, it will be appreciated that the size and position
of holes 10 will vary with module size, fibre packing density,
fibre diameter, fibre pore size and other factors.
[0060] Another way to reduce maldistribution of gas/air is to use a
layer of porous sheet (not shown) which has much smaller pore size
than the holes 10. In this case, the major pressure drop of air
will be across the porous sheet. If the porous sheet has uniformly
distributed pores, the air distribution across the air end of the
module will tend to be evenly spread.
[0061] To further improve distribution of air bubbles, a porous
tube 16 can be inserted in the centre of the cylindrical module 4.
When air passes through porous tube 16, it produces uniform bubbles
which pass out through the array of fibres scouring solids on the
fibre membrane walls. It will be appreciated that more than one
porous tube could be used and such tubes could be distributed
throughout the bundle. Fibres of large pore size or made of
non-woven material could also be used as porous tubes within the
bundle. FIG. 6 illustrates this form of module.
[0062] Referring to FIG. 7, air may be fed into a plenum chamber 17
below the aeration holes 10 by an air supply tube running from
above the feed tank to the bottom of the membrane module. This tube
may run down the centre of the membrane module or down the outside.
The plenum chamber 17 may also be connected to or form part of a
lower manifold 18 which may be used alternately for supply of
aeration gas or as a liquid manifold for removal of concentrated
backwash liquid from the tank during draindown or backwashing from
the bottom of the module.
[0063] FIGS. 8A and 8B shows the trial results of the same module
under different conditions labeled by several zones. The water in
the cylinder tank was drained down every 10 cycles in zones 1 to 4.
The discharge rate of concentrated liquid waste is thus calculated
as 3.2% of the feed volume. Zone 5 was run under the discharge of
liquid waste every 3 cycles at a rate of 10.2% of the feed.
[0064] Zones 1 and 2 compare the effect of using a porous sheet at
the air end on the suction performance for the module with a screen
surrounding the fibre bundle. Initially the suction pressure
decreased (i.e. TMP increased) quickly because of the module was
new. Then both suction pressure and resistance tended to be stable.
By comparison, the increase in suction resistance was faster after
removing the porous sheet as illustrated in Zone 2. These results
illustrate that the air end combined with a porous sheet helps to
distribute air between fibres.
[0065] The use of the screen 8 has a dual effect on filtration. The
restriction of fibre movement by screen facilitates solid
accumulation during suction. On the other hand, limited free space
between fibres reduces coalescence of air bubbles, producing better
scouring effect. It has also been found that the restriction of
fibre movement in conjunction with the movement of gas bubbles
produces high frequency vibrations in the fibres and rubbing
between the closely packed fibre surfaces which further improves
the removal of accumulated solids. Zones 3 and 4 in FIGS. 8A and 8B
represent results for the same modules with and without a
screen.
[0066] During the operation in Zone 3 some by-pass of air bubbles
was observed. This was due to different resistance around the
aeration holes, especially on the border where comparatively less
fibres were distributed around those holes. We therefore used a
porous annulus sheet covering holes at the outer border of the
lower potting head. Results in Zone 4 show the improvement compared
to Zone 3.
[0067] Solid concentration is an important issue to filtration and
fouling rate. When a tank drain was carried out every 10 cycles,
solids were built up quickly, which influenced filtration
performance. When the tank was drained down every 3 cycles, the
increase in suction resistance was significantly reduced as
reflected in Zone 5.
[0068] The frequency of air scrubbing and backwash on the
filtration performance was also investigated. FIG. 9 shows the
resistance increase for 30 minute suction and then backwash and air
scrubbing. Compared with the resistance increase in Zone 5 in FIG.
8, resistance increase was faster when suction time was longer
between backwashes.
[0069] Longer term trials were conducted to compare the effect of
porous sheet on suction performance. FIGS. 10 and 11 show the
resistance increase for more than 6 days operation, with and
without the porous sheet. For the module not connected to a porous
sheet, suction resistance increased slowly by ca. 20% during 8
days, while no obvious resistance increase during 6 days operation
when a porous sheet was used to improve air distribution. These
results and the result shown in Zones 1 and 2 in FIG. 8 suggest
that a porous sheet helps uniform air distribution.
[0070] FIGS. 12-14 are graphs which illustrate the effect of the
bubble scouring on backwash efficiency. The scouring is conducted a
regular intervals as shown the buildup of resistance followed by a
sharp decline at the time of the scouring stage.
[0071] FIG. 12 shows the effect of not using a liquid backwash in
conjunction with the gas scouring. At the beginning of the test a
normal liquid backwash where filtrate is pumped back through the
fibre lumens as a liquid backwash in conjunction with the gas
scouring along the outside of the fibres. The liquid backwash was
then stopped and only regular gas scouring was used. It was found
that even without the liquid backwash a backwash efficiency of
around 90% could be achieved.
[0072] FIG. 13 shows the effect of no gas scouring during the
backwash phase. Again the initial part of the test used a normal
liquid backwash where filtrate is pumped back through the fibre
lumens as a liquid backwash in conjunction with the gas scouring
along the outside of the fibres. The gas scouring was then stopped
between about 9:15 and 10:45. As shown on the graph the backwash
efficiency dropped dramatically from about 96% using gas scouring
to about 41% without gas scouring. The return of gas scouring
showed a marked improvement in backwash efficiency.
[0073] FIG. 14 illustrates the effect of scouring fully within the
bundle as against scouring only the outer fibres. Again the
beginning of the test shows a normal backwash regime with liquid
backwash and gas scouring up until around 9:00. The gas scouring
was then limited to the outside of the fibre bundle. The backwash
efficiency again degraded dramatically from about 98% during normal
operation to 58% with the restricted gas scouring.
[0074] The embodiments relate to membrane filtration systems and
typically to a system using suction to produce transmembrane
pressure, however, it will be appreciated that the scouring system
is equally applicable to any form of fibre membrane filtration
process, including pressurised filtration systems.
[0075] The scouring process and method may be used in conjunction
with any standard backwashing regimes including liquid backwashing,
pressurised gas backwashing, combinations of both, as well as with
chemical cleaning and dosing arrangements.
[0076] The scouring process would normally be used in conjunction
with the backwash stage, however, it may also be used continually
during the filtration and backwash stages. Cleaning chemicals such
as chlorine may be added to the gas providing the bubbles to
further assist the scouring process. Solids removed in the scouring
process may be intermittently or continually removed. With
continual removal of solid a clarifier or the like can be used. The
clarifier can be used in front of the module, in parallel with
module or the module can be in the clarifier itself. Chemical
dosing can be used in conjunction with the clarifier when
required.
[0077] The filter system using such a scouring process may be used
for sewage/biological waste treatment or combined with a
bioreactor, activated sludge or similar system.
[0078] It will be appreciated that further embodiments and
exemplifications of the invention are possible without departing
from the spirit or scope of the invention described.
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