U.S. patent application number 11/722411 was filed with the patent office on 2011-05-19 for cleaning in membrane filtration systems.
Invention is credited to Thomas Beck, Warren Johnson, Rebecca Yeo.
Application Number | 20110114557 11/722411 |
Document ID | / |
Family ID | 36601255 |
Filed Date | 2011-05-19 |
United States Patent
Application |
20110114557 |
Kind Code |
A2 |
Johnson; Warren ; et
al. |
May 19, 2011 |
CLEANING IN MEMBRANE FILTRATION SYSTEMS
Abstract
A method of cleaning permeable, hollow membranes (6) in an
arrangement of the type wherein a pressure differential is applied
across the walls (15) of the permeable, hollow membranes (6)
immersed in a liquid suspension, the liquid suspension being
applied to the outer surface of the porous hollow membranes (6) to
induce and sustain filtration through the membrane walls (15)
wherein some of the liquid suspension passes through the walls (15)
of the membranes (6) to be drawn off as clarified liquid or
permeate from the hollow membrane lumens (7), and at least some of
the solids are retained on or in the hollow membranes (6) or
otherwise as suspended solids within the liquid surrounding the
membranes (6). The method of cleaning comprises the steps of
applying a cleaning solution (14) to one side of the membrane wall
(15); applying a pressure differential across the membrane wall
(15) to cause flow of the cleaning solution (14) through the wall
(15) from the one side of the membrane wall (15) to the other side
of the membrane wall (15) and applying a reverse pressure
differential across the membrane wall (15) to cause flow of the
cleaning solution (14) through the wall (15) from the other side of
the membrane wall (15) back to the one side of the membrane wall
(15). A method of determining the amount of chemical cleaning
solution required is also disclosed.
Inventors: |
Johnson; Warren; (New South
Wales, AU) ; Beck; Thomas; (New South Wales, AU)
; Yeo; Rebecca; (Beijing, CN) |
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20080203016 A1 |
August 28, 2008 |
|
|
Family ID: |
36601255 |
Appl. No.: |
11/722411 |
Filed: |
December 19, 2005 |
PCT Filed: |
December 19, 2005 |
PCT NO: |
PCTAU0501919 |
371 Date: |
June 21, 2007 |
Current U.S.
Class: |
210/636 |
Current CPC
Class: |
B01D 2321/164 20130101;
C02F 1/444 20130101; B01D 63/02 20130101; B01D 2321/162 20130101;
B01D 2321/168 20130101; B01D 2321/16 20130101; B01D 65/02 20130101;
B01D 2315/06 20130101 |
Class at
Publication: |
210/636 |
International
Class: |
B01D 65/02 20060101
B01D065/02 |
Claims
1. A method of cleaning a permeable, hollow membrane, the
permeable, hollow membrane comprising a wall, a filtrate side, and
an outer side, the method comprising: removing liquid from the
filtrate side of the membrane; removing liquid from the outer side
of the membrane; applying a chemical cleaning solution to the outer
side of the membrane; causing flow of the chemical cleaning
solution through the wall from the outer side of the membrane into
the filtrate side to at least partially fill the filtrate side with
the chemical cleaning solution by applying a pressure differential
across the membrane wall; isolating the outer side of the membrane;
causing flow of the chemical cleaning solution back to the outer
side through the membrane wall by applying a pressurized gas to the
filtrate side; accumulating the increased pressure developed on the
outer side of the membrane as a result of the flow of the chemical
cleaning solution; and causing flow of the chemical cleaning
solution though the membrane wall from the outer side to the
filtrate side under the effect of the accumulated pressure on the
outer side of the membrane wall by releasing the pressure applied
by the pressurized gas to the filtrate side of the membrane
wall.
2. (canceled)
3. The method according to claim 1, wherein applying the
pressurized gas to the filtrate side comprises applying gas such
that the filtrate side is substantially drained of the chemical
cleaning solution.
4. The method of claim 1, wherein accumulating the increased
pressure comprises accumulating the increased pressure in a gas
space provided on the outer side of the membrane wall.
5. The method of claim 1, wherein accumulating the increased
pressure comprises accumulating the increased pressure in a bladder
arrangement.
6. The method of claim 1, wherein applying the pressure
differential comprises applying gas pressure to the outer side of
the membrane wall.
7. The method of claim 1, wherein applying the pressure
differential comprises applying a vacuum to the filtrate side.
8. A method of cleaning a permeable, hollow membrane, the
permeable, hollow membrane comprising a wall, a one side, and
another side, the method comprising: applying a chemical cleaning
solution comprising at least one of an acid, a caustic solution,
and an oxidizing solution to the one side of the membrane wall;
causing flow of the chemical cleaning solution though the wall from
the one side of the membrane wall to the other side of the membrane
wall by applying a pressure differential across the membrane wall;
and causing flow of the chemical cleaning solution through the wall
from the other side of the membrane wall back to the one side of
the membrane wall by applying a reverse pressure differential
across the membrane wall.
9. The method of claim 8, wherein applying the pressure
differential comprises applying gas pressure to the one side of the
membrane wall.
10. The method of claim 8, wherein applying the pressure
differential comprises applying a vacuum to the other side of the
membrane wall.
11. The method of claim 8, wherein applying the reverse pressure
differential comprises applying gas pressure to the other side of
the membrane wall.
12. The method of claim 8, wherein applying the reverse pressure
differential comprises applying a vacuum to the one side of the
membrane wall.
13. (canceled)
14. A method of cleaning a permeable, hollow membrane, the
permeable, hollow membrane comprising a wall, a filtrate side, and
an outer side, the method comprising: removing liquid from the
filtrate side of the membrane; removing liquid from the outer side
of the membrane; applying a chemical cleaning solution comprising
at least one of an acid, a caustic solution, and an oxidizing
solution to the outer side of the membrane; causing flow of the
chemical cleaning solution though the wall from the outer side of
the membrane into the filtrate side of the membrane to at least
partially fill the filtrate side with the chemical cleaning
solution by applying a pressure differential across the membrane
wall; and causing flow of the chemical cleaning solution through
the wall from the filtrate side of the membrane back to the outer
side of the filtrate side by applying a reverse pressure
differential across the membrane wall.
15. The method of claim 14, wherein applying the reverse pressure
differential comprises applying a pressurized gas to the filtrate
side of the membrane wall.
16. The method of claim 14, wherein the steps of the cleaning
method are repeated in cycles such that the chemical cleaning
solution is alternately moved from a first side of the membrane to
a second side of the membrane through the membrane wall.
17-24. (canceled)
25. The method of claim 1, wherein the chemical cleaning solution
comprises an oxidizing solution.
26. The method of claim 25, wherein the chemical cleaning solution
comprises chlorine.
27. The method of claim 8, wherein the chemical cleaning solution
comprises citric acid.
28. The method of claim 8, wherein the chemical cleaning solution
comprises chlorine.
29. The method of claim 14, wherein the chemical cleaning solution
comprises citric acid.
30. The method of claim 14, wherein the chemical cleaning solution
comprises chlorine.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to the backwashing of hollow
permeable membranes used in membrane filtration systems and, in
particular, to an improved method of backwashing and cleaning the
hollow permeable membranes.
BACKGROUND ART
[0002] Any discussion of the prior art throughout the specification
should in no way be considered as an admission that such prior art
is widely known or forms part of common general knowledge in the
field.
[0003] Known backwash systems include those described in our
earlier International Application No. WO93/02779 the subject matter
of which is incorporated herein by cross-reference.
[0004] During cleaning of membranes, cleaning solutions are often
flowed through the membranes and their permeable walls to clean
foulants from the membranes. Applying the cleaning solution under
pressure assists the removal of foulants from the surface.
[0005] The typical known cleaning procedure for membranes involves
cleaning the membranes in-situ. This procedure adds a set
concentration of chemical, commonly 2% citric acid followed by
200-1000 ppm sodium hypochlorite, to the membrane in a solution of
filtrate. This usually occurs at the start of the two hour cleaning
period, after which the cleaning solution is filtered through the
membrane and left to soak.
[0006] As the nature of membrane fouling varies according to feed
quality and type, flux through the membrane and hours of operation,
the amount and length of chemical cleaning required in each
situation also varies. This often results in a one process fits all
approach where a standard chemical cleaning stage is applied
regardless of the amount of fouling. This can lead to large amounts
of cleaning solution being used unnecessarily with the effect of
additional cost and environmental impact in disposing of the waste
solution after cleaning is complete.
DISCLOSURE OF THE INVENTION
[0007] It is an object of the invention to overcome or at least
ameliorate one or more of the disadvantages of the prior art or at
least provide a useful alternative.
[0008] According to a first aspect, the present invention provides
a method of cleaning permeable, hollow membranes in an arrangement
of the type wherein a pressure differential is applied across the
walls of the permeable, hollow membranes immersed in a liquid
suspension, said liquid suspension being applied to the outer
surface of the porous hollow membranes to induce and sustain
filtration through the membrane walls wherein:
[0009] (a) some of the liquid suspension passes through the walls
of the membranes to be drawn off as clarified liquid or permeate
from the hollow membrane lumens, and
[0010] (b) at least some of the solids are retained on or in the
hollow membranes or otherwise as suspended solids within the liquid
surrounding the membranes, the method of cleaning comprising the
steps of; [0011] i) removing, at least partially, liquid from the
filtrate side of the membrane; [0012] ii) removing, at least
partially, liquid from the outer side of the membrane; [0013] iii)
applying a cleaning solution to the outer side of the membrane;
[0014] iv) applying a pressure differential across said membrane
wall to cause flow of said cleaning solution through said wall from
the outer side of the membrane into the membrane lumen to at least
partially fill said lumen with cleaning solution; [0015] v)
isolating the outer side of the membrane; [0016] vi) applying a
pressurized gas to the filtrate side of the membrane wall to cause
flow of the cleaning solution back to the outer side through the
membrane wall; [0017] vii) accumulating the increased pressure
developed on the outer side of the membrane as a result of said
flow of cleaning solution;
[0018] viii) releasing the pressure applied by said pressurised gas
to said filtrate side of the membrane wall to cause flow of said
cleaning solution through said membrane from the outer side to the
filtrate side under the effect of said accumulated pressure on the
outer side of the membrane wall.
[0019] For preference the cleaning solution is a chemical cleaning
solution.
[0020] Preferably, in step vi) gas, usually air, is applied such
that the membrane lumen is substantially drained of cleaning
solution. Preferably the pressure is accumulated in step vii) in a
gas space provided on the outer side of the membrane wall or
alternatively in a bladder arrangement.
[0021] The differential pressure of step iv) may be provided by
applying gas pressure to the outer side of the membrane wall or by
applying a vacuum to the filtrate side.
[0022] According to a further aspect the present invention provides
a method of cleaning permeable, hollow membranes in an arrangement
of the type wherein a pressure differential is applied across the
walls of the permeable, hollow membranes immersed in a liquid
suspension, said liquid suspension being applied to the outer
surface of the porous hollow membranes to induce and sustain
filtration through the membrane walls wherein:
[0023] (a) some of the liquid suspension passes through the walls
of the membranes to be drawn off as clarified liquid or permeate
from the hollow membrane lumens, and
[0024] (b) at least some of the solids are retained on or in the
hollow membranes or otherwise as suspended solids within the liquid
surrounding the membranes, the method of cleaning comprising the
steps of; [0025] i) applying a cleaning solution to one side of the
membrane wall; [0026] ii) applying a pressure differential across
said membrane wall to cause flow of said cleaning solution through
said wall from said one side of the membrane wall to the other side
of the membrane wall; [0027] iii) applying a reverse pressure
differential across said membrane wall to cause flow of said
cleaning solution through said wall from said other side of the
membrane wall back to said one side of the membrane wall.
[0028] According to yet a further aspect the present invention
provides a method of cleaning permeable, hollow membranes in an
arrangement of the type wherein a pressure differential is applied
across the walls of the permeable, hollow membranes immersed in a
liquid suspension, said liquid suspension being applied to the
outer surface of the porous hollow membranes to induce and sustain
filtration through the membrane walls wherein:
[0029] (a) some of the liquid suspension passes through the walls
of the membranes to be drawn off as clarified liquid or permeate
from the hollow membrane lumens, and
[0030] (b) at least some of the solids are retained on or in the
hollow membranes or otherwise as suspended solids within the liquid
surrounding the membranes, the method of cleaning comprising the
steps of; [0031] i) removing, at least partially, liquid from the
filtrate side of the membrane; [0032] ii) removing, at least
partially, liquid from the outer side of the membrane; [0033] iii)
applying a cleaning solution to the outer side of the membrane;
[0034] iv) applying a pressure differential across said membrane
wall to cause flow of said cleaning solution through said wall from
the outer side of the membrane into the membrane lumen to at least
partially fill said lumen with cleaning solution; [0035] v)
applying a pressure differential across said membrane wall to cause
flow of said cleaning solution through said wall from the lumen
side of the membrane back to the outer side of the membrane
lumen.
[0036] Preferably the pressure differential in step v) is produced
by applying a pressurized gas to the filtrate side of the membrane
wall to cause flow of the cleaning solution back to the outer side
through the membrane wall.
[0037] The cleaning process can be repeated in cycles such that the
cleaning solution is alternately moved from one side of the
membrane to the other through the membrane wall.
[0038] The process can be applied to membranes submerged in an open
vessel as well as pressurized membrane filtration systems.
[0039] According to another aspect of the present invention there
is provided a method of controlling a chemical clean of a membrane
comprising:
[0040] measuring pH and/or membrane resistance of a membrane for at
least a portion of said clean; and
[0041] ceasing said chemical clean when pH and/or membrane
resistance attains a predetermined value.
[0042] According to another aspect of the present invention there
is provided a method of controlling a chemical clean of a membrane
comprising:
[0043] measuring pH and/or membrane resistance of a membrane for at
least a portion of said clean;
[0044] measuring elapsed time of the clean;
[0045] calculating a rate of change of pH with respect to time
(dpH/dt) and/or a rate of change of membrane resistance (dR/dt)
with respect to time; and
[0046] ceasing said chemical clean when dpH/dt and/or dR/dt attains
a predetermined value.
[0047] According to another aspect of the present invention there
is provided a method of controlling the chemical cleaning of a
filtration system comprising the steps of measuring membrane
resistance of a membrane for at least a portion of said clean;
[0048] measuring elapsed time of the clean;
[0049] calculating a rate of change of membrane resistance (dR/dt)
with respect to time; and
[0050] using dR/dt to calculate a duration for completion of the
clean.
[0051] According to another aspect of the present invention there
is provided a method of controlling a chemical clean of a membrane
comprising:
[0052] increasing the amount of chemical cleaning agent present
during the clean;
[0053] measuring membrane resistance of a membrane for at least a
portion of said clean;
[0054] ceasing the increase in chemical cleaning agent when
membrane resistance attains a predetermined value.
[0055] Preferably, the amount of cleaning agent is increased
incrementally.
[0056] For preference, the predetermined value approximates a
steady-state value of membrane resistance. Preferably, the
membranes are microfiltration or ultrafiltration type
membranes.
BRIEF DESCRIPTION OF THE DRAWINGS
[0057] Preferred embodiments of the invention will now be
described, by way of example only, with reference to the
accompanying drawings in which:
[0058] FIG. 1a shows a simplified sectional side elevation of a
membrane module with a lower portion of the module immersed in a
chemical cleaning solution and suction applied to the membrane
lumens;
[0059] FIG. 1b shows an enlarged sectional view of the membranes in
the indicated region of FIG. 1a;
[0060] FIG. 1c shows an enlarged sectional view of the membranes in
the indicated region of FIG. 1a;
[0061] FIG. 2a shows a simplified sectional side elevation of a
membrane module of FIG. 1 with a lower portion of the module
immersed in a chemical cleaning solution and pressurized gas
applied to the membrane lumens;
[0062] FIG. 2b shows an enlarged sectional view of the membranes in
the indicated region of FIG. 2a;
[0063] FIG. 3a shows a simplified sectional side elevation of a
membrane module of FIG. 1 with a lower portion of the module
immersed in a chemical cleaning solution and suction applied to the
membrane lumens;
[0064] FIG. 3b shows an enlarged sectional view of the membranes in
the indicated region of FIG. 3a;
[0065] FIG. 3c shows an enlarged sectional view of the membranes in
the indicated region of FIG. 3a;
[0066] FIG. 4a shows a simplified sectional side elevation of
another embodiment of a membrane module with a lower portion of the
module immersed in a chemical cleaning solution and suction applied
to the membrane lumens;
[0067] FIG. 4b shows an enlarged sectional view of the membranes in
the indicated region of FIG. 4a;
[0068] FIG. 4c shows an enlarged sectional view of the membranes in
the indicated region of FIG. 4a;
[0069] FIG. 5a shows a simplified sectional side elevation of the
membrane module of FIG. 4 with a lower portion of the module
immersed in a chemical cleaning solution and pressurized gas
applied to the membrane lumens;
[0070] FIG. 5b shows an enlarged sectional view of the membranes in
the indicated region of FIG. 5a;
[0071] FIG. 6a shows a simplified sectional side elevation of
another embodiment of a membrane module with a lower portion of the
module immersed in a chemical cleaning solution and suction applied
to the membrane lumens;
[0072] FIG. 6b shows an enlarged sectional view of the membranes in
the indicated region of FIG. 6a;
[0073] FIG. 6c shows an enlarged sectional view of the membranes in
the indicated region of FIG. 6a;
[0074] FIG. 7a shows a simplified sectional side elevation of the
membrane module of FIG. 6 with a lower portion of the module
immersed in a chemical cleaning solution and pressurized gas
applied to the membrane lumens;
[0075] FIG. 7b shows an enlarged sectional view of the membranes in
the indicated region of FIG. 7a;
[0076] FIG. 8a shows a simplified sectional side elevation of an
embodiment of a membrane module in an open vessel with a lower
portion of the module immersed in a chemical cleaning solution and
suction applied to the membrane lumens;
[0077] FIG. 8b shows an enlarged sectional view of the membranes in
the indicated region of FIG. 8a;
[0078] FIG. 8c shows an enlarged sectional view of the membranes in
the indicated region of FIG. 8a;
[0079] FIG. 9a shows a simplified sectional side elevation of a
membrane module of the embodiment of FIG. 8 with a lower portion of
the module immersed in a chemical cleaning solution and pressurized
gas applied to the membrane lumens;
[0080] FIG. 9b shows an enlarged sectional view of the membranes in
the indicated region of FIG. 9a;
[0081] FIG. 10 shows a graph of transmembrane pressure (TMP)
measured over time for a membrane module of the type illustrated in
FIGS. 8 and 9 undergoing a chemical clean using the method
according to the invention;
[0082] FIG. 11 shows a graph of membrane resistance measured over
time for two types of chemical cleaning process with incremental
increases in the volume of chemical cleaning agent added during the
cleaning process; and
[0083] FIG. 12 shows a graph of membrane resistance measured over
time with incremental increases in the volume of chemical cleaning
agent added during the cleaning process.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0084] Referring to the FIGS. 1 to 7, there is shown a membrane
module 5 having a plurality of hollow fibre membranes 6. The fibre
membranes 6 have their lumens 7 closed at the lower end in a lower
pot 8 and open at the upper end through upper pot 9. The module 5
is contained in a vessel 10 having a controllable valve 11 for
opening/closing the vessel 10 to atmosphere. Upper pot 8 is
connected to a filtrate collection chamber 12 having a port 13.
[0085] One embodiment of the cleaning process according to the
invention will now be described with reference to FIGS. 1 to 3. As
best shown in FIGS. 1a to 1c, liquid remaining in lumens 7 after
filtration is drained while liquid remaining in the vessel 10 is
also at least partially drained. The vessel 10 is then, at least
partially, filled with cleaning solution 14 as best shown in FIG.
1a. A differential pressure is then applied across the membrane
walls 15, in this embodiment by applying a vacuum to port 13, so
that the cleaning solution 14 is drawn through the membrane walls
15 (as shown in FIG. 1b) and up the membrane lumen 7 until it is,
at least partially, filled with cleaning solution.
[0086] As shown in FIGS. 2a and 2b, the valve 11 is then closed to
isolate the vessel 10 while pressurized gas is applied through port
13 to the cleaning solution filling the membrane lumen 7 and
displaced through the membrane wall 15 until the lumen 7 is
substantially drained of cleaning solution. With the vessel 10
isolated, the transfer of cleaning solution 14 through the membrane
wall 15 to the outer side of the membrane 6 results in the pressure
within the vessel to increase as the gas space 16 within the vessel
10 is compressed.
[0087] As shown in FIGS. 3a to 3c, the lumen side of the membranes
are vented to atmosphere. The accumulated pressure in the gas space
16 then forces the cleaning solution 14 to flow back through the
membrane wall 15 as best shown in FIG. 3b.
[0088] FIGS. 4a to 7b illustrate embodiments of the invention where
a bladder arrangement may be used instead of the gas space 16 to
accumulate pressure.
[0089] Referring to FIGS. 4a to 4c, the operation is similar to
that shown in FIG. 3a to 3c, however, in this embodiment when the
lumen side of the membranes is vented to atmosphere through valve
17, the bladder 16 delivers pressure to the feed side of the vessel
10 forcing the cleaning solution 14 through the membrane wall 15
and along the membrane lumen 7 as best shown in FIGS. 4b and
4c.
[0090] Referring to FIGS. 5a and 5b, the pressurising of the
lumen/filtrate side is shown. Pressurised gas is applied to the
lumen/filtrate side of the membranes 6 by feeding pressurised gas
through line 18 and valve 17 to port 13. The pressurised gas causes
the cleaning solution within the lumen 7 to flow through the
membrane wall 15 to the outer side of the membrane resulting in the
pressure within the vessel 10 increasing and being transferred to
the bladder 16 connected to the vessel 10 through line 19 and valve
11.
[0091] FIGS. 6a to 6c show a similar arrangement to FIGS. 4a to 4c
but in this embodiment the gas pressure is applied to the vessel 10
from an external source, rather than the bladder 16, through line
19 and valve 11. Bladder 16 is used to accumulate pressure on the
lumen side of the membranes 6 as shown in FIG. 6a.
[0092] As shown in FIGS. 7a and 7b, when the reverse flow of
cleaning solution is required, the vessel 10 is vented to
atmosphere through line 19 and valve 11 and bladder 16 releases
accumulated pressure to the lumen side forcing the cleaning
solution 14 within the lumens back through the lumen walls 15 (see
FIG. 7b).
[0093] Referring to FIGS. 8 and 9, an embodiment of the cleaning
process according to the invention is illustrated where the vessel
10 is open to atmosphere. In this embodiment flow of cleaning
solution through the membrane wall 15 is provided by alternately
applying suction/vacuum or pressure to the lumen side of the
membranes 6. The membrane module 5 is again immersed at least
partially in chemical cleaning solution 14 and suction is applied
to the open ends of the fibre membrane lumens 7. As best shown in
FIG. 8b, the cleaning solution 14 is drawn through the membrane
wall 15 and into the fibre membrane lumen 7. The cleaning solution
14 is then drawn up through the lumen 7 until it is completely
filled as shown in FIG. 8c. As shown in FIGS. 9a and 9b,
pressurized gas is then applied to the cleaning solution 14 filling
the membrane lumen 7 and the cleaning solution is displaced through
the membrane wall 15 as previously described.
[0094] The process illustrated in the embodiments can be repeated
in cycles such that cleaning solution is alternatively moved from
one side of the membrane wall 15 to the other. This flow of
cleaning solution to and from the membrane lumens 7 and well as
along their length results in an effective chemical clean of the
membrane module 5.
[0095] FIG. 10 shows the results of applying the cleaning regime
according to the invention to a membrane module of the type where
the vessel 10 is open to atmosphere. The cleaning process was
performed as follows:
[0096] 1. The membrane vessel was filled with filtrate via
backfilling from the lumen side to the shell side, with
simultaneous chlorine dosing into the filtrate line. The vessel
filtrate level was about 30%, with a target volume of cleaning
solution (NaOCl) of 30 mL.
2. The filtrate was then recirculated briefly through the system in
order to ensure a well-mixed cleaning solution.
[0097] 3. The lumens were then drained of liquid by 100 kPa air
being applied to the filtrate line. This allowed the cleaning
solution to diffuse through the pores and down the fibre length,
which raised the filtrate vessel level. This step may be ended when
the liquid level stops rising.
[0098] 4. The lumens were then filled with the cleaning solution by
using vacuum air applied to the lumen side of the membranes. During
this step the level in the filtrate tank dropped as the liquid was
pulled into the fibre lumens. This step may be ended when the
liquid level stops falling.
5. The lumen fill and drain steps were repeated until contact time
had reached 1800 seconds.
6. After 1800 seconds of cleaning solution contact, the vessel was
topped up with feed. This allowed the remaining free chlorine in
the cleaning solution to contact with the part of the module that
was exposed during the clean.
7. The system was then aerated to maximise contact of solution with
module.
8. The tank was then drained and flushed with filtrate before
returning to service.
[0099] The data graphed in FIG. 10 shows a period when the cleaning
regime was performed once every 24 filtration hours for 4 days in
succession, with the module operating at 1.7 m3/hr with a 30 minute
backwash interval. Chlorine in the form of sodium hypochlorite
(NaOCl) was used, the average free chlorine concentration during
the clean being 100 ppm. Feed water turbidity was between 60-90 NTU
throughout.
[0100] The data shown in FIG. 10 illustrates the regular reduction
in transmembrane pressure (TMP) flowing each clean.
[0101] One embodiment of the invention seeks to minimise the amount
of chemical required by adding it incrementally to the membrane
tank, whilst monitoring resistance through the membrane during a
recirculation stage in the cleaning process. Chemical additions can
cease when the further addition of chemical leads to change in the
membrane resistance below a predetermined level, hence minimising
the amount of excess chemical agent used in the cleaning
process.
[0102] The resistance value can be monitored during filtrate
recirculation. Typically, during a standard cleaning procedure, the
chemical cleaning solution is recirculated at the start of the
clean only, followed by up to 48 hours of soaking of the membranes.
In the present embodiment, the chemical cleaning solution is
recirculated for several minutes (for example -3 minutes) every
15-30 minutes during the soak/aeration steps, allowing the membrane
resistance to be measured periodically throughout the cleaning
process.
[0103] When the change in resistance per 3 recirculations drops
below a predetermined value (for example -0.1) the cleaning process
has recovered the maximum performance at that chemical
concentration and further chemical agent is added. When the
addition of further chemical agent effects the change in resistance
by less than the predetermined value per 3 recirculations (for
example -0.1), no further recovery can be achieved and the cleaning
process can therefore be terminated immediately. Conversely, the
cleaning potential can be maximized by extending the cleaning
process so that the change in resistance per 3 circulations is
below a certain predetermined value. FIG. 11 shows a graph of
resistance value variation of the duration of the cleaning process
for two different cleaning regimes using citric (CIP1) and chlorine
(CIP2) cleaning agents.
[0104] Referring to the graph shown in FIG. 12, the volume of
chemical agent begins around 100 ml as shown at A resulted in a
significant drop in membrane resistance. The amount of chemical
agent was further increased as shown at B and C resulting in
further decreases in membrane resistance. Once the volume of
chemical agent reached about 250 ml, the membrane resistance change
reached substantially a steady state as shown at D and further
increases (E) in chemical agent had minimal effect. At this stage
the volume of chemical agent added can be ceased without adversely
affecting the cleaning process and recovery in transmembrane
flow.
[0105] It will be appreciated that using the above measurements it
is possible to determine a resistance profile during the cleaning
process for a particular membrane arrangement or configuration. The
resistance profile can then be used to predict the end of cleaning
process time, half-life and reduce chemical use in simultaneous
cleans of similar systems. The resistance profile may be further
used to determine whether chemicals are required to be added during
the cleaning process with the type and amount of chemical being
dependent on feed and foulant quality.
[0106] Typical cleaning solutions which may be used include acids,
caustic solutions and oxidizing solutions (e.g. chlorine).
[0107] The invention may be embodied in a similar apparatus to that
described in the aforementioned International Application No.
WO93/02779 appropriately modified to operate in accordance with the
inventive method.
[0108] 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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