Note: Descriptions are shown in the official language in which they were submitted.
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B&P File No. 4320-49
BERESKIN & PARR CANADA
Title: Aeration System for Submerged Membrane Module
Inventors: Hamid R. Rabie, Pierre Cote, Manwinder Singh and Arnold
janson
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Title: Aeration System for Submerged Membrane Module
FIELD OF THE INVENTION
This invention concerns the use of scouring air bubbles
produced by an aeration system to clean membranes in a submerged
membrane filter.
_ BACKGROUND OF THE INVENTION
Submerged membranes are used to treat liquids containing
solids to produce a filtered liquid lean in solids and an unfiltered retentate
rich in solids. For example, submerged membranes are used to withdraw
substantially clean water from an activated sludge aeration tank containing
wastewater and to withdraw potable water from a tank filled with water
from a lake or reservoir.
The membranes are generally arranged in modules, which
comprise the membranes and headers attached to the membranes, and are
submerged in a tank of water containing solids. A transmembrane pressure
is applied across the membrane walls which causes clean water to permeate
through the membrane walls. Solids are rejected by the membranes and
remain in the tank water to be biologically or chemically treated or drained
from the tank.
Air bubbles are introduced to the tank below the
membrane modules and rise through the membrane modules. The air
bubbles create an air lift which recirculates tank water through the
membrane module. The rising bubbles and tank water produce a cleaning
effect, scouring and agitating the membranes to inhibit solids in the tank
water from fouling the pores of the membranes. There is also an oxygen
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transfer from the bubbles to the tank water which, in wastewater
applications, provides oxygen for microorganism growth.
An aerator to produce scouring air bubbles is typically
attached to a lower header of a membrane module located below the
membranes and connected by conduits to an air blower. The air blower
generally runs continuously to minimize stress on the air blower motors
and to provide a constant supply of air for microorganism growth if desired.
With typical aeration systems, an operator increases the
rate of air flow to the aerators if more cleaning is desired. This technique,
however, has a number of disadvantages. In wastewater applications,
simply increasing aeration may increase the total amount of oxygen
transferred to the tank water but reduces the efficiency of the oxygen
transfer for two reasons. Firstly, as bubble density increases the bubbles
coalesce more readily into larger bubbles which have a reduced ratio of
surface area to volume. Secondly, the retention time of the bubbles is
reduced because a stronger air lift causes the water column around the
membranes to rise faster. Increased aeration also stresses the membranes
and air blower motors and increases the amount of energy used.
E'~nother concern with typical aeration systems is that they
cause the tank v,~ater to move in a generally constant recirculation pattern
in the tank. The recirculation pattern typically includes "dead zones" where
tank water is not reached by the recirculating tank water and bubbles. The
membranes in these dead zones are not effectively cleaned and quickly foul
with solids. A related problem occurs in membrane modules where the
membranes are installed with a degree of slack to allow the membranes to
vibrate and shake off solids. The movement of tank water in the tank often
causes slackened membranes to assume a near steady state position which
prevents significant vibration near the ends of the membranes.
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Yet another concern with current aeration systems is that
they typically foul over time which either reduces the supply of bubbles into
the tank water or requires the operator to clean the aeration system
frequently. For example, aeration is typically stopped from time to time for
backwashing, cleaning or other maintenance procedures. Unfortunately,
simple aerators have no effective mechanism to seal themselves when air-
flow is stopped and tank water may enter the aeration system. Some of the
tank water remains in the aeration system even when the air blowers are
turned back on and interferes with the flow of air. Further, tank water in
the aeration system evaporates rapidly when the air supply is turned back
on leaving deposits of solids in the aeration system which foul the aeration
system. In wastewater applications in particular, the solids build up may
plug many of the holes of the aerators over time.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an
aeration system that Via) has improved cleaning ability for a given amount of
aeration, (b) reduces the size of dead zones around the membranes and
promotes the movement of slackened membranes and (c) more reliably
produces coarse bubbles by inhibiting fouling of the aeration system.
The present invention is directed at an aeration system
which produces a transient flow pattern in the tank water and in which the
aerators foul less frequently, particularly when used to create large bubbles.
The aeration system has at least one but preferably two or
more groups of aerators, further preferably horizontally disposed from each
other. The flow of air through a group of aerators is increased or decreased
in cycles of 120 seconds or less to rapidly increase or decrease the density
of
the tank above the group of aerators. The cycled aeration produces
vertically transient flow which in turn create turbulence and some
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horizontally transient flow. The transient flows and turbulence reduces the
size or duration of dead zones in the tank water, preferably those which
typically form near the headers of membrane modules and, particularly
with membrane modules comprising hollow fibre membranes, promotes
movement of tank water into and out of the group of membranes to help
remove solids from the membrane module. When slackened membranes
are used, the transient flow also encourages movement of the membranes.
Preferably, horizontally transient flow is encouraged by
modulating the flow of air to a first group of aerators while the supply of
air
to at least a second group of aerators is either constant or modulated in a
way that is not in sync with the modulating flow of air to the first group of
aerators. More preferably, the supply of air to both a first and a second
group of aerators are modulated but the two supplies of air are 180 degrees
out of sync. Such an aeration system may be constructed by splitting an air
supply conduit from an air blower into two manifolds, at least one having a
flow limiter such that one manifold carries a higher percentage or all of the
total flow of air. Each manifold has a valve which can be activated to divert
its air to the other manifold. By opening one valve while closing the other,
an alternating supply of air is provided to the manifolds without turning
the air blower motors on and off which drastically reduces their service life.
Further, the total supply of air is generally constant so that the growth of
microorganisms, if present in the tank water, is not significantly disturbed.
Preferably, the varying supply of air to the aerators causes
the aerators to be flooded periodically with tank water (and then at least
partially re-emptied) to wet dried foulants and to wash out foulants.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the present invention will now
be described with reference to the following figures.
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Figure 1 is a schematic drawing of a submerged membrane
reactor according to an embodiment of the invention.
Figure 2 is a plan view schematic of membrane modules
and a portion of an aeration system according to an embodiment of the
invention.
Figure 3 is an elevational schematic of membrane modules
and parts of an aeration system according to an embodiment of the
invention.
Figures 4A, 4B, 4C and 4D are elevational representations
of membrane modules and part of an aeration system according to an
embodiment of the invention.
Figures 5A, 5B, 5C and 5D are schematic drawings of a part
of an aeration system according to an embodiment of the invention.
Figures 6A and 6B are drawings of membrane modules
and a portion of an aeration system according to an embodiment of the
invention.
Figures 7A and 7B are drawings of aerators according to an
embodiment of the invention.
Figures 8A, 8B and 8C are charts showing the results of
tests performed on embodiments of the invention having two groups of
aerators.
Figure 9 is a chart showing the results of tests performed
on embodiments of the invention having a single group of aerators.
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DETAILED DESCRIPTION OF THE INVENTION
Referring now to Figure 1, the general arrangement of a
reactor 10 is shown according to an embodiment of the invention. The
description of the reactor 10 in this section generally applies to various
embodiments to be described below unless modified by or inconsistent with
the description of any particular embodiment.
The reactor 10 has a tank 12 which is initially filled with
feed water 14 through an inlet 16. The feed water 14 may contain
microorganisms, suspended solids or other matter which will be
collectively called solids. Once in the tank, the feed water 14 becomes tank
water 18 which is likely to have increased concentrations of the various
solids, particularly where the reactor 10 is used to treat wastewater.
One or more membrane modules 20 are mounted in the
tank and have headers 22 in fluid communication with a permeate side of
one or more membranes 23. Membrane modules 20 are available in
various sizes and configurations, including hollow fibre and flat sheet, with
various header configurations. For hollow fibre membranes 23, for
example, the membranes 23 may be oriented vertically and held in place
between an upper and a lower header 22 in fluid communication with the
lumens of the membranes 23 or oriented horizontally with horizontally
opposed headers 22 in fluid communication with the lumens of the
membranes 23. For flat sheet membranes 23, the membranes 23 are
typically oriented vertically in a spaced apart pair with headers 22 on all
four
sides in fluid communication with the resulting inside surfaces. A
membrane module 20 may have one or more has microfiltration or
ultrafiltration membranes 23 and many membrane modules 20 may be
joined together to form cassettes, but all such configurations will be
referred
to as membrane modules 20. The membranes 23 in the membrane
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modules 20 preferably have a pore size between 0.005 and 10 microns, such
as those manufactured by Zenon Environmental Inc. and sold under the
ZeeWeed trade mark, but many suitable membranes 23 and membrane
modules 20 are sold by other manufacturers.
During permeation, the tank 12 is kept filled with tank
water 18 above the level of the membranes 23 in the membrane modules
20. Filtered water called permeate 24 flows through the walls of the
membranes 23 in the membrane modules 20 under the influence of a
' transmembrane pressure and collects at the headers 22 to be transported to a
permeate outlet 26 through a permeate line 28. The transmembrane
pressure is preferably created by a permeate pump 30 which creates a partial
vacuum in a permeate line 28. Permeate 24 may be periodically flowed in a
reverse direction through the membrane module 20 to assist in cleaning the
membrane modules 20.
During permeation, the membrane modules 20 reject
solids which remain on the outside of the membranes 23. These solids may
be removed by a number of methods including digestion by
microorganisms if the reactor 10 is a bioreactor or draining the tank 12
periodically or by continuously removing a portion of the tank water 18, the
latter two methods accomplished by opening a drain valve 32 in a drain
conduit 34 at the bottom of the tank.
Cleaning, and aeration if desired, are provided by bubbles
36 from an aeration system 37 having one or more aerators 38 connected by
an air delivery system 40 and a distribution manifold 51 to one or more air
blowers 42. The aerators 38 may be of various types including distinct
aerators, such as cap aerators, or simply holes drilled in conduits attached
to
or part of the distribution manifold 51. The bubbles 36 are preferably made
of air but may be made of other gasses such as oxygen or oxygen enriched air
if required. The bubbles 36 rise upwards through the membrane modules
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20 to clean and inhibit fouling of the membranes 23.
In addition to cleaning the membranes 23 in the
membrane modules 20 directly, the bubbles 36 also decrease the local density
of tank water 18 in or near the membrane modules 20 which creates an air-
lift effect causing tank water 18 to flow upwards past the membrane
modules 20. The air lift effect causes a recirculation pattern 46 in which the
tank water 18 flows upwards through the membrane modules 20 and then
downwards along the sides or other parts of the tank. The bubbles 36
typically burst at the surface and do not generally follow the tank water 18
through the downward flowing parts of the recirculation pattern 46. The
tank water 18 may also flow according to, for example, movement from the
inlet 16 to the drain conduit 34, but such flow does not override the flow
produced by the bubbles 36.
The bubbles 36 have an average diameter between .1 and 25
mm. Larger bubbles 36 are more effective in cleaning membranes, but
smaller bubbles 36 are more efficient in transferring oxygen to the tank
water 18. The bubbles 36 are preferably between 3 mm and 20 mm and more
preferably between 5 mm and 15 mm in diameter. Bubbles 36 of the size
described above provide both effective cleaning of the membranes 23 in the
membrane modules 20 and efficient transfer of oxygen to the tank water 18
without causing Pxcessive foaming of the tank water 18 at the surface of the
tank 12. If the reactor 10 is used to create potable water or for other
applications where oxygen transfer is not required, then bubbles between 5
mm and 25 mm ire preferred.
The bubbles 36 may be larger than a hole in an aerator 38
where the bubble is created according to known factors such as air pressure
and flow rate and the depth of the aerators 38 below the surface of the tank
water 18. If the aerators 38 are located near the bottom of a large tank 12,
such as those used in municipal treatment works, an aerator 38 with holes
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of between 2 mm and 15 mm and preferably between 5 mm and 10 mm
might be used depending on the pressure and flow rate of the supplied air.
In typical systems, there is a pressure drop of between 5 mm and 100 mm
and more typically between 10 mm and 50 mm across the holes of the
aerators 38. Parts of the aeration system 37 located at a distance below the
bottom of the holes of the aerators 38 equal to the pressure drop are
generally free of tank water when the air blower 42 is operating, although
small amounts of tank water 18 may still seep into the aeration system 37.
In a first embodiment, the aeration system 37 is operated
cyclically during permeation as will be described below. Referring still to
Figure 1, the air blower 42 is operated to provide a flow rate of air which
varies from a full rate to a reduced rate and then back to the full rate in
repeated cycles. The rate of air flow preferably varies in a step form through
the cycle meaning that air flow is altered in between the full rate and
reduced rate as~ rapidly or abruptly. The inventors have noticed that when
the rate of air flow is rapidly increased to the full rate, there is a short
burst
of unusually large bubbles and significant transient flow and turbulence.
Further preferably, the step form variation in air flow rate
involves generally equal time at the full rate and reduced rate. This appears
to allow the air lift effect created by the full rate air flow to be reduced
substantially when air flow is supplied at the reduced rate without resulting
in large time periods between successive transitions to full rate aeration.
The air blower 42 can be operated to produce the variation in air flow rate
most simply by turning it on and off at the appropriate times.
Alternatively, the air blower 42 can be operated continuously and
completely or partially vented during the appropriate part of each cycle.
The total cycle time varies with the depth of the tank, the
design of the membrane modules 20, process parameters and the conditions
of the feed water 14 to be treated, but preferably ranges from 10 seconds (5
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seconds at the full rate and 5 seconds at the reduced rate) to 60 seconds (30
seconds at the full rate, 30 seconds at the reduced rate).
The amount of air used is dependant on numerous factors
but is preferably related to the superficial velocity of air flow. The
superficial velocity of air flow is defined as the rate of air flow delivered
by
the air blower 42 at standard conditions (1 atmosphere and 25 degrees
Celsius) divided by the cross sectional area of aeration. The cross sectional
area of aeration is determined by measuring the area effectively aerated by
aerators. Superficial velocities of air flow of between 0.015 m/s and 0.15 m/s
are preferred at the full rate. Air blowers for use in drinking water
applications may be sized towards the lower end of the range while air
blowers used for waste water applications may be sized near the higher end
of the range.
At the reduced rate, the air flow rate preferably ranges from
none to about one third of the full rate. For example, with a ZW500
membrane module produced by ZENON Environmental Inc., airflow of
about 15 scfm per module is often used at the full rate of aeration with air
flow at the reduced rate ranging from 0 scfm per module to 5 scfm per
module. Within these ranges, the reduced rate of air flow is influenced by
the quality of the feed water 14. A reduced rate of air flow of 0 scfm is
generally preferred, but with some feed water 14, the membranes 23 foul
significantly even within the short time of the reduced aeration. In these
cases, it is more efficient for the reduced rate of air flow to range up to
about
one third of the full rate.
Referring now to Figure 2, a plan view of a portion of
another embodiment of the invention is shown. Membrane modules 20
(shown in dashed lines) are arranged within a tank 12. The air delivery
system 40 has a first manifold 50 and a second manifold 52 connected to
aerators 38 each comprised of holes 53 in conduits 54, the holes 53 sized to
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create the desired diameter of bubbles 36 (bubbles 36 not illustrated). The
aerators could also other types of aerators,such as cap aerators
38 be
mounted the conduits54. The objects shownin Figure 2 are
in not
intended be drawn scale and could havevarious dimensions.
to to
Conduits 54 are typically between one to two inches in diameter. Further,
the number of conduits 54 and aerators 38 for a given number and size of
membrane modules 20 may vary and the conduits 54 might also be placed
in other orientations or spacings. For example, the conduits 54 may be
perpendicular to the membrane modules 20 instead of parallel to them as
' 10 shown. Now referring to Figure 3, an elevation of a portion of this
embodiment of the invention is shown.
To create transient flow, the supply of air to at least one of
the first manifold 50 or the second manifold 52 is varied over time relative
to the other. For example, air supply to the first manifold 50 can be cycled
between high and low or off while air supply to the second manifold 52
remains constant. Preferably, air supply to the second manifold 52 is also
cycled between high and low or off but such that there is a phase shift or
time lag, and more preferably a 180 degree phase shift, between the supply of
air to the first manifold 50 and the second manifold 52 so that both
manifolds are not carrying their highest supply of air at the same time.
Since the flow rate of air determines the magnitude of the resulting reduced
density of the tank water 18 above either the first aerators 38 or the second
aerators 39, altering the supply of air to the first aerators 38 or the second
aerators 39 produces variations in the density of the tank water 18 above
one relative to the other. These relative variations in density cause
variations in the vertical velocity of the tank water 18 above the first
aerators 38 or the second aerators 39. Further, the relative variations in
density also causes tank water 18 to flow horizontally between the areas
above the first aerators 38 or the second aerators 39. When the supply of air
to at least one of the first aerators 38 or the second aerators 39 is varied
over
time relative to the other, both the horizontal and vertical movement of
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tank water 18 is generally transient. The resulting transient flow of the tank
water 18 reduces dead spaces in or around the membrane modules 20.
The variation in air supply preferably occurs in a regular
cycle of between two seconds and one hundred and twenty seconds in
length but preferably between 20 seconds and 40 seconds in length. Short
cycles of 10 seconds or less (ie. 5 seconds of air flowing through an aerator
and 5 seconds without air flowing through an aerator) may not be sufficient
to establish regions of different densities with the tank water 18 in a deep
tank 12 where such time is insufficient to allow the bubbles 236 to rise
through a significant distance relative to the depth of the tank 12. Long
cycles of 120 seconds or more may result in parts of a membrane module 20
not receiving bubbles 36 for extended periods of time which can result in
rapid fouling. As described above, a step form variation in rate of air flow
is
preferred.
Preferably, the supply of air to each of the first aerators 38
and the second aerators 39 is varied between a value in the range of 0% and
30% of the total supply of air and a value in the range of 70% to 100% of the
total supply of air. More preferably, the total air supply is switched between
the first aerator and the second aerator. Also preferably, the total supply of
air to the tank 12 remains constant despite the cycling of the air supply to
the first aerator 238 or the second aerator 239. It is further preferred that
each cycle involve aeration at the full rate for one half of the cycle and
aeration at the reduced rate for one half of the cycle and that the area
aerated
by each of the first manifold 50 and the second manifold 52 be similar.
With this arrangement, the air blower 42 or can be operated at a constant
power level while practicing the invention.
With multiple membrane modules 20, a pattern of
membrane modules 220, first aerators 38 and second aerators 39 may be
repeated to control the volume of tank water 18 that is accelerated or
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decelerated as the air supply is cycled from the first manifold 50 to the
second manifold 52. In addition, the techniques described for two sets of
aerators could be extended to three or more sets of one or more aerators
with a variety of patterns of air supplied to each of the three aerators, ie.
air
could be supplied sequentially to each of the three or more sets of one or
more aerators. Further, the first aerators 38 and second aerators 39 or
additional aerators may be of various types including distinct aerators, such
as aerator caps, or conduit aerators of holes in conduits.
' The amount of air used is dependant on numerous factors
but is preferably related to the superficial velocity of air flow. The
superficial velocity of air flow is defined as the rate of air flow delivered
by
the air blower 42 at standard conditions (1 atmosphere and 25 degrees
Celsius) divided by the cross sectional area of aeration. The cross sectional
area of aeration is determined by measuring the area effectively aerated by
aerators but reduced to account for the effect of cycling. For example, where
the entire air flow is cycled between two equally sized sets of aerators for
equal periods of time, the cross sectional area of aeration is the total area
served by aerators divided by two. Superficial velocities of air flow of
between 0.015 m/s and 0.15 m/s are preferred. Air blowers 42 for use in
drinking water applications may be sized towards the lower end of the range
while air blowers 42 used for waste water applications may be sized near the
higher end of the range.
Referring now to Figures 5A, 5B, 5C, and 5D, alternate
embodiments of the air blower 42 are shown. Referring first to Figure 5A,
an embodiment is shown in which an air blower 242 comprises a first air
blower 258 and a second air blower 260. The first air blower 258 is attached
to the first manifold 250 and the second air blower 260 is attached to the
second manifold 252.
In Figure 5B, another embodiment of a part of the aeration
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system 237 is shown. An air blower 242 has a blower 243 which blows air
into a connector 261 which splits the air flow into a low flow line 262 and a
high flow line 264. A valve 266 in the low flow line 262 is adjusted so that
flow in the low flow line 262 is preferably between 0% to 30% of the total air
flow. A controller 268, which could be a timer or a microprocessor or one or
more motors with electrical or mechanical links to the valves to be
described next, controls a low valve 270, which may be a solenoid valve or a
3 way ball valve, and a high valve 272, which may be a solenoid valve or a 3
way ball valve, so that for a first period of time (a first part of a cycle)
air in
the low flow line 262 flows to the first manifold 250 and air in the high flow
line flows to the second manifold 252. For a second period of time (a second
part of a cycle), the low valve 270 and high valve 272 are controlled so that
air in the low flow line 262 flows to the second manifold 252 through cross
conduit 274 and air in the high flow line 264 flows to the first manifold 250
through reverse conduit 276. With 3 way ball valves in particular, the
change between the two conditions described above is smooth and gradual
rather than abrupt. With this arrangement, there is a constant total supply
of air and wear on the air blower 242 from constantly changing speed is
reduced.
Referring now to Figure 5C, another embodiment of a part
of the aeration system 237 is shown. An air blower 242 has a blower 243
which blows air into a connector 261 which splits the air flow into a first
manifold 250 and a second manifold 252. A first manifold valve 280,
preferably a butterfly valve, and a second manifold valve 282, preferably a
butterfly valve are controlled by a valve controller 284. The valve
controller 284 periodically opens and closes the first manifold valve 280 and
second manifold valve 282 but with a phase shift, preferably of 180 degrees
so that one opens while the other closes, between them. The valve
controller 284 may be a microprocessor if the first manifold valve 280 and
second manifold valve 282 are electrically operated. Preferably, the first
manifold valve 280 and second manifold valve 282 are mechanically
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operable by actuators 286 connected by arms 288 to a member 290 on the
valve controller 284 which is a motor turning at the required speed or a
motor attached to a gear set to produce the required speed of rotation of the
member 290.
Referring now to Figure 5D, another embodiment of a part
of the aeration system 237 is shown. An air blower 242 has a blower 243
which blows air into a three way valve 292, preferably a ball valve, with its
two remaining orifices connected to a first manifold 250 and a second
manifold 252. A three way valve controller 294 periodically opens and
closes air pathways from the blower 243 to the first manifold 250 and second
manifold 252 but with a phase shift, preferably of 180 degrees so that the
airway to the first manifold 250 opens while the airway to the second
manifold 252 closes. The three way valve controller 294 may be a
microprocessor if the three way valve 292 is electrically operated.
Preferably,
the three way valve 292 is mechanically operable by handle 296 connected by
connector 298 to a lever 299 on the three way valve controller 294 which is a
motor turning at the required speed or a motor attached to a gear set to
produce the required speed of rotation of the lever 299.
Referring now to Figure 4A, 4B, 4C and 4D, a set of
variations of the embodiment of Figures 2 and 3 are shown wherein the
membrane modules 20 are hollow fibre membrane modules 220. A hollow
fibre membrane module 220 has membranes 223 suspended between
headers 222. An aeration system 237 has an air delivery system 240
comprising a first manifold 250 connected to a first aerator or aerators 238
and a second manifold 252 connected to a second aerator or aerators 239
both of which introduce bubbles 236. The first manifold 250 and the second
manifold 252 are connected to an air blower 242 having two independently
controllable outlets, the first connected to the first manifold 250 and the
second connected to the second manifold 252.
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Although the first aerators 238 and second aerators 239 are
shown beside the lower header 222, they can also be located in other
convenient positions where they are horizontally displaced from each
other. For example, first aerators 238 could be located under a membrane
module 220 and second aerators 239 located beside the same membrane
module 220 as shown in Figure 4B or under an adjacent membrane module
220 as shown in Figure 4D or in other convenient locations. Further, as
shown in Figure 4C, first aerators 238 and second aerators 239 could both be
located under a single membrane module 220. If the membrane modules
220 are rectangular in plan view, the first aerators 238 and second aerators
239, if formed of or attached to elongated conduits, are preferably oriented
parallel to the longer side of the rectangle although they may also be
oriented parallel to the shorter side of the rectangle or in other
orientations.
Particularly regarding the embodiment shown in Figure 4C, if a membrane
module 220 is very wide then the first aerators 238 and second aerators 239
may each be made of multiple conduits 54 alternately attached to the first
manifold 250 and second manifold 252.
With the greater proportion of the total airflow alternating
between the first manifold 250 and the second manifold 252 in cycles, there
is significant transient flow in the tank water 18 and few appreciable dead
zones in or around the membrane modules 220. In addition to the
transient flow, such cycling or pulsing of air creates very large bubbles 236
at
the start of each pulse. These very large bubbles 236 create significant
amounts of local turbulence, especially when flowing into previously still
tank water 18. The resulting transient flow of the tank water 18 helps the
bubbles 36 or 236 penetrate into skeins of hollow fibre membranes 23 or 223
and promotes renewal of tank water 18 within the membrane module 20 or
220. In more complicated systems, additional aerators could be arranged
throughout the tank 12. By cycling the air supply to these aerators, more
complicated transient flows could be established.
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Referring now to Figures 6A and 6B, another effect of
transient flow is illustrated in more detail for membrane modules 220 made
of slackened hollow fibre membranes 278 attached between headers 222.
The degree of slack of the membranes 278 is highly exaggerated for easier
illustration. Further, only two membranes 278 are illustrated for each
membrane module 220 although a membrane module 220 might actually be
constructed of a skein 279 (illustrated with two membranes 278 each). The
skein 279 may be between 3 cm and 6 cm thick and have between 2500 and
3000 hollow fibre membranes 278 at a 10-40% packing density. A ZW500
module produced by Zenon Environmental Inc. for example, has two
skeins 279 each between 6 cm and 7 cm thick and having about 2500 hollow
fibre membranes 278 at about 20% packing density and a total membrane
surface area of about 500 square feet. With such dense packing of the
membranes 278, it is difficult to encourage bubbles 236 to penetrate the
skeins 279 and the fibres on the outer edge of the skeins 279 have
significantly more contact with the bubbles 236.
The natural tendency of the bubbles 236 is to go through
the areas with lowest resistance such as around the membrane modules 220
or through slots between the membrane modules 220. As a consequence,
the tank water 18 in these areas has less density compared to surrounding
tank water 18.
If there is more air supplied to the first manifold 250, the
membranes 278 assume an average shape as shown in Figure 6A with a
local recirculation pattern 280 as shown. If there is more air supplied to the
second manifold 252, the membranes 278 assume an average shape as
shown in Figure 6B with a local recirculation pattern 282 as shown. If
steady state aeration is used in either configuration, the upper 10-20% of the
membranes 278 may be forced into a tightly curved shape and vibrate only
very little so that solids accumulate rapidly in those sections. A smaller
portion at the bottom of the membranes 278 in Figure 6B may also be tightly
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curved and solids could rapidly accumulate in that section. When hollow
fibres membranes are used in other configurations, for example in
horizontal skeins, similar problems occur but in different locations in the
membrane module 220 and with the membranes 278 assuming different
shapes.
In the embodiments of the invention described above,
unsteady non-uniform distribution of bubbles 236 is used to change the
location of the regions of lower density in the tank water 18 so that a
substantial portion of the skein 279 of membranes 278 moves back and forth
along a substantial portion of the length of the membranes 278 and air
bubbles penetrate further into the skein 279. The air supply to the first
manifold 250 and the second manifold 252 is cycled as described above and
the membranes 278 cycle between the two positions shown in Figures 6A
and 6B. Hollow fibre membranes 278 in other configurations such as
horizontal similarly cycle between different positions. The cycling also
creates a reversing flow into and out of the skeins 279 of membranes 278
which forces bubbles 236 to penetrate deeper into the skeins 279.
Now referring to Figure 7A, a conduit aerator 300 is shown
which is of a design preferred for the aerators 38, 39, 238 and 239 described
above. The conduit aerator 300 has an elongated hollow body 302 which is
preferably a circular pipe having an internal diameter between 15 mm and
50 mm but can be made of various other shapes and sizes of conduit
including non-prismatic conduits. A series of holes 304 pierce the body 302
allowing air to flow out of the conduit aerator 300 to create bubbles. The
size, number and location of holes may vary but with each half of a ZW500
membrane module, for example, 2 holes (one on each side) of between 5
mm and 10 mm in diameter placed every 50 mm to 100 mm along the body
302 and supplied with an airflow of between 5 scfm and 20 scfm resulting in
a pressure drop through the holes of between 10 to 100 mm of water at the
depth of the aerator 300 are suitable. The body 302 may be a separate
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component or integrated into the headers of a membrane module. When
used in the aeration systems 37 and 237 described above, the conduit aerator
300 performs the function of the aerators 38, 238 and 239 as well as part of
the functions of parts of the air delivery system 40 and 240, particularly the
conduits 54.
Air enters the conduit aerator 300 at an inlet 306 and exits
at an outlet 308. The highest point on the outlet 308 is preferably located
below the lowest point on the inlet 306 by a vertical distance between the
minimum and maximum expected pressure drop of water at the depth of
the aerator 300 across the holes 304. The minimum expected pressure drop
of water at the depth of the aerator 300 across the holes 304 is preferably at
least as much as the distance between the top of the holes 304 and the
interior bottom of the body 302. An air/water interface 309 between the air
in the aerator 300 and the water surrounding the aerator 300 will be located
below the interior bottom of the body 302 but above the highest point on the
outlet 308. In this way, tank water entering the conduit aerator 300 will flow
to the outlet 308 and not accumulate near the holes 304 and yet air will flow
out of the outlet 308.
Now referring to Figure 7B, another conduit aerator 300 is
shown having holes 304 located at the bottom of the body 302. With this
configuration, it is possible to cap an end of the body 302 with a cap 310
instead of having an outlet 308, although an outlet 308 as described above
may still be used. With the holes 304 at the bottom of the bodv 302, tank
water is discouraged from seeping into the conduit aerator 300 by gravity
and the conduit aerator 300 can be emptied of water through the holes 304.
Conduit aerators 300 such as those described above may
admit some tank water 18, even with air flowing through them, which
dries out leaving an accumulation of solids. When air is not flowing
through a particular conduit aerator 300, air is trapped in the conduit
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aerators 300 at an elevated static pressure but its dynamic pressure is
reduced. This reduction in dynamic allows tank water to enter the conduit
aerators 300. Further, pressure relief valves can be installed and operated to
depressurize the conduit aerators if needed to increase the volume of tank
water entering the conduit aerators 300 or. the rate at which the tank water
18 enters the conduit aerators 300. When air flow resumes through the
conduit aerator, the tank water 18 is pushed back out of the conduit aerator.
When the supply of air is switched between manifolds as described above,
however, the conduit aerator 300 is alternately flooded and emptied. The
resulting cyclical wetting of the conduit aerators 300 helps re-wet and
remove solids accumulating in the conduit aerators 300 or to prevent tank
water 18 from drying and depositing solids in the conduit aerators 300.
Embodiments similar to those described above can be
made in many alternate configurations and operated according to many
alternate methods within the teachings of the invention. In particular,
filtration systems can vary in size and complexity and one or more of any
component described above may be used regardless of whether the
component appears to be singular or plural in the description above.
Examples
Example 1
A cassette of 8 ZW 500 membrane modules were operated
in bentonite suspension under generally constant process parameters but
for changes in flux and aeration. A fouling rate of the membranes was
monitored to assess the effectiveness of the aeration. Aeration was supplied
to the cassette at constant rates of 120 scfm (ie. 15 scfm per module) and 80
scfm and according to various cycling regimes. In the cycled tests, a total
air
supply of 80 scfm was cycled between aerators located below the modules
and aerators located between and beside the modules in cycles of the
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durations indicated in Figure 8A. Aeration at 80 scfm in 30 second cycles (15
seconds of air to each set of aerators) was approximately as effective as non-
cycled aeration at 120 scfm.
Example 2
The same apparatus as described in example 1 was tested
under generally constant process parameters but for the variations in air
flow indicated in Figure 8B. In particular, 70% of the total air flow of 80
scfm was cycled in a 20 second cycle such that each group of aerators
received 70% of the total airflow for 10 seconds and 30% of the total airflow
for 10 seconds. As shown in Figure 8B, cycling 70% of the air flow resulted
in reduced fouling rate at high permeate flux compared to constant aeration
at the same total air flow.
Example 3
2 ZW 500 membrane modules were operated to produce
drinking water from a natural supply of feed water. Operating parameters
were kept constant but for changes in aeration. The modules were first
operated for approximately 10 days with non-cycled aeration at 15 scfm per
module (for a total system airflow 30 scfm). For a subsequent period of
about three days, air was cycled from aerators near one set of modules to
aerators near another set of modules such that each module was aerated at
7.5 scfm for 10 seconds and then not aerated for a period of 10 seconds (for a
total system airflow of 7.5 scfm). For a subsequent period of about 10 days,
the modules were aerated such that each module was aerated at 15 scfm for
10 seconds and then not aerated for a period of 10 seconds (for a total system
airflow of 15 scfm). For a subsequent period of about 10 days, the initial
constant airflow was restored. As shown in Figure 8C, with aeration such
that each module was aerated at 15 scfm for 10 seconds and then not aerated
for a period of 10 seconds (ie. one half of the initial total system airflow),
the
CA 02279766 1999-07-30
membrane permeability stabilized at over 10 gfd/psi whereas with non-
cycled airflow at the initial total system airflow the membrane permeability
stabilised at only about 5 gfd/psi.
Example 4
3 units each containing 2 ZW 500 membrane modules
were operated at various fluxes in a membrane bioreactor. Unit 1 had
modules operating at 15 and 30 gfd. Unit 2 had modules operating at 18 and
27 gfd. Unit 3 had modules operating at 20 and 30 gfd. The units were first
operated for a period of about 10 days with non-cycled aeration at 25 scfm
per module (total system air flow of 50 scfm). The permeability decreased
and stabilized at between 10 and 11 gfd/psi for Unit 1, between 8 and 9
gfd/psi for Unit 2 and between 6 and 7 gfd/psi for Unit 3. For a second
period of about 14 days, a total system airflow of 36 scfm was applied for 10
seconds to aerators below the modules and then for 10 seconds to aerators
beside the modules. Under these conditions, permeability increased and
stabilized at between 14 and 15 gfd/psi for Unit 1 and between 13 and 14
gfd/psi for Units 2 and 3.
Example 5
A cassette of 6 ZW 500 modules was used to treat sewage.
While holding other process parameters generally constant, aeration was
varied and permeability of the modules was measured periodically as
shown in Figure 9. In period A, 150 scfm of air was supplied continuously
and evenly to the modules. In period B, 108 scfm of air was applied for 10
seconds to aerators below the modules and then for 10 seconds to aerators
beside the modules. In Period C. the same aeration regime was used, but
shrouding around the modules was altered. In period D, 108 scfm of air
was applied for 10 seconds to aerators near a first set of modules and then
for 10 seconds to aerators near a second set of modules. In period E, 120
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scfm of air was applied to all of the modules evenly for 10 seconds and then
no air was supplied to the modules for 10 seconds. In Period F, 180 scfm was
applied to all of the modules evenly for 10 seconds and then no air was
supplied to the modules for 10 seconds. In Period G, 90 scfm was applied to
aerators near a first set of modules and then for 10 seconds to aerators near
a
second set of modules.
Example 6
' A single ZW 500 membrane module was used to filter a
supply of surface water. While keeping other process parameters constant,
the module was operated under various aeration regimes and its
permeability recorded periodically. First the module was operated with
constant aeration at (a) 12 scfm and (b) 15 scfm. After an initial decrease in
permeability, permeability stabilised at (a) about 8 gfd/psi and (b) between
11
and 12 gfd/psi respectively. In a first experiment, aeration was supplied to
the module at 15 scfm for two minutes and then turned off for 2 minutes.
In this trial, permeability decreased rapidly and could not be sustained at
acceptable levels. In another experiment, however, aeration was supplied
to the module at 15 scfm for 30 seconds and then at 5 scfm for 30 seconds. In
this trial, permeability again decreased initially but then stabilised at
between 11 and 12 gfd/psi.
