Note: Descriptions are shown in the official language in which they were submitted.
CA 02307492 2000-OS-04
1
Title: AERATED IMMERSED MEMBRANE SYSTEM
FIELD OF THE INVENTION
This invention relates to filtering membranes and
particularly to modules of immersed, suction driven, filtering
membranes used to filter water or wastewater and cleaned in part by
scouring air bubbles.
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 wastewater and to
withdraw potable water from a lake or reservoir.
Immersed membranes are generally arranged in
elements or modules which comprise the membranes and headers
attached to the membranes. The modules are immersed in a tank of
water containing solids. A transmembrane pressure ("TMP") is applied
across the membrane walls which causes filtered 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.
U.S. Patent No. 5,639,373 issued to Zenon
Environmental Inc. on June 17, 1997 describes one such module using
hollow fibre membranes. In this module, hollow fibre membranes are
held in fluid communication with a pair of vertically spaced headers.
TMP is provided by suction on the lumens of the fibres through the
headers. Similar modules are shown in U.S. Patent No. 5,783,083 issued
to Zenon Environmental Inc. on July 21, 1998, PCT Publication No. WO
98/28066 filed on December 18, 1997 by Memtec America Corporation and
CA 02307492 2000-OS-04
2
European Patent Application No. EP 0 931 582 filed August 22, 1997 by
Mitsubishi Rayon Co., Ltd.
To clean such membrane modules, bubbles are
introduced to the tank through aerators mounted below or near the
bottom of the membrane. The bubbles rise to the surface of the tank
water and create an air lift which recirculates tank water around the
membrane module. The rising bubbles and tank water scour and agitate
the membranes to inhibit solids in the tank water from fouling the pores
of the membranes. Further, there is also an oxygen transfer from the
bubbles to the tank water which, in wastewater applications, provides
oxygen for microorganism growth.
One concern with such aerated immersed membrane
modules is that the tank water to move in a generally steady state
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 parts of the membranes in
these dead zones are not effectively cleaned and may be operating in
water having a higher concentration of solids than in the tank water
generally. Accordingly, the affected parts of these membranes quickly
foul with solids. This problem persists even in modules where
membranes are installed with a small degree of slack to allow the
membranes to move and shake off or avoid trapping solids. The
movement of water in the tank encourages the slackened membranes to
assume a near steady state position near the headers which interferes
with the useful movement of the membranes. As a result, the entire
surface of the membranes is not effectively cleaned and parts of the
membrane foul rapidly. In wastewater applications in particular, sludge
often builds up around the membranes in an area directly above the
lower header and an area directly below the upper header. 15% or more
of the surface area of the membranes may quickly become covered in
sludge and lose nearly all of its permeability.
CA 02307492 2000-OS-04
3
PCT Application No. PCT/CA99/00940, filed on October
9, 1998 by Zenon Environmental Inc. describes among other things a
method and apparatus for reducing the build up of sludge on vertical
membranes near the headers. The apparatus includes an aeration system
having a plurality of distinct branches and one or more aerators in fluid
communication with the each distinct branch. An air supply provides
an initial air flow at an initial flow rate and a valve set is provided in
fluid communication with the air supply and having distinct outlets in
fluid communication with the distinct branches of the air distribution
system. The valve set is operable to (i) split the initial air flow such that
at any point in time at least one of the distinct branches of air
distribution system receives air at a higher flow rate and at least one
other of the distinct branches of the air distribution network receives air
at a lower flow rate, the lower flow rate being less than one half of the
higher flow rate, and (ii) switch which branch or branches of the air
delivery network receive air at the higher flow rate and the lower flow
rate in repeated cycles of very short duration. The aerators associated
with a first distinct branch of the air delivery system are interspersed
with the aerators associated with a second distinct branch of the air
delivery system. With sufficiently short cycle times, the water to be
filtered moves horizontally under transient flow. When used with
membranes oriented vertically between upper and lower headers, the
horizontal and transient movement encourages movement of the fibres
and penetration of the tank water into the fibres to help prevent a build
up of sludge around the headers.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an
element or cassette of immersed, suction driven, filtering membranes
used to filter water or wastewater which may be cleaned in part by
CA 02307492 2000-OS-04
4
scouring air bubbles. The present invention is particularly adapted to
filtering water with a high concentration of suspended solids.
In some aspects, the invention is directed at an element
of filtering hollow fibre membranes having an upper header and a lower
header. A plurality of hollow fibre membranes are attached to and
suspended between the headers for collecting permeate through at least
one of the headers. The lower header is movable between a first position
in which the fibres are substantially elongated and second position in
which the two headers are closer to each other by between 0.1 and 5% of
the un-potted length of the membranes. The weight of the lower header
is sufficient to keep the lower header in the first position in substantially
quiescent water while allowing the lower header to rise to the second
position in upwardly flowing air and water.
A module of filtering hollow fibre membranes is made
by attaching one or more of the elements to a frame. The frame fixedly
secures the upper header but merely restrains the lower header while
allowing it to move between the first position and the second position.
Aerators are mounted generally below the elements and supply scouring
bubbles to each element at a higher rate and then at a rate less than one
half of the higher rate in repeated cycles. The cycles are preferably
between 10 seconds and 60 seconds in duration. The lower headers rise
to the second position when bubbles are supplied at the higher rate and
fall to the first position when bubbles are supplied at the lower rate.
In other aspects, the invention is directed at a process
for treating water with filtering hollow fibre membranes of the type that
have a plurality of hollow fibre membranes attached to and suspended
between a pair of headers. The membranes are provided in elements,
each element being a rectangular skein of hollow fibres having an
effective thickness of between 4 and 8 sheets of hollow fibres. Adjacent
elements are horizontally spaced apart, preferably by at least one third of
CA 02307492 2000-OS-04
the width of the headers measured in the direction of the horizontal
spacing. Suction is applied to the interior of the hollow fibre membranes
to withdraw a filtered permeate. During permeation, scouring bubbles
are provided from below the elements. The supply of the scouring
5 bubbles varies between a higher rate and a rate less than one half of the
higher rate in repeated cycles. The cycles are between 10 seconds and 60
seconds in duration. The distance between the headers is between 95%
and 99.9% of the un-potted length of the hollow fibre membranes for at
least a substantial part of the time during which bubbles are supplied at
the higher rate.
In yet other aspects of the invention, at least one header
moves during permeation between the first position and the second
position while scouring bubbles are supplied in repeated cycles as
described above. Preferably the lower header moves upwards to the
second position when bubbles are supplied at the higher rate and moves
downwards to the lower position when bubbles are supplied at the lower
rate.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the present invention will
now be described with reference to the following figures.
Figure 1 is a front elevation of a filtering element.
Figure 2 is a side elevation of the filtering element of
Figure 1.
Figure 3 is a front elevation of a filtering module
containing the filtering elements of Figures 1 and 2.
CA 02307492 2000-OS-04
6
Figure 4 is a schematic representation of an aeration
system.
Figure 5 is a chart of experimental data.
DETAILED DESCRIPTION OF EMBODIMENT
Figures 1 and 2 show front and side elevations
respectively of a filtering element 10. The element 10 has a plurality of
hollow fibre membranes 12 in the form of a rectangular skein 14
suspended between an upper header 16 and a lower header 18. The
rectangular skeins 14 are between four and eight layers of membranes 12
deep (five layers being shown in Figure 2), less frequently up to 12 layers
deep, and are in the range of several tens of membranes 12 wide. The
membranes 12 typically have an outside diameter between 0.4 mm and
4.0 mm. The length of the membranes 12 is chosen to maximize flux for
a given cost according to relationships known in the art and is typically
between 400 mm and 1,800 mm. The membranes 12 have an average
pore size in the microfiltration or ultrafiltration range, preferably
between 0.003 microns and 10 microns and more preferably between 0.02
microns and 1 micron.
The upper header 16 has a permeate channel 20 in fluid
communication with the lumens of the membranes 12. The membranes
are potted into the upper header such that the membranes 12 are all
closely spaced apart from each other. This allows potting resin to
completely surround the outside of the end of each membrane 12 to
provide a watertight seal so that water can only enter the permeate
channel after first flowing though the membranes 12. Suitable potting
resins include polyurethane, epoxy, rubberized epoxy and silicone resin.
One or more resins may also be used in combination to meet objectives
of strength and providing a soft interface with the membranes 26 having
no cutting edges.
CA 02307492 2000-OS-04
7
The inventors prefer to use a potting method like that
described in U.S. Patent No. 5,639,373 which produces layers of
membranes 12, but other potting methods known in the art may also be
used. Regardless of potting method, the thickness of the assembled mass
of membranes 12 is preferably between 18 and 40 mm. Typical headers
16, 18 to accommodate such masses of membranes are 40 to 50 mm wide.
Typical potting densities are between 10% and 40%. A preferred element
uses membranes 12 from a commercially available ZW 500 (TM)
10 module made by Zenon Environmental Inc. which have an outside
diameter of about 2 mm, an un-potted length (meaning the unsupported
length of membrane 12 between the upper header 16 and lower header
18) of 1,600 to 1,900 mm, and a pore size of approximately 0.1 microns.
In the embodiment shown, permeate is not withdrawn
from the lower header 18. The membranes are plugged with resin at
their lower ends and glued into the lower header 18. Preferably, the
width of the lower header 18 is as little as possible more than the width
of the membranes 12. The membranes 12 might also be made of looped
fibres with their looped ends glued into the lower header 18, but if this is
done the loops should be oriented to minimize the depth of the
assembly. If long membranes 12 are desired (for example to better fit in a
deep tank) a permeating lower header may be preferred to reduce head
loss of permeate flow in the lumens of the membranes 12. Membranes
12 would be potted in such a lower header as described for the upper
header 16.
The upper header 16 and lower header 18 are preferably
injection moulded from a suitable plastic such as PE, PP, polyester or
polycarbonate. The lengths of the headers 16, 18 may vary considerably
but lengths less than 1 m are more convenient for injection moulding.
The lower header 18 has pins 22 which extend beyond the membranes 12.
CA 02307492 2000-OS-04
8
The length of the upper header 16 also extends beyond the membranes 12
by about 25 mm to form abutments 24.
A permeate fitting 26 is attached to the top of the upper
header 16. One side of the permeate fitting 26 has a projection 28 with a
groove 30 for an O-ring. The other side of the permeate fitting 26 has a
recess 32 adapted to receive the projection 28 of an adjacent permeate
fitting 26. Thus, adjacent elements 10 can be releasably sealed to each
other. The projections 28 and recesses 32 of the permeate fitting 26 are
made with excess length to allow adjacent elements 10 to be mounted at
varying distances apart, preferably ranging from as closely as one third of
the of the width of the headers 16, 18 (measured in the direction of the
horizontal spacing) to the width of the headers 16, 18.
Figure 3 shows a cassette 40 made of a plurality of
adjacent elements 10. The permeate fittings 26 of adjacent elements 10
seal to each other as described above to create a continuous permeate
header 34. A cap 36 seals one end of the permeate header 34 while a
permeate pipe 38 s connected into the other end of the permeate header
34.
The cassette 40 is made with a steel frame 42 having
stiles 44 and rails 46 bolted together. An upper rail 48 is made of a right
angled section oriented to provide a ledge 50 which receives the
abutments 24 of the upper headers 16. A removable indexed rod 52
covers the upper headers 16 and holds them in horizontally spaced
relationship. Thus the upper headers 16 are fixed to the upper rail 48. By
removing the indexed rod 52, however, the elements 10 can slide
towards or away from adjacent elements 10. The permeate fitting 26 of
any particular element 10 can be disconnected from the permeate header
34 by sliding all adjacent permeate fittings 26 away from it to allow an
element 10 to be removed from the cassette 40.
CA 02307492 2000-OS-04
9
A lower rail 54 is attached to the stiles 44 and has
several openings 56 which admit the pins 22 of the lower headers 18.
The openings 56 restrain the pins 22 of the lower headers 18 within a
selected range of movement between a first position and a second
position. In the first position, the pins 22 are in their lowest possible
position and membranes 12 are substantially elongated. Where the
membranes 12 have sufficient tensile strength, the weight of the lower
headers 18 pulls on the membranes 12 and the lowest point of the
openings 56 can be below the pins 22 in the first position. For
membranes 12 with less tensile strength, however, the lowest point of
the openings 56 contacts the pins 22 in the first position to relieve the
tensile stress on the membranes 12 caused by the weight of the lower
headers. In the second position (shown in Figure 3), the headers 16, 18
are a selected distance closer to each other, the selected distance being
0.1% to 5% of the un-potted or free length of the membranes 12. A
selected distance of 10 mm was used with the ZW 500 membranes
described above.
In the second position, the pins 22 contact the highest
point of the openings. Although the openings 56 are shown as circular,
they may also be rectangular in shape and permit movement from side
to side of the cassette 40 as well as up and down but are preferably slots
permitting mostly up and down movement. The movement from side
to side can be made adjustable by using a C-channel for the lower rail 54
with a long horizontal slot defining the highest and lowest points of the
openings 56 and a series of vertical bolts through the flanges of the C-
channel to define the sides of the openings 56. A space between the
lower rails 54 and the lower headers 18 may also permit some
movement of the lower headers 18 from the front to the back of the
cassette 40. Temporarily unbolting one of the lower rails 54 allows
individual elements 10 to be removed from the cassette 40.
CA 02307492 2000-OS-04
In an alternative embodiment, the lower headers do not
have pins but instead have cavities or attached slotted plates on the sides
of the lower headers. A number of threaded holes are made in the lower
rails in locations corresponding to the cavities or slots in the slotted
5 plates. Bolts are screwed into the threaded holes and protrude into the
cavities or slots. With this construction, any single lower header can be
removed from a frame by un-screwing the relevant bolts.
In another alternative embodiment, the ends of
10 adjacent spaced apart lower headers are fixed together with a bar or plate
so that they move upwards or downwards together and may not rotate
relative to each other. Rotation of the headers can also be controlled by
using two pins for each end of the lower header and a lower rail with
slotted openings.
An aerator rail 58 is bolted to the stiles 44 below the
lower rail 48. The aerator rail 58 supports several conduit aerators 60
connected to an aerator manifold 62 to receive a supply of air. The
conduit aerators are configured to provide a supply of scouring bubbles to
the elements 10 from below them. The conduit aerators 60 have an
elongated hollow body which is a circular pipe having an internal
diameter between 15 mm and 100 mm. A series of holes pierce the body
allowing air to flow out of the conduit aerator 60 to create bubbles. The
size, number and location of holes may vary but 2 holes (one on each
side) placed every 50 mm to 100 mm along the body are suitable.
Scouring bubbles are produced at the holes of the
aerators 60 and agitate the membranes 12 which inhibits their fouling or
cleans them. In addition, the bubbles also decrease the local density of
tank water in or near the membranes 12 which causes tank water to flow
upwards past the membranes. The bubbles have an average diameter
between 3 mm and 50 mm. Bubbles of this size are typically produced in
municipal treatment works with holes between 2 mm and 15 mm in
CA 02307492 2000-OS-04
11
diameter. The scouring bubbles are typically air bubbles but oxygen,
nitrogen or other suitable gases may also be used.
Figure 4 shows an aeration system 70 for use with
cassettes 40 described above. Two aerator manifolds 62 are shown which
each service a set of elements 10 ranging in size from one half of a
cassette 40 to several cassettes 40. The manifolds 62 are connected to a
three way valve 72 which splits the flow from an air blower 74 such that
one manifold 62 receives air at a higher rate and the other manifold 62
receives air at a lower rate. The lower rate ranges from no flow to flow at
one half of the higher rate. A solenoid 76 is connected to the three way
valve 72 and operable to switch which manifold receives air at the
higher and lower rates. A programmable logic controller (PLC) 78
controls the solenoid. Preferably, the PLC is programmed to activate the
solenoid in cycles between 10 seconds and 60 seconds in duration
wherein each manifold 62 receives air at the higher rate for one half of
each cycle.
The air flow provided to a manifold 62 or aerator 60 is
measured by its superficial velocity meaning the rate of air flow in m3/s
at standard conditions (1 atmosphere and 20 degrees Celsius) divided by
the cross sectional area effectively aerated by a manifold 62 or aerator 60
in m2. The higher flow rate preferably has a superficial velocity between
.013 m/s and .15 m/s. A particular higher rate is chosen to inhibit
fouling of the membranes 12 to a desired degree. Supplying air at the
higher rate will, however, be sufficient to cause tank water surrounding
the elements 10 to flow upwards. The weight of the lower headers 18 is
chosen to allow the lower headers 18 to rise to the second position in this
upwardly flowing water. Aeration at the lower rate may also produce
some upward movement of the tank water, but conditions in the tank
are preferably substantially quiescent during aeration at the lower rate.
During aeration at this rate the weight of the lower headers 18 pulls the
lower headers into the first position. Thus the lower headers 18 move
CA 02307492 2000-OS-04
12
upwards when scouring bubbles are supplied at the higher rate and
move downwards when scouring bubbles are supplied at the lower rate.
In operation, one or more elements 10 or cassettes 40 are
immersed in an open tank of water or wastewater to be filtered. Feed
water is supplied to the tank as is known in the art. Suction (provided
for example by a permeate pump), typically between 1 kPa and 150 kPa, is
applied to the interior of the hollow fibre membranes though the
permeate pipe 38, permeate header 34 and permeate fittings 26 to
withdraw filtered permeate. During permeation, scouring bubbles are
provided from below the elements 10 as described above and the lower
headers 18 move between the first position and second position. The
membranes 12 are typically also backwashed and cleaned with chemicals
as is known in the art. Similarly, the tank water is periodically or
continuously deconcentrated as in known in the art.
The process is best suited to filter water having a high
concentration of suspended solids, particularly wastewater, where
aeration is provided throughout permeation. The inventors believe,
however, that the apparatus and method described above may also be
adapted for use with methods of filtering water with lower
concentrations of suspended solids, For example, the method and
apparatus might be used where aeration is provided only during a
portion of the permeation cycle or during backwashing provided that
aeration is used for a long enough period to include multiple cycles.
Example
A new cassette was built as described above except that
adjacent elements were joined to a common upper header. The new
cassette was used to filter wastewater with MLSS of 15 g/ beside a cassette
of ZW500 (TM) membrane modules in a reactor having HRT of 5 hours
and SRL of 20 days. The same membrane size and chemistry were used
CA 02307492 2000-OS-04
13
in both cassettes. The flux from both cassettes was as shown in Figure 5.
Aeration was provided at a superficial velocity of 0.04 m/s in a 20 second
cycle. Permeability of the membranes was measured before backwashing
and corrected to 20 C.
Figure 5 shows the permeability of the new cassette and
the ZW 500 (TM) cassette over time. The new cassette shows a marked
improvement in long term permeability. Weekly visual observations
showed that the new cassette had no solid accumulation adjacent the
bottom header while the ZW 500 (TM) cassette had 4 to 8 inches of solid
accumulation adjacent the bottom header.
It is to be understood that what has been described are
preferred embodiments of the invention. The invention nonetheless is
susceptible to certain changes and alternative embodiments without
departing from the subject invention, the scope of which is defined in
the following claims.
