Note: Descriptions are shown in the official language in which they were submitted.
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B&P File No. 4320-100
BERESKIN & PARK CANADA
Title: MEMBRANE MODULE FOR GAS TRANSFER
Inventors: (1) Pierre Cote
(2) Steven Pedersen
(3) Henry Behmann
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Title: Membrane Module for Gas Transfer
FIELD OF THE INVENTION
This invention relates to membrane modules used to transfer a gas to
or from a liquid.
BACKGROUND OF THE INVENTION
Transferring gases to or from a liquid is most commonly practiced by
providing a bubble diffuser in the liquid. As bubbles rise through the liquid,
gases move across the boundary of the bubble driven by the relative partial
pressures of the gas in the bubble and in the liquid. Such a process has
serious drawbacks including high energy costs, difficulty in independently
controlling mixing of the liquid, foaming on the liquid surface and lack of
control over the gas released by the bubbles as they break at the liquid
surface. Gas permeable membrane modules provide an alternate means for
transferring a gas to or from a liquid and have been used in various reactor
designs. Some examples are described below.
U.S. Patent No. 4,181,604 (issued to Onishi et al. on January 1, 1980),
describes a module having several loops of hollow fibre membranes
connected at both ends to a pipe at the bottom of a tank containing
wastewater. The pipe carries a gas containing oxygen to the lumens of the
membranes. Oxygen flows through the membranes to the wastewater and
to an aerobic biofilm growing on the outer surface of the membranes. In
U.S. Patent No. 4,746,435 (issued to Onishi et al. on May 24, 1988), the same
apparatus is used but the amount of oxygen containing gas is controlled to
produce a biofilm having aerobic zones and anaerobic zones.
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U.S. Patent No. 4,416,993 (issued to McKeown on November 22, 1983),
describes a membrane module in the form of a hollow plate. The plates are
made of a rigid frame wrapped in a porous "netting" made of PTFE
laminated to a woven nylon fabric. The plates are attached to an
overlapping strip which has an inlet port and an outlet port.
In "Bubble-Free Aeration Using Membranes: Mass Transfer Analysis"
(Journal of Membrane Science, 47 (1989) 91-106) and "Bubble-Free Aeration
Using Membranes: Process Analysis" (Journal Water Pollution Control
Federation, 1988, Volume 60, Number 11, 1986-1992), Cote et al. describe the
use of silicone rubber tubes to transfer oxygen to water without creating
bubbles in the water. The apparatus for these studies includes a module
having vertically oriented tubes suspended between an inlet header and an
outlet header. The module is immersed in a tank containing water
recirculated by a pump to provide a horizontal current in the tank.
U.S. Patent No. 5,116,506 (issued to Williamson et al. on May 26, 1992)
describes a reactor having a gas permeable membrane dividing the reactor
into a gas compartment and a liquid compartment. The gas compartment
is provided with oxygen and methane which diffuse through the
membrane to support a biofilm layer in the liquid compartment. The
membrane is made of a teflon and nylon laminate commonly known as
Gore-tex (TM). In one embodiment, the membrane divides the reactor into
lower and upper portions. In another embodiment, the gas compartment
rotates within the liquid compartment.
In "Studies of a Membrane Aerated Bioreactor for Wastewater
Treatment" (MBR 2 - June 2, 1999, Cranfield University), Semmens et al.
describe a membrane module having microporous polypropylene hollow
fibres stitched together to form a fabric. The fabric is mounted between a gas
inlet header and a gas outlet header such that the fibres are oriented
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horizontally. The module is immersed in water in an open reactor with
water recirculated by a pump to provide a horizontal current in the reactor.
Despite the variety of designs available, gas transfer membranes have
not achieved widespread commercial success. Common criticisms of
modules or reactors include (a) that membrane materials lack sufficient
strength to be durable in hostile environments (b) that membrane surface
area is inadequate, particularly for a tank of a fixed and pre-selected size,
(c)
that excessive movement of liquid is required which is costly to implement
in large systems, (d) that biofilm growth on the membranes is difficult to
prevent or maintain at a controlled thickness and (e) that even small leaks
or defects in the membranes cause a significant loss of system capacity.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a
membrane module for transferring a gas to or from a liquid. Such modules
can be used, for example, in supporting and providing oxygen to a biofilm,
in water degassing, in humidification, in pervaporation and to clean air.
In one aspect, the invention provides an apparatus for transferring a
gas to or from a liquid having a flexible and gas diffusive but liquid water
impermeable membrane and a flexible spacer open to gas flow. -The spacer
and the membrane together form a planar element with the membrane
enclosing an inner space containing the spacer. One or more conduits are
provided for transferring gas between the inner space and the atmosphere
or another location outside of the water and the inner space. One or more
tensile members or weights non-rigidly restrain the planar element in a
selected position in a selected reactor. Gases that may be transferred include
oxygen, nitrogen, volatile organic compounds, hydrogen, and water vapour.
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In another aspect, the invention provides a module for transferring a
gas to or from a liquid having a plurality of the apparatus described above
and a gas manifold. The second ends of the gas inlet conduits are connected
in fluid communication with the manifold to admit gas to the planar
elements. The manifold is mounted above the water surface of a reactor
while the planar elements are located below the water surface of the reactor.
The reactor has a tank having a generally straight flow path covering a
substantial portion of the tank between an inlet and an outlet. The planar
elements are restrained in positions in the reactor in which they are
generally parallel to the flow path. In a wastewater treatment applications,
the reactor has a source of agitation for agitating the planar elements to
release accumulated biofilm from time to time.
In another aspect, the invention is directed at a process for
transferring a gas to or from a liquid comprising the steps of (a) immersing
one or more of the planar elements described above in the liquid and (b)
supplying a gas to the planar elements at a pressure which does not create
bubbles in the liquid, the gas leaving the planar elements by diffusion or by
forced circulation using a pump. For some embodiments, the pressure of
the gas is preferably also less than the pressure of the wastewater against
the
planar elements.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the present invention will now be
described with reference to the following figures.
Figures 1 and 2 show a first apparatus in elevation and sectional
views respectively.
Figures 3, 4 and 5 show a second apparatus in elevation, sectional and
front removed views respectively.
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Figures 6 and 7 show a third apparatus in elevation and sectional
views respectively.
Figures 8 and 9 are schematic elevational representations of two
reactors for use with the first, second or third apparatus.
Figures 10 and 11 are drawings of alternative configurations of the
first apparatus.
DETAILED DESCRIPTION OF EMBODIMENTS
A First Embodiment
Figures 1 and 2 show a first apparatus 10 having a membrane 12, a
spacer 14, an inlet conduit 16, an outlet conduit 18, and a non-rigid
restraint
system 20.
The membrane 12 is a sheet material that can be sewed or glued into
a variety of constructions. In the embodiment illustrated, a piece of the
sheet material of an appropriate size, which may be made of several smaller
pieces, is folded in half around the spacer 14 and fastened to itself with a
line of stitching 22 or glue. All lines of stitching 22 of the first apparatus
10
(and all subsequent apparatuses described below) expected to be in contact
with water are sealed by coating them with liquid silicone rubber or another
waterproof adhesive. The membrane 12 thus encloses an inner space 24
containing the spacer 14. The spacer 14 and the membrane 12 together form
a planar element 26.
The membrane 12 is flexible and gas diffusive but liquid water
impermeable. By liquid water impermeable, we mean that a water
molecule may diffuse through the membrane 12 under a suitable driving
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force (for example, if the gas within the inner space 24 is not at 100%
humidity) but that water will not flow in the liquid state through the
membrane 12. A preferred membrane 12 is made of a woven or non-woven
textile fabric, such as nylon, coated or impregnated with a gas permeable but
water impermeable layer. Silicone rubber is preferred for the layer because
of its high permeability to oxygen and availability in liquid and spray forms
but the layer must be inspected carefully to ensure that it is free of voids.
Alternative membranes may be constructed of microporous hydrophobic
materials which do not wet under typical hydrostatic pressures such as
polypropylene or PTFE. The spacer 14 is flexible and open to gas flow
generally parallel to the membrane 12. Suitable materials are sold for use as
spacers in reverse osmosis modules, for example, as Vexar (TM) made by
Valtex.
The inlet conduit 16 and the outlet conduit 18 have first ends 16a and
18a in fluid communication with the inner space 24. The inlet conduit 16
and the outlet conduit 18 each also have second ends 16b and 18b extending
outwardly from the first planar element 26. Waterproof glue is applied to
the point where the inlet conduit 16 and the outlet conduit 18 exit from the
planar element 26 to prevent water from leaking into the inner space 24.
The inlet conduit 16 and the outlet conduit 18 are made of a
composite construction. A part near the second ends 16b and 18b of the
conduits 16 and 18 is a flexible solid tube. The second end 16b of the inlet
conduit 16 has a releasable water tight connector to a header (not
illustrated). The second end 18b of the outlet conduit 18 may be exhausted to
the atmosphere in some applications but may also be collected in a header
(not illustrated). Each flexible tube ends shortly below the start of the
spacer
14. From this point, each of the conduits 16, 18 is made of a section of the
spacer 14 or membrane 12. As illustrated, the conduits 16, 18 are a section of
the spacer 14 rolled to create a porous conduit which admits the flexible tube
and extends along a side of the first planar element 26. Alternatively, the
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spacer 14 may be folded over itself to form the conduits 16, 18 or a flexible
spring can be inserted into a tubular section of the membrane 12 adjacent
the spacer 14 to form conduits 16, 18.
Preferably, the inlet conduit 16 and outlet conduit 18 are located at
opposed sides of the planar element 26 so that oxygen containing gas
entering the inlet conduit 16 will travel across the planar element 26 before
leaving through the outlet conduit 18. Further preferably, each of the
conduits 16, 18 extends substantially along their respective opposed sides of
the planar element 26 and are porous along a substantial portion of their
length inside of the planar element 26. In this way, the gas is encouraged to
flow across the planar element 26 in a well distributed flow pattern.
Optionally, gas can be encouraged to flow downwardly or, preferably,
upwardly by placing the conduits 16, 18 across the horizontal sides of the
planar element 26 rather than the vertical sides of the planar element 26.
A drain tube 28 may also be provided having a first end in fluid
communication with the bottom of the planar element 26 and a second end
extending out of the planar element 26. The drain tube 28 is sealed with
glue where it exits the planar element 26. The second end of the drain tube
28 is provided with a fitting so that it can be connected to a pump for
withdrawing water from the inner space 24 of the planar element 26. Under
ideal conditions, such a drain tube 28 is not required. From time to time,
however, minute defects may develop in the planar element 26 that admit
small amounts of water. Further, under some conditions water vapour may
condense and accumulate in the inner space 24. In either case, the use of a
drain tube 28 avoids the need to periodically remove the first apparatus 10
to remove water from the inner space 24. Alternatively, the drain tube 28
can be inserted into the bottom of the planar element 26 through the outlet
conduit 18.
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The restraint system 20 consists of a series of tensile members in the
form of loops 30, preferably made of the same material as the membrane 12
or another suitable fabric. The loops 30 are sewed or glued to the edges of
the planar element 26 to provide a series of points of attachment.
Grommets, hooks or other fasteners might also be used provided that they
distribute any expected load enough to avoid tearing the edges of the planar
element 26. The restraint system 20 permits the planar element 26 to be
fixedly but non-rigidly restrained in a selected position in a selected
reactor
by passing a wire or rope fixed to the reactor through the loops 30. In some
cases, the wire or rope may assume a curved shape. In these cases, the
lengths of the loops 30 are preferably varied to accommodate the curved
shape and so to transfer the tensile force to the planar element 26 evenly
across the loops 30. Alternately, a larger number of tensioned wires or rope
can be fitted at one end to a reactor and at the other end to the planar
element 26 with clamping connectors such as those used to secure tarps. In
this case, the edge of the planar element serves the purpose of the tensile
member and is reinforced as required.
An alternative version of the first apparatus 10' is shown in Figure
10. In this alternate version, a restraint system 20' has floats 32 sized to
keep
the top of the first apparatus 10' above a water surface. The bottom of the
first apparatus 10' is kept submerged with tensile elements made of wires
34a attached to grommets 36. When the water is lowered or drained for
maintenance etc., second wires 34b attached to grommets 36 perform the
function of the floats 32 in restraining the top of the first apparatus 10'.
The
inlet conduit 16' is a short section at the top of the first apparatus 10' in
which the spacer 14' is exposed to the atmosphere. The outlet conduit 18'
extends down one side and across the bottom of the first apparatus 10' but is
only porous along the bottom of the first apparatus 10'. The outlet conduit
18' is attached to a suction pump to draw air in through the first apparatus
10' from top to bottom. Small amounts of water entering the first apparatus
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10' are withdrawn periodically by increasing suction to the outlet conduit
18'.
A plan view of another alternative version of the first apparatus 10"
is shown in Figure 11. In this version, one or more planar elements 26" of
the first apparatus 10" are wound in a spiral. The layers of the spiral are
separated by one or more loose springs 38 or other open spacers, preferably
spaced apart at regular intervals along the axis of the spiral. Gas enters and
exists through conduits 16" and 18" but the order the relative locations of
the conduits 16" and 18" illustrated may be reversed. The first apparatus 10"
is preferably mounted in a cylindrical vessel 39 which may be a tank or a
large pipe. Flow of water through the vessel 39 may be made to follow the
spiral of the first apparatus 10" by placing one of an inlet and outlet in the
centre of the vessel and the other of the inlet and outlet at the perimeter of
the vessel 39. Alternatively, flow of water through the vessel 39 may be
made to be parallel to the axis of the spiral, for example where the vessel 39
is a pipe, by providing an inlet at one end of the pipe, an outlet at another
end of the pipe and placing the first apparatus 10" in between the inlet and
outlet. Depending on the how tightly the first apparatus 10" is packed in the
pipe, tensile members may not be required to restrain the first apparatus 10"
in position, but tensile members or another restraint system are typically
required where the vessel 39 is a large tank.
A Second Embodiment
Figures 3, 4 and 5 show a second apparatus 110 for supporting and
oxygenating an immersed biofilm. The second apparatus 110 has a
membrane 112, a spacer 114, an inlet conduit 116, an outlet conduit 118, and
a non-rigid restraint system 120.
The membrane 112 and spacer 114 are of the same material described
for the first embodiment. The membrane 112 is similarly folded around the
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spacer 114 and fastened to itself with a line of stitching 122 or glue.
Additional lines of stitching 122 are used to fix the inlet conduit 116,
outlet
conduit 118 and second restraint system 120 in the positions shown. The
membrane 112 thus encloses an inner space 124 containing the spacer 114
and the spacer 114 and the membrane 112 together form a planar element
126.
The inlet conduit 116 and the outlet conduit 118 have first ends 116a
and 118a in fluid communication with the inner space 124. The inlet
conduit 116 and the outlet conduit 118 each also have second ends 116b and
118b extending outwardly from the planar element 126. Waterproof glue is
applied to the point where the conduits 116, 118 exit from the second planar
element 126 to prevent water from leaking into the inner space 124.
The inlet conduit 116 and the outlet conduit 118 are made of flexible
solid tubes. The second end 116b of the inlet conduit 116 has a releasable
water tight connector to a header (not illustrated). The second end 118b of
the outlet conduit 118 may be exhausted to the atmosphere in some
applications but may also be collected in a header (not illustrated). Starting
shortly below the start of the spacer 114 each conduit has a plurality of
perforations 40 to create a porous conduit. As for the first embodiment, the
inlet conduit 116 and the outlet conduit 118 are preferably located at
opposed sides of the planar element 126, extend substantially along their
respective opposed sides and are porous along a substantial portion of their
length inside of the second planar element 126. Optionally, gas can be
encouraged to flow downwardly or, preferably, upwardly by placing the
conduits 116, 118 across the horizontal sides of the second planar element
126 rather than the vertical sides of the first planar element 126. A drain
tube (not illustrated) may also be provided.
The restraint system 120 consists of a tensile member in the form of a
wire or rope 42 sewn or glued around a substantial part of the periphery of
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the planar element 126. The wire or rope 42 sticks out of the planar element
126 at a plurality of locations to provide points of attachment 44.
Preferably,
four points of attachment 44 are provided, one in each corner of the planar
element 126. The restraint system 120 permits the planar element 126 to be
fixedly but non-rigidly restrained in a selected position in a selected
reactor
by connecting the points of attachment 44 to a reactor with ropes or wire.
This attachment may encourage the wire or rope 42 to assume a curved
shape. In these cases, the relevant edges of the planar element 126 are made
in a similar curved shape.
A Third Embodiment
Figures 6 and 7 show a third apparatus 210. The third apparatus 210
has a membrane 212, a spacer 214, an inlet conduit 216, an outlet conduit
218, and a non-rigid restraint system 220.
The membrane 212 is a sheet material as described for the previous
embodiments. The structure of the third apparatus differs, however, in that
the membrane 212 is folded around two layers of spacer 214 separated by a
flexible but impermeable separator 50, preferably a plastic sheet. The edges
of
the membrane are fastened together by waterproof glue or a line of stitching
222 made waterproof with silicone rubber spray or glue. The membrane 212
thus encloses an inner space 224 containing the spacer 214 and the spacer
214 and the membrane 212 together form a planar element 226.
The inlet conduit 216 and the outlet conduit 218 have first ends 216a,
218a in fluid communication with the inner space 24. The inlet conduit 216
and the outlet conduit 218 also have second ends 216b, 218b extending
outwardly from the planar element 226. In the third apparatus 210, the
conduits 216, 218 include a part of the planar element 226 and a header 52.
The planar element 226 is potted in the header 52 with gas impermeable
glue 54 to make an airtight seal with the membrane 212 but leaving the
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spacer 214 in fluid communication with an inlet chamber 56 and an outlet
chamber 58 of the header 52. The inlet chamhPr 5H a"~ ""+m+ .,~.,......,~.._
~o
are separated by the impermeable layer 50. The header 52 provides an upper
mount for fixedly attaching the top of the planar element 226 in a selected
position in a selected reactor.
Gas enters the third apparatus 210 through a tube 62 having one end
in fluid communication with a gas source and a second end in fluid
communication with the inlet chamber 56 of the header 52. From the inlet
chamber 56, the gas enters the planar element 226 through the exposed edge
of the spacer 214. The gas travels first downwards and then upwards
through the spacer 214. The gas exits the planar element 226 through the
other exposed edge of the spacer 214 into the outlet chamber 58 of the
header 52 from which it leaves through several discharge ports 64 or
alternately through a pipe to an outlet header (not illustrated). A drain tube
(not illustrated) may also be provided having a first end in fluid
communication with the bottom of the planar element 226 and a second
end extending out of the planar element 226.
As the header 52 is intended to be mounted above water, a portion of
the membrane 212 is either out of the water or in a depth of water that is
not sufficient to keep the membrane 212 pressed against the spacer 214. In
this portion, preferably less than one half of the area of the planar element
226, glues lines 66 substantially parallel to the primary direction of gas
flow
attach the membrane 212 to the spacer at selected intervals to prevent
ballooning of the membrane 212. Similar glue lines may be used in
appropriate orientations if required in the first apparatus 10 and second
apparatus 110. In those cases, however, it is preferred if the first apparatus
10
and second apparatus 110 are submerged deep enough in relation to the
pressure of gas to be used to allow the water pressure to keep the membrane
212 against the spacer 214.
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The portion of the membrane 212 that is out of the water may permit
some gas to diffuse to the atmosphere. Where the gas flowing within the
membrane 212 is air, particularly air at a pressure below 10 kPa, the length
of membrane 212 that is out of the water can be controlled to the point
where diffusion to the atmosphere is acceptable. Where a pure gas such as
oxygen flows within the membrane 212, however, diffusion to the
atmosphere may be significant and the atmosphere exposed portion of the
membrane 212 is preferably sealed with a gas impermeable coating.
The restraint system 220 consists of the header 52, which may be
fixedly mounted in a reactor, and a weight 68 attached to the bottom of the
planar element 226. For this purpose, the membrane 212 extends below the
bottom of the spacer 214 and the weight 68 is attached in two halves to the
membrane 212 by rivets 70 or other fasteners. The weight is of a sufficient
size to keep the planar element 226 hanging vertically downwards from the
header 52. Alternately, loops can be provided at the bottom of the third
planar element 226 to allow attachment to the bottom of the reactor with
ropes or wires.
Membrane Supported Biofilm Reactors for Wastewater Treatment
Figure 8 shows a reactor 80 having a tank 82, a feed inlet 84 to the
tank 82, an effluent outlet 86 from the tank 82, a flow path 88 between the
feed inlet 84 and effluent outlet 86 and a plurality of the third apparatus
210.
The third apparatus 210 is shown as an example only and the second
apparatus 110 or first apparatus 10 may also be used with suitable
modifications to the reactor 80.
The planar elements 226 are sized to fit the tank 82 and fill a
substantial amount of its volume. The planar elements 226 have no pre-
manufactured or rigid frame and thus are preferably custom made to
provide efficient use of the available space in the tank 82. For example,
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planar elements 226 may range from 0.5 m to 2 m wide and 2 to 10 m deep.
The planar elements 226 are preferably arranged in the tank 82 in a number
of rows, one such row being shown in Figure 8. The planar elements 226
may range from 0.5 to 2 mm in thickness and adjacent rows are placed in
the tank 82 side by side at a distance of 5 to 15 mm to allow for biofilm
growth and wastewater flow between adjacent planar elements 226.
The tank 82 is longer than it is deep and it is preferred to encourage a
generally horizontal flow path 88 with minimal mixing. This is done by
leaving some space near the ends (ie. near the inlet 84 and outlet 86) of the
tank 82 for vertical movement of water and leaving minimal free space at
the top, bottom and sides of the tank 82. A baffle 90 may also be placed
upstream of the effluent outlet 86 to force the flow path 88 to go under it. A
sludge outlet 92 is provided to remove excess sludge.
The flow path 88 is generally straight over a substantial portion of the
tank 82 between the feed inlet 84 and effluent outlet 86. Each third apparatus
210 is held in the tank 82 by its headers 52 attached to a frame 90 and by its
weight 68. The headers 52, frame 90 and weights 68 restrain each third
apparatus 210 in positions in the reactor 80 whereby the planar element 226
of each third apparatus 210 are generally parallel to the flow path 88.
Preferably, a plurality of planar elements 226 are spaced in series along the
flow path 88 so that the reactor 80 will more nearly have plug flow
characteristics. Wastewater to be treated may be partially recycled from the
effluent outlet 86 to the feed inlet 84. Such a recycle can increase the rate
of
gas transfer by increasing the velocity of wastewater along the flow path 88,
but it is preferred if the recycle ratio is small so as to not provide more
nearly mixed flow characteristics in the reactor 80.
Oxygen containing gas is provided to each third apparatus 210
through its inlet conduit 216 connected to an inlet manifold 94 located
above the water to be treated. With the inlet manifold 94 located above the
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water, a leak in any third apparatus 210 will not admit water into the
manifold nor any other third apparatus 210. Gas leaves each third apparatus
210 through its outlet conduit 218 which is connected to an exhaust
manifold 95. Although it is not strictly necessary to collect the gases
leaving
each third apparatus 210, it does provide some advantages. For example, the
gas in the exhaust manifold 95 may have become rich in volatile organic
compounds which may create odour or health problems within a building
containing the reactor 80. These gases are preferably treated further or at
least vented outside of the building.
Preferably, the gas is provided at a pressure such that no bubbles are
formed in the water to be treated and, more preferably, at a pressure of less
than 10 kPa. This pressure is exceeded by the pressure of the water to be
treated from one metre of depth and beyond. Preferably at least half of the
area of the third planar elements 226 is below that depth. The water
pressure thus prevents at least one half of the surface of the membranes 12
from ballooning.
Oxygen diffuses through the membranes 12. The amount of oxygen
so diffused is preferably such that an aerobic biofilm is cultured adjacent
the
planar elements 226, an anoxic biofilm is cultivated adjacent the aerobic
biofilm and the wastewater to be treated is maintained in an anaerobic state.
Such a biofilm provides for simultaneous nitrification and denitrification.
A source of agitation 96 is operated from time to time to agitate the planar
elements 226 to release accumulated biofilm. A suitable source of agitation
is a series of coarse bubble aerators 98 which do not provide sufficient
oxygen to the water to be treated to make it non-anaerobic.
Figure 9 shows a second reactor 180 having a tank 182, a feed inlet
184, an effluent outlet 186, a flow path 188 and a plurality of the first
apparatus 10. The first apparatus 10 is shown as an example only and the
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second apparatus 110 or third apparatus 210 may also be used with suitable
modifications to the second reactor 180.
Each first apparatus 10 is held by its loops 30 wrapped around wires
100 or ropes attached to the tank 182. The loops 30 and wires 100 restrain
each first apparatus 10 in a position in the second reactor 180 whereby the
planar element 26 of each first apparatus 10 is generally parallel to the flow
path 188.
The first planar elements 26 are sized to fit the tank 182 and fill a
substantial amount of its volume. Like the third planar elements 226, the
first planar elements 26 have no pre-manufactured or rigid frame and are
preferably custom made to provide efficient use of the available space in the
tank 182. The first planar elements 26 may range from 0.25 to 1 mm in
thickness and are placed side by side at a distance of 5 to 15 mm to allow for
biofilm growth and wastewater flow between adjacent first planar elements
26.
The tank 182 is deeper than it is long and it is preferred to encourage
a straight and generally vertical flow path 188 over a substantial portion of
the tank 182 with minimal mixing. This is done by leaving minimal space
near the ends and sides of the tank 82 but a substantial amount of space near
the top and bottom of the tank 82. Water to be treated may be partially
recycled from the effluent outlet 186 to the feed inlet 184 but it is
preferred
that the recycle rate be small.
Oxygen containing gas is provided to each first apparatus 10 through
its inlet conduit 16 connected to a manifold 94 located above the water to be
treated. With the inlet manifold 94 located above the water, a leak in any
first apparatus 10 will not admit water into the manifold nor any other first
apparatus 210. The outlet conduits 18 are clipped in a convenient place, for
example to the inlet manifold 94, above the surface of the water to be
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treated. Preferably, the gas is provided at a pressure of less than 10 kPa and
the planar elements 26 are located more than 1 m deep in the tank 182. In
this way, the gas pressure is exceeded by the pressure of the water to be
treated which prevents the membranes 12 from ballooning. Glue lines (not
shown), preferably not effecting more than one half of the area of the planar
elements 26, can be used to reinforce part of the planar elements 26 if they
can not be mounted deep enough.
Alternatively, gas flow through the first element 10 is produced by
applying a suction, preferably of not more than 10 kPa less than atmospheric
pressure, to the outlet conduits 18. The inlet conduits 16 are placed in fluid
communication with the atmosphere. By this method, the rate of gas
diffusion across the membrane 12 is slightly reduced, but no reinforcement
of the membrane 12 (for example, by glue lines) is required regardless of the
depth of the first element 10.
Oxygen diffuses through the membranes 12 preferably such that an
aerobic biofilm is cultured adjacent the planar elements 26, an anoxic
biofilm is cultivated adjacent the aerobic biofilm and the wastewater to be
treated is maintained in an anaerobic state. A second source of agitation 196
is operated from time to time to agitate the first planar elements 26 to
release accumulated biofilm. A suitable source of agitation is a series of
mechanical mixers 102.
Other Reactors
The apparatus described above may also be used in alternative
processes or arrangements. For example, gas transfer into a liquid can be
achieved in a dead end configuration, ie. without an outlet conduit. In this
case, however, it is preferable to provide a small outlet bleed to reduce
condensation in the open space and vent gases transferred from the liquid
into the open space of the apparatus. To remove gases from a liquid, a dead
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end configuration may also be used wherein no inlet conduit is provided.
Use of the apparatus in some other applications is described below.
a) Water Degassing and Pervaporation.
In water degassing, water containing dissolved gases such as nitrogen,
oxygen or carbon dioxide flows into a tank. Planar elements as described
above are immersed in the tank. A sweep gas flows through the planar
element or a vacuum is applied to the planar element (the inlet conduit is
omitted). Gases in the liquid cross the membrane to the inner space of the
planar element from where they are removed through the outlet conduit.
Water lean in dissolved gases leaves the tank. Such a process is useful, for
example, in producing ultrapure water. Pervaporation is accomplished with
a similar reactor but the feed water contains volatile organic compounds
which diffuse to the inner space of the planar elements.
b) Humidification
In humidification, planar elements are immersed in a water bath.
Dry air enters the planar elements. Water vapour crosses the membrane to
the inner space of the planar element and humid air leaves the planar
elements.
c) Air Cleaning
In air cleaning, planar elements are immersed in a water bath
enriched with nutrients and a biofilm is cultured on the planar elements.
Air containing volatile organic compounds flows into the planar elements
and the volatile organic compounds diffuse through the membranes of the
planar elements to the biofilm. Air lean in volatile organic compounds
exits the planar elements.
CA 02300209 2000-03-08
-19-
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, the scope of which is defined by the
following claims.
