CA2300209A1

CA2300209A1 - Membrane module for gas transfer

Info

Publication number: CA2300209A1
Application number: CA 2300209
Authority: CA
Inventors: Steven Kristian Pedersen; Henry Behmann; Pierre Cote
Original assignee: Zenon Environmental Inc
Current assignee: GE Zenon ULC
Priority date: 2000-03-08
Filing date: 2000-03-08
Publication date: 2001-09-08

Abstract

An apparatus to transfer gas to or from a liquid has a flexible and oxygen permeable but liquid water impermeable membrane, a flexible and gas permeable spacer, an inlet conduit, an outlet conduit and a non-rigid restraint system.
The membrane encloses an inner space containing the spacer and the spacer and the membrane together form a planar element. The inlet conduit and outlet conduit allow gases to flow into and out of the planar element. The non rigid restraint system permits the planar element to be fixedly but non-rigidly restrained in a selected position in a selected reactor. A module is made by attaching a plurality of the apparatus to a manifold located above a liquid surface of a reactor, the planar elements being located below the liquid surface. When used for treating wastewater, an aerobic biofilm is cultured adjacent the planar elements, an anoxic biofilm is cultivated adjacent the aerobic biofilm and the wastewater is maintained in an anaerobic state. Other uses include water degassing, humidification, pervaporation and air cleaning.

Description

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 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.

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 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.

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.

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 (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 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 _7_ 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.

_g_ 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 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 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 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 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 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.

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, 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 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 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 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 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.

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.

Claims

1. An apparatus to transfer a gas to or from water comprising:
(a) a flexible spacer open to gas flow;
(b) a flexible and gas diffusive but liquid water impermeable membrane enclosing an inner space containing the spacer to form a planar element;
(c) one or more conduits for transferring gas between the inner space and the atmosphere or another location outside of the water and the inner space;
(d) one or more tensile members for fixedly but non-rigidly restraining the planar element in a selected position in a selected reactor.

2. The apparatus of claim 1 wherein an inlet conduit and an outlet conduit are located at opposed sides of the planar element.

3. The apparatus of claim 2 wherein each of the inlet conduit and outlet conduit (a) extend substantially along their respective opposed sides of the planar element and (b) are porous along a substantial portion of their length inside of the planar element.

4. The apparatus of claim 3 wherein the porous portion of the length inside of the planar element of at least one of the inlet conduit and outlet conduit is made of a section of the spacer or membrane.

5. A module to transfer a gas to or from water in a reactor comprising, (a) a manifold capable of being mounted above the water surface and carrying a supply of a first gas; and, (b) a plurality of the apparatus of claim 1 capable of being mounted below the water surface and each having its inlet conduit connected in fluid communication with the first gas in the manifold.

6. The apparatus of claim 1 wherein the membrane is made of a textile substrate coated or impregnated with a non-porous, gas permeable, liquid water impermeable layer.
6A. The apparatus of claim 1 wherein the membrane is made of a porous but hydrophobic material.

7. The apparatus of claim 6 wherein the layer is silicone rubber.

8. The apparatus of claim 1 further comprising a tube having a first end in fluid communication with the bottom of the planar element and a second end connectable with a pump for withdrawing water from the inner space of the planar element.

9. A reactor for transferring a gas to or from water comprising, (a) a tank having an inlet and an outlet and a generally straight flow path covering a substantial portion of the tank between the inlet and outlet; and, (b) a plurality of apparatus each having (i) a gas permeable spacer, (ii) an oxygen permeable but liquid water impermeable membrane enclosing an inner space containing the spacer to create a planar element, (iii) an inlet conduit having a first end in fluid communication with the inner space and a second end extending out of the planar element and (iv) an outlet conduit having a first end in fluid communication with the inner space and a second end extend out of the planar element, wherein each planar element is restrained in a position in the reactor whereby the planar elements is generally parallel to the flow path.

10. The reactor of claim 9 wherein a plurality of planar elements are spaced in series along the flow path.

11. A process for treating wastewater in a bioreactor comprising the steps of, (a) providing one or more planar elements having no rigid frame and comprising (i) a flexible and gas permeable spacer, and (ii) a flexible and oxygen permeable but liquid water impermeable membrane enclosing an inner space containing the spacer;
(b) fixedly but non-rigidly restraining the one or more planar elements below the surface of the wastewater in the reactor;
(c) flowing an oxygen containing gas through the planar elements at a pressure that does not create bubbles in the wastewater to be treated but permits oxygen to leave the planar elements by diffusion.

12. The process of claim 11 wherein the pressure of the gas is less than 10 kPa.

13. The process of claim 11 wherein the pressure of the gas is less than the lowest pressure of the wastewater against the one or more planar elements.

14. The process of claim 11 wherein an aerobic biofilm is cultured adjacent the planar elements, an anoxic biofilm is cultivated adjacent the aerobic biofilm and the wastewater to be treated is maintained in an anaerobic state.

15. The process of claim 11 further comprising the step of agitating the planar elements from time to time to release accumulated biofilm.

16. An apparatus to transfer a gas to or from a liquid comprising:
(a) a flexible and gas permeable spacer;
(b) a flexible and oxygen permeable but liquid water impermeable membrane enclosing an inner space containing the spacer to form a planar element;
(c) an inlet conduit having a first end in fluid communication with the inner space and a second end extending out of the planar element;
(d) an outlet conduit having a first end in fluid communication with the inner space and a second end extending out of the planar element;
(e) an upper mount for fixedly attaching the top of the planar element in a selected position in a selected reactor; and, (f) a weight at the bottom of the planar element of sufficient size to keep the planar element hanging substantially vertically downwards from the upper mount.

17. The apparatus of claim 16 wherein the inlet conduit and outlet conduit are located at opposed sides of the planar element.

18. The apparatus of claim 17 wherein each of the inlet conduit and outlet conduit (a) extend substantially along their respective opposed sides of the planar element and (b) are porous along a substantial portion of their length inside of the planar element.

19. The apparatus of claim 18 wherein the porous portion of the length inside of the planar element of at least one of the inlet conduit and outlet conduit is made of a rolled section of the spacer.

20. A module to transfer a gas to or from water in a reactor comprising, (a) a manifold capable of being mounted above the liquid surface and carrying a supply of a first gas; and, (b) a plurality of the apparatus of claim 16 each having its inlet conduit connected in fluid communication with the first gas in the manifold.

21. The apparatus of claim 16 wherein the membrane is made of a textile substrate coated or impregnated with a non-porous, gas permeable, liquid water impermeable layer.

22. The apparatus of claim 21 wherein the layer is silicone rubber.

23. The apparatus of claim 16 further comprising a tube having a first end in fluid communication with the bottom of the planar element and a second end connectable with a pump for withdrawing water from the inner space of the planar element.

24. A process for treating wastewater in a bioreactor comprising the steps of, (a) providing one or more planar elements having no rigid frame and comprising (i) a flexible and gas permeable spacer, and (ii) a flexible and oxygen permeable but liquid water impermeable membrane enclosing an inner space containing the spacer;
(b) restraining the one or more planar elements below the surface of the wastewater in the reactor;

(c) flowing an oxygen containing gas through the planar elements at a pressure less than atmospheric.

25. The process of claim 24 wherein the pressure of the gas is not more than 10 kPa below atmospheric pressure.

26. The process of claim 24 wherein an aerobic biofilm is cultured adjacent the planar elements, an anoxic biofilm is cultivated adjacent the aerobic biofilm and the wastewater to be treated is maintained in an anaerobic state.

27. The process of claim 26 further comprising the step of agitating the planar elements from time to time to release accumulated biofilm.