A LOW-PRESSURE, REVERSIBLE AIRLIFT MIXING SYSTEM FOR USE WITH A MEMBRANE AERATED BIOFILM REACTOR

Abstract

The invention relates to the design of an enclosure for a membrane aeration module, which incorporates a reversible, low-pressure, air-lift pump to encourage a vertical water flow through and between the membranes in the module. These enclosed membrane modules are suitable for use in membrane aerated biofilm reactors, which are used to treat water or wastewater.

Claims

1. An enclosure system for use with a membrane module of the type having an upper and lower headspace (104, 106) separated by an array of gas-permeable hollow fibre membranes (102), characterised in that the enclosure system comprises: a plurality of panels (3a, 3b, 3c, 3d) configured to form an enclosed membrane module (120); and an airlift mixing system (4) comprising an airlift channel (7) and a first downcomer (6), which are in fluid communication with each other at their bottom, forming two vertical channels of a substantially U-shaped tube (5), which is open at both ends (12), and configured to transport liquid either from inside the membrane module (120) to outside of the membrane module (120) or from outside of the membrane module (120) to inside the membrane module (120); and wherein the enclosure is open at the top and bottom and which is encased within the enclosure system.

2. An enclosure system according to claim 1, further comprising a modular collar (20) configured to attach to top of the enclosed membrane module and extend vertically above the surface of the liquid.

3. An enclosure system according to claim 1 or claim 2, wherein the plurality of panels (3a, 3b, 3c, 3d) extends vertically above the upper headspace (104) and extending beyond the surface of the liquid within the system.

4. An enclosure system according to any one of the preceding claims in which the airlift mixing system contains a third vertical channel giving the airlift mixing system a substantially W-shape, wherein the third vertical channel is a second downcomer (6a) and wherein one of the vertical channels is the air-lift channel (7), which is in fluid communication with the first and second downcomer (6, 6a).

5. An enclosure system according to any one of the preceding claim, further comprising a vertical return channel (9), in which the vertical return channel (9) is in fluid communication with the top of the vertical channel (6, 6a, 7) of the substantially U-shaped tube (5) or W-shaped tube (50) proximal the vertical return channel (9).

6. An enclosure system according to claim 5, in which the open end of the vertical channel of the substantially U-shaped tube or W-shaped tube proximal to the vertical return channel forms a port with access to outside of the enclosed membrane module.

7. An enclosure system according to any one of claim 5 or 6, in which the vertical return channel is in fluid communication with either the airlift channel or the first or second downcomer and extends the entire height of the enclosed membrane module or to the bottom of a holding tank (200).

8. An enclosure system according to claim 7, in which the vertical return channel is open to the environment outside of the enclosed membrane module and configured to provide a continuous channel to supply liquid from one location within the holding tank, or another holding tank or compartment within a treatment system, to one vertical channel of the airlift mixing system or to another location in the holding tank or treatment system.

9. An enclosure system according to any one of claims 1 to 8, in which the first and second downcomer are in fluid communication with liquid outside the enclosed membrane module and the airlift channel or the vertical return channel is in fluid communication with liquid inside of the enclosed membrane module.

10. An enclosure system according to any one of claims 1 to 8, in which the first and second downcomer are in fluid communication with liquid inside the enclosed membrane module and the airlift channel is in fluid communication with liquid outside of the enclosed membrane module.

11. An enclosure system according to any one of claims 1 to 10, in which the airlift channel further comprises an air injection port (40) configured to accept and deliver air into the airlift channel such that either vertical channel of the U-shaped airlift mixing system can become the airlift channel and reverse the direction of the flow of liquid within the enclosed membrane module.

12. An enclosure system according to claim 11, in which the air that is supplied to the airlift channel is sourced from either exhaust air from the membranes, supplemental air from an external source, or both.

13. An enclosure system according to claim 11 or claim 12, in which the air injection port is configured to release air continuously, in a pulsed or periodic manner, or a combination of both.

14. An enclosure system according to any one of claims 11 to 13, in which the air injection port is connected to an air syphon configured to allow air to accumulate and be released periodically to the air injection port.

15. An enclosure system according to any one of claims 11 to 14, in which the air injection port is configured to introduce air axially, radially, both axially and radially, or at an angle so as to induce turbulent water flow within the airlift channel.

16. An enclosure system according to any one claims 11 to 15, in which the air injection port is less than 3.0 m below the liquid surface within the modular collar of the framing system.

17. An enclosure system according to any one of the preceding claims, in which the airlift mixing system is configured to control the level of liquid within the enclosed membrane module relative to the level of liquid outside the enclosed membrane module.

18. An enclosure system according to any one of the preceding claims, in which at least one panel in a four-sided enclosed membrane module is configured to each accommodate the airlift mixing system.

19. An enclosure system according to any one of the preceding claims, in which at least two, three or all panels in a four-sided enclosed membrane module are each configured to accommodate the airlift mixing system.

20. An enclosure system according to any one of the preceding claims, in which the airlift mixing system is configured to pump liquid in an upward or downward direction through the enclosed membrane module.

21. An enclosure system according to any one of the preceding claims, further comprising a liquid flow distribution means in the headspace of the enclosed membrane module configured to provide uniform liquid flow throughout the enclosed membrane module.

22. An enclosure system according to any one of the preceding claims, in which the enclosure system is modular, and where a plurality of enclosed membrane modules can be stacked one on top of the other.

23. An enclosure system according to any one of preceding claims, in which the lower gas manifold can be purged of liquid that may accumulate as a result of condensation or leakage.

24. An enclosure system according to claim 23, in which a high air flow rate is delivered to the lower manifold either by increasing the airflow through the membranes, or by supplemental air supplied directly to the lower manifold, or a combination of both, so as to transport the accumulated liquid to either the airlift system or to the liquid surface.

25. An enclosure system according to any one of the preceding claims, in which the enclosure system is retrofittable to an existing membrane module.

26. A membrane-aerated biofilm reactor (MABR) of the type comprising: a frame and having an upper and lower headspace; an array of membranes disposed within the frame and extending between the upper headspace to the lower headspace; characterised in that the MABR further comprises an enclosure system as claimed in any one of the preceding claims.

27. An enclosure system for use with a membrane module (120) in membrane supported biofilm reactors, the membrane module (120) of the type having an upper and lower headspace (104, 106) separated by an array of gas-permeable hollow fibre membrane cassettes (100) secured in parallel in the module (120), wherein the cassettes (100) are a linear arrangement of potted hollow fibre membranes, comprising an upper and lower manifold (98, 99) into which are potted a large number of hollow fibre membranes or a number of bunches of hollow fibre membranes (102), characterised in that the enclosure system comprises: (i) a plurality of panels (3a, 3b, 3c, 3d) configured to seal the membrane module (120) to form an enclosed membrane module (1) which is open at the top and bottom but which is encased within the enclosure system; (ii) a modular collar (20) configured to attach vertically to the enclosed membrane module (1), increasing the height of the upper headspace (104); and (iii) a low-pressure airlift mixing system (4), which is integrated into at least one panel of said plurality of panels (3a, 3b, 3c, 3d), which is configured to transport liquid either from inside the membrane module (120) to outside of the membrane module (120), or vice versa, so that liquid is pumped in an upward or downward direction through the enclosed membrane module (1), to encourage good liquid flow through the enclosed membrane module (120) when it is installed in a bioreactor tank (200), wherein the airlift mixing system (4) comprises: an airlift channel (7) and a first downcomer (6), which are in fluid communication with each other at their bottom, forming two sides of a substantially U-shaped tube (5) which is open at both ends; a vertical return channel (9) having its top in fluid communication with the top of the channel of the substantially U-shaped tube (5) proximal the vertical return channel (9); and an air injection port (40) configured to accept and deliver air into the base of the airlift channel (7) to induce upward flow of liquid above the air injection port (40) in the airlift channel (7), causing a corresponding downward flow of liquid in the first downcomer (6); and wherein the open end of the U-shaped tube (5) distal the vertical return channel (7) forms a port (11) with access to inside the enclosed membrane module (1), the open end of the U-shaped tube (5) proximal the vertical return channel (7) forms a port (12) with access to outside of the enclosed membrane module (1) and the lower end of the vertical return channel (9) allows a fluid communication between the vertical return channel (9) and the bottom of said bioreactor or tank (200).

28. An enclosure system according to claim 27, in which the airlift mixing system further comprises a third vertical channel, giving the airlift mixing system a substantially W-shape, wherein the third vertical channel is a second downcomer (6a), and wherein one of the vertical channels is the air-lift channel (7), which is in fluid communication with the first and second downcomer (6, 6a).

29. An enclosure system according to claims 27 and 28, in which the vertical return channel (9) is open to the environment outside of the enclosed membrane module (1) and configured to provide a continuous channel to supply water from one location within the bioreactor or tank (200), or another holding tank or compartment within a treatment system.

30. An enclosure system according to any one of claims 27 to 29, in which the first and second downcomer (6, 6a) is in fluid communication with liquid outside the enclosed membrane module (1) and the airlift channel (7) is in fluid communication with liquid inside of the enclosed membrane module (1).

31. An enclosure system according to any one of claims 27 to 30, in which the air that is supplied to the airlift channel (7) is sourced from either exhaust air from the membranes, supplemental air from an external source, or both.

32. An enclosure system according to any one of claims 27 to 31, in which the air injection port (40) is configured to release air continuously, in a pulsed or periodic manner, or a combination of both.

33. An enclosure system according to any one of claims 27 to 32, in which the air injection port (40) is connected to an air syphon configured to allow air to accumulate and be released periodically to the air injection port (40).

34. An enclosure system according to any one of claims 27 to 33, in which the air injection port (40) is configured to introduce air axially, radially, both axially and radially, or at an angle so as to induce turbulent water flow within the airlift channel (7).

35. An enclosure system according to any one claims 27 to 34, in which the air injection port (40) is 3.0 m or less below the liquid surface within the modular collar (20) of the enclosure system.

36. An enclosure system according to any one of claims 27 to 35, in which the airlift mixing system (4) is configured to control the level of liquid within the enclosed membrane module (1) relative to the level of liquid outside the enclosed membrane module (1).

37. An enclosure system according to any one of claims 27 to 36, in which at least one panel (3a, 3b, 3c, 3d) in a four-sided enclosed membrane module (1) is configured to each accommodate the airlift mixing system (4).

38. An enclosure system according to any one of claims 27 to 37, in which at least two, three or all panels in a four-sided enclosed membrane module are each configured to accommodate the airlift mixing system.

39. An enclosure system according to any one of claims 27 to 38, further comprising a liquid flow distribution means in the headspace of the enclosed membrane module configured to provide uniform liquid flow through the enclosed membrane module (1).

40. An enclosure system according to any one of claims 27 to 39, in which the enclosure system is modular, and where a plurality of enclosed membrane modules (1) can be stacked one on top of the other.

41. An enclosure system according to any one of claims 27 to 40, in which the lower gas manifold (99) can be purged of liquid that may accumulate as a result of condensation or leakage.

42. An enclosure system according to claim 41, in which a high air flow rate is delivered to the lower manifold (99) either by increasing the airflow through the membranes, or by supplemental air supplied directly to the lower manifold (99), or a combination of both, so as to transport the accumulated liquid to either the airlift mixing system (4) or to the liquid surface.

43. An enclosure system according to any one of claims 27 to 42, in which the enclosure system is retrofittable to an existing membrane module.

44. A membrane aeration module (120) of the type comprising: a frame (110) and an upper and lower headspace (104, 106) separated by an array of gas-permeable hollow fibre membrane cassettes (100) fitted in parallel in the fame (110) in the membrane aeration module (120), wherein the cassettes (100) are in a linear arrangement of potted hollow fibre membranes, comprising an upper and lower manifold (98, 99) into which are potted a large number of hollow fibre membranes or a number of bunches of hollow fibre membranes (102).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0089] The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which:—

[0090] FIG. 1A illustrates a cassette of the prior art comprising shaped elements with individual bundles of fibres, while FIG. 1B illustrates a cassette of the prior art in which the membranes are potted continuously directly into the upper and lower manifolds and which require no shaped element connectors. FIG. 1C illustrates an elevation view of the membrane cassettes within a metal frame of the prior art that form a membrane module.

[0091] FIG. 2A illustrates a side elevation view of an enclosed membrane module of the present invention and FIG. 2B illustrates a side elevation view of the enclosed membrane module of the present invention, showing an integrated airlift system comprising an airlift pump, a first downcomer and a return channel. These membrane modules are completely surrounded and protected by the enclosure's panels, but are open at the top and the bottom. FIG. 2C illustrates a plan view of an enclosed membrane module of present invention.

[0092] FIG. 3A illustrates a side elevation of an enclosed membrane module of the present invention, which has been fitted with a modular collar and installed in a holding tank or bioreactor (not shown). FIG. 3B shows that if the liquid (water) level within the modular collar is higher than the surrounding liquid (water) level in the tank, the direction of flow is reversed when compared to the direction of flow in FIG. 3A.

[0093] FIG. 4A and FIG. 4B illustrate how the airlift mixing system provided on a panel of the enclosure system of the invention that can operate as an airlift pump. The airlift mixing system is capable of providing a flow from the upper headspace to the tank or a flow from the tank to the upper headspace, thus inducing an upward or a downward liquid flow, respectively, within the enclosed membrane module of the present invention. In FIG. 4A, the intake port of the airlift mixing system is located within the modular collar and draws liquid from within the enclosed membrane module. The airlift channel discharges liquid outside of the modular collar and liquid may flow downwards through the return channel to the base of the tank within which the entire enclosed membrane module is located. In FIG. 4B, the air injection port is located in the enclosed channel of the other side of the U-shaped tube that forms the airlift mixing system (now the upward flowing channel), and the discharge point of the airlift channel is now located within the modular collar and delivers liquid to the upper headspace above the membranes thus creating a higher liquid level within the enclosed module and forcing a downward velocity of liquid. The liquid within the tank is now drawn up from the bottom of the tank through the return channel or in via opening at point 12. The intake port of the first downcomer and return channel is now located outside of enclosed membrane module.

[0094] FIG. 5A and FIG. 5B show a plan view and side elevation view of the modular collar that is attached to the top of the enclosed membrane module of the present invention or to the top of the uppermost membrane module in a stack.

[0095] FIG. 6A and FIG. 6B illustrate how the enclosed membrane modules of the present invention may be stacked within a tank. FIG. 6A and FIG. 6B are operating in the modes shown in FIGS. 4A and 4B respectively.

[0096] FIG. 7A and FIG. 7B illustrate a plan view and side view in cross section of four membrane modules each enclosed by a framing system of the present invention and which are installed in a tank. The whole treatment system makes up Membrane Aerated Biofilm Reactor which treats inlet wastewater and discharges a treated effluent.

[0097] FIG. 8 illustrates an enclosed membrane module with a liquid flow distribution means in situ in the upper headspace 104 of the enclosed membrane module.

[0098] FIGS. 9A and 9B illustrate a side and front view of an airlift mixing system of the present invention, wherein the airlift mixing system is a substantially W-shape. FIG. 9C illustrates the substantially W-shaped airlift mixing system in an enclosed membrane module of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0099] The invention described herein provides low shear conditions, and the effective delivery of substrates to the biofilms growing on membranes, by providing a low pressure, airlift mixing system, which is integrated into a membrane module enclosed by the enclosure system of the invention and that surrounds the membranes.

[0100] Typically, airlift channels are feasible if the pressure against which the water must be pumped is less than about 300 mm of water. Pressure drops of more than 300 mm of water reduce the efficiency of airlift pumping and the water flow rate drops dramatically. For this reason, it is important that the head losses and pressure drops within the airlift mixing system itself are minimized. The size of the pipes or channels used for the airlift mixing system must be selected to minimize pressure losses and maximize airflow rates.

[0101] The flow rate of liquid that can be achieved in an airlift mixing system is a function of the air flow rate, the depth of the air injection port and the size (effective diameter) of the airlift channel. Typically, the liquid flow rate increases with air flow rate and the depth of the air injection port. Since energy consumption is a major environmental concern and operating cost associated with wastewater treatment, it is important to minimize the energy requirements for mixing and aeration. This can be accomplished by using the air supplied to the membranes for both oxygen transfer and mixing. Also, by using an airlift mixing system with a shallow depth for the air injection port, the air pressure required within the membranes can be kept low and energy consumption can be minimized. The liquid flow rate through an enclosed membrane module can thus be controlled by the design and operating conditions of the airlift mixing system, while the operating air pressure is independent of the depth of submergence of the membranes in stacked membrane modules and only dependant on the depth of the air injection into the airlift mixing system.

[0102] Referring now to the figures, where FIG. 1 illustrates a cassette 100 comprising shaped element connectors 101 with individual bundles of fibres 102, potted into upper 98 and lower 99 manifolds while FIG. 1B illustrates a cassette 100 in which the membrane fibres 102 are potted continuously and directly into the upper 98 and lower 99 manifolds and which require no shaped element connectors 101. FIG. 1C illustrates a view of membrane cassettes 100, being assembled into a membrane module 120 of the prior art. The membrane module 120 comprises a frame 110 into which the cassettes 100 can be fitted.

[0103] Referring now to FIG. 2, where a general embodiment of the present invention is illustrated. Specifically, FIG. 2A illustrates a side elevation view of a membrane module enclosed by the enclosure system of the present invention to form an enclosed membrane module, with the enclosed membrane module generally referred to by reference numeral 1. The enclosed membrane module 1 comprises panels 3a, 3b, 3c, 3d which enclose the membrane module 120, and comprise an airlift mixing system 4 (see FIG. 2B). When in use with the membrane module 120, the panels 3a, 3b, 3c, 3d of the enclosed membrane module 1 completely surround and encloses the cassettes 100, but the enclosed membrane module 1 is open at the top and the bottom. FIG. 2B illustrates a side elevation view of the enclosed membrane module 1, showing the airlift mixing system 4. In the illustrated embodiment, the airlift mixing system 4 is integrated within the panels 3a, 3c. The airlift mixing system 4 could also be integrated within panels 3b, 3d. The airlift mixing system 4 comprises a series of enclosed tubes that form a substantially U-shaped tube 5 with a vertical tube attached thereto. Specifically, the substantially U-shaped tube 5 comprises a first downcomer 6 connected to an airlift channel 7 by a u-bend 8. Additionally, the system may comprise a return channel 9. The enclosed membrane module 1 is shown to further comprise a reinforcing bar 10 and ports 11, 12. The reinforcing bar 10 extends from panel 3a to the opposite panel 3c. A modular collar 20 is attached to the top of the enclosed membrane module 1, and is configured to separate the ports 11 and 12. When water is flowing from an upper headspace 104 to a lower headspace 106, the port 12 provides fluid communication between liquid in the tank and the upper headspace 104 through the substantially U-shaped tube 5. When liquid is flowing from the lower headspace 106 (or from the bottom of a holding tank) to the upper headspace 104, the port 11 provides fluid communication between the upper headspace and the liquid in the bulk tank through the substantially U-shaped tube 5. The port 12 allows for air injected into the airlift channel 7 to escape to atmosphere, and not to become entrained in the liquid flowing downward in the return channel 9 to the bottom of the tank.

[0104] FIG. 2C illustrates a plan view of an enclosed membrane module 1. As illustrated, the upper headspace 104 of the enclosed membrane module 1 and a series of parallel membrane cassettes 100 are visible. The cassettes 100 are arranged in parallel that provide space between each cassette 100 to allow liquid to flow there between.

[0105] FIG. 3A illustrates a side elevation of an enclosed membrane module 1, which has been fitted with the modular collar 20 and installed in a holding tank or bioreactor 200. When the level of liquid inside the modular collar 20 is below the liquid level in the surrounding tank 200, liquid will flow upwards through the enclosed membrane module 1 to equalize the water levels inside and outside of the tank 200. The greater the difference in liquid level between the outside and inside of the modular collar 20 (h mm of liquid as shown in FIG. 3A), the greater the induced liquid velocity through the enclosed membrane module 1. FIG. 3B shows that if the liquid level within the modular collar 20 is higher than the surrounding liquid level in the tank 200, the direction of flow is reversed. Again, the greater the difference in liquid levels (h), the greater the liquid velocity created within the enclosed membrane module. The arrows in FIGS. 3A and 3B illustrate the direction of liquid flow induced by the changes in levels of liquid within the membrane module 120 and the tank 200.

[0106] FIG. 4A and FIG. 4B illustrates the detail of the airlift mixing system 4 within a panel 3a, 3b, 3c, 3d that can operate as an airlift pump, and which is capable of providing either an upward or a downward liquid flow respectively within the enclosed membrane module 1. The airlift mixing system 4 comprises the enclosed substantially U-shaped tube 5 connected to the vertical return channel 9 at one side thereof. In FIG. 4A, the configuration of the channels of the airlift mixing system 4 is shown which illustrate movement of liquid from the bottom of the tank 200 to the top of the enclosed membrane module 1. In this instance, the outer enclosed channel of the airlift mixing system 4 acts as a first downcomer 6, which receives liquid from inside the modular collar 20 (from the upper headspace 104) and acts to reduce the liquid level within the modular collar 20 relative to the liquid level within the tank 200. Air is injected into a middle-enclosed channel, here called the airlift pump 7, via an air injection port 40. The rising bubbles produced by the air injection port 40 induce a vertical liquid velocity flow (indicated by Arrow A), which moves water as illustrated by the arrows shown. At the top of the enclosed membrane module 1, the airlift pump 7 and the return channel 9 merge outside of the enclosed membrane module 1, the air bubbles are vented at the liquid surface and the liquid flows downwards through the return channel 9 to the base of the tank 200. In FIG. 4B, the configuration of the channels of the airlift mixing system 4 is shown which illustrate movement of liquid from the upper manifold 104 within the enclosed membrane module 1 downwards through the cassettes 100 to the bottom of the tank 200. The air injection port 40 is switched to the outer channel of the substantially U-shaped tube 5 and becomes the airlift pump 7, while the inner channel of the substantially U-shaped tube 5 becomes the first downcomer 6 and is physically connected to the return channel 9. The airlift channel 7 supplies liquid to the inside of the modular collar 20 and upper manifold 104, causing an increase in water level above the membrane module 120 relative to the liquid outside of the modular collar 20 in the tank 200. In this operating mode, liquid is drawn from the bottom of the tank 200 via the return channel 9 as illustrated. The arrows A in FIG. 4B illustrate the direction of liquid flow induced by the airlift channel 7. The liquid level in the modular collar 20 covering the enclosed membrane module 1 is separated from the liquid outside in the tank 200 in which the module is immersed due to the seal created by the enclosed membrane module 1, such that the airlift mixing system 4 may raise or lower the liquid level covering the membrane module 1 relative to the liquid level in the tank 200.

[0107] FIG. 5A and FIG. 5B show a plan view and side elevation view of the modular collar 20 that is attached to the top of the enclosed membrane module 1. The modular collar 20 comprises sides 21a, 21b, 21c, 21d and is configured to fit to a frame 110 of a membrane module 120 or to the top of an enclosed membrane module 1, thus providing an upper headspace 104 with an increased height when compared to upper headspace 104 without a modular collar 20 in place. When in use, the modular collar 20 is configured to separate the ports 11 and 12.

[0108] FIG. 6A and FIG. 6B illustrate how a number of enclosed membrane modules 1, 1′ may be stacked one upon the other within a tank 200. The stacked enclosed membrane modules 1, 1′ are configured such that the return channel 9 of module 1 connects to the return channel 9′ of the module F. FIG. 6A and FIG. 6B are operating in the modes shown in FIGS. 4A and 4B, respectively. The substantially U-shaped tube 5′ of module 1′ is cut off by the insertion of a baffle 22 and remains unused. In the embodiment shown in FIG. 6A, the liquid level outside the modular collar 20 of the enclosed membrane module 1 is higher than the liquid level inside the modular collar 20, thus creating an upward velocity of liquid from the bottom of the tank 200 to the upper headspace 104 of the enclosed membrane module 1. In the embodiment show in FIG. 6B, the liquid level outside the modular collar 20 of the membrane module 1 is lower than the liquid level inside the modular collar 20, thus creating a downward velocity of liquid from the upper headspace 104 of the enclosed membrane module 1 to the bottom of the tank 200.

[0109] FIG. 7A and FIG. 7B illustrate a plan view and side view in cross section of four enclosed membrane modules 1, which are installed in a tank 200 to form a treatment system 300. The whole treatment system 300 makes up a MABR, which treats inlet wastewater and discharges a treated effluent. The treatment system 300 comprises the tank 200 having a housing 201 and a series of stacked enclosed membrane modules 1. In the embodiment show in FIG. 7B, the liquid level outside the modular collar 20 in the tank 200 is lower than the liquid level inside the modular collar 20 (indicated by the h mm), thus creating a downward velocity of liquid from the upper manifold 104 of the enclosed membrane module 1 to the bottom of the tank 200.

[0110] In FIGS. 7A and 7B, liquid (water or wastewater (effluent)) enters the system 300 via an inlet waste pipe A and the treated effluent exits the system 300 via outlet B. The liquid is treated by interacting with the cassettes 100 of the enclosed membrane modules 1. The airlift mixing system 4 provides a low-pressure, low-energy mixing system that ensures there is effective contact between as much of the membrane-attached pollutant-degrading biofilm that accumulates on the membranes 102 and the pollutant-rich wastewater to be treated. Generally, the enclosed membrane module 1 is designed to operate with gas pressures inside the hollow membrane fibres 102 which may be higher or lower than the external hydrostatic pressure of the tank 200.

[0111] In FIG. 8, the enclosed membrane module 1 is illustrated with a liquid flow distribution means 60 is shown in situ in the upper headspace 104 of the enclosed membrane module 1. Generally, the water flow distribution means 60 is in fluid communication with port 12 and is configured to ensure uniform flow through the enclosed membrane module 1, that is the velocity of the flow of liquid within the module is equal on a horizontal plane. The provision of a uniform liquid flow through the enclosed membrane module 1 ensures an even upflow liquid velocity throughout the membrane module 1 and prevents short circuiting of the liquid between the port 12 and bottom of the return channel 9. The uniform velocity ensures that all of the membrane supported biofilm is contacted by the wastewater and removes the creation of a dead zone or poorly mixed regions where no flow occurs.

[0112] In FIG. 9A-C, the airlift mixing system 4 is shown to contain a second downcomer 6a giving the airlift mixing system 4 a substantially W-shape, where the central vertical channel is the air-lift channel 7 in fluid communication with the first downcomer 6 and second downcomer 6a on either side thereof. The air is delivered into the central airlift channel 7 at the air injection port 40, while the vertical channels, the first and second downcomer 6, 6a, are connected to the air lift channel 7 by a common water manifold 8a along the bottom, thus forming a substantially W-shape tube 50. Due to the water and air mixture flowing upwards through the central vertical channel 7, a downward waterflow is induced in both the first and second downcomer 6, 6a. Due to the presence of the panels 3a, 3b, 3c, 3d (see FIG. 9C), which extend beyond the surface of the liquid, the liquid discharged through the port 11 and the flow distribution means 60, originating from the airlift channel 7, must flow vertically downward and out of the enclosed membrane module 1 through the open bottom. One of the advantages of the W-shaped mixing system is that there are two inlets to the downcomers in this configuration and liquid can be introduced into the W-Shaped mixing system from two different points in tank. In the preferred configuration if the flow of liquid in the W-shaped tube is to be reversed, then air must be introduced into both vertical channels either side of the central vertical channel to make sure both of the side vertical channels become airlift channels and the central vertical channel becomes the downcomer. The W-shaped airlift mixing system can also be stacked as shown in FIG. 6A and FIG. 6B, one or two vertical return channels can also be installed on either side of the W-shaped mixing system. The W-shaped mixing system is easy to retrofit to existing modules, and can provide for larger area for flow using vertical channels with a smaller cross section.

[0113] One of the advantages of the invention is that the enclosed membrane module both protects the membranes from damage during transit and incorporates a low-pressure airlift system to encourage good liquid flow through the membrane module when the modules are installed in a bioreactor. This means that the effectiveness of the MABR when installed into a large tank is not dependant on the tank mixing, but is independently controlled via the liquid velocity in the enclosed membrane module. Such independent control allows successful installation in tanks of varying depth and shape or which were previously designed for different purposes, e.g. settling tanks, can be upgraded to incorporate the MABR without the need for an installation of an independent mixing system.

[0114] In the specification, the terms “comprise, comprises, comprised and comprising” or any variation thereof and the terms “include, includes, included and including” or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa.

[0115] The invention is not limited to the embodiments hereinbefore described but may be varied in both construction and detail.