Assembly for Water Filtration Using a Tube Manifold to Minimise Backwash

Abstract

A membrane filtration assembly (10) including a membrane module (5) having one or more permeable hollow membranes (6) supported therein by at least one header (7). A filtrate collection chamber (18) is associated with the header (7) and in fluid communication with lumens of the membranes (6) for collecting filtrate withdrawn through said membrane lumens. A filtration chamber (11) is provided for receiving feed liquid to be applied to the surface of membranes (6) within the module (5). The filtration chamber (11) encloses module (5) and extends beyond the height of the module (5).

Description

TECHNICAL FIELD

The present invention relates to membrane filtration systems and, more particularly, to a system that allows for filtration under gravity, under pressure or using a high feed head while minimising the backwash and cleaning solution volume.

BACKGROUND OF THE INVENTION

In a membrane filtration operation, periodically cleaning the membrane by liquid or gas backwash is essential to keep a longer membrane operation time without the need for a chemical cleaning stage. However, during each backwash, a certain amount of liquid waste is produced, which reduces the feed liquid recovery and increases the requirements on post treatment of backwash is waste. The liquid or gas backwash is also supplemented by periodic cleaning of the membranes using a chemical cleaning agent. This process again produces liquid waste which must be further treated or disposed of in an environmentally safe manner. It is thus desirable in any filtration operation to minimise the volume of waste liquid produced during the backwash and chemical cleaning phases of operation in order to reduce costs of operation and any environmental impact.

SUMMARY OF THE INVENTION

According one aspect the present invention provides a membrane filtration assembly including:

• • a membrane module having one or more permeable hollow membranes supported therein by at least one header, a filtrate collection chamber associated with said header and in fluid communication with lumens of said membranes for collecting filtrate withdrawn through said membrane lumens;
  • a filtration chamber for receiving feed liquid to be applied to the surface of membranes within said module, said filtration chamber enclosing said module and extending beyond the height of said module.

Preferably, the membranes extend between a pair of spaced, opposed headers. For preference, a filtrate collection chamber is associated with one or both of said headers. Preferably, said membranes are positioned vertically within said module and said headers are respective upper and lower headers. For preference, a filtrate carrier is provided between and in fluid communication with said filtrate collection chambers. Preferably, said filtrate carrier extends through said module between said headers. For preference, an opening or openings are provided in the upper header to allow the flow of fluid therethrough.

In one embodiment, said filtration chamber is closed at one end and open the other. In another embodiment, the open end of the filtration chamber is provided with a valve which allows gas flow therethrough while only allowing liquid flow into the filtration chamber.

Preferably, the region defined between the filtration chamber and the module is filled with one or more filler elements. For preference an aeration backwash device is provided in fluid communication with said filtration chamber for selectively communicating gas and/or liquid to and/or from said filtration chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

FIG. 1 shows a schematic sectional elevation view of a filtration module according to one embodiment of the invention;

FIG. 2 shows a schematic sectional elevation view of the filtration assembly according to one embodiment of the invention operating in a gravity suction mode;

FIG. 3 shows a schematic sectional elevation view of the filtration assembly according to one embodiment of the invention operating in a pressurised mode; and

FIG. 4 shows a schematic sectional elevation view of the filtration assembly according to one embodiment of the invention operating in a high head mode.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, the membrane filtration module 5 according to one preferred embodiment consists of a bundle of hollow fibre membranes 6 potted at the top and bottom in upper and lower headers 7 and 8 respectively to expose the fibre membrane lumens to collect filtrate. It will be appreciated the fibre bundle may be potted at either or both ends, and further that the fibre lumens may be open at either or both ends. An opening or openings may be provided in the upper header to allow the flow of fluid therethrough.

A filtrate carrier 9 (typically a pipe or tube) extends between the upper and lower headers 7 and 8 to allow filtrate to be collected from either or both ends of the fibre membrane lumens. The carrier 9 may be of any shape in cross section and, though shown in this embodiment in the centre of the module 5, it may be located anywhere in the pot and in some embodiments the filtrate carrier 9 need not be located in the module 5 but can be connected through external connections as described below.

Referring to FIGS. 2 to 4, the filtration assembly 10, according to a preferred embodiment of the invention, consists of a filtration chamber 11 higher than the module 5. If used in the high head filtration mode, as shown in FIG. 4, the filtration chamber 11 may be significantly higher than the module 5. The cross section of the filtration chamber 11 preferably resembles the cross section of the module 5, however, when spacer fillers 12 are used, any cross section of the module may be accommodated. The filtration chamber 11 may be integrally formed as one piece or comprise multiple elements. The filtration chamber 11 preferably has a constant cross-section along its length, however, it may consist of multiple sections with a thicker cross section towards its base.

A spacer filler 12 with an exterior cross section closely resembling the cross section of the filtration chamber 11 and an internal cross section closely resembling the cross section of the module 5 is provided between the module 5 and the filtration chamber 11. The spacer filler 12 is not essential to the operation of the filtration assembly 10 but in the preferred embodiments is used to further reduce backwash and cleaning solution volumes. The spacer filler 12 may be formed from a single or multiple components.

A cap 13 is fitted to one end 14 of the module 5 to allow filtrate flow 15 from the end of the module 5 into the filtrate carrier 9.

A module mounting piece 16 is sealingly attached to lower end of the filtration chamber 11. The mounting piece 16 includes an aeration backwash device 17 and a filtrate collection chamber 18.

As indicated above, the filtrate carrier 9 need not be located within the module 5. Instead, external connections between the cap 13 and the filtrate collection chamber 18 or downstream thereof may be provided to allow for the collection of filtrate from the capped end of the module 5.

The aeration/backwash device 17, at least in part, surrounds a portion of the membrane module 5 above the lower header 8. The aeration/backwash lo device 17 includes a communication chamber 19 having upper and lower through-openings 20 and 21 in fluid communication with the communication chamber 19 and the membrane module 5. It will be appreciated however that the function of the upper and lower through-openings may be performed by one or more elongate openings which are, preferably, varying in width and, more is preferably, are wider at the lower end than at the upper end. The communication chamber 19 is connected via a pipe 22 to a feed, aeration, backwash and draindown header 23. The communication chamber 19 in this embodiment is in the form of annulus 24 which provides for the selective aeration, feed and backwash with through-openings 20 and 21 in the inner wall 25 of the annulus 24 to allow the flow of gas, feed liquid, backwash liquid into the module 5 and draindown of waste liquid from the module 5. The annulus 24 may fully or partially surround the membrane module 5.

It will be appreciated that the functions of the module mounting piece 16 may provided by a single component or by multiple components.

A filtrate header 26 connects the filtrate collection chamber 18 from multiple modules together to a common outlet (not shown).

The aeration, feed, backwash and draindown header 23 connects the aeration gas, feed liquid, backwash and draindown liquids from multiple modules together to a common inlet/outlet (not shown).

Various modes of operation of this embodiment of the invention will now be described, by way of example only, with reference to FIGS. 2 to 4 of the drawings, respectively.

Gravity Suction Mode Operation

This mode of operation is illustrated in FIG. 2 of the drawings which shows the filtration assembly 10 located in a feed tank 28. Although, the assembly 33 is shown located in a tank 28, it will be appreciated that the tank 28 is merely one means of providing feed liquid to the filtration chamber 11 and other means such as pumping, flowing liquid from above and the like are equally applicable.

Filtration

The feed liquid level 29 in the surrounding feed tank 28 is raised higher than the filtration chamber 11 allowing feed liquid 30 to flow into the filtration chamber 11. Feed liquid may also be pumped through the aeration, feed, drain down and backwash header 23 and suction may be applied to the filtrate header 26. Significant pressure can be introduced by high feed levels in the feed tank 28.

The feed liquid flows down the filtration chamber 11 and is diverted into the module 5 by the spacer filler 12. The feed liquid is then filtered through the membranes 6 located in the module 5 and filtrate is recovered from the ends of the module 5. Filtrate from the capped end 31 of the module 5 flows through the filtrate carrier 9 to the bottom end 32 of the module 5 and into the filtrate header 26.

Aeration

During the aeration step, the feed liquid level 29 is lowered in the surrounding tank 28 to lower than the top of the filtration chamber 11. Gas, typically air, is then introduced through the aeration, feed, drain down and backwash header 23 to aerate the membrane module 5. The gas flows into the communication chamber 19 and is directed into the membrane module 5 through the aeration holes 20 in the module mounting piece 16. The gas forms bubbles which scrub the membrane surfaces as they rise through the module 5. The feed liquid level in the filtration chamber 11 is arranged such that, during aeration, no feed liquid is able to escape from the filtration chamber 11 into the feed liquid in the surrounding tank 28.

Feed Backwash

During the feed backwash process, the feed liquid level 29 is lowered in the surrounding tank 28 to lower than the top of the filtration chamber 11. Backwash liquid is then introduced through the aeration, feed, drain down and backwash header 23. The backwash liquid flows into the communication chamber 19 and is directed into the membrane module 5 through the feed backwash holes 21 in the module mounting piece 16. The backwash liquid then scrubs the membrane surfaces as it rises through the module 5. The feed liquid level 29 in the filtration chamber 11 is arranged such that, during backwash, no feed liquid is able to escape from the filtration chamber 11 into the feed liquid in the surrounding tank 28.

In an alternative feed backwash process, backwash feed liquid is introduced into the filtration chamber by raising the feed level 29 in the tank 28 such that it overflows into the filtration chamber. The backwash liquid flows through the module 5 and out through the aeration, feed, drain down and backwash header 23. Suction may be applied to the backwash header.

Permeate Backwash

During the permeate backwash process, the feed liquid level 29 is lowered in the surrounding tank to lower than the top of the filtration chamber 11. Feed is drained from inside the filtration chamber 11 and permeate is introduced through pressurising the filtrate header 26 with permeate. The permeate backwash liquid flows through into lumens and out through the walls of the fibres removing solids from the surfaces of the fibres. During this step, permeate backwash liquid and solids may be drained through the communication chamber 19 and into the aeration, feed, drain down and backwash header. The rate of introduction of the permeate backwash liquid is arranged such that, during permeate backwash, no permeate backwash liquid is able to escape from the filtration chamber 11 into the feed liquid surrounding the tank 28.

Drain Down

After either or both of aeration and backwash the liquid and solids in the filtration chamber 11 are drained out through the holes 20 and 21 in the module mounting piece 16 into the aeration, feed, drain down and backwash header 23.

Pressurised Mode Operation

As shown in FIG. 3, for operation in pressurised mode, an assembly 33 is provided on the top end of the filtration chamber 11 that allows liquid to flow only into the filtration chamber 11 but not out thereof and allows gas to flow in both directions. In this embodiment, the assembly 33 consists of a sealing cone 34 which seals onto the open end 35 of the filtration chamber 11 with an opening 36 therein which is smaller than a sealing ball 37. The sealing ball 37, which has an effective density less than the feed liquid, is located and moves freely within the filtration chamber 11.

It will be appreciated by those in the art that a variety of valve arrangements can be used to achieve the required function.

Although, the assembly 33 is shown located in a tank 28, it will be appreciated that the tank 28 is merely one means of providing feed liquid to the filtration chamber 11 and other means such as pumping, flowing liquid from above and the like are equally applicable.

Filtration

During filtration, the feed liquid is introduced to the filtration chamber 11 through the aeration, drain down and backwash header 23, and suction may be is applied to the filtrate header 26. As the level of the feed liquid in the filtration chamber 11 is raised the sealing ball 37 floats up to seal against the sealing cone 34 and closing opening 36, thereby allowing the filtration chamber 11 to be pressurised.

The feed liquid is then filtered through the membranes 6 in the module 5 and filtrate is recovered from the ends of the module 5. Filtrate from the capped end 31 of the module 5 flows through the filtrate carrier 9 to the bottom end 32 of the module 5 and into the filtrate header 26.

Aeration

During the aeration step, the feed liquid level 29 is lowered in the filtration chamber 11, so that the sealing ball 37 is no longer in contact with the sealing cone 34 and the filtration chamber 11 is depressurised. Gas is then introduced through the aeration, feed, drain down and backwash header 23. The gas flows into the communication chamber 19 and is directed into the module 5 through the aeration holes 20 in the module mounting piece 16. The gas forms bubbles which scour the membrane surfaces as they rise through the module 5. The feed liquid level in the filtration chamber 11 is arranged such that, during aeration, no feed liquid is able to escape from the filtration chamber 11 into the feed liquid in the surrounding tank 28.

Feed Backwash

During the backwash process, the feed liquid level is lowered in the filtration chamber 11, so that the sealing ball 37 is no longer in contact with the sealing cone 34 and the filtration chamber 11 is depressurised. The feed liquid level in the surrounding tank 28 is arranged such that feed liquid overflows into the filtration chamber to backwash the module. The backwash feed liquid flows is through the module removing solids from the surfaces of the fibres. The backwash liquid drains through the aeration, feed, drain down and backwash header 23. Suction may be applied to the backwash header.

Permeate Backwash

During the permeate backwash process, feed is drained from inside the filtration chamber 11 and permeate is introduced through pressurising the filtrate header 26 with permeate. The permeate backwash liquid flows through into lumens and out through the walls of the fibres removing solids from the surfaces of the fibres. During this step, permeate backwash liquid and solids may be drained through the communication chamber 19 and into the aeration, feed, drain down and backwash header. The rate of introduction of the permeate backwash liquid is arranged such that, during permeate backwash, no permeate backwash liquid is able to escape from the filtration chamber 11 into the feed liquid surrounding the tank 28.

Drain Down

After either or both of aeration and backwash the liquid and solids in the filtration chamber 11 are drained out through the holes 20 and 21 in the module mounting piece 16 into the aeration, feed, drain down and backwash header 23.

High Head Mode Operation

As shown in FIG. 4, for high head mode operation the filtration chamber 11 is extended significantly beyond the height of the module 5 such that, when the filtration chamber 11 is filled, a significant pressure is generated across the filtration surfaces of the membranes.

Filtration

During the filtration step, the feed liquid is introduced to the filtration chamber 11, via the module 5, through the aeration, feed, drain down and backwash header 23, or directly into the filtration chamber until the feed liquid level fills the filtration chamber 11. Suction may also be applied to the filtrate header 26.

The feed liquid is then filtered through the membranes 6 located in the module 5 and filtrate is recovered from the ends of the module 5. Filtrate from the capped end 31 of the module 5 flows through the filtrate carrier 9 to the bottom end 32 of the module 5 and into the filtrate header 26. The feed liquid is introduced at such a rate that the level in the filtration chamber 11 stays within controlled limits maintaining sufficient pressure for filtration.

Aeration

Gas is introduced through the aeration, feed, drain down and backwash header 23. The aeration gas flows into the communication chamber 19 and is directed into the module through the aeration holes 20 in the module mounting piece 16. The gas forms bubbles which scour the surfaces of the membranes 6 as they rise though the module 5.

Feed Backwash

Backwash liquid is introduced through the aeration, feed, drain down and backwash header 23. The backwash liquid flows into the communication chamber 19 and is directed into the module 5 through the feed/backwash holes 21 in the module mounting piece 16. The backwash liquid scrubs the surfaces of the membranes as it rises though the module 5.

Permeate Backwash

During the permeate backwash process, the feed liquid level 29 is lowered in the surrounding tank 28 to lower than the top of the filtration chamber 11. Feed is drained from inside the filtration chamber 11 and permeate is introduced through pressurising the filtrate header 26 with permeate. The permeate backwash liquid flows through into the lumens of the fibres 6 and out through the walls of the fibres removing solids from the surfaces of the fibres. During this step, permeate backwash liquid and solids may be drained through the communication chamber 19 and into the aeration, feed, drain down and backwash header.

Drain Down

After either or both of aeration and backwash the liquid and solids in the filtration chamber 11 are drained out through the holes 20 and 21 in the module mounting piece 16 into the aeration, feed, drain down and backwash header 23.

It will be appreciated that the various backwashing processes and steps described above may be performed in a variety of combinations and sequences depending on the operating outcomes desired by the user.

It will be appreciated that further embodiments and exemplifications of the invention are possible without departing from the spirit or scope of the invention described.

Claims

1. A membrane filtration assembly comprising: a membrane module having one or more permeable hollow membranes supported therein by at least one header;a filtrate collection chamber associated with said header and in fluid communication with lumens of said membranes for collecting filtrate withdrawn through said membrane lumens; anda filtration chamber for receiving feed liquid to be applied to the surface of membranes within said module, said filtration chamber enclosing said module and extending beyond the height of said module.

2. A membrane filtration assembly according to claim 1 wherein the membranes extend between a pair of spaced, opposed headers.

3. A membrane filtration assembly according to claim 2 wherein a filtrate collection chamber is associated with one or both of said headers.

4. A membrane filtration assembly according to claim 2 of wherein said membranes are positioned vertically within said module and said headers are respective upper and lower headers.

5. A membrane filtration assembly according to claim 2 wherein said membranes are positioned generally horizontally within said module and said headers are respective side headers.

6. A membrane filtration assembly according to claim 3 wherein a filtrate carrier is provided between and it fluid communication with said filtrate collection chambers.

7. A membrane filtration assembly according to claim 6 wherein said filtrate carrier extends through said module between said headers.

8. A membrane filtration assembly according to claim 1 wherein said filtration chamber is closed at one end and open the other.

9. A membrane filtration assembly according to claim 8 wherein the open end of the filtration chamber is provided with a valve which allows gas flow therethrough while only allowing liquid flow into the filtration chamber.

10. A membrane filtration assembly according to claim 9 wherein said valve comprises a ball valve wherein a valve seat is formed by a truncated conical ceiling of the filtration chamber and said opening is formed in the truncated portion of the cone such that a ball of larger diameter than said opening is located within the filtration chamber and has a buoyancy relative to liquid within the filtration chamber such that the ball rises with the liquid level within the filtration chamber to close said opening by bearing against said valve seat.

11. A membrane filtration assembly according to claim 1 wherein the space defined between the filtration chamber and the module is, at least in part, filled with one or more filler elements.

12. A membrane filtration assembly according to claim 1 wherein an aeration backwash device is provided in fluid communication with said filtration chamber for selectively communicating gas and/or liquid to and/or from said filtration chamber.

13. A membrane filtration system according to claim 1 wherein said filtration chamber is integrally formed in one piece or comprises multiple elements.

14. A membrane filtration system according to claim 2 wherein one or both of said headers is provided with an opening or openings to allow the flow of fluid therethrough.

15. A method of operating a membrane filtration system of the type including a membrane module having one or more permeable hollow membranes supported therein by at least one header, a filtrate collection chamber associated with said header and in fluid communication with lumens of said membranes for collecting filtrate withdrawn through said membrane lumens and a filtration chamber including an opening for receiving feed liquid to be applied to the outer surface of membranes within said module; said filtration chamber enclosing said module and extending beyond the height of said module, said method comprising the steps of: a) providing feed liquid to the filtration chamber so as to produce a pressure differential across a membrane wall;b) filtering liquid through the membrane wall into the membrane lumen as filtrate; andc) withdrawing filtrate from the membrane lumen into the filtrate collection chamber.

16. A method according to claim 15 wherein said filtration chamber is located in a feed containing vessel and said method includes the step of raising the level of feed liquid within the feed containing vessel to above that of the opening in the filtration chamber to provide feed liquid into the filtration chamber.

17. A method according to claim 15 including the further step of performing an aeration operation comprising the step of: d) aerating the membranes within the module by flowing gas bubbles past the surfaces of said membranes.

18. A method according to claim 17 including the further step of: e) preventing liquid flow from the filtration chamber during said aeration step.

19. A method according to claim 17 wherein said filtration chamber is located in a feed containing vessel and said method includes the step of lowering the level of feed liquid within the feed containing vessel to below that of the opening in the filtration chamber before said aeration step.

20. A method according to claim 15 including the further step of performing a feed liquid backwash operation comprising the step of: d) backwashing the membranes within the module by flowing feed liquid past the surfaces of said membranes.

21. A method according to claim 20 including the further step of: e) preventing liquid flow from the filtration chamber during said backwashing step.

22. A method according to claim 20 wherein said filtration chamber is located in a feed containing vessel and said method includes the step of lowering the level of feed liquid within the feed containing vessel to lower than that of the opening in the filtration chamber before said backwashing step.

23. A method according to claim 20 wherein feed liquid is flowed out of said module through an opening or openings in said header.

24. A method according to claim 20 wherein said filtration chamber is located in a feed containing vessel and said method includes the step of raising the level of feed liquid within the feed containing vessel to above that of the opening in the filtration chamber to cause a flow of feed liquid into the filtration chamber before and/or during said backwash step.

25. A method according to claim 15 including the further step of performing a filtrate liquid backwash operation comprising the steps of: d) suspending the filtration operation;e) backwashing the membranes within the module by flowing filtrate liquid into the membrane lumens and through the walls thereof to remove accumulated solids from said membranes;f) removing filtrate backwash liquid containing said accumulated solids from said filtration chamber.

26. A method according to claim 25 wherein said filtration chamber is located in a feed containing vessel and said method comprises the further steps of: g) lowering the level of feed liquid within the feed containing vessel to below that of the opening in the filtration chamber before said backwashing step;h) arranging the rate of flow of filtrate backwash liquid into the filtrate chamber and the rate of removal of filtrate backwash liquid from the filtration chamber such that no filtrate flows from the filtration chamber into said feed containing vessel.

27. A method according to claim 15 including the further step of removing liquid from said filtration chamber following said aeration operation and/or said backwash operation.

28. A method of operating a membrane filtration system of the type including a membrane module having one or more permeable hollow membranes supported therein by at least one header, a filtrate collection chamber associated with said header and in fluid communication with lumens of said membranes for collecting filtrate withdrawn through said membrane lumens and a filtration chamber including an opening for receiving feed liquid to be applied to the outer surface of membranes within said module, said opening being controlled by a valve arranged to allow flow of gas therethrough while only allowing liquid flow into said filtration chamber; said filtration chamber enclosing said module and extending beyond the height of said module, said method comprising the steps of: a) providing feed liquid to filtration chamber such that the valve controlling the opening in the filtration chamber is closed to prevent liquid flow out of the filtration chamber;b) continuing to provide feed liquid to the filtration chamber to pressurize the filtration chamber and produce a pressure differential across a membrane wall;c) filtering liquid through the membrane wall into the membrane lumen as filtrate; andd) withdrawing filtrate from the membrane lumen into the filtrate collection chamber.

29. A method according to claim 28 including the further step of performing an aeration operation comprising the steps of: e) opening said valve to depressurise the filtration chamber;f) aerating the membranes within the module by flowing gas bubbles past the surfaces of said membranes.

30. A method according to claim 29 wherein said filtration chamber is located in a feed containing vessel and said method comprises the further steps of: g) lowering the level of feed liquid within the feed containing vessel to below that of the opening in the filtration chamber before said backwashing step to cause opening of said valve;h) preventing liquid flow from the filtration chamber into said feed containing vessel during said backwashing step.

31. A method according to claim 28 including the further step of performing a feed liquid backwash operation comprising the steps of: e) opening said valve to depressurise the filtration chamber;f) backwashing the membranes within the module by flowing feed liquid past the surfaces of said membranes.

32. A method according to claim 31 wherein said filtration chamber is located in a feed containing vessel and said method comprises the further steps of: g) lowering the level of feed liquid within the feed containing vessel to below that of the opening in the filtration chamber before said backwashing step to cause opening of said valve;h) preventing liquid flow from the filtration chamber into said feed containing vessel during said backwashing step.

33. A method according to claim 28 wherein said filtration chamber is located in a feed containing vessel and including the further step of performing a feed liquid backwash operation comprising the steps of: e) lowering the level of feed liquid within the feed containing vessel to allow said valve to open and depressurise the filtration chamber while maintaining the level of feed liquid level above that of the opening in the filtration chamber to cause a flow of feed liquid into the filtration chamber;f) backwashing the membranes within the module by flowing said feed liquid within the filtration chamber past the surfaces of said membranes.

34. A method according to claim 33 wherein feed liquid is flowed out of said module through an opening or openings in said header.

35. A method according to claim 28 including the further step of performing a filtrate liquid backwash operation comprising the steps of: e) suspending the filtration operation;f) backwashing the membranes within the module by flowing, filtrate liquid into the membrane lumens and through the walls thereof to remove accumulated solids from said membranes;g) removing filtrate backwash liquid containing said accumulated solids from said filtration chamber.

36. A method according to claim 35 wherein said filtration chamber is located in a feed containing vessel, said method comprising the further steps of: g) lowering the level of feed liquid within the feed containing vessel to allow said valve to open and depressurise the filtration chamber before said backwashing step; andh) arranging the rate of flow of filtrate backwash liquid into said filtration chamber and the rate of removal of filtrate backwash liquid from the filtration chamber such that no filtrate liquid flows from the filtration chamber into said feed containing vessel.

37. A method according to claim 28 comprising the further step of removing liquid from said filtration chamber following said aeration operation and/or said backwash operation.