WO1998022204A1

WO1998022204A1 - Microporous membrane filtration and backwashing process

Info

Publication number: WO1998022204A1
Application number: PCT/AU1997/000791
Authority: WO
Inventors: Warren Johnson; Thomas William Beck; Peter Rogers
Original assignee: Memtec America Corporation
Priority date: 1996-11-21
Filing date: 1997-11-19
Publication date: 1998-05-28
Also published as: AUPO377796A0

Abstract

The invention relates to filtration processes of the kind using a microporous membrane, wherein feed containing contaminant matter is applied under pressure to a feed receiving surface of the membrane for passage therethrough and filtrate is withdrawn from the permeate side of the membrane. More particularly, the invention relates to systems that include means for backwashing the filter membranes and to a process and apparatus that enables control of the total volume of backwash fluid resulting therefrom, preferably with minimal detrimental effect on the process operation as a whole. This is achieved by selective reuse of the backwash fluid as part of the sweeping fluid used in the backwash operation and/or as part of the main feed.

Description

TITLE: MICROPOROUS MEMBRANE FILTRATION AND BACKWASHING

PROCESS

FIELD OF THE INVENTION

The present invention relates to filtration processes of the kind using a

microporous membrane, wherein feed containing contaminate matter is applied under

pressure to a feed receiving surface of the membrane for passage therethrough and

filtrate is withdrawn from the permeate side of the membrane. More particularly, the

invention relates to systems that include means for backwashing the filter membranes.

BACKGROUND OF THE INVENTION

In all membrane filtration processes of the kind referred to above, contaminant

matter filtered from the feed continuously builds up on the feed receiving surface of the

membrane. This leads to a decrease in filtration efficiency and a corresponding decrease

in achievable permeate flux or an increase in operating pressure. Accordingly, it is

necessary to periodically clean the feed receiving surface of the membranes.

This is most commonly achieved with a frequent and generally regular

backwashing process, wherein a source of fluid under pressure is applied to the permeate

side of the microporous filter membrane so as to dislodge at least a portion of the

contaminant matter lodged within and/or on the feed receiving surface of the membrane.

The dislodged contaminant matter is then flushed out of the system by passing a

sweeping fluid over the feed receiving surface of the microporous membrane, the

resulting waste then being separately diverted from the system for subsequent disposal or

further treatment. In small filtration units, this backwash comprising the sweeping fluid and

contaminate matter is often disposed of by use directly for irrigation purposes or the like.

However, in large scale membrane filtration plant, the problem of disposing of the

significant volumes of backwash fluid is a major concern.

The preferred solution to date has been to provide backwash settling lagoons.

The levels of the lagoons are controlled to some extent by natural evaporation and

currently by feeding the supernatant from the lagoons back into the main feed on a

continuous basis at a predetermined inclusion proportion of the feed flow.

However, recent tests have shown that this reprocessing of the backwash

supernatant has a surprisingly adverse affect on the overall efficiency of the filtration

system, resulting in significant increases in the rate at which the trans-membrane

pressure (TMP) increases, which ultimately affects achievable permeate flow rates.

Furthermore, simply ceasing to reuse any of the backwash is not desirable, as some

means external to the filtration process will then be required to handle the increasing

volumes of backwash which would then add to the cost and complexity of the process.

It is an object of the present invention to provide a filtration and/or backwashing

method and apparatus of the kind referenced above which overcomes or substantially

ameliorates one or more of the above discussed disadvantages of the prior art or at least

offers a useful alternative thereto.

DISCLOSURE OF THE INVENTION

According to a first aspect of the invention there is provided a method of

backwashing microporous membranes which have been subjected to a filtration

operation wherein feed containing contaminant matter is applied under pressure to a feed receiving surface of the membrane for passage therethrough and filtrate is withdrawn

from a permeate side of the membrane remote the feed receiving surface, said method

comprising the steps of:

(a) terminating the filtration operation by ceasing supply of feed under

pressure to said feed receiving surface of said membrane,

(b) applying a source of fluid under pressure to said permeate side of the

membrane such that said fluid under pressure passes in a reverse direction through said

membrane so as to dislodge at least a portion of contaminant matter lodged within and/or

on said membrane,

(c) passing a sweeping fluid past said feed receiving surface of said

membrane to flush out the dislodged contaminant matter and form a backwash liquid,

and

(d) delivering the backwash liquid to a reservoir,

wherein at least a part of said sweeping fluid of step (c) comprises previously

accumulated backwash liquid from step (d).

Preferably, the backwash liquid used as sweeping fluid in step (c) comprises

supernatant from a backwash settling lagoon.

This method is particularly suited to backwashing systems of the kind comprising

a plurality of hollow elongate fibres having microporous walls which have been

subjected to a filtration operation wherein feed containing contaminant matter is applied

under pressure to the exterior surface of said hollow fibres and filtrate is withdrawn from

the ends of the lumens of the fibres, the fibres being contained within a shell or housing,

said method then comprising the steps of: (a) terminating the filtration operation by ceasing supply of feed under

pressure to said exterior surface of said membrane,

(b) sealing the shell,

(c) applying a source of fluid under pressure to said lumens such that said

fluid under pressure passes through said walls so as to dislodge at least a portion of

contaminant matter lodged within and/or on said fibre walls,

(d) passing a sweeping fluid past said exterior surface of said membrane to

flush out the dislodged contaminant matter to form a backwash liquid, and

(e) delivering the backwash liquid to a reservoir,

wherein at least a part of said sweeping fluid of step (d) comprises previously

accumulated backwash liquid from step (c).

Desirably, the backwash liquid used as sweeping fluid comprises supernatant

from a backwash settling lagoon.

Preferably, the supernatant from the settling lagoon is delivered to the feed

receiving surface of the membranes by direct injection into the feed line. It is presently

believed that some form of plug flow will be achieved, albeit with a certain degree of

mixing at the interfaces. In this manner the switching of the process to backwash

sweeping mode can be triggered on a time or volume flow basis calculated from the

point at which the supernatant is injected. Alternatively, presence of the backwash

supernatant at the membrane can be detected by monitoring the change in the trans-

membrane pressure (TMP). In most cases the TMP will show a shaφ increase when the

supernatant reaches the membrane, as despite the settling process, there is still likely to be a substantial quantity of fine particles that have remained in suspension in the

supernatant.

According to a second aspect of the invention there is provided an apparatus for

backwashing microporous membranes which have been subjected to a filtration

operation wherein feed containing contaminant matter is applied under pressure to a feed

receiving surface of the membrane for passage therethrough and filtrate is withdrawn

from a permeate side of the membrane remote the feed receiving surface, said apparatus

comprising:

(a) means to terminate the filtration operation by cutting off supply of feed

under pressure to said feed receiving surface of said membrane;

(b) means to apply a source of fluid under pressure to said permeate side of

the membrane such that said fluid under pressure passes in a reverse direction through

said membrane so as to dislodge at least a portion of contaminant matter lodged within

and/or on said membrane;

(c) means to deliver a sweeping fluid past said feed receiving surface of said

membrane to flush out the dislodged contaminant matter and thereby form a backwash

liquid;

(d) means to deliver the backwash liquid to a reservoir, and

(e) means to extract previously accumulated backwash liquid from said

reservoir for use as, or at least inclusion with, said sweeping fluid.

Preferably, the backwash liquid used as sweeping fluid comprises supernatant

from a backwash settling lagoon. The method and apparatus of the first and second aspects of the invention

provides a convenient means of at least partially reusing and thereby controlling the

volume of accumulated backwash waste, in a manner that also increases the overall

efficiency of the filtration process when compared to the prior art method of

reprocessing the backwash by incoφoration with the main feed.

According to a third aspect of the invention there is provided a method of

operating a filtration system of the kind having a micro-porous membrane wherein feed

containing contaminant matter is applied under pressure to a feed receiving surface of

the membrane for passage therethrough and filtrate is withdrawn from a permeate side of

the membrane, the system further including means to backwash said membranes by

applying a source of fluid under pressure to the permeate side of the micro-porous filter

membrane so as to dislodge at least a portion of the contaminant matter lodged within

and/or on said feed receiving surface, the dislodged contaminant matter then being

flushed out of the system by passing a sweeping fluid over said feed receiving surface so

as to form a backwash liquid, said method of operation including the step of

accumulating and recycling some or all of said backwash liquid as feed prior to storing

the liquid in a settling lagoon or sending it to waste.

According to a fourth aspect of the invention there is provided a filtration system

of the kind having a micro-porous membrane wherein feed containing contaminant

matter supplied under pressure to a feed receiving surface of the membrane the passage

therethrough and filtrate is withdrawn from a permeate side of the membrane, the system

further including means to backwash said membranes by applying a source of fluid

under pressure to the permeate side of the micro-porous filter membrane so as to dislodge at least a portion of the contaminant matter lodged within and/or on said feed

receiving surface, said system further comprising means to pass a sweeping fluid over

said feed receiving surface to flush the dislodged contaminant matter out of the system in

the form of a backwash liquid, said system including means to accumulate and recycle as

feed some or all of said backwash liquid prior to storing the liquid in a settling lagoon or

sending it to waste.

Preferably, the method and apparatus includes a step of, or means of,

accumulating the backwash in a backwash reservoir and, prior to the scheduled

backwashing step, switching from feed to recycling the backwash liquid from said

reservoir through the system until it has all been filtered.

More preferably, the backwash liquid is recycled a pre-determined number of

times prior to finally being directed in its then more concentrated form to storage or

waste.

Even more preferably, the method of operation includes the step of intermittently

backwashing with feed after a pre-determined number of steps of recycling the backwash

liquid.

The method and apparatus of the third and fourth aspects of the invention

similarly provide a convenient means of at least partially re-using and thereby

controlling the volume of accumulated backwash waste. This not only increases the

overall yield but suφrisingly has little, if any, detrimental effect on the overall efficiency

of the filtration process.

In a further embodiment there is provided a system that incoφorates both the

modified backwash process of the first and second aspects of the invention, in combination with the backwash recycling features of the third and fourth aspects of the

invention.

All aspects of the invention are applicable to both dead end and cross-flow

filtration operations.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of both the first and second as well as the third and

fourth aspects of the invention will now be described, by way of example only, with

reference to the accompanying drawings in which:

Figure 1 is a schematic view of a filtration process in accordance with the first

and second aspects of the invention;

Figure 2 is a graph illustrating the variation in TMP with time for a prior art

process that uses supernatant recycle into the main feed;

Figure 3 is a graph illustrating variation of TMP with time for a process

according to the first and second aspects of the invention that does not use supernatant

recycle into the main feed;

Figure 4 is a graph illustrating variation of TMP with time for processing normal

feed followed by supernatant.

Figure 5 is a schematic view of a prior art filtration process;

Figure 6 is a schematic view of a filtration process in accordance with the third and

fourth aspects of the invention; and

Figure 7 is a graph illustrating the variation in resistance with cumulative volume

treated per unit membrane area. PREFERRED EMBODIMENTS OF THE INVENTION

Referring to Figure 1 , there is shown a schematic representation of a preferred

form of the filtration process according to the first and second aspects of the invention.

The apparatus 1 comprises one or more filtration modules 2 which each include

filtration membranes in the form of a plurality of a hollow elongate fibres 3 having

microporous walls 4 which define central passages or lumens 5. The fibres are housed

within a shell 6 and sealed by means of two end plugs 7. These plugs serve to hold the

hollow fibres in place and provide a barrier between the feed that is delivered to the

exterior surfaces 8 of the fibres and the filtrate which flows out of the lumens.

An inlet port 9 and outlet port 10 are provided intermediate the plugs 7 for

admission of feed from the feed line 11 to and through the module 2. Similarly, two

ports 12 are also provided on the remote side of the plugs 7 for both extraction of the

permeate or admission of pressurised backwashing fluid to the lumens.

The filtration system also includes a backwash liquid reservoir in the form of a

settling lagoon 13 to which backwash from the outlet port 10 is diverted. In the

embodiment illustrated, a recycle circuit 14 is also included for recirculating the feed

during normal cross-flow operation of the filtration module 2. However, the invention in

all aspects is equally applicable to dead end filtration systems.

The system further comprises means shown generally at 15 to extract supernatant

from the reservoir, which in the preferred form is the settling lagoon 13 and direct it via

piping 16 into the main feed line 11 for use in the backwash sweeping operation. A

secondary delivery line 17 connects with the piping 16 for optional addition of some

other form of suitable sweeping fluid. In use, feed is directed under pressure to the inlet port 9 of module 2 via the main

feed line 1 1 , in a manner whereby at least a portion of the feed permeates through the

walls 4 of the fibres 3, the resulting filtrate being extracted from ports 12. The excess

cross-flow feed is diverted from outlet port 10 for recirculation through circuit 14.

Whilst the cross-flow through module 2 helps to limit the build-up of

contaminant matter on the membrane walls 4, the gradual accumulation will nonetheless

result in an increase in trans-membrane pressure (TMP). This results in a decrease in

filtration efficiency and accordingly, a periodic backwashing process is required to

remove the accumulated sludge to keep the process operating effectively.

During the backwashing process, the filtration operation is terminated by ceasing

supply of feed under pressure to the shell 6. In one preferred process, gas is applied

under high pressure to the lumens, in a manner whereby it is caused to pass through the

fibre walls 4 in a reverse to normal direction to dislodge at least a portion of the

contaminant matter lodged within and/or on the membrane. Flushing fluid is then

directed into the shell 6 via inlet 9 to flow past the external surface 8 of the membrane

walls 4 to flush out the dislodged contaminant matter. The sweeping fluid and dislodged

sludge is then diverted from outlet 10 to the settling lagoon 13.

The sweeping fluid in the process according to a preferred form of the first and

second aspects of the invention comprises, at least in part, supernatant extracted from the

settling lagoon 13. The flushing fluid, comprising either supernatant alone or in

combination with another liquid delivered at 17, is preferably injected directly into the

main feed line 11 , where it is believed that a certain degree of plug flow will occur. In this regard plug flow is not critical, the fluid just needs to be circulated long enough to

sweep out the solids.

In order to maximise the usage of supernatant and minimise the use of feed

during this sweeping operation, the supernatant is directed into the main feed line in

5 advance of commencement of the backwashing process. The presence of the supernatant

in the module can be established either on a time and flow rate basis where plug flow is

assumed, or alternatively can be directly detected by monitoring the trans-membrane

pressure (TMP) during the normal filtration process. In this regard it is expected that the

presence of the supernatant will result in a rapid increase in the rate of change of TMP as

o illustrated in Figure 4.

Referring next to Figure 2, there is shown a graph illustrating variation in TMP

with time for prior art systems that incoφorate backwash supernatant into the main feed.

Each step of the graph represents a normal filtration period where the TMP gradually

increases with time, each stage having a backwash operation therebetween. Figure 3

5 illustrates the variation of TMP with time for the same system in which the backwash

supernatant is not added back to the main feed.

Figure 4 illustrates the effect of introducing backwash supernatant into the feed

stream. TMP rise is very slow with no supernatant, but as soon as supernatant is added

there is a sudden increase in TMP. This increase then triggers an automatic backwash.

0 As it can been seen by comparing the graphs, the add back of the backwash

supernatant dramatically reduces the overall filtration efficiency of the process which is

shown by the rate of TMP increase during filtration. It presently appears that use of the supernatant as the backwashing fluid only, will result in an operation efficiency that is

closer to that illustrated in Figure 3 than in the prior art process of Figure 2.

Turning next to the remaining figures 5 to 7, a preferred embodiment in

accordance with the third and fourth aspects of the invention will now be described.

In this regard, figure 5 illustrates a standard filtration system with backwash

facility. The system comprises, in its preferred form, a continuous micro-filtration

(CMF) module 20 having a feed inlet port 21 , a permeate outlet 22 and a backwash

outlet 23.

Connected with the feed inlet port 21 is a feed pump 24 which draws feed from a

break tank 25 via a control valve 26. The backwash outlet port 23 connects to a

backwash tank 27 which itself has an outlet 28 that leads to waste.

In use, feed is directed into the CMF module 20 and the permeate is recovered

from the outlet 22. As the contaminant matter gradually accumulates on the feed

receiving surface of the membrane walls, a periodic backwashing process is required to

remove the accumulated sludge so as to keep the process operating effectively.

As discussed previously, during the backwashing process, the filtration operation

is terminated by ceasing supply of feed under pressure to the module. In one preferred

process, gas is applied under high pressure to the permeate side of the membrane in a

manner whereby it is caused to pass through the membrane walls in reverse to normal

direction to dislodge at least a portion of the contaminant matter lodged within and/or on

the membrane. Flushing fluid, normally in the form of extra feed, is then directed into

the module to flow, not through the membrane, but past the feed receiving surface to

flush out the dislodged contaminant matter and thereby form the backwash fluid. The backwash fluid is then directed from outlet 23 to the backwash tank 27. From the tank

the backwash fluid is then subsequently diverted to waste which in some installations,

may comprise a settling lagoon which may also include supernatant recycle in

accordance with the prior art.

Turning next to figure 6, there is shown the system of figure 5 modified in

accordance with the third and fourth aspects of the present invention. Where

appropriate, like reference numerals have been used to denote corresponding features.

The modified system includes means 30 to divert the backwash fluid from the

backwash tank 25 to the feed inlet port 21 of the CMF module 20. In the embodiment

illustrated, this includes a booster pump 31. The system also includes a waste outlet 32

down stream of the booster pump 31.

In use, the system is operated to process feed, such as river water, in the manner

described above. However, just prior to operating the periodic backwashing process, the

feed is switched to backwash fluid feed until all the backwash fluid in the backwash tank

25 has been filtered. After this step the backwash operation is continued as for the

standard system described above. In the embodiment tested to date, once the backwash

has been filtered about 10 times it is sent to waste and the next backwash conducted

entirely with feed and the process repeated from here on.

Preliminary trials have been conducted to compare systems using recycled

backwash (unit 2) with those using a standard backwash arrangement (unit 1), the results

of which are set out below: Average Feed Water Conditions

As can be seen from reference to figure 7, the method and apparatus of the third

and fourth aspects of the invention provide a means of increasing the overall yield by

partially re-using the backwash fluid, thereby simultaneously controlling the volume of accumulated backwash waste, with suφrisingly little affect on the overall efficiency of

the filtration process. More specifically, figure 7 shows that even with 10 recycles, the

performance of the unit is virtually no different (in fact slightly better in this example)

than the unit operated with no recycle (conventional method). This is thought to arise in

part from the fact that the backwash fluid will contain a particle distribution that will be

the same as the particles within the cake of accumulated contaminants on the feed

receiving surface of the membranes. It is considered that this distribution of particles to

include large particles (which would not be present in, say, supernatant from a settling

lagoon) helps to prevent the filters clogging. Also the particles are filtered onto an

already existing filter cake and are thus more readily backwashed off the surface than if

applied directly to a clean membrane.

Finally, it will be appreciated that the backwash liquid from the backwash tank 25

identified in respect of the third and fourth aspects of the invention can readily be used

as part of the sweeping fluid referred to in respect of the first and second aspects of the

invention, alone or in combination with the step of recycling part of this backwash fluid

as feed.

Although the invention has been described with reference to specific

embodiments, it will be appreciated by those skilled in the art that the invention may be

embodied in many other forms.

Claims

1. A method of backwashing microporous membranes which have been subjected to

a filtration operation wherein feed containing contaminant matter is applied under

filtrate is withdrawn from a permeate side of the membrane remote the feed receiving

surface, said method comprising the steps of:

(a) terminating the filtration operation by ceasing supply of feed under

pressure to said feed receiving surface of said membrane,

(b) applying a source of fluid under pressure to said permeate side of the

on said membrane,

(c) passing a sweeping fluid past said feed receiving surface of said

and

(d) delivering the backwash liquid to a reservoir,

wherein at least a part of said sweeping fluid of step (c) comprises previously

accumulated backwash liquid from step (d).

2. A method of backwashing microporous membranes as claimed in claim 1 wherein

the backwash liquid used as a sweeping fluid in step (c) comprises supernatant from a

backwash settling lagoon.

3. A method of backwashing microporous membranes in membrane filtration systems

of the kind comprising a plurality of hollow elongate fibres having microporous walls which have been subjected to a filtration operation wherein feed containing contaminant

matter is applied under pressure to the exterior of said hollow fibres and filtrate is

withdrawn from the ends of the lumens of the fibres, the fibres being container within a

shell or housing, said method comprising the steps of:

(a) terminating the filtration operation by ceasing supply of feed under

pressure to said exterior surface of said membrane,

(b) sealing the shell,

(c) applying a source of fluid under pressure to said lumens such that said

contaminant matter lodged within and/or on said fibre walls,

(d) passing a sweeping fluid past said exterior surface of said membrane to

flush out the dislodged contaminant matter to form a backwash liquid, and

(e) delivering the backwash liquid to a reservoir,

wherein at least a part of said sweeping fluid of step (d) comprises previously

accumulated backwash liquid from step (c).

4. A method of backwashing microporous membranes as claimed in claim 3 wherein

the backwash liquid used as a sweeping fluid comprises supernatant from a backwash

settling lagoon.

5. A method of backwashing microporous membranes according to claim 2 or claim

4 wherein the supernatant from the settling lagoon is delivered to the feed receiving

surface of the membranes by direct injection into the feed line.

6. A method of backwashing microporous membranes according to claim 5 wherein

switching of the process to backwash sweeping mode is triggered on a time or volume

flow basis calculated from the point at which the supernatant is injected.

7. A method of backwashing microporous membranes according to claim 5 wherein

the switching of the process to backwash sweeping mode is triggered by monitoring the

change in trans-membrane pressure (TMP) due to the presence or absence of the

supernatant.

8. An apparatus for backwashing microporous membranes which have been

under pressure to a feed receiving surface of the membrane for passage therethrough and

surface, said apparatus comprising:

(a) means to terminate the filtration operation by cutting off supply of feed

under pressure to said feed receiving surface of said membrane;

(b) means to apply a source of fluid under pressure to said permeate side of

and/or on said membrane;

(c) means to deliver a sweeping fluid past said feed receiving surface of said

liquid;

(d) means to deliver the backwash liquid to a reservoir, and (e) means to extract previously accumulated backwash liquid from said

reservoir for use as, or at least inclusion with, said sweeping fluid.

9. An apparatus for backwashing microporous membranes according to claim 8

wherein the backwash liquid used as a sweeping fluid comprises supernatant from a

backwash settling lagoon.

10. A method of operating a filtration system of the kind having a micro-porous

membrane wherein feed containing contaminant matter is applied under pressure to a

feed receiving surface of the membrane for passage therethrough and filtrate is

withdrawn from a permeate side of the membrane, the system further including means to

backwash said membranes by applying a source of fluid under pressure to the permeate

side of the micro-porous filter membrane so as to dislodge at least a portion of the

contaminant matter lodged within and/or on said feed receiving surface, the dislodged

contaminant matter then being flushed out of the system by passing a sweeping fluid

over said feed receiving surface so as to form a backwash liquid, said method of

operation including the step of accumulating and recycling some or all of said backwash

liquid as feed prior to storing the liquid in a settling lagoon or sending it to waste.

11. A method of operating a filtration system according to claim 10 including the step

of accumulating the backwash in a backwash reservoir and, prior to the scheduled

reservoir through the system until it has all been filtered.

12. A method of operating a filtration system according to claim 10 or claim 11

wherein the backwash liquid is recycled a pre-determined number of times prior to

finally being directed in its then more concentrated form to storage or waste.

13. A method of operating a filtration system according to any one of claims 10 to 12

including the step of intermittently backwashing with feed after a pre-determined

number of steps of recycling the backwash liquid.

14. A filtration system of the kind having a micro-porous membrane wherein feed

containing contaminant matter supplied under pressure to a feed receiving surface of the

membrane the passage therethrough and filtrate is withdrawn from a permeate side of the

membrane, the system further including means to backwash said membranes by applying

a source of fluid under pressure to the permeate side of the micro-porous filter

and/or on said feed receiving surface, said system further comprising means to pass a

sweeping fluid over said feed receiving surface to flush the dislodged contaminant

matter out of the system in the form of a backwash liquid, said system including means

to accumulate and recycle as feed some or all of said backwash liquid prior to storing the

liquid in a settling lagoon or sending it to waste.

15. A filtration system according to claim 14 including means for accumulating the

backwash in a backwash reservoir and, prior to the scheduled backwashing step,

switching from feed to recycling the backwash liquid from said reservoir through the

system until it has all been filtered.

16. A filtration system according to claim 14 or claim 15 including means for

permitting the backwash liquid to be recycled a pre-determined number of times prior to

finally be directed in its then more concentrated form to storage or waste.

17. A filtration system according to any one of claims 14 to 16 including means to

facilitate intermittent backwashing with feed after a pre-determined number of steps of

recycling the backwash liquid.

18. A method of operating a filtration system according to any one of claims 10 to 13

incoφorating a method of backwashing microporous membranes in accordance with any

one of claims 1 to 7.

19. A filtration system according to claims 14 to 17 including an apparatus for

backwashing microporous membranes in accordance with claim 8 or claim 9.