The present invention relates to concentration of solids in a suspension
using a hollow fibre membrane and, in particular forms, to methods and apparatus
for periodically cleaning by backwashing the hollow fibre membranes.
Prior art methods of concentrating solids in a liquid suspension are
described in WO-A-86/05116 and WO-A-88/00494.
In that prior art, concentration is effected by a filter element
that comprises a bundle of hollow, porous, polymeric fibres in a closed cartridge
or shell. Polyurethane potting compound is used to hold the respective ends of
the fibres in place within the cartridge without blocking the fibre lumens and
to close off each end of the cartridge. The use of gaseous backwashing to cause
explosive decompression through the walls of the fibres is disclosed in WO-A-93/02779.
The transmembrane pressure differential necessary to effect concentration
of the solids in the prior art is achieved by pressurising the feedstock which
necessitates the use of pumps, other ancillary equipment and, of course, a closed
Backwashing of such prior art concentrators involves increasing the
pressure on both sides of the hollow fibres within the closed shell to a relatively
high value before suddenly releasing that pressure on the shell side of the fibre
walls to effect a sudden pressure differential across the walls which causes a
DISCLOSURE OF INVENTION
It is an object of this invention to provide an improved method of
using a reverse-flow mode to dislodge solids retained by filter elements to ensure
rapid removal of those retained solids and in which the separation and dislodgement
modes may be repeated for prolonged periods of time.
The present invention, in at least some embodiments, provides a method
of backwashing a hollow fibre filter which retains some of the features of the
prior art, but optimizes a number of these features to provide improved performance.
Accordingly, in one broad form of the invention, there is provided
a method of backwashing a plurality of hollow fibres having microporous walls which
have been subjected to a filtration operation wherein a liquid feed containing
contaminant matter is applied 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filtrate is withdrawn from the ends of the lumens of the fibres, the fibres being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 comprising:.
- (a) terminating the filtration operation by ceasing supply of feed to said
exterior surface of said fibres,
- (b) sealing the shell or housing and substantially removing filtrate from said
- (c) applying a source of fluid under pressure to said lumens so as to produce
a negative transmembrane pressure before or at the same time as opening the shell
or housing to atmosphere, and to cause explosive decompression through the walls
of the fibres whereby said fluid under pressure passes through said walls, the
time lapse between the start of an increase in negative transmembrane pressure
(TMP) and such negative transmembrane pressure (TMP) reaching a maximum value
corresponding to the explosive decompression, is in the range of from about 0.05
seconds to about 5 seconds;
- (d) maintaining the pressure level in said lumens at a predetermined value
for a sufficient time following said decompression to cause substantial portions
of contaminant matter lodged within and/or on said fibre walls to be dislodged;
- (e) recommencing the filtration operation by introducing said supply of feed
to said exterior surface of said fibres on the shell side of the filter while fluid
pressure is still being applied to said lumens, the flow of feed serving to wash
dislodged contaminant matter away by the flow of liquid over said external surface
of said fibre walls and to rewet said fibres, the period during which feed is
supplied to said exterior of said fibres while fluid pressure is still being applied
to said lumens being between about 1 to about 30 seconds.
In the method of the invention feed liquid is pumped into the shell
side of the filter while fluid pressure is still being applied to said lumens.
This results in liquid/fluid turbulence or frothing around the membrane pores
causing further improved dislodgement of retained solids. The fluid pressure during
this phase preferably should exceed the shell side pressure by about 10kPa to
about 800 kPa.
Preferably, the steps of the method are carried out as a continuous
process utilizing repetitive cycles of solids retention and backwash.
As an alternative preferred from, step (b) is effected by allowing
said remaining filtrate to drain out of said lumens.
When fluid pressure is applied to remove the filtrate from the lumens,
this pressure is typically in the range of about 10 to about 600 kPa. The fluid
pressure applied to the lumens prior to the decompression is typically in the
range of about 100 to about 1200 kPa.
The penetration of gas into the pores of a membrane is resisted by
the surface tension forces of the contained wall-wetting liquid according to well
known theory. Indeed, surface tension is conveniently measured by the breakthrough
pressure needed to force a bubble out of a submerged orifice. For common systems
(such as oil in hydrophobic pores or water in hydrophilic pores) the breakthrough
pressures are much higher than the usual operating pressures of the filter.
Prior art hollow-fibre type ultrafilters are usually fed from the
inside of the fibres for many well known reasons. However, according to the present
invention, feed stock is applied to the outside of the fibres and gas is introduced
into the lumen of the fibre as the back-wash medium. In some cases, the lumen pressure
swells a suitably designed fibre so that the pores are enlarged whereby the particles
are freed and swept away in the expansion of the back-wash gas.
In some cases, especially where very fine-pored interstitial material
is deposited in relatively coarse-pored base fibre, it is advantageous to back-wash
first with a small amount of permeate already in the membrane lumen and follow
with the high pressure gas back-wash. In this way, the small amount of permeate
adequately washes out fine blocking material from within the interstices, and the
overall cleaning is completed by the higher pressure gas swelling the base pores
and erupting around elastic openings. The pores must close again rapidly to reseal
the holes and the base material must not crack by work hardening and must remain
within its modified elastic limit.
Preferably, the fibres are made from thermoplastic polymers such as:
- poly(propylene), poly(4-methylylpent-1-ene), co-polymers of polypropylene,
poly(vinylidenedifluoride), poly(sulphones), poly(phenylene sulphides), poly(phenylene
oxides), phenoxy resins, polyethylene, poly(tetrafluoroethylene) and poly(chlorotrifluoroethylene).
The use of gas as a back-wash medium enables the removal of fouling
species by explosive decompression of the gas through the membrane structure for
the minor part and at the outer membrane surface for the major part. Thus, the
gaseous back-wash step is carried out at a pressure which is sufficient to overcome
the effect of the surface tension of the continuous phase of the feedstock within
the pores of the membrane.
Hitherto, it was felt the gas backwashing phase should be limited
to below 5 seconds to avoid drying out of the fibres and thus difficulty in recommencing
filtration due to gas bubble retention in the fibre pores. The introduction of
improved rewetting techniques has overcome this problem and it has been discovered
that extending the gas backwash phase beyond 5 seconds has significant advantages.
Time periods of up to 60 seconds have been found to be effective. A longer backwash
provides improved removal of trapped solids. Because liquid is reintroduced to
the shell prior to completion of the gas backwash, it enables the overlap where
gas and liquid are both present to be extended. An overlap time of about 1 to about
30 seconds is used. This is desirable in large arrays where it may take considerable
time, with normal pump pressures, to refill the shells with liquid. The extended
time period enables normal pumps to be used to achieve the above overlap while
it also avoids maldistribution of pressure within large filter arrays by allowing
relatively slow refilling of the filter shells.
In another form of the invention, the high pressure fluid application
to the lumens may be pulsed to provide a number of explosive decompressions within
the backwashing phase. These individual pulses are preferably between about 0.1
seconds and about 5 seconds in duration. This provides an advantage of reducing
gas consumption in the backwash phase. The pulsing may be achieved by sealing and
opening the shell at appropriate time intervals sufficient to allow pressure within
the lumens to build up to a required level. Alternatively, the pressure supply
may be pulsed to achieve the same effect. In a further embodiment, the pressure
may be varied between a high and low level without actual total shut off of pressure.
BRIEF DESCRTPTION OF DRAWINGS
Preferred embodiments of the present invention will now be described,
by way of example only, with reference to the following examples and accompanying
drawings, in which:
MODES FOR CARRYING OUT THE INVENTION
- Figure 1 shows a schematic representation of a hollow fibre cross-flow concentrator
to which the present invention is applicable in an operating mode;
- Figure 2 shows the concentrator of Figure 1 in backwash mode;
- Figure 3 shows a graph of transmembrane pressure (TMP) versus time for a standard
- Figure 4 shows a similar graph to Figure 3 using a backwash using a higher than
usual air consumption ;
- Figure 5 shows a normalized flow/TMP versus time graph for a standard backwash;
- Figure 6 shows a normalized flow/TMP versus time graph for the same type of
machine as Figure 5 but using the backwash according to Figure 4;
- Figure 7 shows a graph of TMP versus time for a backwash where feed liquid is
pumped into the filter while the gas backwash is still applied;
- Figure 8 shows a normalized flow/TMP versus time graph for a standard backwash;
- Figure 9 shows a normalized flow/TMP versus time graph for the same type of
machine as Figure 8 but introducing feed liquid during the backwash cycle;
- Figure 10 shows a normalized flow/TMP versus time graph for a standard backwash
at a further installation; and
- Figure 11 shows a normalized flow/TMP versus time graph for the same type of
machine as Figure 10 but introducing feed liquid during the backwash cycle.
The hollow fibre cross-flow concentrator 10 shown in Figs. 1 and 2
includes a cartridge shell 11 within which is positioned a bundle of hollow, porous,
polymeric fibres 12. In this instance, each fibre is made of polypropylene, has
an average pore size of 0.2µm, an internal lumen diameter in the range 250µm to
310µm and a fibre diameter in the range 500µm to 650µm. There may be between 2,800
to 30,000 hollow fibres in the bundle 12 but this number as well as the individual
fibre dimensions may be varied according to operational requirements.
Polyurethane potting compound 13,14 holds the ends of the fibres 12
in place without blocking their lumens and closes off each end of the shell 11.
The liquid feed suspension to be concentrated is pumped into the shell 11 through
feed suspension inlet 15 and passes over the external walls of the hollow fibres
12. Some of the feed suspension passes through the walls of the fibres 12 into
the lumens of the fibres to be drawn off through the lumen outlet port 16 as clarified
The remaining feed suspension and some of the rejected species flows
between the fibres 12 and leaves the shell 11 through outlet 17. The remainder
of the rejected species is held onto or within the fibres or is otherwise retained
within the shell. Lumen inlet port 18 remains closed during the operating mode
of the concentrator shown in Fig. 1.
In order to remove the retained species, lumen outlet port 16 is closed
so that the flow of clarified liquid is stopped. The clarified liquid is then removed
from the lumens by natural drainage or by introducing a pressurized gas through
lumen inlet port 18 to force the liquid from the lumens. Upon completion of the
removal of the filtrate liquid, high pressure compressed gas is introduced through
inlet 18 and the lumens of the fibres 12. The liquid-filled shell is sealed and
gas cannot penetrate the porous walls even though the gas pressure is now raised
well above the normal bubble point of the fibre walls because the liquid within
the shell is relatively incompressible. A reservoir of high pressure gas is thus
accumulated in the fibre lumens.
The shell outlet 17 is then opened which allows gas to penetrate the
pores along the whole length of each fibre. This results in an explosive decompression
of the pressurized gas through the walls of the fibres resulting in the retained
solids in the fibre walls being dislodged from the fibres into the feed side of
the filter. The initial breakthrough of gas through the fibre wall results in a
tendency for pressure to drop in the lumens. It is desirable if this pressure can
be maintained for a short period following decompression to cause increased flow
through the fibre wall and greater removal of retained solids. This is preferably
achieved by providing a large diameter pressure feed to the lumens and/or a higher
pressure to compensate for pressure drop. In some cases, it is desirable to admit
gas through both lumen ports 16 and 18 after carrying out the above described pressurised,
trapped gas operation.
In alternate embodiments, the shell is opened just before or at the
same time as the pressurized gas is applied to the lumens.
Referring to the accompanying graphs, a number of examples will now
be described to illustrate the improved performances provided by embodiments of
An M10C (250µm lumen) filter unit was run using a larger airline to
provide an increased and prolonged pressure to the lumens following the explosive
decompression phase. A 2.5cm airline was used instead of a standard 10mm airline.
There was no pressurize stage used during this improved backwash and the negative
transmembrane pressure (TMP) obtained on the filter unit was 620kPa compared with
380kPa for a standard backwash. The air consumption was higher than that for a
standard backwash. The pressure profiles of the two different backwashes are shown
in Figures 3 and 4.
During the standard backwash shown in Figure 3, it can be seen that
a time of 0.65 seconds elapses between the start of the explosive decompression
phase and the point at which maximum TMP is obtained. Analysis of the similar section
of the improved backwash shows the time to reach maximum negative TMP was only
0.15 seconds. The reaching of maximum TMP corresponds with the air breaking through
the walls of the fibre and expelling the fluid within the wall pores. The period
between the opening of the shell and the breakthrough is a liquid backwash phase
as the liquid within the pores is being moved outwardly from the lumen toward the
shell side. When the air breaks through the fibre wall the liquid backwash phase
is completed. Preferably this period is within the range 0.05 seconds to 5 seconds.
The results of consecutive runs on the test filter unit comparing
the standard and the improved backwash (termed a "mega" backwash herein) procedures
are shown in attached TABLE 1.
As can be seen from TABLE 1 and the performance graphs (Figures 5
and 6), the TMP rise is significantly reduced when the 'mega' backwash is used.
The TMP rise per day for the 'mega' backwash was approximately one quarter of the
TMP rise seen with the standard backwash. This result means that machines could
be run for longer between cleaning cycles, or the machines could give a higher
throughput for the same cleaning interval.
This example relates to the procedure where feed liquid is reintroduced
to shell while the gas backwash is still proceeding. A trial was carried out on
surface water to compare a standard backwash with a backwash stage using pressurized
gas plus feed liquid. This stage is typically referred to as an "air on pump on"
stage (AOPO stage).
Two identical 1M10C filter units were set up to run side by side on
river water. One machine used a standard pressurize backwash cycle, whilst the
other incorporated an extra stage. The extra stage consisted of switching on the
feed pump whilst still applying high pressure air through the hollow fibre walls.
The resultant two phase flow across the fibre bundle appeared to be very effective
in removing fouling from the membrane module.
The two filter units were running at a constant flow of 200h/hr/m2
[200 L/hr/em], using a pump with a variable speed drive to keep the set flow. The
area unit em is related to the surface area of an original Memtec filter module.
Figures 8 and 10 illustrate the results of two consecutive runs of a filter unit
and show that the TMP, when the standard backwash was used, rose to 400kPa within
4 days of operation. At this point the unit could no longer maintain the set flow
of 200 L/hr/em. Figures 9 and 11 show that when the AOPO stage was used the TMP
remained below 150kPa for 7 days. The result of this is that the filter units could
maintain a higher flow rate for a longer period of time when the AOPO stage is
used in the backwash. This is important to filter unit efficiency as the units
require chemical cleaning when the TMP reaches a predetermined value.
Typically during the backwash the decompression stage consists of
the lumens being pressurized to 600kPa, then the shell side valves being released
whilst still supplying air to the lumens (for typically 1 to 3 seconds on most
applications). The AOPO stage would extend the amount of time air is resupplied
to the fibre lumens by typically an extra I to 30 seconds on M10 units.
It will be appreciated that further embodiments and exemplifications
of the invention are possible without departing from the spirit or scope of the
*Instantaneous flow is the average of instantaneous flowrates measured.
M10C (250 µm lumens) comparison of 'mega' and standard backwashes
PP M10C (20,000 fibres)
PP M10C (20,000 fibres)
Feed Turbidity (NTU)
Feed Temperature (°C)
TMP Range (kPa)
82 to 108
86 to 91
Instantaneous Flow* (L/hr/module)
Instantaneous Flow* (L/hr/module) at 20°C