MEMBRANE FILTER CLEANING METHOD AND INSTALLATION FOR
IMPLEMENTING SAME
The invention relates to a method for cleaning
membrane filters comprising hollow fibers with an inner
skin, mounted inside a housing, with delimitation of a
concentrate compartment where the materials retained both
in suspension and on the membranes accumulate, and a
permeate compartment collecting the filtered liquid.
The filtration of liquids, particularly water,
generates a cake composed of particles and fractions of
solutes retained by the membrane. This filter cake carcause
clogging of the membrane, resulting in an increase in
the transmembrane pressure at constant permeate flow rate,
or a drop in permeate flow rate at constant transmembrane
pressure.
Methods for reducing clogging are employed to
remove this filter cake. A backpermeation is generally
performed, with the introduction of permeate in the
opposite direction from the filtration direction applied in
production, the filter cake thereby being entrained by the
one-phase liquid flow. However, the efficiency of these
methods is limited in time, and this can cause the
formation of a filter cake that withstands these detachment
methods. This situation leads to the implementation of
other methods, called chemical regeneration methods,
involving the use of costly and polluting chemical reagents
and which require time for carrying out these washings
(extra operating cost). The production downtimes thereby
caused and the water losses generated require oversizing of
the installation to guarantee the nominal production
throughput, hence an additional investment cost.
Many proposals have been made to improve the
methods for cleaning or for hydraulic unclogging of
membrane filters.
Japanese patent application JP 100089842, filed
25 February 1994, describes a simple backwashing method
comprising steps consisting in:
- draining the concentrate compartment to remove
the liquid to be filtered contained therein and the
suspended matter, then
- performing a backwashing step by passing liquid
from the permeate compartment to the concentrate
compartment through the membranes to detach and remove the
impurities deposited thereon, while circulating a gas in
the concentrate.
According to this document, an air injection is
provided on the concentrate side, supplemented by a water
injection in the opposite direction from the filtration
direction, the air being introduced through one end of the
membrane, and the filter cake removed at the opposite side.
This method serves to improve the unclogging of
ultrafiltration membranes. However, its efficiency needs
to be improved and maintained over time.
FR-A-2 668 078, also, teaches a method using
filtered water, possibly augmented with chlorine, sent in
the opposite direction from the filtration direction with
alternating phases in order to improve the detachment and
removal of the materials forming the filter cake. Here,
also, the efficiency of the method needs to be improved.
It is a primary object of the invention to provide
a method for cleaning membrane filters which serves to
reduce the duration of the unclogging procedure and which
provides an increased and durable gain in permeability,
while remaining simple and economical to implement.
According to the invention, a method for cleaning
membrane filters comprising hollow fibers with an inner
skin, mounted inside a housing for filtering a liquid, with
delimitation of a concentrate compartment where the
materials retained both in suspension and on the membranes
accumulate, and a permeate compartment collecting the
filtered liquid, comprises the steps consisting in:
a) draining the concentrate compartment to remove
the liquid to be filtered contained therein and the
suspended matter, then
b'f performing a backwashing by passing liquid from
the permeate compartment to the concentrate compartment
through the membranes to detach and remove the impurities
deposited thereon, while circulating a gas in the
concentrate,
and is characterized in that backwashing gas and/or
liquid pulses are produced during at least one backwashing
phase.
The number of pulses during backwashing may be
between 1 and 10. The pulse duration may be between 2 ana
60 seconds, and, similarly, the interval between two pulses
can itself last between 2 and 60 seconds.
Preferably, the washing liquid is injected in
pulses from the permeate compartment into the concentrate
compartment while the circulation of gas, particularly air,
is maintained in the concentrate compartment. As a
variant, the gas may be injected in pulses into the
concentrate compartment while the circulation of liquid is
maintained in the concentrate.
During the backwashing step, the rate of passage of
the liquid in the hollow fibers may be between 0.1 ana
1 m3/m2.s (generally expressed in the form of speed in
m/s), while the rate of passage of the gas may be between
and 4 SmJ/m2.s (4 m/s! .
The concentrate compartment drainage step may
comprise the use of a gas stream to accelerate and improve
the drainaae
Generally, the liquid to be filtered (production
phase) is water, the liquid used for the backwashing step
is filtered water, and the gas circulated is air.
The filtered liquid injected during the backwashing
step into the concentrate compartment through the membranes
may previously be augmented with one of the following
products: disinfectant, oxidizing agent (for example
hypochlorite, chlorine dioxide, peroxides, etc.), acidic or
basic chemical compound.
Preferably, the backwashing step comprises at least
one two-phase cycle, that is, one phase with two fluids,
liquid and gas, one of which is pulsed, and another phase
with the fluid that is not pulsed. The number of two-phase
cycles during a single backwashing step is between 1 and
10.
The invention further relates to an installation
for implementing the method defined above; this
installation comprises membrane filters in the form of
hollow fibers with an inner skin mounted inside a housing,
with delimitation of a concentrate compartment and a
permeate compartment, a feed pump and a feed valve for the
concentrate compartment, a drain valve, a backwashing pump
and a backwashing valve, a gas compressor and a valve
connected to the concentrate compartment, characterized in
that it comprises a control means for pulsing the flow of
backwashing liquid and/or gas which can circulate in the
concentrate compartment.
The pulse control means may comprise a solenoid
valve and a circuit for supplying said solenoid valve with
electrical pulses.
Apart from the arrangements described above, the
invention comprises a number of other arrangements more
explicitly described below, with reference to an exemplary
embodiment described with reference to the drawings
appended hereto, but which is non-limiting.
In these drawings:
Fig. 1 is a flowchart of a filtration installation
implementing the method of the invention.
Fig. 2 is a partial operating diagram in filtration
mode.
Figs. 3 to 6 show partial diagrams illustrating
four successive backwashing phases with water with pulsed
air.
Figs. 7 to 10 show partial diagrams illustrating
four successive backwashing phases with air with pulsed
water.
Fig. 11 is an operating diagram of a backwashing
with three pulses; time is plotted on the x-axis and the
ratio of the flow rate/maximum flow rate of the sequence is
plotted on the y-axis.
Fig. 12 is a comparative diagram of the releases of
the various backwashing modes with time plotted on the
x-axis; on the y-axis are plotted the backwashing water
stream as a solid line, and the suspended matter
concentration, expressed in mg/1, as dotted and dashed
curves, and
Fig. 13 is a comparative diagram of the various
backwashing modes with the filtration time expressed in
weeks plotted on the x-axis, and the filtration
permeability expressed as l/h.m2.bar on the y-axis.
Reference to the drawings, particularly to Fig. 1,
shows an installation for implementing a method according
to the invention for unclogging filtration,
ultrafiltration, microfiltration, nanofiltration or
hyperfiltration membranes. The set of M membranes, shown
schematically in Fig. 1, has a tubular geometry, and is
placed in a housing C containing a set of hollow fibers
with an inner skin. The housing C is equipped with two
orifices El, E2, respectively bottom and top, which can
serve as outlets and/or inlets. The orifices El, E2 are
connected to the concentrate compartment formed by the
inner space of the hollow fibers. Inside the housing, the
space around the membranes, and between them, forms the
permeate compartment which comprises an outlet A at
mid-height of the module. As a variant, the outlet A may
be axial with respect to the module diameter.
The membranes M are used for filtering liquids,
typically water, and for retaining particles or solutes
with molecular weights higher than the cutoff threshold of
the membranes concerned.
The installation comprises a feed pump 1 for
pumping the liquid to be filtered, with its discharge
connected, via a feed valve 2, to the orifice El. A
drainage branch between the valve 2 and the orifice El is
provided with a drain valve 3. The line located downstream
of the valve 3 terminates in a waste removal device 13.
A top backwashing discharge valve 4 is connected to
the orifice E2 of the housing C. A line downstream of the
valve 4 terminates in the device 13.
A valve 5 is mounted on a line connecting a gas
compressor 6, particularly an air compressor, to the
orifice E2. A backwashing valve 7 is placed on a line
connecting the outlet of a backwashing pump 8 to the
orifice A. The suction side of the pump 8 is connected to
a filtered liquid tank 9. A pump 10 has its suction side
connected to a tank 11 containing an additive, for example
a disinfectant, oxidizing agent (for example hypochlorite,
dioxide, etc.) solution, or an acidic or basic chemical
compound. The delivery side of the pump 10 is connected to
part of the line located between the valve 7 and the inlet
A.
A production valve or backwashing recirculating
valve 12 is placed on a line located between the valve 7
and the orifice A. The valve 12 is connected downstream to
a line which terminates in the filtered water tank 9. An
overflow 15 is provided to remove the treated water to the
application.
The various valves of the installation are solenoid
valves, most of the control circuits of which have not been
shown for the sake of simplification.
The solenoid valve 5 of the gas compressor and/or
the backwashing solenoid valve 7 are associated with an
electrical pulse control means 5a, 7a for implementing a
predefined sequence of valve openings and closings. The
pulses can be implemented by means of the valves 5 and 7,
associated with distribution devices such as:
- electronic starters or variable-speed units on
the pumps 6 and 8, or
- recycling circuits, with, for example, syncopated
opening of the valve 12.
The operation of the installation according to the
method of the invention is now described.
In filtration mode, illustrated by Fig. 2, the pump
1 is in action and the valves 2 and 12 are open while all
the other valves are closed. The liquid to be processed
enters via the orifice El and the filtered liquid
(permeate) leaves via the orifice A in the direction of the
tank 9.
The cleaning of the membrane M by backwashing can
be performed with water, with pulsed air according to the
steps in Figs. 3 to 6.
In general, in the diagrams, the fluid flow lines
are represented by a thicker line, with an arrow indicating
the flow direction.
The diagram in Fig. 3 corresponds to a concentrate
gravity drainage phase. The pump 1 (Fig. 1) is stopped,
the valve 2 is closed, while the valve 3 is opened, the
other valves are closed, and the pumps 8 and 10 are
stopped.
The drainage phase in Fig. 3 lasts between 5 and 60
seconds.
Optionally, this gravity drainage can be assisted
by gas injection, with opening of the valve 5 and inlet of
the gas via the orifice E2.
The subsequent phases in Figs. 4 and 5, the
succession in time of which can be reversed (sequence 3-4-5
or 3-5-4), constitute a cycle which can be repeated several
times.
The phase illustrated by Fig. 4 corresponds to an
injection of backwashing filtered water via the orifice A,
the pump 8 being activated and the valve 7 being open.
Drainage takes place via the orifice El and the open valve
3.
The phase illustrated by Fig. 5 corresponds to an
injection of backwashing filtered water with, according to
the invention, pulsed air. The backwashing water is again
injected via the orifice A. Moreover, the solenoid valve 5
is successively opened and closed by a series of pulses
corresponding to the pulses 15 in Fig. 11 and 16 in Fig.
12, trapezoidal in this example (but which may also be
square, triangular or sinusoidal).
Surprisingly, the succession of transitory phases
created by the air pulses significantly increases the
removal of the filter cake in comparison to a constant
flow.
The water flow rate during the backwashing step is
typically between 100 and 850 1/h.m2 (liters per hour and
per m2 of membrane area). The preferred values are between
250 and 400 1/h.m2.
The air speed in the concentrate compartment is
typically 0 to 4 Sm3/m2.s. The preferred values are
between 0 and 1 Sm3/m2.s (zero speed corresponding to the
one-phase backwashing periods).
The duration of the water + air phase is typically
between 2 and 60 seconds. The preferred values are between
5 and 30 seconds. The duration of the "water only" phase
is typically between 2 to 60 seconds, the preferred values
also being between 5 and 30 seconds.
The next phase, illustrated in Fig. 6, corresponds
to the end of the backwashing step with injection of
filtered water via the orifice A, flooding of the housing
C; all the valves are closed with the exception of the
valve 7 and the valve 4 used to drain the housing C.
The diagrams in Figs. 7 to 10 illustrate an
operating variant corresponding to a backwashing with air
with water pulse.
The drainage phase illustrated by Fig. 7 is
identical to that of Fig. 3.
The next phases, in Figs. 8 and 9, constitute the
backwashing cycle AA + BB (as defined in Fig. 11), which
can be repeated several times.
The phase illustrated by Fig. 8 corresponds to a
backwashing with air only. The valve 5 (Fig. 1) is open
for the inlet of air via E2 and the valve 3 is also open
for the removal of the filter cake. The other valves are
closed, particularly the valve 7.
According to the phase illustrated by Fig. 9, the
air circulation continues as shown in Fig. 8 but, in
addition, filtered water is injected via the orifice A with
pulses produced by the successive openings and closings of
the solenoid valve 7 (Fig. 1), the pump 8 being activated.
The washing water pulses correspond to the pulses 15 and 16
in Figs. 11 and 12.
The final phase, illustrated by Fig. 10, is
identical to that in Fig. 6, and corresponds to a filtered
water injection with water flooding and drainage.
During a cleaning operation, the cycles
corresponding to the phases in Figs. 4 and 5 or Figs. 8 and
9 can be repeated several times. The number of cycles may
vary between 1 and 10. The number of cycles is preferably
between 2 and 7.
Fig. 11 is a diagram illustrating the operation.
Time T is plotted on the x-axis, and the ratio (expressed
as a percentage) of the flow rate of fluid concerned to the
maximum flow rate of this fluid during the sequence is
plotted on the y-axis.
The zones marked "AA" correspond to pulses, while
the intervals are marked "BB". The pulses are two-phase
with simultaneous injection of gas and water, while the
injections are one-phase, either gas or water. The
trapezoidal shape of the pulses in Fig. 11 is merely
indicative and could equally well be square, triangular or
sinusoidal. In this Fig. 11, the dotted outline represents
the continuous injection of one of the washing fluids,
generally water, and the "sawtooth" 15 correspond to the
introduction of the second fluid, generally gas. Hence
there are clearly periods during which the second fluid is
stopped, hence at zero flow rate, thereby justifying the
speed range of 0 to 4 m/s.
In the example corresponding to Fig. 11, three
pulses are provided for one cycle.
Fig. 12 is a comparative diagram of the releases of
the various backwashing modes. Time T is plotted on the xaxis.
The concentrations of suspended matter in the
releases expressed in mg/1 are plotted on the y-axis. The
dashed curve 17 corresponds to a backwashing with air and
water, without pulsings, for a constant backwashing water
flow rate Q corresponding to the apices of the pulses 16.
The dotted curve 18 corresponds to the concentrations of
suspended matter in the releases during an air backwashing
with pulsed water, according to the pulses 16.
These pulses 16 represent the backwashing water
flow plotted on the y-axis and expressed in 1/h.m2 (liters
per hour and per m2) as a function of time T plotted on the
x-axis. The curve 17 shows that the effective backwashing
phase with air and water without pulsing is limited in time
to a very short period. Analysis of the phenomenon leads
to the conclusion that the mixture of air and water very
quickly tends toward a steady-state condition of the ring
flow type.
The pulses 16 according to the present invention
favor and multiply the two-phase transition periods by
varying the flow rate of water or air injected, and by
repeating this sequence several times.
The curve 18 of releases according to the invention
consists of three peaks corresponding to the three injected
water pulses 16.
However, the backwashing without pulsing (curve 17)
comprises only one peak substantially corresponding to the
first peak of the curve 18. The concentration of suspended
matter in the releases then decreases constantly.
It thereby appears that the solution according to
the invention permits a much more effective unclogging.
Fig. 13 is a comparative diagram of various
backwashing modes. The filtration time D expressed in
weeks is plotted on the x-axis while the filtration
permeability E of the membrane expressed in l/h.m2.bar
(liters per hour, per m2 of membrane and per bar) is
plotted on the y-axis. The more clogged the membrane, the
lower the filtration permeability.
The curve 19 corresponds to the case of
water/pulsed-air backwashing, according to the invention.
The dashed curve 20 corresponds to the water/air
backwashing without pulsings. The dotted curve 21
corresponds to backwashing with water only.
The curve 19 clearly shows that, according to the
invention, the filtration permeability of the membrane is
maintained over time at a substantially constant level,
thanks to the efficient washing and unclogging, markedly
superior to that of the curves 20 and 21, which are
decreasing with a steeper slope for the curve 21.
When the filtration permeability has dropped to the
value 100 l/h.m2.bar, a chemical regeneration of the
membrane is necessary, corresponding to the zone 22.
By combining liquid pulses with a continuous gas
flow or gas pulses with continuous liquid flow during
membrane filter unclogging backwashings, the invention
thereby serves surprisingly and significantly to increase
the removal of the filter cake as compared to a constant
flow.
The interval between chemical regenerations is
thereby significantly increased. By way of non-limiting
example, for similar conditions, the time interval between
two chemical regenerations is multiplied by 5 thanks to the
invention, as compared to a conventional method.
The gain in permeability is increased, thereby
decreasing the frequency of application of this type of
unclogging.
The quantity of water employed is significantly
decreased, thereby improving the productivity of the
system.
Clogging is reduced and permits a decrease in the
frequency of chemical regenerations.

CLAIMS
1. A method for cleaning membrane filters
comprising hollow fibers with an inner skin, mounted inside
a housing for filtering a liquid, with delimitation of a
concentrate compartment where the materials retained both
in suspension and on the membranes accumulate, and a
permeate compartment collecting the filtered liquid,
comprising the steps consisting in:
a) draining the concentrate compartment to remove
the liquid to be filtered contained therein and the
suspended matter, then
b) performing a backwashing by passing liquid from
the permeate compartment to the concentrate compartment
through the membranes to detach and remove the impurities
deposited thereon, while circulating a gas in the
concentrate compartment,
characterized in that backwashing gas and/or liquid
pulses (15, 16) are produced during at least one
backwashing phase.
2. The method as claimed in claim 1, characterized
in that the number of pulses during a phase is between 1
and 10.
3. The method as claimed in claim 1 or 2,
characterized in that the pulse duration is between 2 and
60 seconds.
4. The method as claimed in one of the preceding
claims, characterized in that the washing liquid is
injected in pulses into the concentrate compartment while
the circulation of gas, particularly air, is maintained in
the concentrate compartment.
5. The method as claimed in one of claims 1 to 3,
characterized in that the gas is injected in pulses into
the concentrate compartment while the circulation of liquid
is maintained in the concentrate compartment.
6. The method as claimed in one of the preceding
claims, characterized in that, during the backwashing step,
the rate of passage of the liquid in the membranes is
between 0.1 and 1 m3/m2.s.
7. The method as claimed in one of the preceding
claims, characterized in that, during the backwashing step,
the rate of passage of the gas is between 0 and 4 Sm3/m2.s.
8. The method as claimed in one of the preceding
claims, characterized in that the concentrate compartment
drainage step is terminated by using a gas stream to
accelerate and improve the drainage.
9. The method as claimed in one of the preceding
claims, characterized in that the liquid to be filtered is
water, the liquid used for the backwashing step is filtered
water, and the gas circulated is air.
10. The method as claimed in one of the preceding
claims, characterized in that the filtered liquid injected
during the backwashing step into the concentrate
compartment through the membranes is previously augmented
with one of the following products: oxidizing agents such
as chlorine compounds, peroxides, acidic or basic chemical
compounds.
11. The method as claimed in one of the preceding
claims, characterized in that the backwashing step
comprises at least one two-phase cycle, that is, one phase
with two fluids, liquid and gas, one of which is pulsed,
and another phase with the fluid that is not pulsed.
12. The method as claimed in claim 11,
characterized in that the number of two-phase cycles during
a single backwashing step is between 1 and 10.
13. An installation for implementing a method as
claimed in one of the preceding claims, comprising membrane
filters in the form of hollow fibers with an inner skin
mounted inside a housing, with delimitation of a
concentrate compartment and a permeate compartment, a feed
pump and a feed valve for the concentrate compartment, a
drain valve, a backwashing pump and a backwashing valve,
characterized in that it comprises a gas compressor (6) and
a valve (5) connected to the concentrate compartment, and a
control means (5, 5a; 7, 7a) for pulsing the flow of
backwashing liquid and/or gas which can circulate in the
concentrate compartment.
14. The installation as claimed in claim 13,
characterized in that the pulse control means comprises a
solenoid valve (5, 7) and a circuit (5a, 7a) for supplying
said solenoid valve with electrical pulses.