METHOD FOR WASHING A FILTER HAVING AT LEAST TWO LAYERS
OF FILTERING MEDIA, IN PARTICULAR FOR FILTERING
SEAWATER, AND EQUIPMENT FOR IMPLEMENTING SAID METHOD
5
The invention relates to a method for washing a filter
having at least two layers of filter media for a raw
water containing components that have a tendency to
form cakes or "mud balls" which generally agglomerate
10 at the surface of the filter, notably for seawater. The
layers of filter median are supported by a floor
comprising passage orifices and provided with nozzles
that allow an injection of washing water and/or air,
the raw water flowing from top to bottom through the
15 layers, and the treated water being collected at the
bottom part of the filter, which at the top part
comprises at least one weir for collecting the washing
water.
20 The layers of filter media often comprise at least one
layer of sand and one layer of anthracite or some other
medium such as pumice stone.
When the raw water treated, particularly seawater,
25 contains derived products such as alginates, or natural
polymers, treating the water with coagulant, such as
ferric chloride (FeC13) gives rise to sort of cakes or
"mud balls" caused by the agglomeration of particles of
media which is probably brought about by various
30 natural flocculants contained in the water, notably the
alginates. These cakes may reach dimensions of around
2 cm x 4 cm with a thickness of around 0.5 cm.
'The formation of these cakes disrupts the operation of
35 the filter and entails relatively long rinsing or
washing operations, it being possible for rinsing to
take up to several hours and in some cases it can even
become impossible to remove these cakes. This results
in an appreciable reduction in the filtered water
production output. Furthermore, conventional washing
operations generally give rise to a loss of media as
the media is discharged with the washing water.
5 It is a key objective of the invention to provide a
washing method for filters of the type defined
hereinabove that. makes it possible to reduce the
duration of the washing operation while at the same
time preserving the quantity of media and the way in
10 which such media are arranged through the filter.
The invention is as applicable to filters that operate
under pressure as is it to atmospheric filters.
15 According to the invention, the method for washing a
filter of the kind defined hereinabove is characterized
in that the following successive steps are carried out:
a/ a preparation step in which the raw water inlet
is closed and the filter, where the latter operates
20 under pressure, is depressurized;
b/ the water level in the filter is lowered;
C/ air alone is blown under pressure into the
bottom part of the filter, this air passing from bottom
to top through the layers of the filter and causing the
25 cakes to be reduced to particles, particularly by
friction;
d/ rinsing is carried out using water alone, from
bottom to top, allowing the media to separate and
causing the particles to rise to the surface, the
30 water-rinsing time being short enough that the water
level stops rising below the level of the upper edge of
the washing water collection weir;
e/ the water is emptied out of the filter again by
a discharge at the bottom part;
35 f/ steps c / , d/ and e/ are repeated at least once,
and a final rinsing is performed by passing
through a quantity of backwash water which is injected
at the bottom part of the filter and discharged from
the top part of the filter by a pipe.
For preference, steps c/, d/ and e/ are repeated at
5 least twice.
The blowing-in of air alone is also referred to as the
bubbling step. Advantageously, the duration for which
air alone is blown in is less than 10 min.
10
The duration of the rinsing with water alone may be
less than 60 s, in particular around 45 s.
Prior to each rinsing with water alone, the cushion of
15 air under the floor is not discharged. It is only
before the last rinse referred to as the final rinse
that the cushion of air is discharged.
Advantageously, the level of the water at the end of
20 rinsing with water alone lies at least 5 cm below the
level of the upper edge of the washing water collection
weir, preferably between 5 cm and 10 cm below said
level.
25 Having repeated the draining, bubbling and rinsing
sequence a number of times, a final rinse is carried
out after the air from the cushion situated under the
floor has been discharged, this final rinsing
corresponding to 5 to 10 covolumes and the dirty
30 washing water is discharged to a sludge treatment
facility.
The washing method is as applicable to a filter of
horizontal axis as it is to a filter of vertical axis.
35
The invention also relates to a filtration
installation, particularly for seawater, implementing
the method defined hereinabove, and comprising a filter
having at least two layers of filter media which are
supported by a floor equipped with nozzles, a pipeline
equipped with a valve for admitting air underneath the
floor and a pipeline equipped with a valve for
5 admitting backwash water underneath the floor, a
pipeline equipped with a valve for discharging the
filtered water underneath the floor, a branch to a
discharge, which branch is equipped with a valve used
to drain the filter down to a level LSL (or LSLL) and
10 an air extraction pipeline situated at a high point of
the filter and equipped with a valve, a raw water inlet
pipeline equipped with a valve, at the top part of the
filter, above the layer of media, and a pipeline for
discharging the washing water collected by at least one
15 weir at the top part of the filter and equipped with a
valve, this installation comprising a valve control
unit for performing the steps of the method defined
hereinabove.
20 Apart from the provisions set out hereinabove, the
invention consists in a certain number of other
provisions that will be covered more explicitly
hereinafter with reference to one exemplary but
entirely nonlimiting embodiment described with
25 reference to the attached drawings. In these drawings:
Figure 1 is a schematic vertical section through a
horizontal cylindrical filter, for implementing the
method of the invention, during the step of injecting
30 under pressure the flocculated raw water that is to be
treated.
Figure 2 shows, in a similar way to figure 1, a
bubbling step with air alone being blown in under
35 pressure at the bottom part of the filter, and
Figure 3 shows, in a similar way to figure 1, a step of
rinsing for a brief period.
With reference to the drawings, notably to figure 1,
there may be seen a pressurized filter F for seawater
or, more generally, for raw water containing components
5 that have a tendency to form cakes or "mud balls" that
agglomerate generally at the surface of the filter.
According to the example depicted in the drawings, the
filter F is a horizontal filter comprising a
10 cylindrical jacket 1, of horizontal axis, into which
jacket the raw water, having undergone a flocculation
treatment, is introduced under pressure via a pipeline
2 equipped with a valve 3 opening at the top of the
cross section of the jacket 1. All the valves mentioned
15 in the description are of the remote controlled
electrovalve type.
The filter comprises at least two layers of filter
media. In the example depicted, the filter F is a trio-
20 layer filter comprising a lower layer 4 of sand and an
upper layer 5 of anthracite. The layers are supported
by a horizontal floor 6 comprising passage orifices
situated in the bottom part of the filter and extending
in cross section along a chord of the circular cross
25 section. The floor is equipped with nozzles 6a allowing
the injection of air and/or of washing water. The air
is admitted at the bottom part, underneath the floor 6,
via a pipeline 7 equipped with a valve 8, whereas the
washing water is introduced, in the bottom part of the
30 filter, by a pipe 9 equipped with a valve 10.
The raw water introduced into the top part of the
filter flows from the top to the bottom through the
layers 5 and 4 then through the orifices in the floor
35 6, and in the bottom part forms a layer 11 of filtered
water which can be discharged via a pipe 12 equipped
with a valve 13. A branch 12b, equipped with a valve
13b, is connected to the pipe 12 upstream of the valve
13, to discharge to a discharge.
The washing water injected in the bottom part of the
5 filter rises from the bottom to the top through the
layers 4 and 5, performing a backwash, is collected at
the top by at least one weir D and is discharged from
the top part of the filter by a pipe 14 connected to
the weir and equipped with a valve 15.
10
The air is discharged from the top part of the filter
by a pipe 16 equipped with a valve 17.
During the pressurized filtration operation illustrated
15 in figure 1, the raw water is admitted by the pipe 2,
the valve 3 being open as is the filtered-water
discharge valve 13. The other valves are closed. The
upper part of the filter, situated above the filtering
layers, is filled with pressurized raw water.
20
The water flows through the layers of the filter and
the filtered water collected at the bottom part is
discharged by the pipe 12.
25 The seawater contains components such as alginates
which agglomerate with particles of media and form
cakes or 'mud balls" M generally at the surface of the
filter. These cakes M may have dimensions of around
2 cm x 4 cm with a thickness of around 0.5 cm and
30 increasingly slow down the filtration as the number of
them increases.
The washing method according to the invention makes it
possible to reduce the volume of the cakes M by
35 subjecting them to a phenomenon of friction by bringing
them into contact with air in order to dry them, and to
discharge the fines formed toward the upper part of the
filter.
For the washing, in a first, preparation, step a/, the
raw water inlet valve 3 is closed then, after a certain
time, so is the valve 13. The pressure is then reduced,
5 notably by opening the valves 15 and 17. The filter
cake is destroyed by breaking it up by sending a small
quantity of water countercurrentwise, namely from the
bottom to the top, by opening the valve 10, for example
for a few seconds. At the end of this step the valves
10 15 and 17 are kept open, or are opened, until the end
of the final rinse.
During the next step b/, the water level falls as
filtered water is discharged through the pipe 12b and
15 the valve 13b, which has been opened, until it reaches
a level 5 to 10 cm above the medium, ~b~hich level is
detected by two level (LSL and LSLL) sensors. The valve
13b is then closed.
20 For the next step c/, the flushing-air inlet valve 8 is
opened (see figure 2) and air alone is thus blown in
under pressure at the bottom part of the filter, this
air passing from bottom to top through the layers 4 and
5 of the filter and causing the cakes M to be reduced,
25 particularly by friction, to fine particles.
The duration of this injection of air, or bubbling, is
short, notably less than 10 min (generally 5 to 6
minutes), after which time the valve 8 closes again.
30
Such bubbling for a relatively short length of time,
lasting a feb~ minutes, makes it possible to avoid or
limit the mixing of the media of the two layers 4 and
5.
35
During the bubbling operation c/, as illustrated in
figure 2, the washing-water discharge valve 15 is open
as is the air discharge valve 17.
By aeration, the injected compressed air alloris the
cakes to dry out making them easier to turn into fine
particles.
5
The next step d/ consists in a short rinse with water
alone, from the bottom to the top, by opening the
backwash water valve 10 and the washing-water discharge
valve 15 (see figure 3). The air discharge valve 17 is
10 also open, >~hile the other valves are closed. This step
d/ allows the sand and anthracite media to separate and
the layers 4 and 5 to be reconstituted without
appreciable mixing. Furthermore, the rinsing allows the
particles derived from the cakes M to rise to the
15 surface.
The rinsing in step d/ is carried out for a length of
time that is short enough that the water level H
(figure 3) remains below the level of the upper edge of
20 the washing water collection weir D. Advantageously,
the water level H at the end of a rinse is a distance J
of at least 5 cm and preferably of between 5 and 10 cm
below the level of the upper edge of the weir. This
then avoids the loss of media that may be carried away
25 by the washing stream into the discharge pipe 14.
The duration of a rinse with water alone is short,
generally less than 60 s and notably of the order of 30
to 45 s.
30
During the next step e/, the water is drained off for
filter again, discharging the water using the pipe 12b,
the valve 13b being open, until a level of 5 to 10 cm
above the medium.
35
Steps c/, d/ and e/ are repeated at least once and
preferably twice.
Not until just before the final rinse is the air from
the cushion 18, situated underneath the floor 6,
discharged via an outlet that has not been depicted, so
as to avoid a mixing of air bubbles and rinse water
5 during the rinse.
When the sequences involving short-duration rinsing,
generally lasting less than 60 s, have been carried
out, a final rinse is performed, the air from the
10 cushion 18 being discharged. During this rinse, a
quantity of backwash water corresponding to several
covolumes (a covolume corresponding to the total volume
of sand and anthracite medium), notably 5 to
10 covolumes, is passed through, with the dirty washing
15 water being discharged to a sludge treatment facility
by the pipe 14, the valve 15 of which is open. The
backwash water inlet valve 10 is' also open as is the
air discharge valve 17, while the other valves are
closed.
2 0
An installation for implementing the method comprises
the filter F, the various pipelines and valves, and a
valve control unit U (figure 1) for performing the
sequences of opening and closing of the valves, in
25 accordance with the method.
The solution of the invention thus consists in draining
the filter, bubbling air alone, then rinsing for a
short duration with water alone in order to avoid any
30 overspill onto the discharge weirs or channels and the
loss of medium. The cushion of air 18 under the floor 6
is removed just before the final rinse only.
This sequence of draining, bubbling and short-duration
35 rinsing is repeated a number of times, two repeats
generally sufficing.
The steps of preparing for the washing of the filter
mean that depressurization can be achieved, the filter
cake broken up, and the water level lowered in order to
avoid any loss of medium during the step of bubbling
5 with pressurized air.
The invention is as applicable to filters which are
horizontal (total height from the drain valve to the
vent 4 m max., for transport and manufacturing reasons)
10 as it is to filters which are vertical (for economic
reasons).
The method requires no modification to equipment and
makes it possible to improve the efficiency of the
15 filter, notably by increasing the filtration time.
The method can work with an atmospheric filter.
CLAIMS
1. A method for washing a filter (F) having at least
5 two layers of filter media for a raw water containing
components that have a tendency to form cakes or "mud
balls" (M) which generally agglomerate at the surface
of the filter, notably for seawater, the layers of
filter media being supported by a floor (6) comprising
10 passage orifices and provided with nozzles (6a) that
allow an injection of washing water and/or air, the raw
water flowing from top to bottom through the layers,
and the treated water being collected at the bottom
part of the filter, which at the top part comprises at
15 least one weir for collecting the washing water,
characterized in that the following successive steps
are carried out:
a/ a preparation step in which the raw water inlet
is closed and the filter, where the latter operates
20 under pressure, is depressurized;
b/ the water level in the filter is lowered;
c/ air alone is blown under pressure into the
bottom part of the filter, th~s air passing from bottom
to top through the layers of the filter and causing the
25 cakes to be reduced to particles, particularly by
friction;
d/ rinsing is carried out using water alone, from
bottom to top, allowing the media to separate and
causing the particles to rise to the surface, the
30 water-rinsing time being short enough that the water
level stops rising below the level of the upper edge of
the washing water collecting weir;
e/ the water is emptied out of the filter again by
d discildrye d i iile boiiorr~ p d ~ i ;
35 f/ steps c/, d/ and e/ are repeated at least once,
and a final rinsing is performed by passing
through a quantity of backwash water which is injected
at the bottom part of the filter and discharged from
the top part of the filter by a pipe (14).
2. The method as claimed in claim 1, characterized in
5 that steps c/, d/ and e/ are repeated at least twice.
3. The method as claimed in claim1 or 2,
characterized in that the duration for which air alone
is blown in is less than 10 min.
10
4. The method as claimed in any one of the preceding
claims, characterized in that the duration of the
rinsing with water alone is less than 60 s, in
particular around 45 s.
15
5. The method as claimed in any one of the preceding
claims, characterized in that prior to each rinsing
with water alone, the cushion of air under the floor is
not eliminated, this cushion being eliminated just
20 prior to the final rinse.
6. The method as claimed in any one of the preceding
claims, characterized in that the level ( H ) of the
water at the end of rinsing with water alone lies at
25 least 5 cm below the level of the upper edge of the
washing water collection weir (D).
7. The method as claimed in claim 6, characterized in
that the level (H) of the water at the end of rinsing
30 with water alone lies between 5 cm and 10 cm below the
level of the upper edge of the washing water collection
weir (D) .
8 . The method as claimed in any one of the pie~edil~cj
35 claims, characterized in that, having repeated the
draining, bubbling and rinsing sequence a number of
times, a final rinse is carried out after the air from
the cushion situated under the floor has been
discharged, this final rinsing corresponding to 5 to
10 covolumes and the dirty washing water is discharged
to a sludge treatment facility.
5 9. A filtration installation, particularly for
seawater, comprising a filter having at least two
layers (4, 5) of filter media which are supported by a
floor (6) equipped with nozzles (6a), a pipeline (7)
equipped with a valve (8) for admitting air underneath
10 the floor and a pipeline (9) equipped with a valve (10)
for admitting backwash water underneath the floor, a
pipeline (12) equipped with a valve (13) for
discharging the filtered water underneath the floor, a
branch (12b) to a discharge, which branch is equipped
15 with a valve (13b) used to drain the filter down to a
level LSL (or LSLL) and an air extraction pipeline (16)
situated at a high point of the filter and equipped
with a valve (17), a raw water inlet pipeline (2)
equipped with a valve (3), at the top part of the
20 filter, above the layer of media, and a pipeline (14)
for discharging the washing water collected by at least
one weir (D) at the top part of the filter and equipped
with a valve (15), this installation comprising a valve
control unit (U), characterized in that the control
25 unit (U) is designed to perform the sequences of
opening and closing of the valves according to the