FILTER WITH CLOSELY-SPACED VERTICAL PLATES

Abstract

The filter (1) according to the invention with vertical plates (2) includes plates (2) each including a tube (200) connected to an external manifold (5, 50, 51, 52), and said filter (1) is characterized in that it includes at least two external manifolds (5, 50, 51, 52), in that, for all the plates (2), two adjacent plates (2) are connected to two different external manifolds (5, 50, 51, 52) and in that each plate (2) includes at least one filter element (22), each filter element (22) being constituted of at least two drains (220) and a cloth (221), the (221) having compartments (222) in each of which a drain (220) is inserted. By not discharging two adjacent plates (2) at the same time, only one of the two facing cakes (6) is detached rather than both of them. The construction of these plates (2) enables low inflation of the cloth (221) during discharge and therefore limited movement of the cake (6). It is therefore possible to reduce the space provided between the plates (2) with no risk of causing jams fatal to the operation of the filter (1) when the cakes (6) fall off and with no risk of pressing the discharged cakes (6) onto the plates (2) during filtration. This makes it possible to increase the number of plates (2) in the filter (1).

Claims

1. A filter for use with a drain, the filter comprising: at least two external manifolds; and multiple vertical plates that each includes a tube connected to one of the at least two external manifolds, wherein two adjacent vertical plates of the multiple vertical plates are connected to different manifolds of the at least two manifolds, each of the multiple vertical plates also including at least one filter element, each of the at least one filter element including: at least two drains; and a cloth defining compartments configured such that the drain is insertable therein.

2. The filter according to claim 1, wherein the plates are disposed as a star.

3. The filter according to claim 2, including an alternation of double plates with two filter elements including a long tube and single plates with one filter element including a short tube.

4. The filter according to claim 3, wherein the single plates are connected to a double plate, thus forming pairs of plates.

5. The filter according to claim 4, wherein the short tube of a single plate is connected to the long tub of a double plate by a pipe connecting the two plates.

6. The filter according to claim 4, wherein the pair of plates is connected to one and only one external manifold through the tube of the double plate.

7. The filter according to claim 3, further including N external manifolds, wherein: N is greater than or equal to three.

8. The filter according to claim 7, wherein the pairs of plates, N consecutive pairs of plates are connected to the N external manifolds and wherein the single plate and the double plate of the same pair are spaced by an even number of plates.

9. A method of discharging cakes from a filter for use with a drain, the filter comprising: at least two external manifolds; and multiple vertical plates that each includes a tube connected to one of the at least two external manifolds, wherein two adjacent vertical plates of the multiple vertical plates are connected to different manifolds of the at least two manifolds, each of the multiple vertical plates also including at least one filter element, each of the at least one filter element including: at least two drains; and a cloth defining compartments configured such that the drain is insertable therein, comprising: passing through external manifold a flow of filtrate that is reversed in accordance with a predefined cycle; and performing discharge such that the cycles of all the external manifolds are identical with a phase difference between them

10. The method according to claim 9, wherein the cycles of reversing the filtrate flows of all the external manifolds are phase-shifted by the same time interval.

11. The method according to claim 9, wherein the flow of filtrate is reversed by a pump through a blower circuit that is able to connect the outlet of the pump to each of the external manifolds.

12. The method according to claim 9, wherein the flow of filtrate is reversed by an over pressurized balloon filled with the filtrate through a blower circuit able to connect the outlet of the over pressurized balloon to each of the external manifolds.

13. The method according to claim 11, wherein the blower circuit is equipped in parallel with a safety overflow (70) between 1 and 5 m inclusive high above the filter and connected to the interior of the filter in the upper part of the tank of the filter.

14. The method according to claim 9, wherein the flow of filtrate is reversed by a balloon filled with the filtrate under load on the filter and connected to the tank of the filter in their upper part, this reversal of the flow being effected through a blower circuit adapted to connect the outlet of the balloon to each of the external manifolds.

15. The method according to claim 9, wherein the flow of filtrate is reversed by balloons filled with the filtrate under load on the filter and connected to the tank of the filter in their upper part, this reversal being effected through blower circuits each able to connect the outlet of one and only one the balloons to one and only one of the external manifolds.

16. The method according to claim 14, wherein the upper part of the balloons is situated between 1 and 5 m above the filter.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0027] Other advantages of some embodiments will be apparent to the person of ordinary skill in the art on reading the following examples, illustrated by the appended figures, which are provided by way of example:

[0028] FIG. 1 is a perspective view of a double star filter according to some embodiments without its cover,

[0029] FIG. 2 is a view from above of a related art double star filter the tank of which is cut off at the level of the tubes of the plates of the filter,

[0030] FIG. 3 is a view from above of a related art single star filter the tank of which is cut off at the level of the tubes of the plates of the filter,

[0031] FIG. 4 is a view from above of the double star filter according to some embodiments of FIG. 1 without its cover,

[0032] FIG. 5a is a view from above of a single star filter according to the invention without its cover,

[0033] FIG. 5b is a view from above of a filter according to some embodiments without its cover and with the plates disposed in parallel,

[0034] FIG. 6a is a detail perspective view of single plates and double plates and the pipe connecting them in some embodiments of FIG. 1,

[0035] FIG. 6b is a view of a plate from FIG. 6a in section on the plan A,

[0036] FIG. 7 is a section showing the detail of the connection between the tube of a filter plate and a connecting pipe from FIG. 6a,

[0037] FIG. 8a is a developed diagrammatic sectional view of four adjacent plates discharged simultaneously using a related art technique,

[0038] FIG. 8b is a developed diagrammatic sectional view of four plates, two non-adjacent plates of which are discharged by the method according to the invention,

[0039] FIG. 9 is a diagram of a first variant of the method according to the invention,

[0040] FIG. 10 is a diagram of a second variant of the method according to the invention,

[0041] FIG. 11 is a diagram of a third variant of the method of the invention,

[0042] FIG. 12 is a diagram of a fourth variant of the method of the invention.

BRIEF DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0043] It is considered in the remainder of the description that the term high refers to the top parts and the term low to the bottom parts of FIG. 1 and FIGS. 6 to 12. The same elements bear the same references.

[0044] A related art filter 1 shown in FIG. 2 or FIG. 3 is a filter press with vertical plates 2 for the liquid/solid separation of a charged suspension. It is constituted of a cylindrical tank 3 with a conical bottom wall 30 and a domed removable cover 31 shown in FIGS. 9 to 12. The plates 2 are suspended vertically inside the tank 3 and disposed radially in a star arrangement.

[0045] The small diameter related art filters 1 shown in FIG. 3 are equipped with identical plates 2 connected through the wall 32 of the tank 3 by bent tubes 4 termed filtrate outlets to an exterior manifold 5 that extends around the filter (single star disposition).

[0046] The large diameter related art filters 1 shown in FIG. 2 are equipped with plates 2 of two different types, plates 2 with one filter element 22, termed single plates 20, and plates 2 with two filter elements 22, termed double plates 21. The single plates 20 and the double plates 21 are connected through the wall 32 of the tank 3 by the filtrate outlets 4 to an external manifold 5 (double star disposition). In these cases, all the plates 2 are grouped in pairs, a single plate 20 and an adjacent double plate 21 having a common filtrate outlet 4.

[0047] The filter 1 from FIG. 1 is a double star filter according to some embodiments in which the vertical plates 2 are connected in independent groups to three distinct external manifolds 5. One third of the single plates 20 and of the double plates 21 are therefore connected to a first external manifold 50, a second of those thirds to a second external manifold 51, and the third of those thirds to the third external manifold 52. Each single plate 20 is connected to a double plate 21 by a pipe 23, thus forming a plate pair. The double plate 21 of each pair is connected to one and only one external manifold 5 via a filtrate outlet 4.

[0048] In this example the number N of external manifolds 5 is three. For N external manifolds 5, N being greater than or equal to three, there will be N independent groups of plates 2 each connected to one of the N external manifolds 5.

[0049] As can be seen in FIG. 4, in all the pairs of plates 2 three consecutive pairs of plates 2 are therefore connected to the three external manifolds 50, 51 and 52. The single plates 20 and the double plates 21 of the same pair are spaced by two plates 2. The pairs of plates 2 connected to the same external manifold 50, 51 or 52 thus form an independent group of non-adjacent plates 2. The filter therefore includes three independent groups of plates 2. A first independent group of plates 2 is represented in black in FIG. 4, a second group cross-hatched, and the third group in white. As can be seen, a double plate 21 of the first group (black) follows on from a single plate 20 of the third group (white), then a double plate 21 of the second group (cross-hatched), then a single plate 20 of the first group (black), then a double plate 21 of the third group (white), then a single plate 20 of the second group (cross-hatched), then a double plate 21 of the first group (black), and so on. The single plates 20 and the double plates 21 of the same pair, because of the distance between them, are no longer connected to one another through the wall 32 of the tank 3 as in the related art, but instead by a removable pipe 23 represented in FIG. 4 in the color of the group of independent plates to which the pipe 23 belongs. In this arrangement, the filter elements of the same group are never or rarely adjacent. In the case of FIG. 4, the three modes of representation used (black, cross-hatched and white) highlight the various flow circuits through the tubes 200, the pipes 23 and the filtrate outlets 4.

[0050] In this example the number N of external manifolds 5 is three and the single plate 20 and the double plate 21 of the same pair are spaced by two plates 2. For N external manifolds 5, N being greater than or equal to three, the single plate 20 and the double plate 21 of the same pair will be spaced by two or another even number of plates 2.

[0051] It can be seen in FIGS. 6a and 7 that all the single plates 20 and all the double plates 21 respectively include one and two filter elements 22 each constituted of drains 220 and a cloth 221. These filter elements 22 are connected to the tubes 200 of the plates 2. Each connecting pipe 23 connects the tubes 200 of the pairs of plates 2, forming a bridge between them. Seals 230 placed between the tubes 200 and each of the two ends of the pipes 23 are compressed by wire springs 231 that exert sufficient force to guarantee a seal. As shown in FIG. 6a the filter elements 22 include a plurality of drains 220, each drain being disposed in a compartment 222 formed in the cloth 221. These compartments 222 may be produced for example by seams or welds 223 on the cloth 221. These filter elements 22 are described in detail in the patent EP226478 hereby incorporated by reference. FIG. 6b shows the filter element 22 in section and it can be seen that, because they are narrow, the compartments 222 are inflated relatively slightly and therefore enable the filter elements to be closer to one another than conventional filter elements not including compartments 222.

[0052] The filter 1 from FIG. 5a is a single star filter according to the invention, in which the identical plates 2 are connected in independent groups to two distinct external manifolds 5. Thus one half of the plates 2 are connected to a first external manifold 50 and the second half to the second external manifold 51.

[0053] The filter 1 from FIG. 5b is a filter according to some embodiments in which the plates 2 are disposed in parallel and connected in independent groups to two distinct external manifolds 5. Thus one half of the plates 2 are connected to a first external manifold 50 and the second half to the second external manifold 51. As can be seen in FIG. 5a and FIG. 5b, in all the plates 2 two consecutive plates 2 are therefore connected to the two external manifolds 50 and 51. The plates 2 connected to the same external manifold 50 or 51 therefore form an independent group of non-adjacent plates 2. The filter therefore includes two independent groups of plates 2. A first independent group of plates 2 is represented in black in FIG. 5a and in FIG. 5b and the second independent group of plates 2 is represented cross-hatched. As can be seen, a plate 2 of the first group (black) follows on from a plate 2 of the second group (cross-hatched), then a plate 2 of the first group (black), and so on. In this arrangement, the filter elements 22 of the same group are never or rarely adjacent. In the case of FIG. 5a and in the case of FIG. 5b the two modes of representation used (black and cross-hatched) highlight the various flow circuits through the tubes 200 and the filtrate outlets 4.

[0054] In the related art the filtration cycle of the filter 1 is divided into two phases: the filtration phase and the discharge phase. During the filtration phase the tank 3 of the filter 1 is filled with the suspension to be filtered. the tank 3 is then pressurized whereas the plates 2 are subjected to a reduced pressure via their tube 200. The liquid phase of the suspension then passes through the cloths 221 of the filter elements 22 and the solid phase remains on the surface of the cloths 221, forming a cake 6. During the filtration phase the cloths 221 are pressed onto the drains 220 of the filter element 22. The filtration phase ends when the resistance of the cakes 6 to the flow of the liquid becomes too high. It is then desired in this embodiment to detach the cakes 6 from the cloths 221, which is the discharge phase. A number of discharge methods enable the cakes 6 to be detached from the filter elements 22 but, as can be seen in FIG. 8a, this often involves directing simultaneously into all the filter elements 22 a contraflow fluid that will inflate their cloths 221 and pass through the cloths without passing through the cake 6, therefore blowing the cake 6 off the filter element 22. During blowing, the cloth 221 is inflated and moves away from the drains 220. The fluid used to detach the cakes may in particular be the filtrate, water, air or vapor.

[0055] The commonest discharge process is termed filtrate contraflow discharge. The filtrate used is most often stored during the filtration phase in a balloon termed a blowing balloon situated above the filter 1. During the discharge phase, after venting the tank 3 of the filter 1 to the atmosphere the filtrate is sent back by gravity alone into all the filter elements 22 of the filter 1. Once detached from their filter element 22, the cakes 6 in the suspension between the plates 2 settle to the bottom of the filter 1 before being extracted.

[0056] It is seen in FIG. 8a that in a related art filter 1, during the discharge phase, the discharged cakes 6 face one another. It is desired in this embodiment to ensure that the cakes 6 do not come into contact with one another when they are detached from the cloths 221 inflated by the filtrate.

[0057] The minimum distance E1 between the vertical axes of two adjacent filter elements 22 of the related art filter 1 typically correspond to the sum of: [0058] the maximum inflation distance e1 between the vertical axis of a first filter element 22 and its cloth 221 inflated by the filtrate, [0059] the detachment distance e2 of the plate 6 from the filter element 22, [0060] the maximum allowed thickness e3 of the cake 6, [0061] the maximum inflation distance e1 between the vertical axis of the second filter element 22 and its cloth 221 inflated by the filtrate, [0062] the detachment distance e2 of the cake 6 from the second filter element 22, [0063] the maximum allowed thickness e3 of the cake 6, [0064] a safety distance e4 between facing discharged cakes 6.

E1=(2e1)+(2e2)+(2e3)+e4

For example there will be a minimum distance E1 of 124 mm between the vertical axes of two adjacent filler elements 22 for a maximum inflation distance e1 of 25 mm, a detachment distance e2 of 10 mm, a maximum allowed thickness e3 of 25 mm and a safety distance e4 of 4 mm.

[0065] As shown diagrammatically in FIG. 8b, in which the compartments 222 from FIG. 6a are not represented, in a filter 1 of some embodiments the discharged cakes 6 are not face-to-face during the phase of discharging a group of independent plates. Each discharged cake 6 faces a cake 6 that is being formed, with a smaller thickness that can be calculated as a function of the thickness of the cake 6 discharged, not detached and under which the cloth 221 has not been inflated. To ensure that a discharged cake 6 does not come into contact with the facing cake 6 being formed, a safety distance e5 must or should be provided between them. During settling this therefore prevents the discharged cake 6 from being pressed onto the facing cake 6 being formed by the flow of suspension penetrating into the latter.

[0066] This safety distance e5 is calculated using the following formula:

e5=hVf/Vd [0067] h is the height of the filter element 22, [0068] Vf is the velocity of the flow of suspension penetrating into the cake 6 being formed, [0069] Vd is the velocity of the settling of the cake 6 in the suspension.

[0070] The minimum distance E2 between the vertical axes of two adjacent filter elements 22 of the filter 1 of some embodiments corresponds to the sum of: [0071] the maximum inflation distance e1 between the vertical axis of a first filter element 22 and its cloth 221 inflated by the filtrate, [0072] the detachment distance e2 of the cake 6 from the first filter element 22, [0073] the maximum allowed thickness e3 of the cake 6, [0074] the maximum distance e6 between the vertical axis of the second filter element 22 and its cloth 221 pressed onto the filter element 22, [0075] the thickness e7 of the cake 6 being formed, [0076] the safety distance e5.

E2=e1+e2+e3+e5+e6+e7

[0077] In the case of a filter 1 of some embodiments with two external manifolds, there will for example be a minimum distance E2 of 96 mm between the vertical axes of two adjacent filter elements 22 for a maximum inflation distance e1 of 25 mm, a detachment distance e2 of 10 mm, a maximum allowed thickness e3 of 25 mm, a height h of 3000 mm, a velocity Vf of the flow of suspension of 0.8 mm/s, a settling velocity Vd of 200 mm/s, a maximum distance e6 of 6 mm and a maximum allowed thickness e7 of 18 mm. E2 is therefore more than 20% less than E1.

[0078] The above example shows that in a filter 1 of some embodiments the minimum distance between the vertical axes of two adjacent filter elements 22 is reduced relative to the minimum distance in a related art filter 1. It is therefore possible in a filter 1 of some embodiments for the plates 2 to be closer together, the number of the plates 2 to be increased and the filter area of the filter 1 therefore to be maximized.

[0079] The FIG. 9 diagram shows a first variant of the method of discharging a filter 1 of some embodiments with three external manifolds 5. In this configuration, during the phases of discharging the independent groups of plates 2 the difference between the pressure inside the filter elements 22 and the pressure outside the filter elements 22, measured by pressure transmitters PT, is regulated by the rotation speed of the pump 7. A swan-neck safety overflow 70 connects the outlet of the pump 7 to the high part of the tank 3 of the filter 1 via an automatic valve 71. The overflow 70 is connected at its top to the cover 31 of the filter 1 to allow free circulation of air trapped between the cover 31 of the tank 3 of the filter 1 and the overflow 70. The height of the overflow 70 will physically determine the maximum difference allowed between the pressure inside the filter elements 22 and the pressure outside the filter elements 22. During the discharge phase, at all points of the filter elements 22 of the group of independent plates 2 the difference between the pressure inside and the pressure outside the filter elements 22 will therefore be less than the pressure corresponding to the height of the overflow 70 filled with blowing fluid.

[0080] During the phase of filtration of the independent group of plates 2 that is connected to it, each of the three external manifolds 50, 51, 52 communicates with the general filtrate pipe 8 via an automatic valve 720, 721, 722. During the phase of discharging the independent group of plates 2 that is connected to it, that of the three external manifolds 50, 51, 52 concerned communicates with the outlet of the pump 7 via an automatic valve 730, 731, 732. If the discharge pressure of the pump 7 were such that the maximum allowed difference between the pressure inside the filter elements 22 and the pressure outside the filter elements 22 were to be reached, all risk of an overshoot would be avoided thanks to the excess blowing fluid overflowing into the tank 3 of the filter 1 via the overflow 70.

[0081] The FIG. 10 diagram shows a second variant of the method for discharging a filter 1 of some embodiments with three external manifolds 5. In this configuration, during the phases of discharging independent groups of plates 2, the difference between the pressure inside the filter elements 22 and the pressure outside the filter elements 22 is regulated by a self-driven compressed air differential pressure regulator 74 installed on the compressed air feed of the over-pressurized balloon 75 filled with filtrate. The low-pressure pulse pipe 741 of the regulator 74 is connected to the cover 31 of the filter 1. The high-pressure pulse pipe 742 of the regulator 74 is connected to the compressed air supply of the over-pressurized balloon 75 downstream of the regulator 74 and upstream of the automatic isolation valve 752. A swan-neck safety overflow 70 connects the bottom of the pressurized balloon 75 to the top part of the tank 3 of the filter 1 through an automatic valve 71. The top of the overflow 70 is connected to the cover 31 of the filter 1 to allow free circulation of compressed air between the cover 31 of the tank 3 of the filter 1 and the overflow 70. The height of the overflow 70 will physically determine the maximum allowed difference between the pressure inside the filter elements 22 and the pressure outside the filter elements 22. During the discharge phase, at any point on the filter elements 22 of the group of independent plates 2 the difference between the pressure inside and the pressure outside the filter elements 22 will therefore be lower than the pressure corresponding to the height of the overflow 70 filled with blowing fluid.

[0082] During the filtration phase of the independent group of plates 2 that is connected to it, each of the three external manifolds 50, 51, 52 communicates with the general filtrate pipe 8 via an automatic valve 720, 721, 722. Before beginning the phase of discharging the independent group of plates 2 connected to it, that of the three external manifolds 50, 51, 52 concerned is connected to the over-pressurized balloons 75 via an automatic valve 730, 731, 732. The over-pressurized balloon 75 is filled with filtrate, the automatic vent valve 751 is open and automatic isolation valve 752 is closed. When the level switch LS is reached, the automatic vent valve 751 is closed and the automatic isolated valve 752 is opened to enable regulation of the difference between the pressure inside the filter elements 22 and the pressure outside the filter elements 22, thus beginning the discharge phase. If the pressure of the compressed air coming from the self-driven compressed air differential pressure regulator 74 were such that the maximum difference allowed between the pressure inside the filter elements 22 and the pressure outside the filter elements 22 were to be reached, all risk of an overshoot would be avoided thanks to the excess blowing fluid overflowing into the tank 3 of the filter 1 via the overflow 70.

[0083] The FIG. 11 diagram shows a third variant of the method of discharging a filter 1 of some embodiments with three external manifolds 5. In this configuration, during phases of discharging independent groups of plates 2, the difference between the pressure inside the filter elements 22 and the pressure outside the filter elements 22 stems from the difference between the level of the filtrate in the balloon 76 and the level of the suspension in the filter 1. The top of the balloon 76 is connected to the cover 31 of the filter 1 via an automatic balancing valve 77 to allow free circulation of compressed air between the cover 31 of the tank 3 of the filter 1 and the balloon 76 during the discharge phases. The height of the balloon 76 will physically determine the maximum difference allowed between the pressure inside the filter elements 22 and the pressure outside the filter elements 22. During the discharge phase, at any point on the filter elements 22 of the group of independent plates 2 the difference between the pressure inside and the pressure outside the filter elements 22 will therefore be lower than the pressure corresponding to the height of the balloon 76 filled with blowing fluid.

[0084] During the phase of filtration of the independent group of plates 2 that is connected to it, each of the three external manifolds 50, 51, 52 communicates with the general filtrate pipe 8 via an automatic valve 720, 721, 722. Before starting a phase of discharging the independent group of plates 2 that is connected to it, that of the three external manifolds 50, 51, 52 concerned is connected to the balloon 76 via an automatic valve 730, 731, 732. The balloon 76 is filled with filtrate, the automatic vent valve 78 is open and the automatic balancing valve 77 is closed. When the level switch LS is reached, the automatic vent valve 78 is closed and the automatic balancing valve 77 is opened to balance the pressure at the top of the filter 1 and the pressure at the top of the balloon 76, thus beginning the discharge phase. During this phase, the balloon directs the filtrate by gravity alone into the external manifold 50, 51, 52 concerned. The level of suspension in the filter 1, measured on the level transmitter LT, increases. A setpoint suspension level in the filter 1 is re-established after each discharge phase by injecting compressed air at the top of the filter 1 via the automatic valve 9. The height of the balloon 76 is such that the maximum difference allowed between the pressure inside the filter elements 22 and the pressure outside the filter elements 22 is never or rarely exceeded.

[0085] The FIG. 12 diagram shows a fourth variant of the method of discharging a filter 1 of some embodiments with three external manifolds 5. In this configuration, during phases of discharging independent groups of plates 2, the difference between the pressure inside the filter elements 22 and the pressure outside the filter elements 22 stems from the difference between the level of the filtrate in one of the balloons 760, 761, 762 and the level of the suspension in the filter 1. During the discharge phase, the tops of the balloons 760, 761, 762 are connected to the cover 31 of the filter 1 via an automatic balancing valve 770, 771, 772 to allow free circulation of compressed air between the cover 31 of the tank 3 of the filter 1 and one of the balloons 760, 761, 762. The height of the balloons 760, 761, 762 will physically determine the maximum difference allowed between the pressure inside the filter elements 22 and the pressure outside the filter elements 22. During the discharge phase, at any point of the filter elements 22 of the group of independent plates 2 the difference between the pressure inside and the pressure outside the filter elements 22 will therefore be lower than the pressure corresponding to the height of the balloons 760, 761, 762 filled with blowing fluid.

[0086] During the phase of filtration of the independent group of plates 2 that is connected to it, each of the three external manifolds 50, 51, 52 communicates with the general filtrate pipe 8 via its own balloon 760, 761, 762, the overflow 7600, 7610, 7620 of the balloon 761, 762, 763 and the filtrate outlet valve 790, 791, 792 of the balloon 761, 762, 763. Before beginning the phase of discharging the independent group of plates 2 that is connected to it, that of the three external manifolds 50, 51, 52 concerned is in direct communication with its balloon 760, 761, 762. The automatic vent valve 780, 781, 782 and the automatic filtrate outlet valve of the balloon 760, 761, 762 are closed and the automatic balancing valve 770, 771, 772 of the balloon 760, 761, 762 is opened to balance the pressure at the top of the filter 1 and the pressure at the top of the balloon 760, 761, 762 thereby beginning the discharge phase. During that phase the balloon 760, 761, 762 directs the filtrate by gravity alone into the external manifold 50, 51, 52 concerned. The level of the suspension in the filter 1, measured on the level transmitter LT, increases. A setpoint suspension level in the filter 1 is re-established after each discharge phase by injecting compressing air at the top of the filter 1 via the automatic valve 9. The height of the balloons 760, 761, 762 is such that the maximum difference allowed between the pressure inside the filter elements 22 and the pressure outside the filter elements 22 is never or rarely exceeded.