GB1589600A

GB1589600A - Separating solid particles from a liquid suspension

Info

Publication number: GB1589600A
Application number: GB36633/77A
Authority: GB
Original assignee: SIGMA Z NA VYROBU CERPACICH ZA
Current assignee: SIGMA Z NA VYROBU CERPACICH ZA
Priority date: 1976-09-09
Filing date: 1977-09-01
Publication date: 1981-05-13
Also published as: JPS5652604B2; DE2740308C2; JPS5341860A; IT1084446B; DE2740308A1; NL7709939A; CS189242B1

Description

(54) IMPROVEMENTS RELATING TO SEPARATING SOLID PARTICLES FROM A LIQUID SUSPENSION (71) We, SIGMA, ZAVODY NA VYROBU CERPACICH ZARIZENI A ARMATUR GENERALNI REDITEL STVI of Olomouc, Czechoslovakia. a corporation organised and existing under the laws of the Czechoslovak Socialist Republic, do hereby declare the invention. for which we pray that a patent may be granted to us, and the method bv which it is to be performed, to be particularly described in and by the following statement: This invention relates to methods and apparatus for separating solid particles from a liquid suspension (comprising a liquid contaminated by the solid particles in suspension thereinj, with or without the addition of flocculation or clarification agents, the method being performed in a plurality of successive working stages.

Methods and apparatuses for this purpose have previously been proposed in which separation takes place within spaces defined between walls, inclined to the vertical, within a cylindrical tank. Such separation is accomplished by gravity sedimentation, the density of the solid particles being greater than that of the carrier liquid, and takes place in one stage or in a plurality of hydraulically interconnected stages, within one or more sedimentation tanks. The maximum upward velocity of the carrier liquid is limited by the settling velocity of the finest-grained particles to be separated.

these being the slowest to settle. These fine-grained particles settle on the inclined surfaces of the above-mentioned walls, the latter being in the form of inclined plates which may typically be assembled as a unit to form a multiple-plate insert or matrix.

Periodically these plates have to be cleaned of the very fine particles adhering to them; and the need to remove the plates from the sedimentation tank, for cleaning purposes.

results in closing down the apparatus with consequent loss of production.

Methods and apparatuses have also been proposed for removing fine particles (or agglomerates thereof) from a liquid suspension by deep filtration instead of sedimentation, filtration being effected by a filter consisting of discrete, bouyant, granular particles which may float on the surface of the carrier liquid. or which may be kept at a predetermined depth below the surface by means of a screen. Such a filter becomes, in use, contaminated by accumulation of fine particles separated from the liquid and adhering to the buoyant particles which constitute the filter. The filter may typically be cleaned by backwashing, effected by a sudden flow of liquid backwardly through the filter, causing the buoyant filter particles to tend to move downwardly. As disclosed in, for example. British Patent specification No. 1359199, this backwashing may be achieved by diverting the flow of the carrier liquid from its normal path, in such a way as to increase the pressure in the filter, and then, by operation of suitable valves in the flow circuit of the liquid, decreasing the pressure suddenly so as to induce a back pressure which. in turn, causes a sudden stream of liquid to flow backwards through the filter.

The buoyant filter particles may for example be of polystyrene foam. In methods of filtration such as that described above. the output of the apparatus and the time taken to filter a given volume of suspension are dependent on the concentration of suspended solids in the carrier liquid. Whilst some solid matter is separated within the filter itself. there is a tendency for much of it to form a cake-like deposit on the buoyant particles at the inlet end of the filter. thus eventually causing clogging of the latter and so seriously reducing the flow rate. Thus the greater the concentration of solids in the liquid suspension, the greater will be the frequency with which it has to be cleaned.

Furthermore. where the filter is large there occurs, due to an unsteady flow pattern of the liquid entering the filter. a non-uniform distribution of the separated fine particles deposited on the filter. resulting in serious deterioration in performance of the apparatus.

According to the invention in a first aspect, a method of separating solid particles from a liquid suspension comprises the successive stages. all carried out in a single vessel, of: introducing the suspension at an- interme diate level in the vessel; separating larger particles from the suspension by simple sedimentation: then passing the liquid suspension in an upwardly-inclined path through upwardlyinclined. relatively narrow sedimentation spaces, in which suspended particles of intermediate size are separated by further sedimentation; and finally passing the liquid suspension upwardly beyond the sedimentation spaces and through a filtration layer of buoyant particles contained below a perforate filterretaining screen. so that the finest suspended particles are separated by filtration whilst the liquid. now substantially free of suspended particles, passes upwardly through the screen. to be led off by overflow at a level spaced above the screen by a depth at least as great as the thickness of the filtration layer. the separated solids from each of said stages being allowed to fall freely through the liquid suspension to accumulate below said intermediate level.

Preferablv. the filtration laver is cleaned periodically by sudden backflow of the treated liquid from above the screen, causing temporary backward expansion of the filtration layer and release of filtered-out particles therefrom through the sedimentation spaces to the bottom of the vessel. The backward expansion of the filtration layer during cleaning preferably increases its depth to at least twice its normal thickness.

Again, during cleaning. buoyant particles from the filtration layer preferably penetrate the inclined sedimentation spaces to be cleaned by contact with each other and with walls defining said spaces.

According to a preferred feature of the invention. backflow is caused bv suction induced upstream of the inclined sedimenta- tion spaces by ejection of a flow of pressurised air to atmosphere along a path com municating with the interior of the vessel.

According to the invention in a second aspect, apparatus for separating solid particles from a liquid suspension. by a method according to said first aspect of the invention. comprises a vessel having: a sludge chamber with draw-off means at the bottom thereof: a sedimentation chamber above the sludge chamber and containing an array of upwardly-inclined plates defining between them relatively narrow sedimentation spaces open at top and bottom: an inlet chamber for the liquid suspension. communicating with the sludge and sedimentation chambers through a distributor. disposed at an intermediate level below the sedimentation chamber and above the sludge chamber, the distributor having an outlet in the form of an elongate slot; a perforate filter-retaining screen extending across the tank below the top of the tank but above the sedimentation chamber, being spaced above the latter by a filter space: a filtration layer of buoyant particles held down in the filter space by the screen; and an overflow trough adjacent to the top of the tank, spaced above the screen by an amount such that. when the tank contains liquid spilling over into the trough, the screen lies at a depth at least equal to the thickness of the filtration layer.

In one form, the said distributor has the shape of a slender, horizontal, hollow pyramid having two of its sides inclined convergently downwards towards the elongate slot at the bottom. said slot being straight and separating the two said sides and facing a reflection shield disposed below the slot.

In another form, the distributor is annular, being constituted by part of a cylindrical side wall of the vessel. a frusto-conical, downwardly directed intermediate wall extending inwardly from the side wall, and a frusto-conical shield extending inwardly from the side wall above and toward the intermediate wall, the elongate slot being an annular gap separating the intermediate wall from the bottom of the shield and facing down the slope of the intermediate wall. the distributor including at least one fixed vane in the slot, inclined at an angle of at least 10" with the line contained by the surface of the intermediate wall in a radial plane intersecting the vane. so as to impart swirl to the liquid suspension passing through the slot.

Apparatus embodying the present invention will now be described. by way of example only. with reference to the accom panving diagrammatic drawings, in which: Figures 1 and 2 show the apparatus in endwise vertical section: Figure 3 is a perspective view of a modification of a part of a sludge chamber of the apparatus: Figure 4 shows in perspective a slotted distributor of the apparatus of Figure 1; Efteitre 5 and 6 show altemative forms of the single tank of the apparatus; and Figttre 7 and K show alternative forms of air chambers as used with double sedimen tation and sludge chambers.

Referring first to Figures 1 to 4, the apparatus according to the present invention comprises a single vessel in the form of tank 1 having vertical side walls 2 and 16, and end walls, not shown, at least the upper parts of which are vertical. The end walls, together with downwardly-convergent walls below the walls 2 and 16, together constitute a convergent bottom portion, which encloses a sludge chamber 15 and which may be in the form of an inverted pyramid 4, Figures 1 and 2, or in a wedge shape as in Figure 3, in which an auger 6, or a shaft with straight or curved blades, may be arranged for delivering thickened sludge to a sludge discharge drawoff branch 7 at the bottom.

Above the sludge chamber 15, a sedimentation chamber 12 is arranged between an inclined wall 3 and a further wall 8, part of which is also inclined and which forms one side of an inlet chamber 9 for the liquid suspension to be treated. Within the sedimentation chamber 12 is a multi-plate insert piece 14, consisting of an array of upwardly-inclined lamellar plates 13, defining between them relatively narrow sedimentation spaces. The plates 13 are arranged at an angle to the horizontal of 30 to 80". A further, similar, insert piece 14 may be provided in the upper part of the sludge chamber, as shown in Figure 1.

The inlet chamber 9 communicates with the sludge chamber 15 and sedimentation chamber 12 through a distributor 10 disposed at an intermediate level, immediately below the sedimentation chamber and above the sludge chamber. The distributor 10 is shown in Figure 4 and has the shape of a slender, horizontal, hollow pyramid having two downwardly-divergent upper sides 25 and two further sides which are inclined convergently downwards towards an elongate slot 26 at the bottom of the distributor.

The larger end of the distributor is open and constitutes an inlet 100 for the liquid suspension. Facing the slot 26 and immediately below it, is a fixed horizontal baffle 11, which may be either a flat piece of sheet steel, or a rolled T-section as shown.

A horizontal, perforate filter-retaining screen 18 extends across the full width of that part of the tank 1 lying between the wall 8 and the outer wall 16 of the tank, so that any liquid passing upwards through the sedimentation chamber 12 has to pass through the screen 18. The latter is spaced above the chamber 12 to define a filter space, containing a filter 17 in the form of a layer of buoyant particles 19 which are held down in the filter space by the screen 18 when the tank 1 is full of liquid. The particles 19 may for example be small balls of expanded perlite or polystyrene foam.

Adjacent to the top of the tank 1 is an overflow trough 21, which is spaced above the screen 18 by a headspace 20 whose depth is at least as great as the value of the vertical thickness which the filter layer 17 has during normal operation, i.e. whilst liquid is flowing steadily upwards through the filter layer and into the headspace 20 to spill over from the latter into the overflow trough 21.

It should be noted that the filter particles 19 are buoyant in the sense that, if the screen 18 were absent, they would float on the surface of the liquid. In normal operation, with the surface of the liquid level at the overflow trough 21, the filter particles are of course submerged.

It is also to be understood that the surface area inside each of the sedimentation spaces between the plates 13 is only a very minute proportion of the total surface area within the sedimentation chamber.

An air chamber 22, within the tank 1, is bounded by the side wall 16, the two end walls of the tank, and the inclined wall 3, the latter having a downward extension, as seen in Figures 1 and 2, towards the side walls 16 but being separated from the latter by a draw-off gap 24 at the bottom of the air chamber 22. The gap 24 forms the lower end of channel 23 extending upwardly to bring the air chamber into communication with the lower part of the sedimentation chamber 12 and with the upper part of the sludge chamber 15. Alternatively, the channel 23 may be omitted, the air chamber then communicating directly with the sludge chamber 15 through the slot 24, and with the sedimentation chamber via the upper part of the sludge chamber.

Referring to Figure 2 (from which all parts within the tank 1, except the inclined wall 3 and its downward extension, are omitted), the air chamber 22 is connected via piping 42 with a source 41 of compressed air provided with a reduction valve 40. A venting valve 39 and a pressure sensor 38 are connected with the piping 42.

In operation, liquid containing, in suspension, solid particles which may represent a wide range of particle sizes from very fine upwards, is introduced at the intermediate level into the distributor 10 via the inlet chamber 9, to be discharged downwardly through the slot 26 of the distributor. In the inlet chamber 9, any accidentally entrapped air bubbles out of the liquid. If any flocculating and/or clarifying agent is to be added, this also takes place in the chamber 9, so that any floccules will be formed before entering the distributor.

The suspension is distributed evenly by the slot 26 over the whole width of the sedimentation chamber, the flow rate being approximately constant over the length of the distributor due to the convergent form of the latter. The baffle 11 redirects the flow laterally, at both sides of the distributor.

The resulting substantially uniform distribution of liquid entering the sedimentation chamber 12 in a substantially horizontal plane, enables considerable economy to be made in the height of the tank 1.

The downward flow velocity of the liquid, as it leaves the distributor and is redirected laterally by the baffle 11, is reduced. The liquid then flows upwards into the sedimentation chamber 12. The larger suspended particles, however, being heavier, have a lower upward component of velocity; consequently the larger suspended particles become separated from the carrier liquid at this stage by simple sedimentation. The liquid, still carrying the remaining suspended matter, then flows upwardly along the inclined sedimentation spaces within the insert piece 14, where particles of intermediate size are separated from the carrier liquid by further sedimentation on to the walls of the inclined plates 13. The relatively narrow sedimentation spaces, and their inclination to the vertical, provide advantageous hydrodynamic conditions for effective separation of the intermediate-size particles.

These fall comparatively quickly down the plates 13, and so into the sludge compartment 15, without significant effect on the flow of the suspended particles. Since primary separation (of the larger particles) has already taken place before the carrier liquid enters the inclined sedimentation spaces, the larger particles have already fallen into the sludge chamber; their absence from the sedimentation chamber minimises the length of the flow path necessary for effective sedimentation between the distributor 10 and the outlet end of the sedimentation chamber 12.

A further advantageous effect of the use of the relatively narrow inclined sedimentation spaces, is that the relatively large surface area of the plates 13 (with respect to the width of each space) enables sedimentation to take place over substantially the whole wall surface of the sedimentation space. Thus, the larger the suspended particle, the lower down the space it will separate from the carrier liquid; but the intermediate-size particles are in fact of a wide range of sizes and the smaller particles in this range will be separated higher up the sedimentation spaces, so that the flow of liquid is not unduly disturbed by concentration of sediment in the lower part of these spaces.

The liquid leaving the top of the sedimentation spaces now contains only the finest suspended particles. These are removed by the filter particles 19. The carrier liquid, now free or substantially free of suspended particles, continues upwards through the screen 18 into the headspace 20, whence it is continuously led off by the overflow trough 21.

Particles filtered out of suspension in the filter fall, like those removed earlier by primary sedimentation and those removed by the secondary sedimentation in the chamber 12, freely through the liquid suspension to accumulate in the sludge chamber. The latter, of course, contains contaminated carrier liquid, but the matrix or insert 14 therein, if provided, helps to retard any re-suspension of the falling particles, besides assisting the primary sedimentation.

Sludge is periodically drawn off through the branch 7.

The filter 17 retains some of the particles filtered from the carrier liquid and therefore requires periodic cleaning. This is effected by automatic backwashing in the following manner. Accumulation of solid matter in the filter 17 gradually changes the pressure of the liquid in the sedimentation chamber and therefore in the air chamber 22. When this pressure reaches a set value detected by the sensor 38, the latter causes the venting valve 39 to be opened, causing a sudden release of air from the source 41 to atmosphere. This causes suction whereby liquid from the sedimentation chamber 12 is drawn into the air chamber 22, and clean liquid from the headspace 20 is at the same time drawn in backward flow down through the filter 17, washing the latter of the fine solid matter adhering to it so that the solid matter is carried down through the sedimentation spaces, to fall freely into the sludge chamber 15. During backwashing, the filter particles 19 tend to be driven downwardly, but the effect is such that the thickness of the filtration layer is in fact at least doubled, and some of the particles 19 are drawn into the sedimentation spaces, and so into contact with the plates 13, where their cleaning is assisted by scouring.

Using the above method, backwashing can be effected in the comparatively short time of 20 to 100 seconds, so that the normal process of decontaminating the carrier liquid is not seriously interrupted. We have found that fully automatic operation of the apparatus can be maintained for a working cycle whose duration, depending on the degree of contamination of the carrier liquid and on the quality and size of the suspended solid matter, may be from 2 to 24 hours.

Referring now to Figure 5, in this embodiment the tank 1 has a cylindrical side wall 27 terminating at the bottom in an inverted frusto-conical base 28 having the sludge draw-off branch 7 at its apex and containing the sludge chamber 15. A secondary sedimentation matrix, constructed and operating in the same way as the matrix 14 in Figure 1, is indicated in Figure 5 by full and broken hatching, below the filtration layer of buoyant particles 19, screen 18, headspace 20 and overflow trough 21. The air chamber 22 is arranged above, and in direct communication with, the sludge chamber 15, and is bounded at its top by an internal, inverted, frusto-conical intermediate wall 29 having a central hole from which depends a cylindrical skirt 30 extending into the sludge chamber. The air chamber thus communicates with the sedimentation chamber through the sludge chamber.

Extending inwardly from the side wall 27, above the wall 29, is a frusto-conical shield 33, of sharper cone angle than the wall 29 and separated from the latter by an annular gap 26, which constitutes an elongate slot.

Contaminated liquid is introduced through an inlet branch 31, shown diagrammatically but in practice connected tangentially to the side wall 27. The liquid is thence passed into the space above the wall 29 through the annular distributor comprising the walls 27, 29, shield 33 and slot 26, the space 9 enclosed thereby serving the purpose of an inlet chamber. The slot 26 faces down the slope of the wall 29, and is provided with at least one fixed vane 34 which imparts swirl to the fluid introduced tangentially through the port 31 and passing along the chamber 9 and so inwardly through the slot 26. The (or each) vane 34 lies in a plane which makes an angle of at least 10" with the line contained by the surface of the wall 29 in a radial plane intersecting the vane. The liquid is thus caused to enter through the slot 26 in a helical path, primary sedimentation taking place on the top surface of the wall 29.

Liquid then rises up towards the sedimentation chamber, for secondary sedimentation (of intermediate-size particles) and filtering as already described.

An alternative arrangement of the air chamber 22 is shown in Figure 6, in which the cylindrical side wall 27 of the tank extends down to a flat bottom 35 having the sludge discharge branch 7, A frusto-conical intermediate wall 28, corresponding in function to the wall 29 of Figure 5, extends between the wall 27 and bottom 35 to enclose the air chamber 22, which communicates with the sedimentation chamber above the wall 28 by means of a vertical syphon pipe 23 having its lower end 36 directed tangentially.

In the further modifications shown in Figures 7 and 8, the tank is bifurcated in a manner symmetrical about the vertical axis of the tank. The tank has two sedimentation chambers, each with its associated filter, screen, sludge chamber and sludge outlet 7.

For this purpose, the two halves of the tank are upwardly divergent. In Figure 7, the space between the two convergent sludge chambers being occupied by respective, enclosed air chambers 22, separated by a partition 37, each air chamber communicates through a syphon pipe 23 with the corresponding sedimentation chamber. In Figure 8 the arrangement is the same, except that the partition 37 is omitted, the air chamber thus being common to both halves of the apparatus and communicating with the sedimentation chambers through a single syphon pipe 23 having its top end bifurcated to form two respective branches.

A control valve 43 may be provided at the junction of these branches.

Methods such as those described have been tested and have shown great advantages over previously-proposed methods, in respect of operation and of capital cost, not in waste water treatment plant, but also in process equipment in the chemical, food and pharmaceutical industries. One particular advantage is quick starting. Another is the self-contained and compact nature of the apparatus, due partly to the savings in height associated with improved control of liquid flow, mentioned hereinbefore, and partly to the fact that flocculation and/or clarification can be carried out within the same tank as the remainder of the process.

The apparatus is thus readily capable of being made mobile, so that it may be used on a temporary basis (e.g. for purification of water at seasonal sites such as recreation centres etc.), besides being suitable for permanent installation.

WHAT WE CLAIM IS: 1. A method of separating solid particles from a liquid suspension, comprising the successive stages, all carried out in a single vessel, of: introducing the suspension at an intermediate level in the vessel; separating larger particles from the suspension by simple sedimentation; then passing the liquid suspension in an upwardly-inclined path through upwardlyinclined, relatively narrow sedimentation spaces, in which suspended particles of intermediate size are separated by further sedimentation; and finally passing the liquid suspension upwardly beyond the sedimentation spaces and through a filtration layer of buoyant particles contained below a perforate filterretaining screen, so that the finest suspended particles are separated by filtration whilst the liquid, now substantially free of suspended particles, passes upwardly through the screen, to be led off by overflow at a level spaced above the screen by a depth at least as great as the thickness of the filtration layer, the separated solids from each of said stages being allowed to fall freely through the liquid suspension to accumulate below said intermediate level.

2. A method according to Claim 1,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. and broken hatching, below the filtration layer of buoyant particles 19, screen 18, headspace 20 and overflow trough 21. The air chamber 22 is arranged above, and in direct communication with, the sludge chamber 15, and is bounded at its top by an internal, inverted, frusto-conical intermediate wall 29 having a central hole from which depends a cylindrical skirt 30 extending into the sludge chamber. The air chamber thus communicates with the sedimentation chamber through the sludge chamber. Extending inwardly from the side wall 27, above the wall 29, is a frusto-conical shield 33, of sharper cone angle than the wall 29 and separated from the latter by an annular gap 26, which constitutes an elongate slot. Contaminated liquid is introduced through an inlet branch 31, shown diagrammatically but in practice connected tangentially to the side wall 27. The liquid is thence passed into the space above the wall 29 through the annular distributor comprising the walls 27, 29, shield 33 and slot 26, the space 9 enclosed thereby serving the purpose of an inlet chamber. The slot 26 faces down the slope of the wall 29, and is provided with at least one fixed vane 34 which imparts swirl to the fluid introduced tangentially through the port 31 and passing along the chamber 9 and so inwardly through the slot 26. The (or each) vane 34 lies in a plane which makes an angle of at least 10" with the line contained by the surface of the wall 29 in a radial plane intersecting the vane. The liquid is thus caused to enter through the slot 26 in a helical path, primary sedimentation taking place on the top surface of the wall 29. Liquid then rises up towards the sedimentation chamber, for secondary sedimentation (of intermediate-size particles) and filtering as already described. An alternative arrangement of the air chamber 22 is shown in Figure 6, in which the cylindrical side wall 27 of the tank extends down to a flat bottom 35 having the sludge discharge branch 7, A frusto-conical intermediate wall 28, corresponding in function to the wall 29 of Figure 5, extends between the wall 27 and bottom 35 to enclose the air chamber 22, which communicates with the sedimentation chamber above the wall 28 by means of a vertical syphon pipe 23 having its lower end 36 directed tangentially. In the further modifications shown in Figures 7 and 8, the tank is bifurcated in a manner symmetrical about the vertical axis of the tank. The tank has two sedimentation chambers, each with its associated filter, screen, sludge chamber and sludge outlet 7. For this purpose, the two halves of the tank are upwardly divergent. In Figure 7, the space between the two convergent sludge chambers being occupied by respective, enclosed air chambers 22, separated by a partition 37, each air chamber communicates through a syphon pipe 23 with the corresponding sedimentation chamber. In Figure 8 the arrangement is the same, except that the partition 37 is omitted, the air chamber thus being common to both halves of the apparatus and communicating with the sedimentation chambers through a single syphon pipe 23 having its top end bifurcated to form two respective branches. A control valve 43 may be provided at the junction of these branches. Methods such as those described have been tested and have shown great advantages over previously-proposed methods, in respect of operation and of capital cost, not in waste water treatment plant, but also in process equipment in the chemical, food and pharmaceutical industries. One particular advantage is quick starting. Another is the self-contained and compact nature of the apparatus, due partly to the savings in height associated with improved control of liquid flow, mentioned hereinbefore, and partly to the fact that flocculation and/or clarification can be carried out within the same tank as the remainder of the process. The apparatus is thus readily capable of being made mobile, so that it may be used on a temporary basis (e.g. for purification of water at seasonal sites such as recreation centres etc.), besides being suitable for permanent installation. WHAT WE CLAIM IS:

1. A method of separating solid particles from a liquid suspension, comprising the successive stages, all carried out in a single vessel, of: introducing the suspension at an intermediate level in the vessel; separating larger particles from the suspension by simple sedimentation; then passing the liquid suspension in an upwardly-inclined path through upwardlyinclined, relatively narrow sedimentation spaces, in which suspended particles of intermediate size are separated by further sedimentation; and finally passing the liquid suspension upwardly beyond the sedimentation spaces and through a filtration layer of buoyant particles contained below a perforate filterretaining screen, so that the finest suspended particles are separated by filtration whilst the liquid, now substantially free of suspended particles, passes upwardly through the screen, to be led off by overflow at a level spaced above the screen by a depth at least as great as the thickness of the filtration layer, the separated solids from each of said stages being allowed to fall freely through the liquid suspension to accumulate below said intermediate level.

2. A method according to Claim 1,

wherein the filtration layer is cleaned periodically by sudden backflow of the treated liquid from above the screen, causing temporary backward expansion of the filtration layer and release of filtered-out particles therefrom through the sedimentation spaces to the bottom of the vessel.

3. A method according to Claim 2, wherein the backward expansion of the filtration layer during cleaning increases its depth to at least twice its normal thickness.

4. A method according to Claim 2 or Claim 3, wherein, during cleaning, buoyant particles from the filtration layer penetrate the inclined sedimentation spaces to be cleaned by contact with each other and with walls defining said spaces.

5. A method according to any one of Claims 2 to 4, wherein the backflow is caused by suction induced upstream of the inclined sedimentation spaces by ejection of a flow of pressurised air to atmosphere along a path communicating with the interior of the vessel.

6. Apparatus for separating solid particles from a liquid suspension by a method according to any one of the preceding claims, the apparatus comprising a vessel having: a sludge chamber with draw-off means at the bottom thereof; a sedimentation chamber above the sludge chamber and containing an array of upwardly-inclined plates defining between them relatively narrow sedimentation spaces open at top and bottom; an inlet chamber for the liquid suspension, communicating with the sludge and sedimentation chambers through a distributor, disposed at an intermediate level below the sedimentation chamber and above the sludge chamber, the distributor having an outlet in the form of an elongate slot; a perforate filter-retaining screen extending across the tank below the top of the tank but above the sedimentation chamber, being spaced above the latter by a filter space; a filtration layer of buoyant particles held down in the filter space by the screen; and an overflow trough adjacent to the top of the tank, spaced above the screen by an amount such that, when the tank contains liquid spilling over into the trough, the screen lies at a depth at least equal to the thickness of the filtration layer.

7. Apparatus according to Claim 6, having an air chamber and means interconnecting the air chamber with a lower part of the sedimentation chamber and an upper part of the sludge chamber, and, via piping, with a source of compressed air provided with a reduction valve, the piping being provided between the reduction valve and the air chamber with both a venting valve and a pressure sensor.

8. Apparatus according to Claim 7, wherein the air chamber is interconnected through the upper part of the sludge chamber with the sedimentation chamber.

9. Apparatus according to Claim 7 or Claim 8, wherein the interconnecting means comprises a syphon.

10. Apparatus according to Claim 8, wherein the interconnecting means comprises a draw-off opening.

11. Apparatus according to any one of Claims 6 to 10, wherein the distributor has the shape of a slender, horizontal, hollow pyramid having two of its sides inclined convergently downwards towards the elongate slot at the bottom, said slot being straight and separating the two said sides and facing a reflection shield disposed below the slot.

12. Apparatus according to any one of Claims 6 to 10, wherein the distributor is annular, being constituted by part of a cylindrical side wall of the vessel, a frustoconical, downwardly directed intermediate wall extending inwardly from the side wall, and a frusto-conical shield extending inwardly from the side wall above and toward the intermediate wall, the elongate slot being an annular gap separating the intermediate wall from the bottom of the shield and facing down the slope of the intermediate wall, the distributor including at least one fixed vane in the slot, inclined at an angle of at least 100 with the line contained by the surface of the intermediate wall in a radial plane intersecting the vane, so as to impart swirl to the liquid suspension passing through the slot.

13. Apparatus according to Claim 12 when dependent on Claim 7, wherein the air chamber communicates with the sedimentation chamber through the sludge chamber, being bounded at its top by the frustoconical intermediate wall.

14. Apparatus according to any one of Claims 6 to 13, wherein the sludge compartment is provided in an upper part thereof with a further array of downwardly-inclined plates defining between them relatively narrow spaces open at top and bottom.

15. Apparatus for separating solid particles from a liquid suspension by a method according to Claim 1, said apparatus being constructed, arranged and adapted to operate substantially as hereinbefore described with reference to, and as illustrated in, Figures 1, 2 and 4 of the accompanying diagrammatic drawings.

16. Apparatus according to Claim 15, modified substantially as hereinbefore described with reference to, and as illustrated in, Figure 3 of the drawings.

17. Apparatus for separating solid particles from a liquid suspension by a method according to Claim 1, said apparatus being constructed, arranged and adapted to operate substantially as hereinbefore described with reference to, and as illustrated in, Figure 5 or 6 or 7 or 8 of the accompanging diagrammatic drawings.

18. A method of separating solid particles from a liquid suspension, said method being substantially as hereinbefore described with reference to the accompanying drawings.

19. A liquid which has been treated by a method as claimed in any one of Claims 1 to 5 or Claim 18 or in apparatus as claimed in any one of Claims 6 to 17.