GB2031295A

GB2031295A - A de-watering plant for removing water from sludges and similar substances

Info

Publication number: GB2031295A
Application number: GB7904930A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-10-13
Filing date: 1979-02-12
Publication date: 1980-04-23
Also published as: DE2844716A1; DE2844716C2; GB2031295B

Abstract

In a de-watering plant for sludges, particularly for water purification, a sludge chamber 1 has an inlet 2 and an outlet 5, the bottom wall of the chamber comprises a continuous filter band 6 carried on a carrier belt 7, and mounted in the sludge chamber are a plurality of rotatably driven filter drums 8, each drum 8 having an associated cleaning device. The filter drums are arranged to urge the sludge towards the sludge chamber outlet and also serve to conduct filtrate externally of the sludge chamber. The cleaning device may comprise a brush 30 mounted inside the drum co-axially for relative rotation and a stationary scraper element 9 outside the drum. Internal spray nozzles may be provided for the drum. The band 6 leads to at least one pressing stage where the sludge on the band is pressed by a second endless band 11. The band 6 is supported on a carrier band 7 which has channels 18. <IMAGE>

Description

SPECIFICATION A de-watering plant for removing water from sludges and similar substances This invention relates to a de-watering plant for removing water from sludges and similar substances, particularly from the sludges which occur in water purification plant, which require further and more intensive concentration of solids, e.g. in filter belt presses, chamber-type filter presses, vacuum filters or centrifugal filters.

It is common practice with sludges and substances of this kind, to subject the sludge to a preliminary, primary gravitational or settlement de-watering stage which may involve, either the provision of horizontal filter paths of revolving filter belts which hold back the sludge whilst the water is gravitationally discharged, or the introduction of flocculated sludge into the advancing pouches or chambers of chamber-belt filters, where the sludge is held in the filter pouches from which the filtrate discharges by gravity.

A typical known plant for de-watering sludge prior to feeding it to a sludge press comprises a settling tank with a screen or sieve drum which is rotated to separate the water from the sludge, and the water filters through into the interior of the drum. In this known construction, the screen drum is partially immersed in the sludge and inside the drum there is an array of jet sprays directed against the non-immersed region of the drum to clean the effective sieve or screen surface. The water has been extracted from the sludge is then conducted laterally out of the tank. In this arrangement, the sludge outlet is located in the region of the upper end of the sloping base or bottom wall of the settling tank.The plant is operated intermittently, the watery sludge is first introduced into the settling tank and then the sieve drum is set in rotation when the level of watery sludge in the tank has reached the underside of the drum. The concentrated sludge which sinks and collects at the funnel-shaped lower end of the settling tank is then lifted over the upwardly inclined bottom by a chain conveyor, which is followed by a cascade-type sceen belt conveyor whereby the sludge is constantly turned over and taken down to a station from which it goes on for further processing. The chief drawback of this known arrangement resides in its comparatively low efficiency due to the limited area of available filter surface.Another disadvantage must be seen in that, as a direct result of the chosen manner of handling the sludge, the effect of the flocculating agent is, at least partially, destroyed by this treatment in as much as the sludge particles which are held together by flocculation tend to break apart under this treatment, and this is a considerable drawback with regard to the subsequent pressing process.

Other known devices of this kin#d, whilst enabling higher throughput rates to be achieved in the associated, aforementioned main de-watering plant, have the disadvantage that the size of filter surface area available for gravitational water discharge is directly related to the amount of water discharged. This means that high throughout rates can only be obtained with comparatively large structural dimensions which take up a great deal of space.Another disadvantage appertaining to such known pre-dewatering plant systems resides in that the flocculated sludge may suffer adverse effect either on being introduced into the filter pouches of a pouch belt filter or on being loaded on the filter belts, in as much as the flocculated particle structure is easily damaged or destroyed by such transfer operations so that the eventual final dewatering process is not effective at desired performance rates.

It is the aim of this invention to provide a dewatering plant wherein the described disadvantages are avoided, or at any rate greatly reduced.

According to this invention, there is provided a de-watering plant for removing water from sludges and similar substances, particularly from the sludges which occur in water purification systems, said de-watering plant comprising a sludge chamber to which sludge is supplied through an inlet and moved towards an outlet for further treatment for further concentration of the solids in the sludge, characterised in that the sludge chamber has a bottom wall comprising a continuous advancing filter band and that within the sludge chamber either wholly or partially submerged in the sludge, there are disposed and arranged a plurality of rotatably driven filter drums, each filter drum having an associated cleaning device and an outlet for filtrate arranged to conduct the filtrate outside the sludge chamber.

The main advantage arising from this invention resides in that the sludge can be introduced gently into the sludge chamber and that, moreover, it is now possible to dispense with spray-cleaning jets for the filter elements. It is considered to be a further advantage that, especially with the filter elements wholly submerged in the sludge, a certain pressure level is guaranteed by the sludge level above the filter elements which enhances and accelerates the de-watering process.

A particular advantage of the invention can be realised by arranging for the filter band to be carried and supported on a co-advancing carrier belt which is preferably a plastics or rubber belt with comparatively high lateral rigidity.

For acceleration of filtrate evacuation, the carrier belt is preferably provided with a plurality of mutually parallel, open-top, transversely extending channels, the filter band resting on the strips or bands of material which extend between these channels.

It is also preferred that these channels should be obliquely directed relative to the longitudinal axis of the carrier belt at an angle of less or more than 900, preferably 450. Such an arrangement, in conjunction with a sloping bottom of the sludge chamber considerably accelerates and improves filtrate evacuation.

In a preferred embodiment of the invention, supporting and return rollers which carry the filter band and the carrier belt are provided with lateral guide means for the belts/bands which ensure track-holding, and the guide means may comprise simple rims or flanges at the ends of the rollers.

Due to the strong lateral rigidity of the carrier belt a simple provision like this is only required.

According to a further development of this invention the filter drums are provided with cleaning devices comprising internal revolving brushes. Such brushes have the special effect that their bristles during rotation penetrate through the holes in the filter material, and they impart a pushing impetus to any sludge particles which may adhere around these holes thereby keeping the filter element free of material adhering thereto at all times. The same penetration of the bristles through the holes of the filter material also considerably accelerates water discharge.

In one arrangement, the filter drums rotate in a common direction of rotation and this creates a conveying movement carrying the sludge towards the sludge chamber outlet.

In this particular arrangement of the invention, it is also possible to provide stationary cleaning devices, externally or internally of the filter drums.

Preferably, the internal cleaning devices comprise brushes arranged on the inside of the filter drums as aforementioned.

This particular embodiment of the invention may be further developed by arranging for the internal brushes to be continuously rotated in the opposite sense to that of the filter drum elements, whereby the rate of water discharge can be further increased.

The brushes may preferably be mounted on a hollow shaft which is provided with spray nozzles or jets.

According to an advantageous further development of the invention, the bottom wall of the sludge chamber adjoins or leads to a further de-watering stage. In this stage, a further continuously advancing band is applied to the topside of the sludge which is carried on the filter band of the bottom wall of the sludge chamber.

This further band preferably comprises flexible transverse blades or vanes directed towards the filter band so that the upper band is capable of exerting a certain amount of pressure on the sludge.

In a preferred embodiment of the invention, the sludge chamber comprises an ante-chamber which is connected to a filtrate or rinsing water return flow line. For preference, this ante-chamber is of circular form in cross-section and houses a filter drum element of smaller diameter. The filter drum element is preferably rotated and comprises transverse fins or blades which engage with the internal wall of the anti-chamber and act as valves in such a way that in each and any rotational position of the filter drum element, the filtrate inlet is closed from the sludge chamber.

In a particularly preferred form of execution, the preliminary de-watering device is associated in successive combination with a continuously operating filter press including a pair of continuous advancing filter bands, wherein the filter band which forms the bottom wall of the sludge chamber is one of said pair of filter bands.

In another embodiment of the invention, one of the walls of the sludge chamber may comprise a revolving combination or assembly comprising a carrier belt and a filter band. In such embodiment of the invention, for the carrier belt/filter band combination assembly may provide the bottom as well as one of the walls of the sludge chamber.

It is obvious that due to such an arrangement, filter surface area of maximal dimensions can be made available because only the side walls wherein the filter drums are mounted, and the top cover of the sludge chamber are not made of a pervious filter material.

In further development of the invention, the sludge chamber is closed on all sides and adapted to be charged with the pumping pressure of a sludge feed pump which pumps the sludge to the inlet of the chamber. This arrangement offers the distinct advantage of enabling the plant according to this invention to operate like a chamber-type filter press which, due to the available filter surface area, is capable of removing considerable amounts of water from the sludge, because filtrate yield rate depends on the magnitude of applied pressure.

Conveniently, in this embodiment of the invention co-advancing lateral sealing means which provide effective lateral seals for the space between the filter bands in the region of the sludge outlet of the sludge chamber can be provided so that a fully enclosed system is created wherein substantial pressures can be applied which, for various practical considerations, would be in the region of 1 kg/cm2.

Preferably, the lateral sealing means comprise flexible hollow bodies made of rubber or like material, with a circular cross-section. As such cross-sectional configuration and material permit deformation there are no problems during advancement of the bands and belts. Furthermore, such hollow bodies have enough flexibility to make a reliable pressure seal along the lateral edges of the filter bands.

According to a further development of the invention, the sludge chamber may be provided with a second inlet which is connected to a rinsing water and/or filtrate return flow line associated with further equipment stages or with the filter bands. Such an additional inlet is preferably provided with a pressure-gate control means.

The invention is hereinafter more particularly described with reference to some examples of embodiments of the invention shown in the accompanying drawings wherein: FIGURE 1 is a schematic sectional side elevation of a first embodiment of the invention; FIGURE 2 is a detail section of Figure 1; FIGURE 3 is a cross-sectional view of Figure 2; FIGURE 4 depicts a modification, and the view is a detail section as in Figure 2; FIGURE 5 is a cross-sectional view of Figure 4; FIGURES 6 and 7 show details of the filter bands and carrier belts; FIGURES 8 and 9 are both schematic sectional side elevations and show examples of this invention in combination with further de-watering plant; and FIGURES 10 and 1 1 are both schematic sectional side elevations and show a particularly preferred embodiment of the invention.

With reference to Figure 1 , the plant according to this invention comprises a sludge chamber 1 provided with a sludge inlet 2 at one end thereof.

As shown, the sludge inlet is below the upper sludge level, indicated by arrow 3, in the sludge chamber 1 so that the sludge is very gently and carefully introduced into the chamber. The bottom 4 of the sludge chamber 1 is formed by a continuous filter band 6 resting on a continuous carrier belt 7 therebeneath, and the bottom 4 extends upwardly from the inlet 2 to an outlet 5.

Inside the sludge chamber 1 there are a number of filter elements 8, partially or wholly submerged in sludge 12. These elements 8 are driven to rotate in a common direction to convey or move the sludge from the inlet 2 to the outlet 5 of the sludge chamber 1. In this embodiment, each filter element 8 is associated with fixed, i.e. nonrotating, stripper 9 which strips any sludge material adhering to the filter material of the drum elements 8 so as to keep the filter holes open.

Within the sludge chamber 1 there is provided a pre-de-watering region 10 which is next to the filter drums 8 as viewed in the direction of sludge advancement. In this region 10 a further belt 1 1 overlies the uphill portion of the filter band 6 and carrier belt 7. This further belt 1 1 comprises flexible transverse blades 13 which engage the filter band 6 and carry the sludge 12 along the filter band 6. The blades 13 also co-operate with the side walls of the sludge chamber 1 to form fully enclosed chambers or compartments, which, by virtue of theirflexibility, can at the same time apply pressure to the sludge in these compartments or chambers since the space between filter band 6 and belt 1 1 is of slightly tapered configuration. The extent of the belt 1 1 is inclined relative to the underlying extent of the filter band 6.

The continuous filter band 6 runs with its carrier belt 7 over and around a pair of end rollers 14, 15, one of which is driven. The roller 15 is spring-loaded in order to tension the belt and the band. Along that distance of their advancement which corresponds to the length of the chamber bottom 4, the belt 7 and band 6 are supported by a number of rollers 1 6 which prevent them from sagging under the weight of their load. A rinsing or cleaning station 17 for the filter band 6 is provided.

Due to the substantial lateral rigidity of the carrier belt 7, the precise configuration and makeup of which is explained later, it is possible to dispense with special guide means for keeping the band and belt straight and true because this can be done by simple track-holding rings, rims or flanges on opposed ends of the various rollers 14, 15, and 16.

With reference to Figures 6 and 7, the details and special aspects of the filter band 6 and carrier belt 7 will now be described.

Figure 6 is a sectional view of the assembly or combination with the sludge 12 supported thereon. As shown, the carrier belt 7 is provided with open-topped transverse channels 18 which open to the side facing the filter band 6, and the filter band 6 rests on the strip-like lands 19 which extend between the channels 18. As shown in Figure 7, the channels 18 extend obliquely to the direction of advancement of the carrier belt 7, and preferably are inclined at an angle of 450 to the direction of advancement, that being the major axis of the carrier belt 7.

The water contained in the sludge 12 can therefore pass through the filter band 6 and enter into the channels 18, as indicated by arrows 20 in Figure 6, and due to the gradient of the filter band/carrier belt combination, the water can be quickly conducted to the side and may be collected in a suitable manner. The carrier band 7 is made of a plastics or rubber material and has high lateral rigidity.

Figures 2 and 3 depict one form of the filter elements 8 which are partly or wholly submerged in the sludge within sludge chamber 1. As shown, the filter element 8 consists of a cylinder 21 made of pervious filter material supported both ends with short, open-ended pipes 22. These pipes 22 are mounted by means of flanges 23 in the side walls 24 of the sludge chamber 1 in such a way as to provide both ends of the filter elements 8 with outlets 25 leading from the sludge chamber 1. As indicated by arrows 26, the water in the sludge 12 can penetrate through into the interior of the filter drums 8 through the filter material and be diverted through the outlets 25 as shown by arrows 27.

To assist this effect and to keep the filter material constantly clear, each pipe 22 of the drum 8 mounts a bearing 28 for a rotationally driven shaft 29. The shaft 29 is fitted with three brushes 30 with bristles of nylon or the like of such size that as the shaft 29 rotates, the bristles will penetrate slightly through the holes in the filter material of the cylinder 21 and detach any sludge particles and assist in water discharge.

It will be understood that whilst water penetrating the filter drum 8 is discharged, at the same time, the water which penetrates through the filter band 6 atthe bottom 4 of the sludge chamber 1 is also laterally diverted and evacuated through the channels 18 as indicated by arrows 31.

Figures 4 and 5 are views corresponding to Figures 2 and 3 showing a modified form of filter element 8. The pipe or tubular ends 22 of the filter body 8 are mounted in gland-type seals 33 in the side walls 24 of the sludge chamber 1. The elements 8 are rotated by a drive transmitted through gear 34 on a hollow shaft 35. The central shaft 29 is driven in the opposite sense at an independent speed so that it is possible to vary the frequency at which the bristles of brushes 30 penetrate through the holes in the filter material.

This enables the control of the intensity of water discharge in accordance with requirements. A further stripper element 36 is provided on the outside of the filter body 8 to assist in the cleaning action of the brushes 30, but this is an optional provision.

Figures 2 to 5 further show that the shaft 29 is preferably a hollow or tubular shaft and comprises a row of spray nozzles or jets 51 which permit intermittent additionally cleaning of the filter material by water under pressure, should this be needed.

Figure 8 is a schematic sectional side elevation of another de-watering plant according to this invention for use with a simple band filter press.

As shown, the bottom 4 of the sludge chamber 1 has a double gradient which increases the effective filter surface area of the bottom 4. There are a plurality of the filter elements 8 as aforedescribed in the sludge chamber 1. The filter element which is at the lowest point of the sludge chamber, is provided with relatively spaced rubber rims or flanges 37 which convert this element into a return pulley or roller for the assembly of the filter band 6 and carrier belt 7 which forms the bottom of the sludge chamber 1. These rims 37 do not impede the flow and advancement of the sludge 12.

At the outlet 5 the sludge which has been preliminarily de-watered in chamber 1 is transferred to another filter band 38 with its own carrier belt 39, identical with the filter band 6 and carrier belt 7 combination hereinbefore described.

This continuous filter band 38 with its carrier belt 39 is driven around opposed rollers 40, 41. Along the extent of the filter band 38 which is opposite to the filter band 6 with carrier belt 7, the band 38 and belt 39 are supported by rollers 42. The filter bands 6 and 38 define a wedge-shaped space 43 which is determined by spring-loaded rollers 44 which bias the filter band 6 and carrier belt 7 towards the filter band 38 and carrier belt 39. As a result increasing pressure is applied to the sludge which had been pre-drained in sludge chamber 1 and this pressure application stage is continued right up to the ejector station 45. Behind the ejector station 45 the filer bands 6 and 38 are conducted over suitable pulleys or rollers through respective rinsing and cleaning stations 47, 46.

Figure 9 shows the combination of a preliminary de-watering plant according to this invention with a band filter press of the kind comprising a medium pressure stage 48 and a high pressure stage 49. In the arrangement of Figure 9 additional filter surface area is achieved by the fact that filter band 6 with carrier belt 7 also forms one of the end walls of the sludge chamber 1, and this provides an additional de watering area. The filter element 550 in the sludge chamber 1 also functions as a return pulley element, as described with reference to Figure 8.

The examples of Figures 8 and 9 are given to show that in the event of such combinations, the filter band and associated carrier belt which form the bottom wall of the sludge chamber will also be one of the filter bands of the associated band filter press.

Figures 10 and 11 depict a preferred variant of the invention. As shown, the sludge chamber consists almost entirely, that is to say, with the exception only of its side walls and a comparatively small upper wall region, of revolving filter bands with associated carrier belts.

Filter band 6 with its carrier belt 7 forms the bottom 4 and the rear wall 55 of the sludge chamber. The front wall 54 of the chamber 1 is formed by a further revolving combination assembly of a filter band 52 with carrier belt 53 of the same constructional type as previously described. The upper end of the sludge chamber 1 is completely closed as the filter bands 6 and 52 enter into the chamber through sealing elements 64. The cover part for the chamber is provided by a device 65 for processing and recovering the effluent of the rinsing stations 46,47 for the filter bands 52, 6, or further parts of the equipment.

This processor or recovery device 65 also comprises a driven, rotating filter drum 8 with associated cleaning means and an additional inlet 62 communicating to the sludge chamber 1. This embodiment was designed in order to submit the sludge chamber 1 to a substantial pressure provided by a sludge feed pump (not shown) which pumps the sludge towards and through the inlet 2. For this reason, the effluent recovery device 65 is provided with a pressure gate 63, which consists of a vaned rotor of star-section rotating in a designed housing so that pressure within the sludge chamber 1 cannot be applied to the recovery device 65 to which the dirty effluent is fed gravitationally. For the same reason a coadvancing lateral sealing element 60 is provided along each of the lateral edges of the filter bands 6, 52. This is described below with reference to Figure 11.

The two lateral sealing elements 60 are conducted externally alongside and past the sludge chamber 1, and are respectively inserted between the filter bands 6,52 in the region of the sludge outlet and there firmly held in place between the filter bands. In this way the sludge in chamber 1 is confined and sealed in by a pressuretight seal comprising the sealing elements 60 and by means of the sludge trapped between the filter bands 6, 52 in the following pressure stage 66.

The pressure stage 66 comprises spring-loaded pressure rollers 63 which bias the carrier belt 53 with filter band 52 towards and relative to the assembly of the filter band 6 and carrier belt 7 which run on rollers 16. On leaving the pressure stage 66, the de-watered sludge is conveyed to the elevated ejector station 45.

Figure 11 shows details of one of the two lateral, co-advancing sealing elements 60. The lateral sealing element 60 comprises a hose-like hollow body of rubber or similar material so that the element 60 can be freely deformed under the pressure in pressure stage 66 and does not prevent or in any way impede an alteration in the distance between filter band 6 and filter band 52.

If desired, the hose element 60 may be filled with compressed air in order to enhance deformability and especially in order to prolong its service life, in which event suitable valves would be provided for topping up or refill purposes.

Claims

1. A de-watering plant for removing water from sludges and similar substances, particularly from the sludges which occur in water purification systems, said de-watering plant comprising a sludge chamber to which sludge is supplied through an inlet and moved towards an outlet for further treatment for further concentration of the solids in the sludge, characterised in that the sludge chamber has a bottom wall comprising a continuous advancing filter band and that within the sludge chamber either wholly or partially submerged in the sludge, there are disposed and arranged in a plurality of rotatably driven filter drums, each filter drum having an associated cleaning device and an outlet for filtrate arranged to conduct the filtrate outside the sludge chamber.

2. A de-watering plant according to claim 1, characterised in that the filter band is supported on a co-advancing carrier belt.

3. A de-watering plant according to claim 2, characterised in that the carrier belt comprises plastics or rubber material band and has comparatively high lateral rigidity.

4. A de-watering plant according to claim 2 or 3, characterised in that the carrier belt comprises a plurality of open-top, transverse channels and the filter band rests on the strips or lands of material extending between said channels.

5. A de-watering plant according to claim 4 characterised in that the channels are inclined at an angle which is larger than 900, preferably 450, to the major axis of the carrier belt.

6. A de-watering plant according to any one of claims 2 to 5, characterised in that lateral band guide means are provided on supporting and return rollers which carry the carrier belt and the filter band.

7. A de-watering plant according to claim 6, characterised in that the guide means on a roller are track-holding rims or flanges at the ends of the roller.

8. A de-watering plant according to any one of the preceding claims, characterised in that the cleaning device provided on a filter drum is arranged for rotation internally or externally relative to the filter drum.

9. A de-watering plant according to claim 8, characterised in that the cleaning device comprises an internally revolving brush.

10. A de-watering plant according to any one of the preceding claims, characterised in that the filter drums rotate in a common direction of rotation which induces a conveying movement of the sludge towards the outlet of the sludge chamber.

11. A de-watering plant according to claim 10, characterised in that the cleaning devices comprise stationary externally arranged scrapers or blades.

12. A de-watering plant according to any one of claims 8 to 10, characterised in that the brushes rotate in the opposite sense to the direction of rotation of the filter drum.

13. A de-watering plant according to any one of the preceding claims 8 to 12, characterised in that the brushes are mounted on a tubular shaft that comprises spray nozzles or jets.

14. A de-watering plant according to any one of the preceding claims, characterised in that the bottom of the sludge chamber adjoins or leads to a further de-watering treatment stage.

15. A de-watering plant according to claim 14, characterised in that in the further de-watering stage a further continuous advancing band engages with the top side of sludge carried on the filter band of the bottom wall of the sludge chamber.

1 6. A de-watering plant ac#cording to claim 15, characterised in that said further band includes flexible transverse blades or vanes which are directed towards the filter band.

1 7. A de-watering plant according to any one of the preceding claims, characterised in that the sludge chamber is provided with an ante-chamber which is connected to a filtrate or rinsing water return flow line.

18. A de-watering plant according to claim 17, characterised in that the ante-chamber has a circular cross-sectional configuration.

19. A de-watering plant according to claim 18 characterised in that a filter drum of smaller diameter than the ante-chamber diameter is mounted inside the ante-chamber.

20. A de-watering plant according to claim 18 or 19, characterised in that the filter drum is rotatably driven.

21. A de-watering plant according to any one of claims 18 to 20, characterised in that the filter drum comprises outwardly projecting transverse fins or blades which are arranged to engage with the ante-chamber walls and act as valves in such a way that the filtrate or rinsing water return inlet is closed from the sludge chamber in each and any rotational position of the filter drum.

22. A de-watering device according to any one of the preceding claims, characterised by the provision, in successive combination therewith, of a continuously operating filter press including a pair of continuous advancing filter bands, and in that the filter band which forms the bottom of the smudge chamber is one of the pair of filter bands of said press.

23. A de-watering plant according to any one of the preceding claims, characterised in that one of the walls of the sludge chamber is formed by an advancing combination or assembly of the carrier belt and filter band comprising the bottom wall of the sludge chamber.

24. A de-watering plant according to any one of the preceding claims, characterised in that a further combination assembly comprising a filter band and a carrier belt forms another wall of the sludge chamber.

25. A de-watering plant according to claim 23 or 24, characterised in that the sludge chamber is fully enclosed and adapted to be charged by the pumping pressure of a sludge feed pump which feeds sludge to the sludge inlet.

26. A de-watering plant according to any one of claims 23 to 25, characterised by the provision of co-advancing lateral sealing elements which, in the region of the sludge outlet of the sludge chamber, provide a lateral seal between a pair of opposed spaced filter bands.

27. A de-watering plant according to claim 26, characterised in that the lateral sealing elements are flexible hollow elements made of rubber or the like having a circular cross-section.

28. A de-watering plant according to any one of the preceding claims characterised in that said inlet for the supply of sludge to the sludge chamber is disposed in the bottom of the sludge chamber, and said outlet of the sludge chamber is at a higher level with the bottom wall of the sludge chamber being inclined upwardly towards said outlet.

29. A de-watering plant according to any one of claims 23 to 28, characterised in that the sludge chamber comprises a second inlet which is connected with a return flow line of rinsing water and/or filtrate from other downstream equipment or from one or more of the filter bands.

30. A de-watering plant according to claim 29, characterised in that the second inlet is provided with a pressure gate device.

31. A de-watering plant substantially as hereinbefore described and as illustrated with reference to any one of the embodiments depicted in the accompanying drawings.