CA1048854A - Method for dewatering sludge - Google Patents

Abstract

Abstract of the Disclosure There is disclosed a method of dewatering sludge such as residuals in sewage discharge processing installations comprising the steps of feeding a partially dewatered sludge through a preliminary pressure zone in the form of two spaced apart relatively moving filter bands by guiding the sludge be-tween said two bands and causing the bands to exert pressure upon the sludge while the latter is moving along and between the bands, then guiding the two filter bands with the sludge therebetween about part of the periphery of a rotary first main dewatering pressure zone and applying pressure to the outer one of said bands and thus to the sludge while being guided by and on said first main pressure zone, whereby to effect dewatering of the sludge through both said bands, then guiding the two filter bands with the sludge there-between from said first main pressure zone to and partly over the periphery of a rotary second main dewatering pressure zone and applying pressure to the filter bands and thus to the sludge therebetween to further dewater the sludge through both bands while rotating the second main pressure zone in opposition to the first main pressure zone and so as to reverse the inner and outer por-tions of the filter bands relative to the main pressure zones. This method better overcomes the prior art problem of insufficient break-up of the sludge during the dewatering operation.

Description

-The invention relates to a method of dewatering sludge-type material as it is produced as a by-product or residual product in sewage discharge processing or similar installations.
There are known devices for dewatering sludge of the general type above referred to in which cakes formed of sludge to be processed are guided `~ between two filter bands about the circumferential wall of a drum. As a result, a certain percentage of the water contained in the cakes is removed ` as the cakes are guided about the drum.
A method using this mode of dewatering sludge and also a device :~
for carrying out the method is described, for instance, in German DTAS
1,960,787. This patent describes a dewatering press in which filter bands are guided partly about the circumference of a single drum, the sludge to be processed being carried between two filter bands. It has been found that such an arrangement does not result in an economically acceptable dewatering efficiency.
~ There are also known devices for the purpose from United States ., ~.
Patent 2,111,720 and German Patent 689,090, in which guidance of filter bands used for causing dewatering of the sludge is similarly unfavorable with respect to efficiency.
; 20 Tests have shown that the shortcomings of dewatering devices as previously described are caused primarily by an insufficient break-up of the sludge during the dewatering operation itself.
According to the invention there is provided a method of dewatering sludge, comprising the steps of feeding a partially dewatered sludge through a preliminary pressure zone in the form of two spaced apart relatively moving filter bands by guiding the sludge between said two bands and causing the bands to exert pressure upon the sludge while the latter is moving along and between the bands, then guiding the two filter bands with the sludge there-between about part of the periphery of a rotary first main dewatering pressure zone and applying pressure to the outer one of said bands and thus to the L~

;, t ~ , : .-. - , , sludge while being guided by and on said first main pressure zone, whereby to effect dewatering of the sludge through both said bands, then guiding the two filter bands with the sludge therebetween from said first main pressure zone to and partly over the periphery of a rotary second main dewatering pressure zone and applying pressure to the filter bands and thus to the sludge there-between to further dewater the sludge through both bands while rotating the second main pressure zone in opposition to the first main pressure zone and so as to reverse the inner and outer portions of the filter bands relative to the main pressure zones.
Thus, the invention combines a high dewatering efficiency with minimal space requirements. These advantages of high dewatering efficiency and minimal space requirements are obtained by increasing the active length of the filter bands in relation to the total length of the filter bands which, in all dewatering devices as heretofore known, was well below 50%. Modern demands with respect to the quality of processing sewage water discharges require much higher efficiency for environmental reasons coupled with economically acceptable costs.
Conveniently, at least two drums are provided as dewatering drums and guiding filter bands are provided between which the sludge to be dewatered is carried, at least over part of the circumferential walls of these drums.
In this connection it is advantageous to drive the two drums in opposite directions and to guide the filter bands over the circumferential walls of the drum so that they define a generally S-shaped configuration in upright position.
The circumferential walls of the drums may be made either of corrugated or perforated sheet metal such as steel, or they may be composed of tubes extending lengthwise of the axes of the drums. The latter arrange-ment assures that the filtrate as it is extracted from passing sludge will be discharged into the interior of the drums from which it can be conveniently removed.

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' ' ' ' ' :' ' ' .'. : , 1~48854 The dewatering drums define within the range of their circum-ferential walls about which the filter bands are guided several pressure zones within which an S-shaped guidance of the filter bands causes pulling, kneading and deforming action upon the generally cake-shaped sludge portions placed between the filter bands. As a result, the internal consistency of the cake is loosened thereby facilitating extraction of water therefrom.
In this connection, it may be mentioned that the concept of the invention is not limited to two drums, but in some instances it may be pre-ferable to provide more than two drums in side-by-side arrangement and to guide the filter bands accordingly over the circumferential walls of the drums.
According to another embodiment of the invention, sludge in the form of sludge cakes of inhomogenous consistency, after having been guided through one or more straining and/or preliminary dewatering zones (-n) supported on a filter band, is guided to a further filter band while being simultaneously mixed, if desired, assisted by gravitational force. There-upon the sludge cakes are guided between the two filter bands which are disposed in parallel relationship and preferably along a linear path through a preliminary pressure zone. At the end of this zone the sludge is guided over the first one of the afore-referred to dewatering drums. At the same time, the sludge cakes may be subjected to vibratory pressure and continuing shear forces. After having been subjected to the action of this composite force resulting in a thorough loosening of the sludge cakes, the filter bands and thus also the sludge cakes therebetween are deflected after leaving the first dewatering drum into the opposite direction and then guided upon the second dewatering drum, that is, the sludge cakes are now subjected to a pressure zone curved in opposition to the curvature on the first drum since, as previously stated, the two drums being driven in opposite direction.
The afore-referred to pulsing or vibrating pressure is obtained by guiding the filter bands and thus also the sludge cakes between pressure 1048~54 rolls and the circumferential wall of the dewatering drum l/n. The just-described aspect of the invention may also provide that the pressure force applied to the sludge cakes via the filter bands, and more specifically by the outer filter band, is increasing in the driving direction of the bands.
In a preferred embodiment of the invention, the envelopment angle of the filter bands on the drums which obviously contTols the length of the pressure zones is larger than 180. It is further advantageous that the pressure or planet rolls be provided at least along the circumferential range of the drum as determined by the afore-mentioned angle of more than 180.
As it is evident, the longer the pressure zone is, the more inten-sive is the action of the shear forces upon the sludge cake as these shear forces are generated when and while the filter bands envelope the circum-ferential walls of the dewatering drums. Obviously, the outer filter band must travel a longer distance than the inner filter band as they are driven by the rotation of the drums. Accordingly, assuming equal driving speed for both bands, the outer band will be retarded relative to the inner band by a distance which is equal to the product of the radial differential, that is, the radial thickness of the cakes between the two bands times the angle of envelopment. This retarding of the outer filter band causes displacement or deformations within the cakes. As a result~there is a change in the consist-ency of the cakes, and particularly a loosening thereof, so that liquid can be more readily driven out of the cakes. Moreover, the already described reversal of the direction of stresses due to the opposite rotational direc-tions materially increases the dewatering effect. ~-The invention also allows the pressure of the planet rolls, both for individual rolls and for a group thereof, can be controlled. Such con- ~ -trol can be effected in a particularly simple manner by means of a tension member such as a rope. This tension member is in contact with lever arms provided according to the invention, which in turn are linked to a stand -. ' ' . ' . .

8~54 or frame structure for the drums or to another suitable support.
The rope or other tensioning member which is tensioned along the circumferential outline of the dewatering drums may coact for example with adjustable noses or dogs on the lever arms or other pivotal elements can be used. By tensioning of the rope the component of force composed of the force of the rope in a direction normal to the lever arms is enlarged. As a result, the pressure exerted by the planet roll or rolls toward the respective dewatering drum and thus the filter bands increases corresponding to the effective length of each lever arm between its pivot point or the setting of the nose or dog.
According to still another embodiment of the invention, the pressure or planet rolls which are preferably staggered relative to each other in the driving direction of the filter bands, can be increased by several pressure plates overlying the rolls and also made more uniform. Such pressure plates can be provided on both sides of the filter bands which are disposed substantially parallel to each other within the preliminary pressure zones, and if desirable, the pressure plates are so mounted that they can be jointly pressed toward the bands whereby additional control of the effective pressure exerted by the rolls is obtained.
The pressure rolls can also be in the form of parts of continuously rotating pressure bands, etc. These parts may be coupled by chain links. In actual practice, it depends on the specific conditions under which a particular device is operated whether the pressure bands are directly driven or driven indirectly by being placed in driving contact with the filter bands.
It is also possible to arrange pressure bands or similar members so that they in turn will drive the filter bands without special drive for the filter bands. The control of the bands can be effected in an efficient manner by means of deflection rolls, or as it is described in the present invention, by means of the dewatering drums.
In a preferred device for carrying out the method according to the . ~,, i - 5 -~8854 invention, one of the filter bands is extended to the location of a feed chute or hopper, and it extends from the point of feed about horizontally to a deflection roll. This distance between the feed point and the deflection roll may be utilized as a straining or pre-dewatering zone.
After passing the deflection roll, the aforementioned extended filter band now jointly with the second filter band which is only in tangential relation-ship with the deflection roll, is thus moving in opposition to its initial direction of movement through the preliminary pressure zones. At the end of the movement in this opposite direction, both filter bands are jointly guided upon the first one of the dewatering drums. This drum is enveloped by the two filter bands with an envelopment angle which according to the embodiment is between 220 to 240.
After having passed about the first dewatering drum, the two filter bands are guided to a further dewatering drum which is disposed side-by-side and approximately horizontally of the first drum. The envelopment angle of the filter bands about the second drum is preferably larger than the envelopment angle for the first drum.
After leaving the second dewatering drum, or if there are more than two drums after leaving the last drum, the now fully processed or worked sludge cake is discharged and each filter band ls separately guided back to the straining and/or preliminary pressure zone. While moving toward these zones, the filter bands are preferably guided into contact with at least some of the pressure rolls on the sides thereof distant from the respective drum thus increasing the effective pressure exerted by these rolls.
A dewatering device according to the invention with two dewatering drums having a drum diameter of 1 meter provide about 16 meters of active band length and the total length of the device is not more than 2.5 meters.
The thus obtained space utilization rationis more than 70%, and this is very much in excess over the utilization ration as can be obtained with dewatering devices as heretofore known.

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1~)48854 -In the accompanying drawing, several embodiments of the invention are shown by way of illustration and not by way of limitation.
Fig. 1 is a diagrammatic elevational section of a dewatering device including two substantially side-by-side disposed dewatering drums;
Fig. 2 is a detail view of Fig. 1 on an enlarged scale;
Fig. 3 is a view of a modified detail of the device according to Fig. 1 on an enlarged scale;
Fig. 4 is an elevational sectional view of a modification of a dewatering device; and Fig. 5 is a detail view of Fig. 4 on an enlarged scale.
Referring now to the figures in greater detail, and first to Fig. 1, this figure shows an exemplification of a dewatering device R which includes two large dewatering drums 1 and 2. Two filter bands 4 and 5 which are dis-posed for part of their length in parallel spaced apart relationship, envelop the two drums for part of the circumferential walls thereof.

~ 1~34~3854 More specifically, the envelopment angle for drum 1 is m (for instance 235 ), and n ~for instance 285 ), whereby the two filter bands approximately define an upright S-shape.
A cake Q formed of sludge to be processed is fed in the direction t upon first filter band 4 in the conveying direction i. This band guides the cakes Q by means of support rolls 6 through straining zone D to effect preliminary dewatering of the cakes. Water extracted from the cakes is removed through the mesh openlngs or holes in the pulling side 4 of band 4 and drops into a catch trough 7.
Each preliminæily dewatered cake Q falls at a deflectior roll 8 for filter band 4 upon the pulling side 5 of filter band 5 which is disposed approxlmately tangentially with respect to the lower sid- of deflectlon roll 8. As a result, the cakes are ~uccessively turned around and also mixed. Between the idling side 4 o the first filter band 4 and the pullinq side 5 of the second filter band S the sludge cakes are gulded through a prellminary press zone D3. In this zone pressure rolls 9 whlch are disposed staggered on both ~ides of fllter band sides 4 and 5O process the sludge caXes- Both rows of pressure rolls 9 are via pressure plates P pressed against each band side 4u and 5O by means of power sources 10 coacting with pressure plates P.
The deformation such as bends introduced by pressure rolls 9 in the ~ilter bands are ignored in the drawing to simplify the lllustratlon.
After leaving the prellmlnary pressure zone D3 the two f~lter bands 4 and 5 with sludge cakes Q therebetween are gulded upon the circumferent~al wall of dewatering drum 1 in the ~ eFtion i, within the range of the eAvelopment , , ' ' .
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~ 1~14~354 angle m. ~hls angle controls the effective length o the main pressure zone D in which the filter bands 4 and 5 are guided between the circumferential wall 11 of the drum on one hand and planet~rolls 40 disposed in juxtaposition to the outer side of the circumferential wall of the drum.
The drum wall 11 consists either of grooved or perforated metal sheets such as steel sheets, or according to ! Fig. 2, out of tubes 41 which are disposed parallel to therotational axis of the drwm. The filtrate which is pre~sed out of cakes flows between the tubes into the interior ~ of the i drum. The tubes 41 are bored at their ends ~not shown) so that any ~lltrate accumulating in the lower part of the space U
within the drum flows first lnto the space within the tubes and can then be removed from the tubes at the end~ thereof.
The planet or pressure rolls 40 are blased by the action of power supplies 42 toward the outside wall 11 of the drum. Such biasi may be obtained either for each one of the planet rolls 40 or for groups of these rolls. Each roll 40 is pivotally supported by means of a link 44 hinged to a pivot 46 as it i8 ~chematically indicated by supports 45 or similar means.
These plvotal links 44 mount dogs or noses 47 for placing there-upon a pull rope 49. The pu?ling force exerted by this rope can be àd~usted by the power supply 42 shown as a rotary pulley.
The aforedescribed pressure means ~42 to 48) permit by utilizat1on of the leverage to effect individual regulation of the pressure for each one of the planet rolls. By tens~oning rope 48 the component of force as produced by the rope pull in a direction normal to the pivotal lin~ 44 is increased, whereas the pres,sure of the planet roll 40 toward drum wall 11 varies ~. '.

¦~ 104Bg54 the respective lever arm between the pivot point 46 of linX 44 and the respective nose for supporting the pull rope 48.
The pressure of the planet rolls toward the respective drum wall i8 further enlarged by the tension of the returning filter band portlons 4e and 5e within each range C in which the planet rolls 40 ln toto or ind~vidual planet rolls 40 are utilized or e~fecting the deflecting of the filter bands. The point at which the filter ~ands 4 and 5 leave drum 1 ~d thus the pressure zone D5 is controlled by the lower leg of the envelopment angle m.
At this stage of processing each sludge caXe Q which ,~ ~ ~o~espo(~ d~
~i was bent to a curvature somewhat less than core~pondingato the radius r of dewatering drum 1, is suddenly flattened between filter bands 4, 5. As a result, tXe previously inner parts of the cake ar- stretched and the prevlou~ly outside lying cake parts are pushed together whereby the stru~ural consistency of the cake is correspondingly loosened and prepared for further pressing operations.
As i6 now apparent, the action upon the sludge cakes is similar to the one to whlch the cakes have been subjected in the straining zone Dl and the preliminary pressure zone D3 due to the actlon~of the gravitational force.
When the outer wall surface 11 of the second dewatering drum 2 ~8 reached at the end of the comparatively pressure-free zone F the sludge cakes Q which are now loose~ed as described are again bent but now in the opposite direction i3. As a result, the effect due to the simultaneous ~ction of the pressure due to the tension of the filter bands and due to the first planet roll6 40a, is further considerably increased. Due to ~uch preparation, it is possibl2 to extract from the sludge ~ 10~

cakes, which have now a totally different structural consist-ency, considerable quantities of liquid as they are guided about the dewatering drum 2. Such further extraction of water would not be possible without the aforedescribed relocation o the sludge particles forming the cake. The continuous shear action as it is obtained by the more rapidly moving inner filter band 4 further substantially as~ists the dewatering operation.
The dewatering drums 1 and 2 can be driven with synchronou# speed or with at least a minimum differential o~
circumferential speed. For this purpose, a power drive with gearings which have different transmiss~on ratios as herein-after described can be used.
Filter bands 4, 5, after leaving dewa~ering drum 2 and the range of roll~ 40z together with the dewatered sludge cakes now designated by 0 are guided over a diverter roll 50 common to both filter bands to small individual rolls 52. The roll 50 i8 disposed at the apex o~ an imaginary triangle. This triangle is defined by a line L which extends through the axes of the drums, as it is shown ln ~ig. 1 and legs defining an angle v of about 35 . The roll 50 is associated with planet rolls 51; in addition, some of the planet rolls 40 associated with the adjacent dewatering drum i touch part of the filter band sides 4 and 5e~
The two indlvidual rolls 52 are mounted spaced apart from each other and define a discharge gap 53 between filter bands 4,5 ~or e~ecting dewatered sludge cakes 0. The return or pulling band portions 4e and 5e respectively, extend from discharge gap 53 first over the planet rolls 40 (range C), then via the upp-r deflector rolls 55, back to the straining zone D

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I 1 1~48854 and to the preliminary pressure zones D3, respectively. Band 1ushing means 56 are interposed between the deflector rolls 55 and zones Dl and D3.
Fig. 3 shows another exemplification of the preliminary pressure zones D3. According to this exemplification, pressure rolls 9u are dlsposed undernea~h band sides 4u and 5 and also rolls 90 over which via roller bearings 60, engaged by chain linXs 61 are joined to form endless pulling band portions 620 and 62U in the direction of arrow S. Accordingly, a structure is formed which is similar to the flexible cage o~, for instance, a needle bearing.
Driving of each préssure roll 9 $s e~ected by frictional driving of filter bands 4,5. The pressure rolls 9 are pressed withln the pressure 7ones D3 upon the inner side of filter bands 4 and 5. Diametrically opposite to pressure rolls 9 there is disposed a pressure plate Pl and P2, respect-ively. When now the chain links 61 are guided over the sprocket teeth 63,64 in the driving direction i of the fiLter bands 4 and 5, the pressure rolls 9 are rolling of~ on the stationary pressure plate P.
The rotational speed on the opposite ~ide ~s now twice a8 high as the running ~peed of the chains formed by links 61.
This has the advantage that by means of relatively thin-walled and light pressure rolls 9, high pressure6 can be transmitted.
This is due to the fact that there is no strain due to the absence of bending forces normal to the axis of the rolls. The bending forces arè compensated by pressure plates P overlying the rolls.
The pressure plates P are spring tensioned at 65 80 that no forces need to be transmitted by an external support structure.
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1~348~354 In particular, if the bands are fairly light, the sprocket~ 63,64 for the chain links can be driven and then in turn drive the~filter band~ 4 and 5 in the direction i via the pulling band sides 62, that ~s, the drive of filter bands 4 and 5 is effected no longer by one of the deflection rolls 8, 50, 55 or the dewatering drums 1 and 2 which would cause heavy 6trains within~the range of the transmission of force upon the respective filter band 4 or 5, which must be pulled by a multi-tude o gaps between rolls. Instead of this transmission of L0 force there i8 obtained at eaah of the gaps between the rolls a transTnission of force corresponding to the prevailing pressure, and tbis transmission of force is uniformly distri-buted over a large surface.
By selecting different diameters d and at axial æpacings f ~or the upper and lower pressure rollers 9 sub-stantially more or less pronounced sine-shaped deflectors can be obtained at the surfaces of the filter bands between which sludge cakes Q are placed. Moreover, relative movement can be superimposed by differential speeds of the upper and lower pulling sides 620 and 62U of the bands.
According to the exçmplification shown in Fig. 4, the device ~1 does not include the ~training zone Dl, but the sludge caXes Q drop immediately into preliminary pressure zones D3 which is disposed below dewatering drums 1 and 2. Pressure rolls 9 are loaded by means of pressure plates P. These plates in turn are ~ngingly supported on catch trough 7.
The sludge cakes Q are guided from the preliminary pressure zone D3 to the first one of the dewatering drums.
After l-aving this drum, cakos a are guided through pressure ' ,' , ,,",, .. ~ .

1~ 1048854 ~ :
zone D5 and subsequently to pressure zone D7. After having pa~sed through the last-mentioned zone, filter ~and 4 guides the now dewatered cake O to deflection roll 90 which is con-tinued by a discharge chute 91. Filker band 4 after having been guided over roll 90 passes through cleaning zone 56 and a subsequent catch trough 93. This catch trough is formed by two rolls 92 on band 4 itself. Finally, band 4 is guided by deflection rolls 55 and 8 bacX to preliminary pressure zone D3.
The second filter band S is guided over and between the dewatering drums l and 2 by the lower band 4 and it is also lifted by this band above the drums and either immediately at planet rolls 40 or over further de1ection rolls and cleaning station 56 back to preliminary pressure zone D3. Cleaning station 56 includes discharge elements 94 which accumulate the cleaning water and discharge the same via pipes 95.
` If lt is de~ired to effect preliminary dewate~g also with the exemplification Rl, filters may be provided ahead of the feed-in points ~not shown).
Referring now to ~lg. 5, according to this figure a further belt 3 formed by filter bands 4 and 5 is guided over dewatering drum 1 in the driving direction i. Slu~ cakes Q
f are again placed between the filter bands. The two filter bands 4 and 5 are passed about the circumferential wall surface 11 of drum 1 wlthin an enveloping angle m of about 235 , said angle being the sum total of the so-called roller angles m1.
The filter bands leave dewatering drum l which rotates in the , direction i at the lower branch 50 of enveloping angle m.
A plurality of planet rolls 40 press the outer or - upper filter band 4 toward the circumferential surface wall of drum l within pressure zone~ D5 which in turn is controlled ,, , , ,,' .
i f~f 1 ~ ~)48~59~
by the envelopment angle m.
The axle 70 of each planet or pressure roll 40 is journalled at both ends by radially disposed bars 71. The end -~ of these bars which faces the drum shaft F mounts a lock nut 72, A coil spring 74 which i6 positioned to expand in radial ~ direction is disposed between nut 72 and abutment plates 73 ; which in turn are supported by diagrammatically indicated drum holders 45. Each of these springs biases the respective pressure roll 40 toward the circumferential wall 11 of the drum.
- 10 The axles 70 of the rolls are staggered in circum-ferential direction by pull links 75. The axle 70a which is ; the first one in the driving direction i is suspended by means of radially disposed guide bars 76 on a cross bar 77 on the drum support 45.
The effective length c of the guide parts 76 can be adjusted by settlng an adjustment nut 78. Similarly, the effective length of radially disposed bars 71 can be adjusted by means of nuts 72.
A dewatering devlce as exemplified, that is, a device which lncludes two dewatering drums 1 and 2, provides that wlth a drum diameter of 1 meter about 16 meters of effective filter band length are available. Yet the total length of the device is not more than 2.5 meters. The resulting utilization of the filter bands is more than 7~/O~ and such high efficiency ~; 25 factor i8 far above the efflciency factor of any dewatering dovice ~ the nd herein-r-ferred to au now kn~ n.

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While the invention has been described in detail with respect to certain now preferred examples and embodiments : of the invention, it will be understood by those skilled in the art, after understanding the invention, that various changes and modifications may be made without departing from the spirit and scope of the invention, and it is intended, therefore, ~ to cover all such ~changes and m~difications in the appended cl~

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Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of dewatering sludge, comprising the steps of feeding a partially dewatered sludge through a preliminary pressure zone in the form of two spaced apart relatively moving filter bands by guiding the sludge be-tween said two bands and causing the bands to exert pressure upon the sludge while the latter is moving along and between the bands, then guiding the two filter bands with the sludge therebetween about part of the periphery of a rotary first main dewatering pressure zone and applying pressure to the outer one of said bands and thus to the sludge while being guided by and on said first main pressure zone, whereby to effect dewatering of the sludge through both said bands, then guiding the two filter bands with the sludge therebet-ween from said first main pressure zone to and partly over the periphery of a rotary second main dewatering pressure zone and applying pressure to the filter bands and thus to the sludge therebetween to further dewater the sludge through both bands while rotating the second main pressure zone in opposition to the first main pressure zone and so as to reverse the inner and outer por-tions of the filter bands relative to the main pressure zones.

2. The method according to claim 1 wherein said two main pressure zones are in the form of rotary dewatering drums, and comprising the further step of guiding said two parallel filter bands about said drums so as to form the shape of the letter S.

3, The method according to claim 1 and comprising the further step of applying pressure upon the outer one of said parallel filter bands while it is being guided upon the peripheries of the drums of said main pressure zones.

4. The method according to claim 2 wherein said parallel first and second filter bands are substantially linearly guided prior to being guided to the main pressure zones.

5, The method according to claim 1 and further comprising the step of filter bands guiding the sludge cakes therebetween along a substantially straight path when and while moving from said first main pressure zone to the second main pressure zones.

6. The method according to claim 1 and further comprising the steps of subjecting the sludge cakes, while passing through said main pressure zones, to repetitive pressure and shear forces, by reversing the direction of said forces when and while the sludge cakes are passing between said main pressure zones.

7. The method according to claim 1 wherein the pressure to which the sludge cakes are subjected is varied when and while the cakes are passing through said preliminary and main pressure zones.

8. The method according to claim 1 and further comprising the steps of subjecting the sludge cakes while being guided by said parallel filter bands and passing said main pressure zones to the action of the gravitational force and turning the cakes upside down for changing the structural consist-ency of the sludge cakes.