CA1055776A - Device for dewatering sludge - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
There is disclosed a device for dewatering sludge such as residuals in sewage discharge processing installations comprising a device for dewatering sludge in the form of cakes, said device comprising in combin-ation: a first rotary dewatering drum having a porous circumferential wall and a second rotary dewatering drum also having a porous circumferential wall disposed side-by-side of said first drum, said drums being rotatable in opposite directions; two parallel spaced apart filter bands for supporting and conveying sludge cakes placed therebetween, said filter bands being guided about part of the circumferential wall of the first drum for conveying the bands in the rotational direction of the first drum and also guided about part of the circumferential wall of the second drum for conveying the bands in the rotational direction of the second drum and thus in opposition to the moving direction of the bands while being conveyed about the first drum. This device better overcomes the prior art problem of insufficient break-up of the sludge during the dewatering operation.

Description

~5S7~6 This invention relates to a device for dewatering sludge and also compacting the dewatered sludge.
This application is a clivisional of our copending application Serial No 202,3113 filed June 12~ 19740 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
1~960~7870 This patent describes a dewatering press in which filter bands are guided partly about the circumference of a single drum, the sludge to be pro cessed 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 2,111,7~0 and German Patent 689~090~ in which guidance of filter bands used for causing dewatering of the sludge is similarly Imfavorable with respect to efficiency.
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 itselfO
According to the invention there is provided a device for dewatering sludge in the form of cakes~ said device comprising in combination: a first rotary dewatering drum having a porous circumferential wall and a second ro-tary dewatering drum also having a porous circumferential wall disposed side- -by-side of said first clrumg said drums being rotatable in opposite directions;
two parallel spaced apart filter bands for supporting and conveying sludge cakes placed therebetween~ said filter bands being guided about part of the circumferen~l wall of the first drum for conveying the bands in the rotational '.'' ~, .' . ~ ~

~5~776 direction of the first drum and also being guided about part of the circum-ferential wall of the second drum for conveying the bands in the rotational direction of the second drum and thus in opposition to the moving direction of the bands while being conveyed about the first drum, and said filter bands being guided about and by said drums so as to define an approximately S-shape disposed in an upright plane; pressure rolls disposed at the walls of the drums for pressing the filter bands against the circumferential wall of at least one of the drums; said filter bands being guided to constitute a straining and preliminary pressure zone for causing partial dewatering of sludge cakes upstream of said first rotary drum; and guide rolls provided at the inlet end of said straining and preliminary pressure zone for guiding the filter band into said zone.
Thus, the invention combines a high dewatering efficiency with minimal space requirements. These advantages of high dewa*ering 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 econ-omically acceptable costs.
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.
The dewatering drums define within the range of their circumferen-tial walls about which the filter bands are guided several pressure zones within which an S-shaped guidance of the filter bands causes pulling, knead-ing 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.

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~155776 In this connection, it may be mentioned ~hat the concept of the invention is not limited to two drums, but in some instances it may be preferable to provide more than two drums in side-by-side arrangement and to ,"~ ..:,. . ,' ~. . .
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. ~ ' 1~35~776 guide the filter bands accordingly over the circumferential walls of the drums.
According to another embodiment of the inventiong sludge in the form of sludge cakes of inhomogenous consistency~ after having been guided through one or more straining and/or pre~;minary 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. mere-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. A* the end of this 30ne 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 forcesO 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 ~one curved in opposition to the curvature on the first drum since~ ~ -as previously stated~ the two drums being driven in opposite direction.
~0The afore-referred to pulsing or vibrating pressure is obtained by guiding the filter bands and thus also the sludge cakes between pressure rolls and the circumferential wall of the dewatering drum l/nO 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 bandsO
In a preferred embodiment of the invention~ the envelopment angle of ~he filter bands on the drums which obviously controls 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 180o ~C~5577~

As it is evident~ the longer the pressure zone is~ the more intensive 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 circumferential walls of the dewatering drumsO 0bviOuslyg 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 bands3 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 bet~een the two bands times the angle of envelopmentO This retarding of the outer filter band causes displacement or deformations within the cakes~ As a result there is a change in the consistency 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 directions 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 controlledO Such control can be effected in a particularly simple manner by means of a tension member such as a ropeO This tension member is in contact with lever arms provided according to the invention~ which in turn are linked to a stand or frame structure for the drums or to another suitable supportO
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 usedO 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 dogO .

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~55776i The pressure rolls can also be in the form of parts of contin-uously rotating pressure bands~ etcO These parts may be coupled by chain linksO 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 bandsO me 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 drumsO
In a preferred device of the 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 rollO This distance between the feed point and the deflection roll may be utilized as a straining or pre-dewatering zoneO After passing the deflection rolla the aforementioned extended filter band now jointly with the second filter band which is only in tangential relationship with the deflection roll~ is thus moving in opposition to its initial direction of movement through the preliminary pressure zonesO At the end of the movement in this opposite direction~ both filter bands are jointly guided upon the first one of the dewatering drumsO
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 drumO The envelopment angle of the filter bands about the second drum is preferably larger than .
the envelopment angle for the first drumO :
After leaving the second dewatering drumJ or if there are more than two drums after leaving the last drum, the now fully processed or worked - - . .. .

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sludge cake is discharged and each filter band is separately guided back to the straining and/or preliminary pressure zoneO 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 e~erted by these rollsO
A dewatering device according to the invention with two de-watering drums having a drum diameter of 1 meter provide about 16 meters of active band length and the total length of the devioe is not more than 2.5 meters. The thus obtained space utilization rationis more than 70%9 and this is very much in excess over the utilization ration as can be obtained with dewatering devices as heretofore knownO
In the accompanying drawing~ several embodiments of the inven-tion are shown by way of illustration and not by way of limitationO
Figure 1 is a diagrammatic elevational section of a dewatering device according to the invention including two substantially side-by-side disposed dewatering drums;
Figure 2 is a detail view of Figure 1 on an enlarged scale;
Figure 3 is a view of a modified detail of the device according to Figure 1 on an enlarged scale;
Figure 4 is an elevational sectional view of a modification of a dewatering device according to-the invention, and Figure 5 is a detail view of Figure 4 on an enlarged scale~ ~ :
Referring now to the figures in greater detailt and first to Figure 1, this figure shows an exemplification of a dewatering device ~ ~
which includes two large dewatering drums 1 and 20 Two filter bands 4 and 5 :
which are disposed for part of their length in parallel spaced apart relation_ ship~ envelop the two drums for part of the circumferential walls thereof~
More specifically7 the envelopment angle for drum 1 is m (for instance 235)3 ~
and n (for instance 285)~ whereby the two filter bands approximately define an upright S-shape : ' ~ ~5577~

A cake Q formed of sludge to be processed is fed in the direc-tion t upon first filter band 4 in the conveying direction io This band guides the cakes Q by means of support ro:Lls 6 through straining zone D
to effect preliminary dewatering of the cakesO Water extracted from the cakes is removed through the mesh openings or holes in the pulling side 40 of band 4 and drops into a catch trough 70 Each preliminarily dewatered cake Q falls at a deflection roll 8 for filter band 4 upon the pulling side 500f filter band 5 which is disposed approximately tangentially with respect to the lower side of deflection roll 80 As a result~ the cakes are successively turned around and also mixed~
Betveen the idling side 4u of the first filter band 4 and the pulling side 50 of the second filter band 5 the sludge cakes are guided through a pre-liminary press zone D30 In this zone pressure rolls 9 which are disposed staggered on both sides of filter band sides 4u and 50 process the sludge cakesO Both rows of pressure rolls 9 are via pressure plates P pressed against each band side 4 and 50 by means of power sources 10 coacting with pressure plates P0 The deformation such as bends introduced by pressure rolls 9 in the filter bands are ignored in the drawing to simplify the illustrationO
After leaving the preliminary pressure zone D3 the two filter bands 4 and 5 with sludge cakes Q therebetween are guided upon the circum- ::
ferential vall of dewatering drum 1 in the rotational direction i~ within the range of the envelopment angle m. mis angle controls the effective length of the main pressure zone D5 in which the filter bands 4 and 5 are guided between the circumferen*ial wall 11 of the drum on one hand and planet rolls 40 disposed in juxtaposition to the outer side of the circum-ferential wall of the drumO :
The drum wall 11 consists either of grooved or perforated metal sheets such as steel sheets3 or according to Figure 2~ out of tubes 41 which are disposed parallel to the rotational axis of the drum, me filtrate which ~

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lOS5776 is pressed out of cakes flows between the tubes into the interior J of the drum. The tubes 41 are bored at their ends (not shown) so that any filtrate accumulating in the lower part of the space U within the drum ~lows first into the space within the tubes and can t'hen be removed from the tu~es at the ends thereof.
The planet or pressure rolls 40 are biased by the action of power supplies 42 toward the outside wall 11 of the drum. Such bias 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 li~c 44 hinged to a pivot 46 as it is schematically indicated by supports 45 or similar means~ These pivokal links 44 mount dogs or noses 47 for placing thereupon a pull rope 49. The p~ ;ng force exerted by this rope can be adjusted by the power supply 42 shown as a rotary pulleyO
The aforedescribed pressure means (42 to 48) permit by utili- '-zation of the leverage to effect individual regulation of the pressure for each one of the planet rollsO By tensioning rope 48 the component of force ' as produced by the rope pnll in a direction normal to the pivotal linlc 44 is increased~ whereas the pressure of the planet roll 40 toward drum wall 11 varies the respective lever arm between the pivot point 46 of li~c 44 and the respective nose for supporting the pull rope 480 me pressure of the planet rolls toward the respective drum ' ~
wall is further enlarged by the tension of,the returning filter band portions ~ , 4 and 5e within each range C in which the planet rolls 40 in toto or in-dividual planet rolls 40zare utilized for effecting the deflecting of the ~, filter bands~ The point at which the filter bands 4 and 5 leave drum l and thus the pressure zone D5 is controlled by the lower leg of the envelopment angle m.
At this stage of processing each sludge cake Q which was bent to a curvature somewhat less than corresponding to the radius r of dewatering ---drum l~ is suddenly flattened between filter bands 49 50 As a result~ the -~iO55'776 previously inner parts of the cake are stretched ~nd the previously outside lying cake parts are pushed together whereby the structural consistency of the cake is correspondingly loosened and prepared for further pressing operations.
As is now apparent, the action upon the sludge cakes is similar to the one to which the cakes have been subjected in the straining zone Dl and the preliminary pressure zone D3 due to the action of the gravitational force.
When the outer wall surface 11 of the second dewatering drum

2 is reached at the end of the comparatively pressure-free zone F the sludge cakes Q which are now loosened as described are again bent but now in the opposite direction i3. As a result~ the effect due to the simultaneous action of the pressure due to the tension of the filter bands and due to the first planet rolls 40a, is further considerably increased. ~ue to such preparation~ it is possible to extract from the sludge cakes~ which have now a totally different structural consistency~ 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 of the sludge particles forming the cakeS The continuous shear action as it is obtained by the more rapidly moving inner filter band 4 further substantially assists the dewatering operation.
The dewatering drums 1 and 2 can be driven with synchronous speed or with at least a minimum differential of circumferential speedO For this purpose~ a power drive with gearings which have different transmission ratios as hereinafter described can be usedO ~ -Filter bands 4~ 5~ after leaving dewatering drum 2 and the range of rolls 40z together with the dewatered sludge cakes now designated by O are guided over a diverter roll 50 conmlon to both filter bands to small individual rolls 520 The roll 50 is disposed àt the apex of an imaginary triangle.
This triangle is defined by line L which extends through the axes of the drums~

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~S5776 as it is shown in Figure 1 and legs defining an angle v of about 35 0 The roll 50 is associated with planet rolls 51; in addition, some of the planet rolls 40 associated with the adjacent dewatering drum 1 touch part of the filter band sides ~e and 5 0 me two individual rolls 52 are mounted spaced apart from each other and define a discharge gap 53 between filter bands 4~ 5 for ejecting dewatered sludge cakes 00 The return of pulling band portions 4e and 5 respectively, extend from discharge gap 53 first over the planet rolls 40 (range C)~ then via the upper deflector rolls 55~ back to the straining zone Dl and to the preliminary pressure zones D3~ respectively. Band flushing means 56 are interposed between the deflector rolls 55 and zones Dl and D30 Figure 3 shows another exemplification of the preliminary pressure 7ones D3. According to this exemplification~ pressure rolls 9u are disposed underneath band sides 4u and 50 and also rolls 90 over which via roller bearings 60, engaged by chain links 61 are joined to form endless pull;ng band portions 620 and 62U in the direction of arrow SO Accordingly~
a structure is formed which is similar to the flexible cage of~ for instance~
a needle bearingO
Driving of each pressure roll 9 is effected by frictional driving ~-of filter bands 4~ 50 The pressure rolls 9 are pressed within the pressure zones 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, respectivelyO Nhen 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 rol ;ng off on the stationary pressure plates PO
The rotational speed on the opposite side is now twice as high as the running speed o-f the chains formed by links 610 This has the advantage that by means of relatively thin-~alled and light pressure rolls 9~ high pressures can be transmitted. mis is due to the fact that there is no strain due to the absence of bending forces normal to the axis of the rollsO

3~55~76 The bending forces are compensated by pressure plates P overlying the rollsO
The pressure plates P are sp:ring tensioned at 65 so that no forces need to be transmitted by an external support structure~
In particular, if the bands are fairly light, the sprockets 63, 64 for the chain links can be driven and then in turn drive th~ filter bands

4 and 5 in the direction i via the pulling band sides 629 that isg the drive of filter bands 4 and 5 is effected no longer by one of the de~lection rolls 8~ 50~ 55 or the dewatering drums 1 and 2 which would cause heavy strains within the range of the transmission of force upon the respective filter band 4 or 5~ which must be pulled by a multitude of gaps between rollsO Instead of this transmission of force there is obtained at each of the gaps between the rolls a transmission of force corresponding to the prevailing pressure~
and this transmission of force is uniformly distributed over a large surfaceO
By selecting different diameters d and at axial spacings f for the upper and lower pressure rollers 9 substantially more or less pro-nounced sine-shaped deflectors can be obtained at the surfaces of the filter bands between which sludge cakes Q are placedO Moreover, relative movement can be superimposed by differential speeds of the upper and lower pulling sides 620 and 62U of the bandsO
According to the exemplification shown in Figure 4~ the device does not include the straining zone Dl~ but the sludge cakes Q drop immed-iately into preliminary pressure zones D3 which is disposed below dewatering drums 1 and 2. Pressure rolls 9 are loaded by means of pressure plates PO
mese plates in turn are swingingly supported on catch trough 7O
The sludge cakes Q are guided from the preliminary pressure ~one D3 to the first one of the dewatering drumsO After leaving this drum~ cakes Q are guided through pressure zone D5 and subsequently to pressure zone D
After having passed through the last-mentioned zone~ filter band 4 guides the now dewatered cake 0 to deflection roll 90 which is continued by a dis charge chute 91o Filter band 4 after having been guided over roll 90 passes ~L055776 through cleaning zone 56 and a subsequent catch trough 93. mis catch trough is formed by two rolls 92 on band 4 itselfO Finally~ band 4 is guided by deflection rolls 5S and 8 back to preliminary pressure zone D
The second filter band 5 is guided over and between the de-watering 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 o~er further deflection rolls and cleaning station 56 back to preliminary pressure zone D3. Cleaning station 56 includes discharge elements 94 which accun~ate the cleaning water and discharge the same via pipes 95.
If it is desired to effect preliminary dewatering also with the exemplification ~, filters may be provided ahead of the feed-in points (not shown).
Referring now to Figure 5~ according to this figure a further belt 3 formed by filter bands 4 and 5 is guided over dewatering drum l in the driving direction iO Sludge cakes Q are again placed between the filter bands. The two filter bands 4 and 5 are passed about the circumferential wa71 surface 11 of drum l within an enveloping angle m of about 235 ~ said angle being the sum total of the so-called roller angles ml. me filter bands -leave dewatering drum l which rotates in the direction il at the lower branch -50 of enveloping angle m-0 A plurality of planet rolls 40 press the outer or upper Mlter -band 4 toward the circumferential surface wall of drum l within pressure zones D5 which in turn is controlled by the envelopment angle m~
The axle 70 of each planet or pressure roll 40 is journalled at both ends by radially disposed bars 710 The end of these bars which faces the drum shaft F mounts a lock nut 72~ A coil spring 74 which is positioned to expand in radial direction is disposed between nut 72 and abutment pla~es 73 which in turn are supported by diagrammatically indicated drum holders 450 Each of these springs biases the respective pressure roll 40 toward the circumferential wall ll of the drum.

- 12 _ 1~5776 The axles 70 of the rolls are staggered in circumferential direction by pull links 75. me 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 setting an adjustment nut 78-~ Similar:Ly3 the effective length of radially disposed bars 71 can be adjusted by means of nuts 720 A dewatering device as exempLified~ that is, a device which includes two dewatering drums 1 and 2~ provides that with 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 205 meters. The result-ing utilization of the filter bands is more than 70%~ and such high effic-iency factor is far above the efficiency factor of any dewatering device of the kind herein-referred to as now known.
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 sh-Lled in the art~ after understanding`the invention9 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 modifications in the appended claimsO

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Claims (23)

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

1. A device for dewatering sludge in the form of cakes, said device comprising in combination: a first rotary dewatering drum having a porous circumferential wall and a second rotary dewatering drum also having a porous circumferential wall disposed side-by-side of said first drum, said drums being rotatable in opposite directions; two parallel spaced apart filter bands for supporting and conveying sludge cakes placed therebetween, said filter bands being guided about part of the circumferential wall of the first drum for conveying the bands in the rotational direction of the first drum and also being guided about part of the circumferential wall of the second drum for conveying the bands in the rotational direction of the second drum and thus in opposition to the moving direction of the bands while being conveyed about the first drum, and said filter bands being guided about and by said drums so as to define an approximately S-shape disposed in an upright plane; pressure rolls disposed at the walls of the drums for pressing the filter bands against the circumferential wall of at least one of the drums; said filter bands being guided to constitute a straining and preliminary pressure zone for causing partial dewatering of sludge cakes upstream of said first rotary drum; and guide rolls provided at the inlet end of said straining and preliminary pressure zone for guiding the filter band into said zone.

2. The device according to claim 1 wherein the angle of envelopment through which said filter bands are guided by and about said drums is more than 180° for at least one drum.

3. The device according to claim 1 and comprising further guide rolls guiding said filter bands along a substantially linear path while constituting said straining and preliminary pressure zone.

4. The device according to Claim 1 wherein one of said filter bands is extended past the other filter band, said extended filter band portion constituting the feed station for feeding sludge cakes to the device, and comprising preliminary dewatering means for removing water from cakes, said extended filter band portion being guided past said preliminary dewatering means for coaction therewith, deflector means for deflecting said extended filter band portion into said parallel relationship with the other filter band, and a preliminary pressure zone, said filter bands being guided through said preliminary pressure zone prior to being guided to said rotary drums.

5. The device according to Claim 4 wherein said preliminary pressure zone includes substantially linearly disposed pressure means.

6. The device according to Claim 4 wherein the filter band having said extended portion is guided over part of the circumferential wall of the first rotary drum prior to feeding sludge cakes upon said extension.

7. The device according to Claim 1 and comprising a preliminary pressure zone for extracting water from the sludge cakes, said filter bands being guided through said preliminary pressure zone, and one of said filter bands being guided parallel to the other filter band while passing through said preliminary pressurze zone and also while being guided over part of the circumferential walls of said drums, said one filter band being de-flected away from the other filter band after being guided over said wall part of the second drum to define a discharge opening for dewatered sludge cakes.

8. The device according to Claim 7 wherein deflection means for deflecting said one filter band away from the other filter band are provided between said two drums.

9. The device according to Claim 7 wherein said filter bands are guided about the circumferential wall of the second drum through an angle of at least 270°.

10. The device according to Claim 1 and comprising pressure rolls for pressing said filter band against the circumferential wall of at least one of said drums.

11. The device according to Claim 10 wherein said pressure rolls in conjunction with the circumferential wall of the respective rotary drum constitutes a pressure zone.

12. The device according to Claim 10 comprising setting means for individually varying the pressure of each of said pressure rolls toward the wall of the respective drum.

13. The device according to Claim 10 and comprising means ganging a plurality of said pressure rolls for joint action, and setting means coacting with said ganging means for varying the pressure of each plurality of ganged pressure rolls toward the wall of the respective drum.

14. The device according to Claim 10 and comprising adjustment means for varying the positions of the pressure rolls relative to the wall of the respective drum thereby correspondingly varying the pressure applied by said rolls upon the filter bands and thus upon sludge cakes between the filter bands.

15. The device according to Claim 14 wherein said adjustment means comprise lever arms each mounting at one end a pressure roll, the other end of each lever arm being hinged to a suspension means.

16. The device according to Claim 15 wherein said suspension means comprise a common carrier for a plurality of said lever arms, and comprising yieldable tension means biasing said lever arms toward the wall of the respective drum, and setting means coacting with said tensioning means for varying the bias force thereof.

17. The device according to Claim 10 and comprising connection means interconnecting said pressure rolls in positions substantially parallel to the circumferential wall of the respective drum, and also connecting the interconnected rolls to a stationary point.

18. The device according to Claim 17 wherein said connection means comprise strips extending between each two adjacent pressure rolls and being attached to the axles of the respective rolls.

19. The device according to Claim 17 wherein said stationary point is located at the first pressure roll as seen in the rotational direction of the respective drum.

20. The device according to Claim 17 wherein said pressure rolls are interconnected in the circumferential direction of the drum and also with a common stationary suspension point, and said bias means are coupled to the pressure rolls to apply a radially directed force on the rolls.

21. The device according to Claim 1 and comprising preliminary pressure means disposed ahead of said dewatering drums for effecting partial dewatering of sludge cakes fed to said preliminary pressure means by said filter bands, said preliminary pressure means including pressure rolls disposed on opposite sides of said filter bands staggered in the direction in which said filter bands are driven.

22. The device according to Claim 21 wherein pulling bands connect said pressure rolls on one side of said filter bands.

23. The device according to Claim 1 and comprising drive means driving said dewatering drums at different circumferential speeds.