DEVICE FOR DEWATERING A SLURRY

Abstract: A device for dewatering sludge comprises a plurality of screening means (11) that are located at different levels, said screening means comprising hole-provided tube pieces which communicate with a common manifold (12) and through which water may pass in a direction from the outside and inwards, without the solid constituents of the sludge being carried along by the water. When necessary, the screening means may be back-flushed in order to keep them clean, shut-off valves (24) acting between the differently high situated screening means with the purpose of individually interrupting the communication between the manifold (12) and the screening means (11) that land(s) up above the surface (9) of the slurry (8), according as its level sinks during a dewatering operation.

Description: DEVICE FOR DEWATERING A SLURRY Background of the Invention This invention emanates from problems related with the handling of the sludge in sinks in connection with smaller houses. Conventionally, this is done by means of slurry exhausting vehicles that comprise a container in the form of a pressure tank, into which the sludge may be sucked from the sink and then be transported to and poured out into a central wastewater treatment plant. In practice, the sludge has a dry content within the range of 1 to 3%. This implies that extremely large amounts of water are transported from a sink to a wastewater treatment plant; something that requires a lot of time and is costly, since the distances between the respective destinations usually are long. In order to remove the above-mentioned problems, attempts have recently been made to create devices, by means of which the sludge may be dewatered before the transport to a wastewater treatment plant . These devices may by divided into two main categories, viz . on one hand devices that are mounted in or in the immediate proximity of the pressure tank of the vehicle and on the other hand devices based on intermediate storing of dewatered sludge substance in a special container that is separate from the slurry exhausting vehicle per se, before the sludge substance is transported to the wastewater treatment plant. However, a disadvantage of the first category of devices is that the devices as such are comparatively voluminous and require a large space - and, thereby, a large loading capacity - on the vehicle, either within the pressure tank or directly adjacent thereto. Furthermore, in practice, previously known devices of this type require an addition of chemical flocculants to the sludge. Such an addition of chemical substances involves that the water that is separated from the solid substances is not suitable for being returned to the sink, in that the bacterial flora then runs the risk of being decomposed. An inconvenience of the second category of devices (see SE 9102906-6) is inter alia that the slurry exhausting vehicle per se has to cover distances that are admittedly short, but large in number, between individual sinks in a living area. Such transports also take a considerable time. Moreover, the water separated from the dry substances must be handled in a special order, more precisely by being poured into another receiver than the next sink, which is in turn to be emptied, by the fact that, in practice, these devices also require the addition of a flocculant to the raw sludge. Objects and Characteristics of the Invention The present invention aims at removing the above- mentioned disadvantages of previously known technique and at creating an improved device for the dewatering of slurries. Thus, a primary object of the invention is - in a wide aspect - to provide a dewatering device that continuously guarantees an effective separation of the solid substances in a slurry. In a narrower aspect, the invention aims at providing a dewatering device that is particularly suited for slurry exhausting vehicles and that may be built-in in a pressure tank without obviously encroaching upon the loading capacity of the tank. A further object of the invention is to provide a dewatering device intended for slurry exhausting vehicles, which device does not require the addition of any flocculants or any other chemicals to the raw sludge for its function. Yet another object is to provide a dewatering device that is suited for slurry exhausting vehicles and is capable of utilizing the conventional standard equipment of the slurry exhausting vehicles to obtain a negative or a positive pressure, respectively, within the pressure tank, in order to continuously uphold a good separation effect during a dewatering operation. Further, it is an object of the invention to provide a dewatering device that makes possible a direct return of the separated water to the sink, out of which the raw sludge has been sucked up. Still another object is to provide a dewatering device that may be built-in after- wards into existing pressure tanks of slurry exhausting vehicles . According to the invention, at least the primary object is attained by means of the characteristics that are given in the characterizing clause of claim 1. Furthermore, advantageous embodiments of the invention are defined in the dependent claims . Brief Description of the Appended Drawings In the drawings Fig 1 is a schematic side view of a slurry exhausting vehicle with a pressure tank that comprises two dewatering devices according to the invention, Fig 2 is a cross-section through the pressure tank of the vehicle according to fig 1, Fig 3 is an amplified longitudinal section through a portion of the cylinder wall of the pressure tank, and two screening means comprised in the device according to the invention, Fig 4 is a perspective view showing said screening means which is comprised in a screening unit mounted at a certain level in the tank, Fig 5 is a perspective, extremely amplified partial section showing two portions of a screw element comprised in the individual screening means, Fig 6 is a side view of a slurry exhausting vehicle with a pressure tank, within which is mounted a dewatering device according to the invention, Fig 7 and 8 are cut-out sections from fig 6 showing details of the device according to the invention, and Fig 9 is a cross-section through the pressure tank of the vehicle in the region of two devices according to the invention. Detailed Description of Preferred Embodiments of the Invention In fig 1 and 2, reference numeral 1 designates generally a container in the form of a pressure tank that is mounted on a vehicle 2 in the form of a car. The tank 1 is in a conven- tional manner composed of a cylinder or a cylinder wall 3 and two gables 4, 4' . To the tank are connected two dewatering devices 5, 5' according to the invention, which are identically the same, although mirror- inverted. The two devices are connec- ted via main conduits 6 to a common outlet 7 for the outfeed of separated water. In this context it may be pointed out that in its practical realization, the pressure tank also comprises a not shown tube stud for the intake of raw sludge and the outfeed of dewatered sludge substance together with a not shown, compressor-driven unit in order to alternately obtain a negative and a positive pressure within the tank. In fig 2 reference numeral 8 designates a raw sludge or a slurry whose surface or liquid level is designated 9. Above the surface 9 of the sus- pension there is an air-filled space 10. Characteristic for the invention is that the individual dewatering device, e.g., the device 5, comprises a plurality of screening means 11 located at different levels within the tank 1, each one of which means communicating with a common manifold 12. In the example, this manifold transposes into the previously mentioned main conduit 6. According to the preferred embodiment, screening means 11 are provided at every level along the manifold 12, of which one protrudes backwards from the manifold and the other in a direction forwards. Each pair of screening means forms a screening unit at a separate level within the tank. Reference is now made to fig 3 to 5 , which illustrate in more detail the nature of the screening means. As may be seen in fig 4, the individual screening means comprises a stiff or shape-permanent tube piece 13, which at its one end is connected to a coupling in the shape of a T-coupling 14 and which at its opposed end is closed and has a threaded tap 15. According to the example, the tube piece has an outside with a polygonal, more specifically hexagonal, cross-sectional shape. In the planar, longitudinally extending surfaces are recessed a large number of relatively small holes 16, through which water may pass, more precisely in a direction from the outside inwards, during the operation of the screening means . Besides the tube piece 13 , the screening means 11 comprises an externally arranged, filter-forming casing designated generally 17. It consists of a screw-shaped, elastically flexible element 18, which along its screw-shaped extension (see fig 5) has shallow protrusions 19, between which are delimited microscopically small canals 20, through which water (or possibly a flushing medium) may pass. According to the preferred example shown in fig 5, the screw element 18 has the shape of a comparatively thin belt of a rectangular cross-sectional shape, the protrusions 19 being stamped (in a special machine) along the one side of the belt, while the opposed side is substantially smooth or unmachined. At production in series, the protrusions 19 are placed equidistantly from each other, whereby the openings between the protrusions become equally wide. By giving the protrusions 19 different heights on different screw elements, the height of the individual canal 20, and thereby the crosssectional area, may be varied for different purposes. In other terms, filter casings with different dewatering capacities may be produced in a simple manner. In practice, the width of the individual canal may amount to 0,5 to 2, suitably 1 mm, while the height may vary within the range of 0,01 to 0,50, suitably 0,10 to 0,20 mm. In other terms, the cross-sectional area of the canal in the region of the inner edge of the belt element may amount to fractions of a square millimeter, e.g. 0,1 to 0,2 mm 2 . In this context it should be noted that the height of the individual canal may further be gradually diminished in the direction of the outer edge of the belt element, whereby the outwardly directed inlet side of the canal (the liquid passes in a direction from the outside and inwards through the filter casing) may be even smaller. As may be clearly seen in fig 4, the individual filter casing 17 may be threaded on the hexagonal tube 13 and secured thereon between the T-coupling 14 and a clamping mechanism, which according to the example comprises a sleeve 21 and a clamping nut 22 that may be tightened on the threaded tap 15. By means of the clamping mechanism 21, 22, the screw-shaped belt element 18 may be compressed, so that the opposed sides of the belt are tightly pressed against each other, i.e., the protrusions 19 on the one side of the belt will be distinctly pressed against the planar side of an adjacent belt portion. The indi- vidual screening unit that comprises two screening means 11 connected via the T-coupling 14, is connected with the manifold 12 via a branch conduit 23. In accordance with the principle of the invention, a valve designated 24 in its entirety is provided adjacent to this branch conduit, by means of which valve the communication between the screening unit and the manifold may be interrupted, i.e., the individual screening unit may be shut off. According to the shown example, the branch conduit 23 intersects the cylinder wall of the tank in a radial direction, the different valves, the manifold 12 and the main conduits 6 being placed externally. It should also be noted that the two screening means 11 are oriented axially, i.e., placed parallel to the cylinder wall 3, and located at short distance from it. Concerning the dimensions of the individual screening means 11, it may be pointed out that the externally situated filter casing 18 has a cylindrical basic shape, the length of the casing advantageously, although not necessarily, being at least ten times larger than its diameter. In practice, the outer diameter of the filter casing 17 may be within the range of 30 to 70, suitably 40 to 60 mm, the minimum length of the casing being 300 mm. The distance between the cylinder wall 3 and the next portion of the filter casing or the screening means may advantageously be smaller than the diameter of the screening means. In relation to the total volume of the pressure tank, which may amount to 5 to 15 m 3 , the screening means comprised in the device according to the invention have a total volume that is extremely small, e.g. within the range of 20 to 50 dm 3 . As may be seen in fig 2, the lower screening units or means in each device 5, 5' are located in the bottom region of the pressure tank, while the upper screening units are located near the central top part of the tank. The intermediate screening units are advantageously placed equidistantly on different levels. In practice, the level difference between adjacent screening units may lie within the region of 100 to 400, suitably 200 to 300 mm. The Function of the Device According to the Invention Raw sludge from the sink to be emptied is led via a not shown tube stud into the tank 1 by means of the conventional compressor of the vehicle and the appurtenant suction/pressure unit (not shown) . More precisely, the suction of the sludge takes place by said unit effecting a negative pressure in the tank. After suction having been accomplished, e.g. up to full height in the tank, the two dewatering devices 5, 5 1 are started. More specifically, at this stage all valves 24 are kept open, and the outlet 7 (to which a return flexible tube is connected) is opened. In addition, the function of the suction/- pressure unit is reversed, so that the compressor brings about a positive pressure within the tank. By this positive pressure, the water contained in the sludge is pressed through the filter casings 18 of the different screening means in a direction from the outside inwards and further through the holes in the carrying tube pieces 1, in order to be fed to the manifold 12 via the branch conduits 23, and further through the main conduit 6 to the outlet 7 that is common for the two devices. Thereafter, the water can be led back to the emptied sink via the appurtenant flexible tube. When the water passes in through the individual filter casing 18, the solid constituents that are larger than the canals 20 are stopped. By the fact that these are extremely small, also very small solid constituents are prevented from being carried by the water into the interior of the screening means. The constituents that are stopped in this way, without being carried away by the water, of course deposit on the outside of the screening means, thus forming a gradually thickening particle layer outside the means. On a level herewith, the liquid feeding capacity through the screening means is reduced. In order to guarantee a good separation effect during a tank emptying operation, the screening means should be intermittently back-flushed. This is realized by the compressor of the vehicle briefly altering its working mode, in order to during a short period of time, e.g. 5 to 15 seconds, accomplish a negative pressure within the tank. In this way, a flushing medium in the form of the air in the interior of the screening means is brought to pass the individual filter casing from the inside outwards, whereby the particle layer on the outside of the screening means is removed. Gradually as the water rid of solid particles is poured out from the tank, the sludge or liquid level in the tank, i.e. the surface 9, will sink, while uncovering the top screening units within the tank. If the branch conduit 23 from the individual screening unit always would be open to the manifold 12, the previously mentioned back- flushing of the screening means would be impossible to realize, since the negative pressure in the tank then would suck air through the exposed screening units without having any manifest effect on the screening units located in the slurry. However, by the presence of the valves 24, the individual screening units may be shut off individually from the manifold 12 gradually as the sludge level 9 sinks. In this way, leak air cannot pass through the screening units located above the slurry level. In other terms, it is guaranteed that an efficient back-flushing effect is attained in all screening units located in the slurry, which still have their valves open. Reference is now made to fig 6 to 9 , which show an alternative embodiment that in practice is to be preferred at those latitudes where temperatures below the freezing-point of water may prevail. In this case, the two dewatering devices 5, 5 ' of the pressure tank 1 are placed within the tank in their entirety, i.e., not only the different screening means 11, but also the main conduit 6 and the manifold 12. This internal placing guarantees that the separated water - just as little as the raw sludge in the tank - does not run the risk of freezing. The internal placing also has the advantage that the individual dewatering device may be produced as a cartridge that may be installed later into existing pressure tanks, in that some branch conduits do not need to intersect the cylinder wall of the tank. Thus, the screening means 11 may be supported within the tank by being connected with their one end to the manifold 12, which in a suitable manner is fastened on the inside of the cylinder wall, and with their other end being supported by a cantilever or a strut 25 (see fig 8) . In spite of the fact that the manifold is placed in the interior, the individual dewate ¬ ring device still has a volume that constitutes only a small fraction of the whole tank volume. In fig 7 may be seen that each one of the two screening means 11 constituting a screening unit on each individual level is coupled to the manifold 12 via a valve 24. Instead of two valves, it is also feasible to use only one valve which is arranged in the manifold 12 between two adjacent screening units at different levels. Feasible Modifications of the Invention The invention is not restricted solely to the embodiments as described and shown in the drawings. Hence, the invention is applicable also in connection with such slurries that are not kept in a pressure tank, or not even in a container. Thus, it is possible to accomplish a back-flushing effect in another way than by bringing about a negative pressure in a pressure tank. Thus, to the individual dewatering device may be connected a special back-flushing unit, by means of which a flushing medium, e.g., air and/or water, may be flushed through the filter casings of the screening means, in a direction from the inside outwards. However, also in this case it is essential that shut-off valves exist between the screening means of different levels, so that an efficient back-flushing effect may be upheld, also when the level of the slurry sinks. Albeit the filtering casing outside the appurtenant supporting tube piece, as described in connection with fig 3 to 5, is preferred in practice, it is feasible per se to use other shapes of filtering casings, which may be cleaned by back-flushing . In this context, it should also be noted that the term "hole-provided tube piece", such as used in the claims, shall be interpreted in its widest sense. Thus, the described filtering casing is also to be considered as a hole-provided tube piece, in that this casing may be carried in another way than by the tube piece 13 as shown in the drawings, e.g. by a number of stiff rods or bars installed within the casing. Furthermore, the device according to the invention is usable to separate other constituents from water or similar liquid than those fecal particles which are comprised in normal sink sludge, for instance oil or fat particles.

Claims: Claims 1. A device for dewatering a slurry, c h a r a c t e r i z e d in that it comprises a plurality of screening means (11) that are locatable at different levels, said screening means comprising hole-provided tube pieces (13) which communicate with a common manifold (12) and through which water may pass in a direction from the outside and inwards, without the solid or non-water-containing constituents of the sludge being carried along by the water, it when necessary being possible to back- flush the means in order to keep them clean, and in that between the differently high situated screening means act shut-off valves (24) with the purpose of individually interrupting the communication between the manifold and the screening means (11) that land(s) up above the surface (9) of the slurry (8), according as its level sinks during a dewatering operation. 2. Device according to claim 1, c h a r a c t e r i z e d in that it is mounted within a container in the form of a pressure tank (1) . 3. Device according to claim 2, the pressure tank (1) constituting a part of a slurry exhausting vehicle (2) and having the shape of a lying cylinder with opposed gables, c h a r a c t e - r i z e d in that the different screening means (11) being comprised in the device are oblong and placed on the inside of the cylinder with a lower screening means located near a bottom part of the tank and an upper screening means located near the upper part of the tank, all screening means being oriented substantially axially, i.e. parallel to the wall of the cylinder, and located at a short distance from the cylinder wall . 4. Device according to any of the preceding claims, c h a r a c t e r i z e d in that the individual screening means (11), besides said tube piece (13), comprises a filter- forming casing (17) provided outside said tube piece, said casing consisting of a screw-shaped, elastically flexible element (18), which along its screw-shaped extension has shallow protrusions between which are delimited microscopically small canals (20) through which water and/or a flushing medium may pass, but not solid particles or non-water-containing consti- tuents that are larger than the canals . 5. Device according to claim 4, c h a r a c t e r i z e d in that the screw element (18) is arranged between a fixed stop means (14) , e.g. a T-coupling, at one end of the tube piece and a clamping mechanism (15, 21, 22) arranged at the opposed end of the tube piece, by means of which mechanism the screw- shaped portions of the element abutting against each other may be tightly pressed against each other, the element being dismoun- table by removing the clamping mechanism. 6. Device according to any of the preceding claims, c h a r a c t e r i z e d in that the tube piece (13) has an outside of a polygonal, e.g. hexagonal, cross -sectional shape, and that the surrounding filter casing (17) has a cylindrical basic shape, the length of the casing being at least ten times larger than its diameter.

Inventors: GUSTAFSSON BERNT, POUSETTE KURT, GUSTAFSSON, BERNT, POUSETTE, KURT
Applicants: GUSTAFSSON BERNT, POUGUS AB, POUSETTE KURT, GUSTAFSSON, BERNT, POUSETTE, KURT
Application year: 1998
Main classification: C02F11
IPC codes: B01D29, B01D35, C02F9, C02F11