The subject invention concerns a dewatering device of the kind comprising an essentially cylindrical drum screen arranged for rotational movement inside a housing, means arranged to supply medium to be dewatered into the interior of the drum screen, means to remove from the drum screen interior material retained therein and thus not having passed through the drum screen perforations, and means to receive and to remove water passing through the drum screen. Devices of this kind are used for instance in sewage treatment plants for treatment of e.g. waste water from the community households. Dewatering devices of this kind are likewise used in industry, for instance within the paper industry, to dewater fibrous matter. Also household effluents contain fibrous substances, such as hair.

Obviously, devices of the kind referred to could also be used in other applications for dewatering purposes.

Prior-art devices of this kind usually consist of a cylindrical drum screen having a perforated jacket and arranged for continuous rotation inside a housing, the screen interior being formed with feeder devices partici¬ pating in the drum rotation and usually consisting of a feed screw which is secured to the inner face of the drum screen jacket and upon rotation of the drum screen conveys the material that does not pass through the perforations towards one end of the drum screen where partially dewatered sludge and the like is discharged and sometimes conveyed further for additional dewatering, for instance through compacting. These prior-art dewatering devices often tend to clog, despite flushing of the drum screen external face by means of jet nozzles directed against said face, and

if needed supplemented with the use of rotating brushes acting on the drum screen exterior.

These devices are also generally very difficult to clean, with the result that the plant often must be closed temporarily.

The main purpose of the subject invention is to eliminate the drawbacks outlined in the aforegoing and to provide an efficiently operating dewatering device that does not particularly tend to clog and by means of which the material may be dewatered to a very high degree of dryness, in addition to which the device, despite its high dewatering effenciency, requires but a small space. This purpose is achieved in a dewatering device of the kind outlined above which is essentially character- ized in that the interior of the drum screen is formed with lengthwise carrier protrusions projecting inwardly from the screen jacket face, and in that the means for removal of the material retained inside the drum screen consists of an upwardly open material discharge duct extending in the lengthwise direction of the drum screen in the upper part of said drum screen, at least one side edge of said duct, e.g. the first side edge thereof in the direction of rotation of said drum as seen radially relatively to the drum screen, extending to the vicinity of said carrier protrusions, a series of flushing nozzles distributed evenly in the longitudinal extension of the drum screen and directed towards the drum screen being arranged above the material discharge duct and externally of the drum screen for the purpose of flushing material adhering inside the drum screen into the discharge duct, a discharge means being disposed in said material discharge duct in order to convey, in the lenghtwise direction of the material discharge duct, material falling from the screen drum over the edge of the material discharge duct as well as material flushed into said duct from the drum screen, and to discharge said material from the drum screen interior and out of the

housing enclosing the drum screen. Owing to this arrangement the lower part of the drum screen may be used more efficiently for the screening function since the drum screen need no longer be provided with space-requir- mg material-conveyance devices that form part of the screen. Feed-in of the medium to be dewatered may also be carried out comparatively unrestrictively into the drum screen without the presence of devices that interfere with the flow. Preferably, the discharge means comprise a compression section for additional dewatering through compacting of the material. Because the discharge device positioned at the screen drum top also comprises a compression zone it becomes possible to effect the dewatering to extreme dry contents by means of a device which as a whole requires a minimum of space, since the compression zone need not increase the external dimensions of the dewatering device.

Preferably, the discharge means consist of a feed screw resting in the material discharge duct and having its discharge end penetrating into a tubular screen channel having a perforated jacket face, the outlet end of said screen channel equipped with a counter-pressure element whereby to oppose the discharge force exerted by the feed screw, at least during a pressure build-up stage. The result is an efficiently operating device in which the discharge force to a large extent is counteracted by the friction exerted by the material against the pipe walls. Preferably, said first side edge of the material discharge duct continues in a downwardly sloping slide plate extending m the opposite direction to that of the drum screen direction of rotation, essentially tracing the circular arc described by the inner edge of the carrier protrusions upon rotation of the drum screen, whereby material carried by said protrusions is caught in the area before the material discharge duct in a pocket

formed by the drum screen, the carrier protrusions, and the slide plate, to be emptied into the disharge duct. Owing to this arrangement dewatered material will be efficiently conveyed to the discharge device wherein any adhering material will easily be dislodged during the flushing. Upon further rotation of the drum screen the now carefully cleaned portion of the drum screen jacket will again move downwards upon rotation of the drum screen and the entire surface thereof will serve as an efficient screen.

Further characteristics and advantages obtained with the device in accordance with the invention will become apparent from the subsequent claims and description illustrating the invention by means of non-restricting embodiments with reference to the accompanying drawings, in which:

Fig. 1 is a vertical, longitudinal section along line I-I in Fig. 1 through a dewatering device in accordance with the invention; Fig. 2 is vertical, sectional view along line II-II i Fig. 1 through the same dewatering device; and

Fig. 3 is a perspective view showing a part of the same dewatering device from within.

In Fig. 1 numeral reference 1 designates generally a housing enclosing a rotatable drum screen 2, a dewatering device 3 and a flushing device .

The drum screen 2 is constructed from an essentially cylindrically curved perforated sheet-metal element 5 formed with a number of indentations 6 distributed evenly around the drum periphery and forming ridges projecting into the drum screen interior and extending in the longi¬ tudinal direction of the drum. The sheet-metal screen 5 s supported at one of its ends by an end wall member 41 having a cog-wheel 7 extending around its circumference in driven engagement with a gear 8 driven by a prime mover, preferably consisting of a electric motor 9 and an angular gear 10 mounted on the exterior of the housing 1.

Numeral reference 11 designates an end element rigidify- ing the sheet-metal screen at the opposite end of the drum screen and having an annular configuration, leaving this end of the drum screen essentially entirely open. At its base, the drum screen is supported on a number of support rollers 12 which are rotatably mounted in the housing 1 and run in peripheral tracks 13 and 14, respectively, on the end wall part 41 and 11, respective¬ ly. A slide ring 15 of Teflon or some other wear- resistent material having a low friction coefficent may be provided for sealing purposes between the housing and the end wall parts 11. Similarly, a corresponding slide ring 40 may be provided on the opposite end wall part 41 to support and seal the drum screen against the associated end wall of the housing. The sealing effect could be further improved by providing an annular seal to seal between the drum screen and the associated slide ring, preferably a peripherally extending lip-type of slip ring. This ring is not illustrated in the drawings but could be of any conventional type. The slide ring 40 is positioned radially interiorly of the end wall element 41 in accordance with the embodiment illustrated whereas the slide ring 15 is provided radially exteriorly of the track of the end wall part 11. In accordance with the embodiment illustrated the discharge device 3 consists of a material discharge duct 16 positioned at the upper part of the interior of the drum screen 2 and the cross-sectional configuration of which, as most clearly apparent from Fig. 2, comprises an essentially circular bottom portion 16a continuing in one side flange 16b and in one side flange 16c extending obliquely upwards, the latter flange 16c continuing at its top in a downwardly directed slide plate 16d extending arcuately close to the circular orbit A of the ridges 6 upon rotation of the drum screen 2. Side flange 16b preferably extends sufficiently far upwards to ensure that also the upper edge of said flange will be close to

the ridge orbit A. Letter A does also denotes the direction of rotation of the drum screen. The material discharge duct 16 supports a feed screw 17, which may be of a type formed without a shaft, e.g. a feed screw formed entirely by the helically wound flange and having no central shaft. The feed screw 17 extends through the end wall 11 into a compacting device, illustrated in the drawing figures generally by reference 18. The compacting device consists of a pipe 19 the jacket face of which is formed by perforated sheet metal. The feed screw 17 projects somewhat into the pipe 19 the inner diameter of which equals or slightly exceeds the external diameter of the feed screw 17.

Numeral reference 20 designates a counter-pressure means consisting of a flap lid 20a which is hingedly mounted at its top in a pivot pin 20b and formed at its lower end with a weight 20c acting as a lever. The per¬ forated pipe 19 is enclosed by a casing 21 which is opened to the left and downwards as seen in Fig. 1, externally of the flap lid 20a. Numeral reference 22 designates an outlet line leading from the part of the casing positioned underneath the pipe 19 and debouching at its lower end into the interior of the housing 1. In Fig. 2, the mouth of pipe 22 in the end wall 11 is designated by reference 23. In Fig. 2, numeral reference 24 designates an inlet opening in the end wall 11 which continues on the external face of the end wall in the form of a pipe stub 25 to be connected to an inlet line supplying the medium to be dewatered in the dewatering treatment. Numeral reference 26 designates a level guard and 27 an overflow. Numeral reference 28 designates a drive motor which by means of an angular gear 29 operates the feed screw 17 via a shaft 30 extending through the end wall 30 but not illustrated in the drawings. The opposite end wall of the housing is designated by 31. Owing to the perforations of the pipe 19 the latter function as a screen. Preferably, the perforations in the

pipe 19 as well as in the drum screen 2 are circular, a shape involving less risks that elongate fibres will pass through the screen perforations. Numeral reference 32 designates a spray pipe extending above the discharge duct 16 exteriorly of the drum screen and having a number of spray nozzles 33 distributed along the pipe length and thus along that of the drum screen and directed towards the external face of the drum screen. Numeral reference 42 designates a stop valve by means of which water supply to the spray pipe 32 from a supply line 34 may be inter¬ rupted. At the opposite end, the spray pipe 32 may be connected to the screen pipe 19 by means of a further stop valve 35 via a line 36, to flush said pipe clean according to need. Numeral reference 37 designates a outlet pipe stub positioned at the bottom of the housing 1 for the purpose of carrying off separated water. Just as the left-hand end of the drum screen according to Fig. 1 is open also its right-hand end is open to allow the drive shaft extending into the housing 1 from the drive assembly 28, 29 to run freely without interference from the rotating drum screen.

Preferably, the dewatering device is provided with control means adapted to control the operation of the drum screen and the feed screw in such a manner that these are set in rotary motion upon a signal issued for instance from the level guard 26 to the effect that the water level has risen above a predetermined threshold value, for instance as a consequence of the perforations at the lower part of the drum having become clogged. In connection with the rotation of the drum and the feed screw, valve 42 is opened, which valve may be controlled automatically by means of a solenoid valve. The length of operation of the drum screen and of the feed screw as well as the flushing times for the high pressure water supplied from the flushing pipe 32 preferably are controlled by means of an adjustable timer which could be of a kind known per se and therefore need not be desribed

here. Preferably, means may also be provided to return separated water to the spray pipe 32, preferably via an intermediate filter and a high pressure-pump inserted in the connection line. The dewatering device functions in the following manner. A medium to be dewatered, e.g. sewage or other effluents, for instance from the pulping industry, is introduced through the housing wall 31 via an inlet opening 24, preferably while the drum screen is in a non- rotating condition. The medium which is flowing freely into the drum thus is separated by the lower part of the drum screen. Consequently, water flows out through the drum perforations and leaves the housing 1 via the outlet 37 whereas fibres and other matter having a dimension exceeding that of the drum perforations are retained inside the drum. As the perforations of the drum are becoming clogged because material collects on its inner face, the water level rises. When, as a consequence thereof, the water level has risen to a predetermined height the drive means operating the drum screen and feed screw are rendered active, for instance in response to a signal from the level guard 26, whereupon the drum and the screw are set in a rotating motion. At the same time also the devices controlling the supply of rinsing water to the flushing pipe 32 are made operational. Material collected at the lower part of the drum screen thus is set in motion by the rotational movement of the drum screen, as most clearly apparent from Fig. 3 and is lifted by the carrier ridges 6. Upon the movement upwards of the carrier ridges the entrained material is retained between said ridges, the inner face of the drum screen and the release plate 16d until the material passes the upper edge of the side wall 16c of the discharge duct 16 and thus falls into the discharge duct. Material that does not automatically fall down but continues together with the drum screen is flushed off additionally with the aid of heavy water jets issued from the spray nozzle 33

and entering from the drum screen exterior ensuring that this material, too, falls into the discharge duct. Because the side walls 16b and 16c of the discharge duct diverge upwards it becomes possible to perform the clean- flushing over a comparatively large angular area. The inclination of the discharge duct side walls preferably is chosen to ensure that the material falling downwards slides easily down said walls. The feed screw 17 conveys material falling into the discharge duct towards the compacting section 18. Because the counter-pressure means 20 initially covers the outlet opening of the pipe 19 the material will be compacted inside the latter. When the desired degree of compaction has been reached and thus the desired degree of dewatering of the compacted material, which dewatering degree may be set by choosing the weight of the lever 20c or possibly the magnitude of a spring force acting on the counter-pressure means or the like, the counter-pressure means is opened and compacted and dewatered material is discharged through the outlet opening of the pipe 19. When the compaction is completed, the friction of the material against the pipe walls contributes to sustaining the compaction state, also if the counter-pressure means were to be removed. Discharged material preferably is allowed to fall into a collection container or conveyer positioned below the outlet opening for transportation of dewatered material. The water that has been squeezed out of the material in the compaction section 18 flows via the pipe 22 into the interior of the drum screen and from there further through the outlet 37. Screenings collected in the drum screen interior are also subjected to washing from the spray nozzles 33 prior to being compacted by means of the feed screw 17.

Because the lower part of the drum screen lacks devices preventing inflow through the opening 24 the screening capacity becomes very high, in addition to which components could contribute to rapid clogging of

the drum screen have been reduced to a minium. In other words, the separation operation may be effected while the drum screen is immobile and the drum capacity is based on the static pressure of the medium to be treated against the inner face of the drum screen jacket. In accordance with the embodiment illustrated the level guard 26 is positioned approximatetly at midheight of the drum screen, which means that the drum is set in rotary motion when filled to about 50%. Preferably, the feed screw 17 is started and the solenoid valve 42 affecting the rinsing water is opened. It is of course within the scope of the invention to position the level guide at other heights, should this be desired. Obviously, the drum screen, the feed screw, and the flushing of the drum screen could also be started manually. Material falling into the discharge duct could be compressed to dry contents of approximately 35-40%. The operation cycle may be set according to wish, depending on the proportion of particles or fatty contents in the water. Preferably, the timer of the device is set to ensure that the rotation of the drum screen and the supply of rinsing water continue until the feeder screw has emptied the contents in the discharge duct. Should any malfunction occur with respect to the level guard 26, resulting in the drum screen not being emptied in the desired manner, surplus liquid may flow through the overflow 27.

In order to allow access to the interior of the housing 1 and to the drum screen positioned therein, for instance for service purposes, the upper portion of the housing is configured as a lid 38 which may be pivoted about a hinge 39. In addition, the drum screen preferably has a jacket section that may be unscrewed and removed to form an access opening into the drum screen interior. The invention is not limited to the embodiment illustrated in the drawing figures merely as an example but could be varied with respect to its details within the scope of the appended claims. For example, the

discharge device and the compacting section could be of another type than shown. For instance, the feed screw may be replaced by other conveyance means, such as scrapers, rakers, pistons and other conveyance means. Likewise, the compacting device may be of a different kind. The provision of the compacting device in the intermediate vicinity to the discharge duct does, however, allow a highly compact design of the dewatering device, with the result that the dewatering device, although its large capacity, may be positioned in narrow spaces. The slip rings 15 and 40 could, by themselves, form the mounting means on which the drum screen 2 is supported. In this case the support roll ers 12 may be eliminated.