SCREENING DEVICE

Abstract: The invention is a screening device suitable for screening a liquid material having solid matter suspended in it, which comprises a number of sets of generally planar discs, the discs of each set being rigidly mounted on a shaft, with the discs on each shaft being separated from one another axially with the plane of each disc being substantially perpendicular to the axis of the associated shaft, means mounting the shafts about parallel axes, with the discs of adjacent shafts being interleaved with one another and means to cause the shafts to rotate independently in the same rotational sense such that successive shafts rotate progressively faster to cause the solid matter suspended in the liquid material to be moved in a direction from the slowest shaft to the fastest shaft. The device is used particularly in sewage processing plants.

Description: The present invention relates to the screening of liquid material having solid matter suspended in it, in particular to the screening of sewage waste. Many industries need to handle large volumes of liquid material which requires processing or treatment. In some cases that treatment involves screening, or coarse sifting, the liquid to remove solid matter suspended in it. The processing of sewage is one such example. In plants involving treatment works of this type, the standard method used to separate solid matter from the liquid material is to provide a belt screen which consists of a wire mesh passing continuously around two rollers like a conveyor belt. The belt screen may be arranged at an angle from the vertical, with the belt moving in an upwardly direction relative to the liquid flow, such that the liquid has to pass through the wire mesh screen twice. This presents an undesirable degree of resistance to the flow of liquid being processed. Added disadvantages are the tendency of the wire mesh to clog up and further impede the liquid flow, and the fact that the individual wires of the mesh are prone to deform upon impact by large solids. DE-A-3617665 discloses a screening system suitable for screening water having solid matter suspended in it or floating in it, including fish, which comprises a feed channel for the liquid material, a screening device located in the channel and including a number of sets of generally planar discs having parallel flat faces, the discs of each set being mounted on a horizontal shaft, and separated from one another axially, the shafts being mounted to extend on parallel horizontal axes, the discs of adjacent shafts being interleaved with one another, and means to cause the shafts to rotate independently in the same rotational sense such that successive shafts rotate progressively faster, wherein the discs of each set are rigidly mounted on their associated shaft with the faces of each disc substantially perpendicular to the axis of the associated shaft. This apparatus is designed to remove the solid matter from the inlet to a power station colling system. It is unlikely to operate very satisfactorily, especially as regards to removing the solid material once it has risen above the water level. With these problems in mind, the present invention is characterised in that the axes of the shafts are mounted vertically and in that the said faces are closely adjacent to the faces of the discs of the adjacent shaft or shafts, to define parallel-sided slots therebetween. Thus the liquid material having solid matter suspended in it is screened such that the solid matter suspended in the liquid material is moved generally horizontally to one side of the channel, in a direction from the slowest shaft to the fastest shaft, while remaining in the flow of liquid applied to the discs. The discs of a given shaft are substantially parallel to each other and closely adjacent to the discs of each adjacent shaft, with faces of the discs of adjacent shafts closely adjacent and the peripheries of the discs of each shaft being very close to the adjacent shaft itself. The spacing between the faces of the overlapping disc is typically from 0.5 to 10 mm, for example from 1 mm to 6 mm. A spacing of 1 mm, 3 mm or 6 mm is particularly preferred. The diameter of each disc is generally large compared with the diameter of the associated shaft. The disc diameter is typically in the range from 150 to 300 mm, preferably from 180 to 250 mm. The shaft diameter, that is to say the minor diameter of the shaft space, is typically from 50 to 125 mm, preferably from 80 to 100 mm, and is selected according to the diameter of the discs to be mounted on the shaft, to give a close spacing between the periphery of each disc and the adjacent shaft itself. The spaces provided between the discs, and between the discs and the adjacent shafts, is thus the only passageway for material not retained by the screening device. One or more scrapers are suitably included in the device attached to the discs, to remove material from the discs. At least some of the discs may be provided with serrations on their peripheries. In one such embodiment of the invention the discs of the last shaft are not provided with serrations. Advantages of the system are that it allows the liquid being processed to flow with minimum hindrance. Also, the discs are more resistant to impact by solid matter than are the individual wires of a mesh. The progressively faster motion of successive shafts means that the discs self-clean and the system has no tendency to become blocked up. The system also provides a finer screen than conventional systems; the spacing between adjacent bars in a conventional sewage screen is generally of the order of 20mm, whereas in the present invention the spacing between adjacent discs is usually no greater than about 6 mm. The present invention also provides a method of screening, macerating or conveying a liquid material having solid matter suspended in it, comprising passing the liquid material along the channel of the system of the invention, whereby the liquid material having solid matter suspended in it is screened, such that the solid matter is moved generally horizontally to one side of the channel in a direction from the slowest shaft to the fastest shaft while remaining in the flow of liquid applied to the discs. The shafts of the discs may extend with a plane including the axes of the shafts either perpendicular to or at an angle across the feed channel, with the faster moving discs then being further downstream. The system enables the screenings to be removed and treated further. In the case of sewage treatment the screenings contain both biological matter and non-biodegradable material such as plastic, rubber and rags. The screenings are typically macerated and washed and the biological matter returned to the liquid flow for subsequent treatment and degradation downstream. For sewage treatment in particular the fact that the biological matter contained in the solids is not removed from the sewage flow altogether, as it is when a conventional mesh screen is used, is an advantage since it remains available for the downstream treatment. Rather than being returned to the liquid flow, the disintegrated and washed screenings of a non-biodegradable nature are removed from the system altogether, in which case they are in an acceptably clean state for disposal. The system need not necessarily include macerating biological matter returned to the liquid flow for subsequent treatment and degradation downstream. The invention will be further described with reference to the accompanying drawings in which: Figure 1 shows part of a screening, macerating and conveying system according to the invention for sewage; Figure 1a shows the interleaved discs of the device in closer detail; Figure 2 shows a screening and conveying system according to the invention for a liquid containing solid matter. Referring first to Figures 1 and 1a, the crude sewage containing solids 10 flows along a feed channel 12 in the direction of arrow A to the device 14, which is mounted to extend at an angle across the feed channel. The device 14 comprises upper and lower members 19, 20 and discs 18 rigidly mounted on shafts 16 which are mounted for rotation about parallel axes in bearings supported by members 19 and 20. The discs, with the exception of the discs 18′ mounted on the extreme right-hand shaft 16′, bear serrations 22 on their peripheries. The shafts rotate and the discs all move in the same rotational sense, indicated by arrow B. Successive shafts move progressively faster with the right-hand shaft 16′ moving the fastest. The liquid part of the sewage is able to flow through the device 14 without significant resistance so that screened sewage therefore continues along the feed channel 12 in the direction of arrow A′. A barrier to the solid matter in the sewage is presented by the rotating discs however. Owing to increasing speed of rotation of successive shafts, a positive displacement of the solids towards the fastest moving shaft 16′ is promoted, whereupon the solids are either disintegrated in the flow or removed to a solids grinder 24 comprising two parallel shafts 26, mounted with discs 28, which rotate in opposite senses. Once through the grinder, the material (screenings) is moved by an inclined upwardly helical conveyor 30 mounted above a screen 31. Water jets (not shown) spray water to wash the macerated screenings, which comprises both faecal matter and non-biodegradable material such as plastic and rubber. The macerated faecal matter is returned to the flow in the feed channel 12 through the screen 31 and is transported downstream for biological treatment, whereas the remaining material passes down a pipe 32 on to a conveyor belt 34 and is removed from the system in the direction of arrow C. Figure 1a shows the interleaved arrangement of the discs 18, which in this embodiment are provided with serrations 22. In operation the discs 18 rotate in the sense indicated by arrow B. In Figure 2 the liquid material containing solid matter 10 flows in the feed channel 12 in the direction of arrow A. A device 14 as described in connection with Figure 1 is mounted with the shafts extending vertically at an angle across the feed channel. Whilst liquid material flows past the device, solid matter 10 is taken up on the discs 18, passes from left to right towards the fastest moving shaft 16′, proceeds, as in Figure 1, into an inclined helical conveyor 30 and continues down a pipe 32 on to a conveyor belt 34 which carries it out of the system. One face of each disc may be provided with a radial rib to act as a moving wiper to clean the adjacent interleaved disc.

Claims: A screening system which comprises a feed channel (12), a screening device (14) located in the channel and including a number of sets of generally planar discs (18) having parallel flat faces, the discs of each set being mounted on a shaft (16) and separated from one another axially, means (19,20) mounting the shafts upon parallel axes, the discs of adjacent shafts being interleaved with one another, and means to cause the shafts to rotate independently in the same rotational sense such that successive shafts rotate progressively faster, the discs (18) of each set being rigidly mounted on their associated shaft, with the faces of each disc substantially perpendicular to the axis of the associated shaft, characterised in that the axes of the shafts are mounted vertically, in that said faces are closely adjacent to the faces of the discs of the adjacent shaft or shafts, to define parallel-sided slots therebetween. A system according to claim 1, characterised in that at least some of the discs are provided with a scraper or scrapers arranged to remove material from the discs. A system according to claim 1 or 2, characterised in that peripheries of at least some of the discs are provided with serrations (22). A system according to claim 3, characterised in that the discs of the last shaft are not provided with serrations. A system according to any preceding claim, characterised in that a plane including the axes of the shafts extends at an angle across the feed channel, with the fastest moving discs then being further downstream. A system according to any preceding claim, characterised in that a macerator (24) is positioned to receive the solid matter from the discs of the last shaft. A system according to any preceding claim, characterised in that there is provided, downstream of the device, a means for the biological treatment or degradation of the liquid material flowing in the feed channel. A method of screening, macerating or conveying a liquid material having solid matter suspended in it, comprising passing the liquid material along the channel of the system of any preceding claim, whereby the liquid material having solid matter suspended in it is screened, such that the solid matter is moved generally horizontally to one side of the channel in a direction from the slowest shaft to the fastest shaft while remaining in the flow of liquid applied to the discs. A method according to claim 8 in which the liquid material is raw sewage or sewage sludge.

Inventors: STEVENSON, CHRISTOPHER, STEVENSON, CHRISTOPHER, C/O WASTE-TEC
Applicants: MONO PUMPS LIMITED
Application year: 1990
Main classification: B01D33
IPC codes: B01D33