WO1992013626A1

WO1992013626A1 - Filtering apparatus

Info

Publication number: WO1992013626A1
Application number: PCT/GB1992/000159
Authority: WO
Inventors: Edward Victor Byers; Michael Richard Byers
Original assignee: Severn Trent Water Ltd
Current assignee: Severn Trent Water Ltd
Priority date: 1991-01-30
Filing date: 1992-01-28
Publication date: 1992-08-20
Anticipated expiration: 1993-07-30
Also published as: GB9315297D0; IE920296A1; GB2267658A; GB2267658B; PT100066A

Abstract

Filtration apparatus for continuous separation of residues of solids from liquid filtrates comprises a rotatably mounted drum (1) of cylindrical form with a plurality of filtering elements (5) in each end wall (2). Materials to be filtered are introduced into the drum (1) through a duct (8) and filtrate passes out through one or more of the filter elements (5) at a lower part of the drum (1). Spraying nozzles (16) are disposed adjacent to the end walls (2) to direct jets of liquid on to the filtering elements (5) at an upper part of the drum (1), thus backwashing the elements (5). Backwashed material is collected within the drum (1) and led away through the duct (8) to waste. Continuous rotation of the drum ensures that each element (5) alternately filters liquid and is backwashed. A filtering element (5) suitable for use with the apparatus has a mesh (M) supported at its periphery between curved bearing surfaces (40; 42) to minimise cyclic stresses in the mesh caused by continuous filtering and backwashing cycles.

Description

FILTERING APPARATUS

The present invention relates to filtering apparatus for separating residues of solids from liquid filtrates, such as in sewage treatment or in filtration of raw water as part of a potable water treatment process.

Where filtering apparatus is required to filter a continuous flow of liquid, steps must be taken to ensure that material removed from the liquid is not allowed to build up on the filtering element, or elements, as this would increasingly impede the flow of liquid through the apparatus and might ultimately cause complete blockage.

It is known for a filtering element, e.g. in the form of a mesh of fine strands of wire supported by a peripheral frame, to be cleaned by backwashing; that is, by forcing liquid back through the element, in the opposite direction to the normal flow,to dislodge matter which has accumulated on the upstream (as in normal use) face of the mesh.

In situations where a flow of liquid to be filtered cannot be interrupted, a rotary arrangement may be used. In a known rotary liquid filtration apparatus, a disc filter comprising a mesh secured around its periphery to a stiff annulus is mounted in a liquid-carrying duct. A seal between the periphery of the disc filter and the duct causes all liquid flowing in the duct to pass through the filter disc. In normal use, only a lower portion of the disc is submerged in the liquid to be filtered. To prevent the filter mesh becoming blocked with matter, the filter disc is rotated to raise previously-immersed portions out of the liquid for backwashing, any matter so dislodged from the mesh being carried away to waste.

The known disc filter system has disadvantages. Particularly, it is difficult to maintain an effective seal between the periphery of the disc and the duct.

The present invention provides, in a first of its aspects, filtering apparatus for separating residues of solids from liquid filtrates comprising a rotatably mounted filtration vessel, filtering means disposed about the axis of rotation in one or more walls of the vessel to provide filtering outlets from the vessel, feeding means enabling materials to be filtered to be introduced into the vessel, means for collecting filtrate emanating from the filtering outlets at a lower level, cleaning means for dislodging residues from the filtering means at a higher level, and means within the vessel for collecting residues dislodged from the filtering means by the cleaning means, portions of the filtering means being conveyed successively between the lower and higher levels by rotation of the drum.

The vessel may be rotated continuously or periodically. Intermittent rotation might be used, for example, where the filtration process was part of a water treatment or similar process.

By mounting the filtering means in the walls of a rotatable vessel the need for a seal between relatively moving components can be removed.

The cleaning means may operate by backwashing the filtering means. In a preferred embodiment, the cleaning means comprises one or more liquid-spraying nozzles arranged externally of the vessel and operative to cause a spray of liquid to impinge on the filtering means.

The or each filtering means may be in the form of a mesh screen stretched across a frame. It is preferable to provide a plurality of discrete filtering elements which are individually mounted in the walls of the filtration vessel. In this way, damage to a small section of the filtering means is restricted in its effect to the element in which the damage has occurred.

The filtration vessel may be a generally cylindrical drum, filtering elements being mounted in at least one end wall of the drum.

In a preferred embodiment, the feeding means and the residue-collecting means comprise ducting which extends through the vessel, generally along the rotational axis, for the delivery into the vessel of materials to be filtered and the removal of residues from the vessel. The vessel may be rotatably mounted on a single duct which has one or more discharge ports for delivery of materials to be filtered into the vessel, and one or more reception ports for receipt of residues cleaned from the filtering means, the discharge ports being separated from the reception ports by a sealing partition within the duct. The residue-collecting means may comprise funneling means to feed residues backwashed from the filtering means into a stationary collecting trough which discharges into the one or more reception ports.

It has been found that filtering elements constructed from a mesh clamped at its periphery to a stiff annular frame can be subject to early failure. This is primarily due to the repeated forward flow and backwashing causing cyclic stresses in the strands of the mesh close to the frame which lead to fatigue in the strands.

The invention provides, in a second aspect, a filtering element comprising a mesh screen, and peripheral means supporting the screen, the screen being retained between opposed convex bearing surfaces of the supporting means which form a convergent entry for the screen.

In a filtering element such as this, the strands of the screen are prevented by the bearing surfaces from being bent sharply as the screen is deflected due to pressure exerted by fluid flowing through it. Thus, the magnitude of the cyclic stresses in the strands, and consequently the tendency of the strands to fatigue, is reduced.

This aspect of the invention can be of particular benefit where the mesh is composed of metal strands-

The convex bearing surfaces may extend to each side of a line of closest proximity between the opposed surfaces.

In a preferred embodiment, the supporting means comprises a frame to which the mesh screen is secured, retained within a box-section housing. The bearing surfaces are formed along the walls of an opening in the box housing forming the convergent entry. In one embodiment the box housing is formed of two parts (which may be adjustable relative to one another so as to maintain the bearing surfaces in contact with the strands of the mesh) .

In an alternative embodiment, the bearing surfaces are presented by components of an elastomeric material which will yield as the strands bear against it to an extent dependent upon the amount of applied cyclical pressure. In effect, variable curvilinear bearing surface portions are provided by the elastic properties of the elastomeric material which adapt to variations in the pressure on the mesh.

A filtering element in accordance with the second aspect of the invention can, most advantageously, be used in a filtering apparatus according to the first aspect of the invention.

Embodiments of the invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which:-

Figure 1 is a diagrammatic view of one end wall of a rotary filtration vessel within a collection tank, viewed on a section through the vessel;

Figure 2 is a longitudinal vertical section through the apparatus of Figure 1;

Figure 3 shows, in part section, a first filtering element;

Figure 4 is a sectional view along the line A-A of Figure 3; and Figure 5 is a cross-sectional view corresponding to Figure 4 but showing a modified construction of filtering element.

As shown in Figures 1 and 2 of the drawings, a filtration chamber of a filtering apparatus embodying the invention is provided by a filtration vessel in the form of a drum 1. The drum comprises two rigid circular end plates 2 at opposite ends of a cylindrical wall 3 secured and sealed to the end plates 2. The end plates 2 are relatively located and attached to each other by annularly-arranged tie rods 4 to provide a rigid structure.

The end plates 2 are provided with a plurality of apertures over which filtering elements 5 are secured by, for example, bolts which are not shown, the elements being arranged in uniform circular distributions around the plates. Each of the discrete filtering elements 5 (described in more detail hereinafter) consists of a peripheral supporting ring 6 to which the periphery of a filtering wire mesh screen M is secured, the mesh completely covering the aperture which is surrounded by the ring 6. The apertures in the end plates, covered by filtering elements, provide filtering outlets from the drum.

The filtering elements 5 are shown as having a circular form, this being preferred for ease of manufacture, but other shapes may be used.

The end plates 2 are mounted on bearings 7 to enable the drum 1 to rotate about a horizontal axially-extending cylindrical duct 8 which, in part, forms a stationary feed duct through the drum.

Materials to be filtered can be admitted into one end of the duct 8 (as indicated by an arrow 9) the materials falling into the interior of the drum 1 through discharge ports in the form of slots 10. Axial progression of the materials in the duct 8 beyond the slots 10 is prevented by an internal sealing partition 11 in the duct.

A tubular axial extension 12 on the drum 1 permits the drum 1 to be rotated through a pulley or gear wheel 13 of the extension by suitable driving means (not shown) .

The duct 8 is supported outside the drum 1, adjacent to the opposite ends of the drum, by walls of a collection tank 14 in which liquid filtrate issuing from the drum 1 through the filtering elements 5 can collect. The liquid can be withdrawn from the tank via an outlet tube 15 which is telescopically extendable upwardly (not shown) to permit alteration of the filtrate outlet level.

A proportion of the filtered liquid (up to about 10%, depending on working conditions and degree of effluent contamination) can be pumped through spraying devices 16 held above the tank 14 in fixed positions adjacent to both end plates 2 of the drum 1. The devices 16 are provided with nozzles to cause the filtered effluent to be sprayed in jets 17 which impinge on opposed screens of the filtering elements 5, so flushing residues of solids which may have accumulated on the screens back towards the interior of the drum 1.

Catchment plates 18 forming V-shaped funneling devices, one for each filtering element 5, are attached to the inside surface of each end plate 2 adjacent to the elements 5. The apex of the V is directed towards the axis of rotation of the drum 1 while the arms of the V pass to either side of a corresponding filtering element 5 to collect backwashed residues. Each funneling device forms a spout 19 at the apex of the V-shape. The spouts 19 feed residues backwashed from the filtering elements into a stationary collection trough 20 attached to the uppermost portion of the central duct 8 along its length within the interior of the drum 1. The trough 20 conducts collected material to a reception port 21 in the trough whence it falls through an aligned aperture in the duct 8 on the far side of the partition 11 from the slots 10. The material can then drain from the duct 8 (as indicated by an arrow 22) .

In use of the apparatus, material to be filtered flows into the drum 1 through feeding means comprising the duct 8 and slots 10. Filtrate passes out of the drum 1 through those of the filtering elements 5 at a lower level into the collection tank 14. The level of liquid in the collection tank 14 is maintained, by adjustment of the outlet tube 15, at a level such, that a filter element 5 in its lowermost position is fully submerged. Hydrostatic pressure of liquid in the drum 1 is sufficient to cause the filtrate to flow through the filter elements 5.

The drum 1 is continuously rotated so that each filtering element 5 is successively cycled between a lower level, in which it filters liquid, and a higher level at which it passes closely in front of the spray jets 17 and is backwashed. A first construction of filtering element 5 will now be described in more detail with reference to Figures 3 and 4.

The supporting ring 6 of each filtering element 5 comprises a circular annular rigid frame 30, across which the mesh screen M is stretched.

The frame 30 is mounted within an annular box-section housing 32 formed of two parts 34, 36, secured together by bolts in bolt holes 38 through the parts.

The two parts 34 and 36 of the housing 32 are provided with annular bearing surfaces 40 which are convex in cross-section as shown in Figure 4 to form a convergent entry for the screen. The surfaces extend to each side of an annular line of closest proximity between them.

When the two parts 34 and 36 of the housing 32 are secured together as shown in Figure 4 the bearing surfaces 40, opposed to one another, lie against the individual wires of the mesh screen M adjacent to the point of emergence of the mesh M from the support ring 30.

In the embodiment shown in Figure 4, the bearing surfaces 40 lie on the circumference of a circle, the radius of which is chosen such that as the wires of the mesh screen M flex they conform to the shape of the surfaces 40 thereby limiting bending stresses to militate against the onset of metal fatigue.

In a modified construction shown in Figure 5, the bearing surfaces are formed by elastomeric rings 42 of circular cross-section positioned in accommodating recesses 44 in the parts 34,36 of the housing 32.

When the mesh screen M is in flexure the surfaces of the elastomeric rings 42, being resilient, assume a radius which is somewhat greater than their radius when at rest to an extent which depends on the elastic properties of the material and to the tension in the wires of the mesh. In this way allowance is created for variable amounts of flexure in the mesh screen M. This results in the reduction of bending stresses over a wide range of cyclical movement of the wires of the mesh screen M.

The advantages of the invention thus described may be illustrated by consideration of the stresses present in the individual wires of the mesh screen of a filtration device when in use. These comprise a tensile force which is a sum of the stress induced by the initial tension applied to the mesh, the stress additionally induced by the elongation of the wires of the mesh screen owing to distension and a further superimposed stress arising as a consequence of the wires being bent at their support points under operational pressure.

By virtue of the introduction of the bearing surfaces this superimposed stress is comprised of the tensile stress at the convex edge of the bent wires (indicated by M' in Figures 4 and 5) and the compressive stress of equal magnitude at the concave edges measured with respect to the point of contact with the opposed bearing surfaces.

It is the additional tensile stress so induced which the alternative embodxment compensates for, its magnitude depending upon wire diameter and the radius of the curved bearing surfaces.

The magnitude of the additional stress may be calculated using a knowledge of the theory of elasticity and the wire properties.

Without the provision of curvilinear bearing surfaces of correct radius the stress increment may be very high and sufficient to cause the total wire stress to rise beyond the limit which will safely resist fatigue cracking over an unlimited number of stress fluctuations.

This safe stress limit will be known and predetermined for wires of a particular material. As an example, for a stainless steel wire of 0.1 mm diameter it will usually be adequate to use a bearing surface radius of 6 mm.

Claims

1. Filtering apparatus for separating residues of solids from liquid filtrates comprising a rotatably 5 mounted filtration vessel (1) , filtering means (5) disposed about the axis of rotation in one or more walls of the vessel to provide filtering outlets from the vessel, feeding means (8,10) enabling materials to be filtered to be introduced into the vessel, means

¹⁰ (14) for collecting filtrate emanating from the filtering outlets at a lower level, cleaning means (16) for dislodging residues from the filtering means at a higher level, and means (18,19,20) within the vessel for collecting residues dislodged from the filtering

15 means by the cleaning means, portions of the filtering means being conveyed successively between the lower and higher levels by rotation of the drum.

2. Apparatus according to claim 1 in which the 20 cleaning means (17) enables backwashing of the filtering means (5) .

3. Apparatus according to claim 2 in which- the cleaning means comprises one or more spraying nozzles

25 arranged opposite the filtering means (5) at the higher level and operative to cause a spray of liquid to impinge on the filtering means (5) .

4. Apparatus according to any preceding claim in •30 which the filtering means (5) comprises a plurality of discrete filtering elements disposed about the axis of rotation of the vessel.

5. Apparatus according to claim 4 in which each 5 filtering element comprises a mesh screen (M) .

6. Apparatus according to any preceding claim in which the filtration vessel (1) is a generally cylindrical drum, the filtering means (5) being mounted in at least one end wall (2) of the vessel.

7. Apparatus according to any preceding claim in which the feeding means and the residue-collecting means comprise ducting (8) which extends through the vessel, generally along the rotational axis, for the delivery of materials to be filtered into the vessel (1) and the removal of residues from the vessel.

8. Apparatus according to claim 7 in which the ducting comprises a single duct (8) which has one or more discharge ports (10) for delivery of materials into the vessel (1) and one or more reception ports (21) for receipt of residues cleaned from the filtering means (5) , there being a sealing partition (11) within the duct between the one or more discharge ports (10) and the one or more reception ports (21) .

9. Apparatus according to any preceding claim in which the residue-collecting means comprises funneling means (18) secured within the vessel adjacent to the filtering outlets to collect residues dislodged from the filtering means and to feed the residues into a collecting trough.

10. A filtering element comprising a mesh screen (M) and peripheral means supporting the screen, the screen being retained between opposed convex bearing surfaces (40) of the supporting means which form a convergent entry for the screen.

11. A filtering element according to claim 10 in which the convex bearing surfaces (40) extend to each side of a line of closest proximity between the opposed surfaces.

12. A filtering element according to claim 10 or claim 11 in which the supporting means comprises a frame (30) to which the mesh screen (M) is secured and a housing (32) in which the frame (30) is retained, the bearing surfaces being presented by the housing (32) .

13. A filtering element according to claim 12 in which the housing (32) is of box section.

14. A filtering element according to claim 13 in which the housing is constructed from two parts (34,36) secured together, one bearing surface (40) being presented by each part (34,36) .

15. A filtering element according to any one of claims 10 to 13 in which the bearing surfaces (40) are presented by components (42) of circular section.

16. A filtering element according to any one of claims 10 to 15 in which the bearing surfaces (40) are presented by an elastomeric material.

17. Filtering apparatus according to any one of claims 1 to 9 in which the filtering means comprises a plurality of filtering elements (5) in accordance with any one of claims 10 to 16.