GB2362837A

GB2362837A - Sludge clarifier with shear inducing elements

Info

Publication number: GB2362837A
Application number: GB0006651A
Authority: GB
Inventors: Alistair Martin
Original assignee: United Utilities PLC
Current assignee: United Utilities PLC
Priority date: 2000-03-21
Filing date: 2000-03-21
Publication date: 2001-12-05
Also published as: GB0006651D0

Abstract

A sludge clarifier comprises a tank 1 with a rotatable support member 3 and at least one shear inducing element 14 supported by the rotatable support member which is longer in the direction of rotation than it is across the direction of rotation. The shear inducing elements may be pivotally mounted on a second rotatable support member 12 which is mounted on the first rotatable support member, while drive means may be provided to rotate the first and second support members. The first and second support members may be geared such that each shear inducing element does not return to the same part of the tank after a full revolution of the first support member. The shear inducing elements may have an aerofoil cross section.

Description

1 SLUDGE CLARIFIER 2362837 The present invention relates to a sludge

clanfier, particularly to a sludge clarifier having a raking mechanism to improve consolidation of the sludge.

Sludge thickeners comprising a gravity settling tank designed to encourage suspended solid particulates to separate out from the liquid due to the action of gravity are well known. The speed of consolidation of the sludge is dependent on the penneability of the sludge, the permeability being the ability of the liquid phase to escape the matrix of the solid phase.

As sludge thickens or consolidates due to gravity, it tends to forin a gel-like suspension and exhibits a compressive yield stress. This tends to prevent further compaction until the gel can be caused to fail to allow egress of liquid and hence further consolidation of the solid matter. Such failure of the gel may be induced due to the shape and depth of the thickener tank or by raking the sludge.

A known raking arrangement to induce failure of the gelled mass incorporates a rake known as a "picket fence" rake, so called because it resembles a picket fence. The rake is suspended from a rotatable bridge that extends above the level of the sludge from the centre of a circular tank to its circumference. As the bridge is rotated, the picket fence rake is swept through the sludge and creates channels behind the uprights, into which liquid can escape and rise upwardly. The escape of the liquid induces a failure of the gelled suspension so that it collapses under its own weight and thus consolidates. This is referred to as the density driving force of consolidation, and the resistance to collapse increases as the sludge consolidates.

It is an object of the present invention to provide an improved sludge clarifier.

According to a first aspect of the present invention there is provided a sludge clarifier comprising a tank having a rotatable support member and at least one vertically mounted shear inducing element supported by the rotatable support member, the shear inducing element being longer in the direction of rotation of the support member than across the direction of rotation.

The use of shear inducing elements that are longer in the direction of rotation than across the direction of rotation creates a shear effect in the sludge. This allows the liquid to escape in a much more efficient manner than the creation of channels for the liquid to flow into, and also means that the consolidation of the sludge is much quicker than with prior art devices. For example, with known picket fence clanifiers, the faces of the uprights that form the picket fence rake effectively represent bluff bodies to the Sludge. This creates a turbulent wake as the rake is moved through the sludge, and the rake thus imparts random and uncontrolled shear effect to the sludge.

According to a second aspect of the present invention, there is provided a sludge clarifier comprising a tank having a first rotatable support member, a second rotatable support member rotatably mounted on the first support member, driving means to rotate the first and second support members, and at least one raking element mounted on the second support member.

The at least one raking element can be positioned on the second rotatable support member to trace a desired path through the sludge. For example, the raking element can be caused to create a looping path around the circular tank, similar to pattems that may be created with a--Spirograph"toy (Spirograph is a registered trade mark). This ensures that a large proportion of the tank may be swept by the raking element. The pattern created by the raking element as it is rotated in the tank may be such that it will not return to its starting point in a very large number of rotations, if at all.

Preferably a plurality of raking elements are mounted on the second support member. The or each raking element is preferably a shear inducing element. The rotational speeds of the raking elements mounted on the second support may be matched to create a uniform pattem of shear determined to be optimum for the particular sludge conditions.

Preferably, the first and second support members are geared so that the raking element does not return to the same part of the tank after a full revolution of the first support member. The gearing of the rotatable support member ensures that the raking can be made to cover more of the sludge than was previously possible, allowing more even raking.

The first and second aspects of the present invention are preferably used together in one sludge clarifier, or may be used separately.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings, in which:

3 Figure 1 is a schematic top view of a thickener tank incorporating a sludge clarifier according to the present invention; Figure 2 is a schematic part cross-sectional view of the sludge clarifier of Figure 1; and Figure 1; and Figure 3 is a side view of a rake suitable for use in the sludge clarifier of Figure 4 illustrates cross-sectional views of different embodiments of rakes.

Referring to the Figures 1 and 2 of the drawings, there is illustrated a gravity thickener tank 1 for use in the separation of sewage sludge. The tank 1 is cylindrical in shape and has a central feed well 2 that extends from the upper portion of the tank to a depth close to the top of the height of the expected sludge "blanket". Fresh sludge may be pumped into the tank via feed well 2. A rotatable radial bridge 3 is provided at the top of the tank and extends from a pivot point 4 at the centre of the tank to the outer wall 5 of the tank. A draw off hopper 6 is provided at the centre of the tank 1, underneath feed well 2, to allow separated solids to be removed from the tank. Separated liquids may be run off via a weir (not shown) that may be provided around the upper edge of the tank. The surface of the liquid in normal use is shown by a dotted line in Figure 2.

Depending ftom the radial bridge 3 are three vertical support members 7, 8 and 9. Support member 7 is close to the feed well 2 at the centre of the tank, support member 8 is adjacent to the outer wall 5 of the tank and support member 9 is mid-way between support members 7 and 8.

The three support members 7, 8 and 9 support a horizontal support member 10 close to the bottom of tank 1. Depending from support 10 are a plurality of blades 11 that extend towards the bottom of the tank. Blades 11 are generally known as "plough shares" and are arranged at an angle to the direction of rotation of bridge 3, such that solids collected at the bottom of the tank 1 are gradually swept towards the draw off hopper 6 at the centre of the tank by the plough share blades 11 as the bridge 3 is rotated.

Two horizontal support members 12 and 13 are pivotally mounted on central vertical support member 9. Support member 12 is positioned so that it will be close to the top of the height of the expected sludge "blanket", that is, below the level of the 4 separated liquids, and is rotatable about vertical support member 9 by means of driving means (not shown). Support member 13) is positioned just above horizontal support 10. Extending between support members 12 and 13 are a plurality of vertically mounted vanes 14.

'anes 14 are described in greater detail with reference to Fipres 3 and 4 of the accompanying drawings. Each vanc 14 is pivotally mounted on a vertical spindle 15 that is placed forwards (in the direction of rotation of support member 12 illustrated by arrows C) of the centre of drag 16 of the vane. The placing of the spindle ahead of the centre of drag is important as otherwise the vanes may flip around as the support member is rotated and the vanes 14 are dragged through the sludee.

C, The vanes 14 generally comprise a flat or profiled sheet of material having sufficient rigidity to resist deformation under the influence of applied shear forces. Using terminology from aeroplane wing design, the length of the surface of an t t> aeroplane wino from leading edge to trailing edge is known as a chord. Vanes 14 are t:l C> C shaped to present a chord 17 which is much longer than the width of the vane so as to create a shear force as the vane is moved through sludge, and to minimise turbulence Z:1 which would reduce the shearing effect. The ratio of the chord length to the width of the vane is determined for each application, and may typically range from about 4:1 to about 10: 1 (an example ratio may be 10: 1). An example of a suitable shape for vanes 14 is to have both sides of the vane having an aerofoil shape, i.e. shaped similarly to the upper surface of an aeroplane wing W, In use of the thickener tank 1, fresh sludge is introduced into tank 1 by means of feed well 2, with the sludge flowing out of the bottom of the feed well 2 so as to 1 minimise turbulence in the tank. The bridge -3) is rotated in the direction of the arrow A as shown in Figure 1, with the horizontal support member 12 (and hence vertical vanes 14) being rotated at the same time in the direction of arrow B of Figure 1. The relative velocities of the rotation of the bridge 3 and the horizontal supports 12 and 13 may be varied according to the requirements of the particular application, which may be achieved by gearing the drive arrangement. The two different rotating supports may for instance be arranged to ensure that the vanes 14 trace an irregular path through the sludge, the gearing being such that the outermost vanes 14 supported by the supports 12 and 13 trace a looped pattern around the tank, and do not rake the same portion of the tank within a very large number of rotations of the bridge arm 3, if at all. This ensures that the vanes 14 rake the whole volume of sludge, greatly enhancing the consolidation.

The relative rotational speeds of the bridge arm 3 and supports 12, 13 may be controlled so that the vanes 14 travel through the sludge at substantially the same speed and thus creating substantially the same shear effect throughout the tank. The rotational speed of the bridge arm 3 and the support member 12 can thus be chosen such that the speed of the individual vanes 14 show a desirable degree of uniformity, creating a uniform pattern of shear determined to be optimum for the particular sludge conditions.

It is believed that the sludge clarifier according to the present invention works due to the creation of shear effects in the sludge allowing liquid to escape the solid matrix rather than being due to the creation of channels for the egress of liquid as in the prior art, Faster consolidation rates of the sludge are achieved than with the prior art, meaning that a more dense sludge product may be achieved in the same or a shorter period of time than was previously possible, In addition, a more dense sludge product may be achieved at the same or an increased underflow solids flux. As the consolidation process is much quicker than with prior art devices, the solid sludge "blankeC is lower in the tank than with the prior art devices, meaning that the liquid may be drawn at a higher overflow flux, thus increasing through-put speed.

The three main parameters affecting the amount of shear created in the sludge are the speed of rotation of the bridge arm 3, the speed of rotation of support member 12, and the chord length of the vanes 14 along support member 12. Longer vanes 14 need to be placed further apart than shorter vanes due to their rotation about spindle 15. However, the vanes could be fixed in position on support member 12 if they were curved in the direction of rotation. Longer vanes travelling slowly will induce the same shear as shorter vanes travelling more quickly, and so this can be taken account of in the design of the raking mechanism for a particular application.

The vanes 14 may be simply fixed along the length of the rotating bridge, rather than on a second rotatable support, although this would mean that the outer vanes would move more quickly than the inner vanes, thus creating more shear.

6 However, such an arrangement may be advantageous in certain circumstances (for example for case of retrofitting existing settling tanks). The amount of shear created may, be too great for the particular sludge rheologgy, but the creation of shear by the vanes 14 would still be advantageous compared to the previous "picket fence" rakes that create turbulent channels, even without the use of the second rotating supports 12 and 13.

The outer vanes may, have different length chords than the inner vanes. The chords may be altered by changing the cross-sectional shape of the vanes or by changing their lengths. More than one rotatable bridge arm may be provided, with the vanes being staggered so that vanes on each arm do not pass through the same area of the tank in order to create shear in as much of the tank as possible. The vanes may also be staggered along support member 12.

The vanes 14 may be of any suitable cross-sectional shape, examples of which are illustrated in Figure 4 of the drawings.

Other parameters which may determine the exact configuration of vanes for a particular application include the power consumption. Faster consolidation of the sludge could be achieved by rotating the vanes more quickly, but at the expense of higher power consumption. In some circumstances it may be acceptable to have a lower speed of consolidation with a lower power consumption.

The use of two rotatable supports would be advantageous with the use of "picket fence" style rakes in place of the vanes 14. Although not providing the shear effect found to be particularly effective it would nevertheless enable the speed of rotation of the "picket fence" rakes to be optimised through the tank.

It should be appreciated that the lower horizontal support 10 and the "plough share" blades may be replaced by any suitable device such as a hollow arm with a plurality of bored apertures in its leading face or edge so that solids are collected by the arm and fed to the draw off hopper 6. The arm may be oriented such that its axis describes a portion of a radius of the tank, or alternatively the arm may describe a portion of a line which terminates as the tangent of a circle, the radius of which lies between zero and that of the tank. The arm may also be curved in either or both of the vertical and horizontal planes.

7

Claims

1. A sludge clarifier comprising a tank having a rotatable support member and at least one shear inducing element supported by the rotatable support member, the shear inducing element being longer in the direction of rotation of the support member than across the direction of rotation.

2. A sludge clarifier according to claim 1, wherein the shear inducing element is pivotally mounted on the rotatable support member.

3. A sludge clarifier according to claim 2, wherein the shear inducing element is pivotally mounted on a spindle attached to the rotatable support member.

4. A sludge clarifier according to claim 2 or 3, wherein the shear inducing element is pivotally mounted on the rotatable support member forwards of the centre of drag of the shear inducing element in the direction of rotation of the support member.

A sludge clarifier according to any preceding claim, wherein a plurality of shear inducing elements are supported by the rotatable support member.

6. A sludge clarifier according to claim 5, wherein the shear inducing elements have different lengths in the direction of rotation of the support member to one another.

7. A sludge clarifier according to claim 5 or 6, wherein the shear inducing elements have different cross sectional areas to one another.

8. A sludge clarifier according to any preceding claim, wherein the or each shear inducing element has an aerofoil cross section.

8

9. A slud c clarifier comprising a tank ha-ving a first rotatable support member, a 9:> In second rotatable support member rotatably mounted on the first support member, driving means to rotate the first and second support members, and at least one raking element mounted on the second support member.

10. A sludge clarifier according to claim 9, wherein a plurality of raking elements are mounted on the second support member.

11. A sludge clarifier according to claim 9 or 10, wherein the first and second support members are geared so that the raking element does not return to the same 1 part of the tank after a full revolution of the first support member.

12. A sludge clarifier according to any of claims 9 to 11, wherein the second support member is an elongated horizontal member pivotally mounted about its CI centre.

13. A sludge clarifier according to any of claims 9 to 12, wherein the or each raking element is a shear inducing element and is longer in the direction of rotation of the second support member than across the direction of rotation of the second support member.

14. A sludge clarifier according to claim 13, wherein the or each raking element is pivotally mounted on the second support member.

15. A sludge clarifier according to claim 14, wherein the or each raking element is Z-1 pivotally mounted on a spindle attached to the second support member.

16. A slud c clarifier according to claim 14 or 15, wherein the or each raking 9:> element is pivotally mounted on the second support member forwards of the centre of drag of the raking element in the direction of rotation of the support member.

9

17. A sludge clarifier according to claim 10, wherein the plurality of raking elements are positioned in a staggered arrangement on the second support member such that as the second support member is rotated the raking elements do not pass through the same area of sludge.

18. A sludge clarifier according to claim 10 or 17, wherein the raking elements have different lengths in the direction of rotation of the second support member to one another.

19. A sludge clanfier according to claim 10, 17 or 18, wherein the raking elements have different cross sectional areas to one another.

20. A sludge clarifier according to any of claims 9 to 19, wherein the or each raking element has an aerofoil cross section.

21. A sludge clarifier substantially as hereinbefore described, with reference to the accompanying drawings.