GB2267839A - Sedimentation tank - Google Patents

Abstract

A sedimentation tank comprises a rotary bridge and an annular scum barrier, extended at one location to form a scum discharge zone from which an overflow pipe extends downwardly. The top of the pipe is normally closed by a valve 20, which is lifted by a contact member 7 on the bridge each time the bridge rotates. Valve 20 may be spring-loaded to the closed position and may be slidable or pivotally mounted as shown. <IMAGE>

Description

A SEDIMENTATION TANK This invention relates to a sedimentation tank.

Sedimentation tanks are used to allow heavier components suspended in a liquid to settle, so that they can be removed as sludge. Such tanks are used in a number of applications including, for example sewage treatment, chemical processing, and food processing.

Certain materials suspended in a liquid do not settle, but float on the surface of the liquid as a scum. The scum must be removed from the surface.

The problem of removing scum has been addressed using valves having inlets at about (generally just below) the intended level of the surface of liquid in the reservoir when the tank is in use. The valves can be permanently open, but this gives rise to difficulty because the rate of flow of liquid through the inlet can not be controlled.

It is also known to provide such valves with closing mechanisms, but difficulties have been encountered with such valves because of the low reliability of the mechanisms in what can often be a harsh operating environment in the liquid in the tank.

The present invention provides a sedimentation tank with a discharge conduit for removal of scum from the surface of collected liquid, which includes a closing member for the discharge conduit which is operated by a rotating bridge as it rotates.

Accordingly, in one aspect, the invention provides a sedimentation tank which comprises: (a) a reservoir, (b) a bridge element mounted for rotation in a plane approximately parallel to the surface of liquid in the reservoir, (c) a discharge conduit having an inlet at about (generally just below) the intended level of the surface of liquid in the reservoir when the tank is in use, (d) a member for closing the inlet to the discharge conduit, and (e) contact portions on the bridge element and the closing member which can engage one another on rotation of the bridge member to cause the closing member to move to open the discharge conduit.

The tank of the present invention has the advantage that flow of liquid into the discharge conduit is controlled by the closing member, which is caused to move to open the inlet to the discharge conduit as a result of rotation of the bridge.

Sedimentation tanks, especially circular tanks, frequently include such a bridge to carry vanes which move sludge through the tank, towards a central collecting hopper at the centre of the tank. The control of flow of scum can therefore be achieved simply as a result of modification of existing tank equipment. Simplicity in the operation of the closing member results from the use of the rotating bridge element to open the inlet to the discharge conduit, so overcoming difficulties associated with poor reliability in other previously used valve arrangements.

The closing member might be mounted on a wall of the reservoir, or on some other component of the tank. Preferably, however, the closing member is mounted on the discharge conduit.

The closing member might be arranged to move in a direction approximately perpendicular to the plane of the inlet to the discharge conduit; for example, it might move in a sliding sense along the discharge conduit. In a preferred arrangement, the closing member is mounted so that it can move pivotally relative to the inlet to the discharge conduit, so as selectively to open or to close the inlet. When the closing member moves pivotally, it is preferred that the fulcrum about which it moves be above the inlet to the discharge conduit, for example on an arm, so that the portion of the closing member that engages the inlet is lifted out of the inlet. In this arrangement, it can be appropriate for the contact portion on the closing member to be spaced from the fulcrum in a direction parallel to the surface of liquid in the tank (although generally not in the same plane as the fulcrum) When the closing member moves pivotally, it can be preferred for the contact portion on the closing member to comprise a crank arm. The crank arm extends from the fulcrum for the closing member appropriately for it to be struck by the contact portion on the bridge element as the element rotates, and then to cause the closing member to pivot to open the inlet to the discharge conduit.

It is preferred that the closing member is biassed towards the position in which the inlet to the discharge conduit is closed.

In this way, the reliability of the seal between the closing member and the inlet can be enhanced, and the closing member can be made to move from an open position to the closed position more reliably.

Preferably, the contact portion on the bridge element is provided by a striker plate, which engages the contact portion on the closing member as the element rotates. Preferably, the striker plate includes a portion which is inclined to the direction of travel of the bridge element and which contacts initially the contact portion of the closing member. The inclined portion of the striker plate can ensure that the closing member moves smoothly from the closed position. The striker plate can include another inclined portion to ensure that the closing member moves smoothly from its open position back to the closed position as the bridge element moves away from the closing member. The striker plate can include a portion between the two inclined portions which extends approximately parallel to the surface of the liquid in the tank.

The position of the contact portion on the bridge element, relative to the bridge element may be adjustable, especially in a direction perpendicular to the surface of the liquid in the tank.

Preferably, one of the contact portions, on the bridge element or the closing member, is provided by at least one roller, for example a roller made from an inert polymeric material.

Certain nylons are suitable for use in sedimentation tanks used in the treatment of sewage. For example, when the contact portion on the bridge element comprises a striker plate, and the contact portion on the closing member comprises a crank arm, a roller can be provided on the end of the crank arm to engage the striker plate as the bridge element rotates.

The inlet to the discharge conduit is positioned suitably relative to the intended level of the surface of liquid in the reservoir when the tank is in use for drainage of scum which has collected on the surface of liquid in the tank. The inlet will generally be mounted just below that surface. The discharge conduit can be arranged to move with changes in the level of liquid in the tank to ensure that the inlet remains positioned appropriately relative to the liquid level. This can be achieved conveniently by means of floats.

The tank can include more than one discharge pipe, each with an associated closing member. For example, there might be two discharge pipes with associated closing members, diametrically opposite one another across a circular sedimentation tank. The tank might be provided with one bridge element, which engages the closing members in turn. The bridge element might be arranged to rotate about twice each hour.

A handle can be provided, mounted detachably on the closing member for manual operation of the closing member.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an isometric view of a sedimentation tank; Figure 2 is a schematic cross-section through the discharge outlet and a portion of the bridge element of the tank shown in Figure 1, with the closing member in the closed position; and Figure 3 is a schematic cross-section through the components shown in Figure 2, with the closing member in the closed position.

Referring to the drawings, Figure 1 shows a sedimentation tank, according to the invention and generally indicated by the reference numeral 1. The tank comprises a reservoir 2 for liquid to be treated; a boom or barrier 3 to retain surface deposits, which may be in the form of a scum; four rotating bridge elements 4; and a closure member 5 with an actuation member 6, leading to a discharge pipe. The inlet to the discharge pipe is located just below the liquid surface, and the discharge pipe has a diameter of about 150 mm. The bridge elements rotate in a plane substantially parallel to the surface of the liquid contained within the tank. A striker plate 7 is provided on one of the rotating bridge members 4 such that the striker plate 7 periodically engages with the actuation member 6 to move the closure member 5 from a position in which it closes the inlet to the discharge pipe to a position in which the inlet is open. As a result, scum collected in the rectangular trap 8 is allowed to flow down the opened valve and into a discharge pipe from which the scum may be collected in an appropriate receptacle.

Figure 2 shows the valve in the closed position with the closing member 20 engaged with the inlet to the discharge pipe.

In the closed position liquid is prevented from entering the discharge pipe 21. The closing member 20 seals the discharge pipe. Liquid pressure acting on the closing member 20 enhances the seal, and it is possible to enhance the seal between the closing member and the inlet to the discharge pipe further by biassing the closing member towards the inlet, for example by means of a spring connecting the arm portion 22 to the support member 23.

The striker plate 7, fixed on at least one of the rotating bridge elements, periodically engages with the end 24 of the crank arm 25. On engagement the crank arm 25 is forced to pivot about the fulcrum point 26 and disengages the closing member 20 from the inlet to the discharge pipe 21. As a result of the fulcrum for the closing member being above the inlet to the discharge pipe, the portion of the closing member that engages the inlet is lifted out of the inlet. Scum is allowed to flow down the discharge pipe while the striker plate 7 and crank arm end 24 are engaged. The striker plate 7 has inclined surfaces 27 at its leading and trailing ends to facilitate smooth engagement and disengagement of the striker plate 7, and a portion 30 between the two inclined portions which extends approximately parallel to the surface of the liquid in the tank. The inclined leading surface ensures that the closing member is, initially, opened gradually and liquid begins to enter the discharge taking scum with it. A roller 28 is provided on the crank arm end 24 to facilitate movement of the striker element over the end. The roller 28 may be formed from an inert plastics material, for example, a nylon.

As the striker plate continues to move over the roller 28, its central portion 30 comes into contact with the roller 28 (see Figure 3). In this position, the closing member 20 is held at its point of maximum opening for as long as the closing member is in contact with the central portion of the striker plate 7.

The time that the inlet to the discharge pipe is open can be controlled by selection of a striker plate with a central portion of appropriate length. As the trailing inclined surface of the striker plate passes, the closing member is lowered gradually onto the inlet to the discharge pipe. The valve remains closed until the next pass of the striker plate 7.

A handle may be provided on the crank arm by which the closing member 20 can be moved manually from its closed position to its open position.

Claims (9)

CLAIMS:

1. A sedimentation tank which comprises: (a) a reservoir, (b) a bridge element mounted for rotation in a plane approximately parallel to the surface of liquid in the reservoir, (c) a discharge conduit having an inlet at about the intended level of the surface of liquid in the reservoir when the tank is in use, (d) a member for closing the inlet to the discharge conduit, and (e) contact portions on the bridge element and the closing member which can engage one another on rotation of the bridge member to cause the closing member to move to open the discharge conduit.

2. A tank as claimed in claim 1, in which the closing member is mounted on the discharge conduit.

3. A tank as claimed in claim 1 or claim 2, in which the closing member is mounted so that it can move pivotally relative to the inlet to the discharge conduit, so as selectively to open or to close the inlet.

4. A tank as claimed in claim 3, in which the contact portion on the closing member comprises a crank arm.

5. A tank as claimed in any one of claims 1 to 4, in which the closing member is biassed towards the position in which the inlet to the discharge conduit is closed.

6. A tank as claimed in any one of claims 1 to 5, in which the contact portion on the bridge element is provided by a striker plate.

7. A tank as claimed in claim 6, in which the striker plate includes a portion which is inclined to the direction of travel of the bridge element and which contacts initially the contact portion of the closing member.

8. A tank as claimed in any one of claims 1 to 7, in which one of the contact portions, on the bridge element or the closing member, is provided by at least one roller.

9. A tank as claimed in any one of claims 1 to 8, which includes a handle mounted detachably on the closing member, for manual operation of the closing member.