Apparatus for the removal of sludge

Apparatus for removing sludge from an underwater deposit comprises a blade having a cutting edge that points forwards and a blade surface to the rear of the cutting edge that faces the interior of a housing. Nozzles are positioned and oriented to direct jets of water onto the blade surface to fluidize sludge that has been cut by the blade. The jets have a component of motion in the rearward direction relative to the blade surface, whereby the fluidized sludge is effectively contained within the housing and high collection rates can be achieved. The apparatus can be adapted for horizontal use over the surface of a sludge deposit or for movement in any direction through a bulk sludge deposit.

BACKGROUND

The invention relates to the removal of sludge from underwater deposits.

Sludge can build up over time on the floors of artificial containers for water such as tanks or ponds or on the floors of natural bodies of water such as lakes and oceans. Removal of the sludge may be desirable to increase the volume or flow of water, to remove pollutants contained in the sludge, to collect the sludge for use as a resource or for other reasons.

The source of the sludge may be natural, e.g. from settlement of particles suspended in the water, or may be artificial, e.g. as a result of mining or drilling. It may comprise particles of a well-defined size or of a range of sizes. The size or range of sizes may vary substantially between different deposits. The sludge may contain varying proportions of water within its structure. As a result of the pattern of deposition, the sludge may be uniform or stratified. As a result of physical or chemical processes following deposition, the particles may be bonded together in a consolidated mass or they may be held together only by gravity. For the purposes of this specification, the term “sludge” should also be taken to include dense liquids and semi-solids such as wax that are not particulate in nature but have similar properties and, in particular, are susceptible to fluidization by the use of pressurized water.

One example of sludge that requires removal is an accumulation on the floors of the ponds in which nuclear waste is stored. The ponds are artificially constructed so the floors can be assumed to be flat and even but the presence of larger items dropped into the ponds cannot be ruled out. Because such sludge is radioactive, it requires efficient collection and careful disposal, and this must be achieved remotely from the presence of a human operator. Such sludge tends to have built up slowly over a long period and is therefore stratified and may be relatively strongly bonded. Previous attempts to remove sludge in this situation have achieved poor collection rates of less than 15%.

Another example of sludge that requires removal is an accumulation on the sea floor around oil or gas wells as a result of materials that have been spilled during the drilling and extraction processes. Sludge in this situation tends to be soft but poorly structured and it may form deep deposits. The natural sea floor on which the deposit rests may be uneven and its contours will likely be unknown.

SUMMARY OF THE INVENTION

The invention provides an apparatus for removing sludge from an underwater deposit as defined in claim1.

The invention further provides a method of removing sludge from an underwater deposit as defined in claim18.

Preferred but non-essential features of the invention are defined in the dependent claims.

The invention uses a blade to cut into the sludge first, followed by water jets acting against the surface of the blade to fluidize the sludge and convey it towards the rear of the collection chamber. Because the sludge is not fluidized until already inside the chamber, the fluidized sludge is effectively contained and very high collection rates can be achieved.

FIGS. 1 to 4illustrate a first embodiment of the invention, which is an apparatus for removing stratified sludge from the flat floor of an artificial tank such as a nuclear waste storage pond. The apparatus comprises a housing2, which defines a forward direction (indicated by an arrow3) and an opposite rearward direction. In this embodiment the forward and rearward directions are horizontal, parallel to the floor of the pond (not shown).

FIG. 1differs fromFIGS. 2 to 4in having a larger number of cutting discs4and jets14,16. These differences do not affect the principle of operation of the apparatus andFIGS. 1 to 4will be described as a single embodiment of the invention.

The operation of this embodiment of the apparatus will first be described in outline. As the apparatus moves forwards over the surface of the sludge, a transverse row of cutting discs4at the front of the apparatus slices vertically into the surface of the sludge. A transverse cutting blade5projects from the underside of the housing2so that, as the apparatus moves forwards, a cutting edge7of the blade5cuts horizontally into the surface of the sludge to separate a layer of sludge and direct it into the interior of the housing2.

An inlet6delivers water at high pressure from an external source to a manifold8that extends transversely across the apparatus near to the cutting blade5. A first set of nozzles10is arranged along the manifold to direct a first row of jets14of the pressurized water rearwards onto the surface of the blade5. The first jets14fluidize the collected sludge and carry it towards the rear of the housing. A second set of nozzles12is arranged along the manifold to direct a second row of jets16of the pressurized water rearwards towards the roof of the housing2. The second jets16assist with carrying the fluidized sludge towards the rear or the housing2. A jet pump18removes water and the fluidized sludge from the rear end of the housing2and delivers it to an outlet20to be further processed externally of the apparatus.

The housing2provides structural support for all the other components. A front wall22, top and bottom walls23,24and side walls25,26define a collection chamber28. The collection chamber28tapers towards a port30at the rear of the housing2. In the illustrated embodiment, both the top and bottom walls23,24and the left and right side walls25,26converge towards the port30but it is not essential that they should all do so. The housing will typically be formed from steel sheets but for particular applications the choice of material may be influenced by the properties of the sludge, for example its pH or its radioactivity.

The cutting blade5projects at a slight angle (e.g. 10°) from an opening in the bottom wall24of the housing2. Preferably the cutting blade5is a separate component mounted on the housing, which permits it to be made of a different and wear-resistant material. It may be possible to replace the blade5when it has become worn. Alternatively, the blade5may be formed from a down-turned portion of the bottom housing wall24. The cutting edge7of the blade5is preferably straight but an arcuate blade is also possible. The triangular gap between the blade5and the main plane of the lower housing wall24may be closed at each side by a triangular side wall (not shown) to prevent the escape of fluidized sludge before it enters the chamber28.

The row of cutting discs4is arranged along a transverse, horizontal axis at the front of the housing2so that each disc4is in a generally vertical plane. The discs4may be mounted on a common axle or each disc4may be mounted so that it can rotate independently. The cutting discs4may be driven to rotate by turning the axle or they may rotate passively in response to the forward movement of the apparatus over the surface of the sludge. The cutting discs4comprise blades around their periphery, which may be continuous or segmented into teeth. The effect of the cutting discs4is to slice the top layer of sludge into longitudinal strips before it is lifted by the horizontal cutting blade5of the apparatus. The lower rim of the cutting discs4should therefore project at least as far as the cutting blade5below the plane of the bottom wall24of the housing2. The spacing between the discs4partly determines the size of the lumps of sludge that are lifted by the blade5and fluidized by the jets14. The spacing may therefore need to be smaller in the case of tougher, more consolidated sludge deposits.

A grid (not shown) of suitable spacing may be provided to prevent objects greater than a certain size from entering the collection chamber28through the opening above the blade5, including solid objects that might have been dropped into the pond and be resistant to cutting by the discs4.

The manifold8extends across the width of the housing, following the line of the blade5. The first set of nozzles10is arranged along the manifold8to direct jets14of pressurized water from the manifold through the collection chamber28to impinge on the surface32of the blade5that faces into the interior of the housing2. Each nozzle10is in the form of a slit so that the jet is fan-shaped, having a cross-section of much greater extent in a direction parallel to the cutting edge7of the blade5than its extent in the perpendicular direction. The divergence of the fan-shaped jet14from the nozzle10is typically more than 30° but it may be smaller if adjacent nozzles are closer together. The jets14should meet the surface32of the cutting blade5in a continuous line with no gaps between them. As seen inFIG. 4, the line of jets14preferably hits the surface32of the blade5close to its tip, for example within 10 mm of the edge7.

The jets14hit the blade surface32at an incident angle that imparts a rearward component of motion relative to the surface. The incident angle may be chosen to suit the nature of the sludge that is to be fluidized and carried away by the jets. For strongly bonded sludge, a steeper incident angle, approaching perpendicular to the surface32, may be desirable to promote good fluidization. For unconsolidated sludge, the emphasis may be on sweeping the sludge towards the rear of the chamber28so a shallower incident angle may be more efficient. InFIG. 4, the angle is 130°, falling in the typical range of 115° to 135°. Any angle less than 95° is unlikely to have sufficient momentum to carry the fluidized sludge to the rear of the chamber and any angle greater than 175° is unlikely to provide sufficient impact against the surface32. Note that these incident angles are measured between the centre line of the jet14and the surface32of the blade, using the convention that an angle of 0° represents a jet parallel to the blade surface directed towards the front of the apparatus and 180° represents a jet parallel to the blade surface directed towards the rear of the apparatus.

The choice of pressure at which the water is delivered to the nozzles10is determined primarily by the consolidation of the sludge, as well as by the configuration of the nozzles10and the length of water through which the jet14has to pass from the nozzle to the blade surface32. For unconsolidated sludge, a pressure of 5 bar (0.5 MPa) may be sufficient, while for strongly consolidated sludge a pressure in excess of 1000 bar (100 MPa) may be required. The pressure can be adjusted during use to adapt to the conditions discovered, subject to the rating of the nozzles10.

For certain types of sludge (e.g. wax) the pressurized water may be heated to assist with the fluidization process. Similarly, the water may contain additives or a fluid such as a solvent may be used in place of water.

A second set of nozzles12is arranged along the manifold8to direct jets16of pressurized water from the manifold through the collection chamber28towards the top wall23of the housing2. Again, it is preferred that the nozzles12should form fan-shaped jets16that merge into a continuous line. However, it is not essential that the jets16should impinge on the top wall23because no significant fluidization of sludge occurs at this location. The second nozzles12need not be of the same design as the first nozzles10. The second nozzles12could be provided along a second manifold (not shown) which would allow their position or the pressure of the water supply to them to be independently chosen. More conveniently, the first and second nozzles may be arranged alternately along the manifold8, as shown inFIG. 2. The second jets16have a rearward component of motion relative to the top wall23, whereby the first and second jets together create a flow of water and fluidized sludge towards the port30at the rear of the collection chamber28. The two rows of fan-shaped jets14,16effectively prevent any fluidized sludge from escaping to the front of the housing or through the opening in the bottom wall24of the housing2.

The pump18is connected between the port30and the outlet8to remove water and fluidized sludge from the collection chamber28. The pump18inFIG. 1is a jet pump, driven by pressurized water from the inlet6that is delivered through a branch34. Other types of pump such as centrifugal pump or a macerating pump may alternatively be used. The rate of flow of the pump should be adjustable to accommodate the variable flow of the jets14,16. The size of the port30and the capacity of the pump should be chosen to accommodate the largest lumps that are capable of entering the collection chamber28and surviving the fluidization process.

The apparatus normally rests with the bottom wall24of the housing2on the surface of the sludge deposit. A remotely operated tractor unit (not shown) drives the apparatus forwards over the surface so that the top layer of sludge is scraped up by the blade5, fluidized and collected in the chamber28. The tractor unit may push or pull the apparatus; pushing is preferred because it helps to embed the blade5in the sludge. It has been found effective for the tractor to drive the apparatus in a reciprocating motion, repeatedly pushing it forwards by a few centimeters then back by a shorter distance.

When the sludge deposit has been almost completely removed, it is important that the cutting edge7of the blade5should not ground on the floor of the pond. The apparatus is therefore provided with front and rear support wheels36,38that can run along the floor. The front wheels36are not shown inFIG. 3but they rotate about the axle39. The height of the axle39may be adjustable to ensure the correct offset40between the tip of the blade5and the floor.

FIGS. 5 and 6illustrate a second embodiment of the invention, which is an apparatus for removing a sludge from deep deposit that may not be stratified and may not lie on a level floor. This apparatus is particularly suitable for use with soft, unconsolidated sludge.

The apparatus comprises a housing52, which defines a forward direction (indicated by an arrow53) and an opposite rearward direction. In this embodiment the forward and rearward directions are typically vertical, the apparatus being lowered from a crane under its own weight or actively pushed down into the bulk sludge deposit, but if the apparatus is pushed it may in fact be used in any orientation. As with the first embodiment, reciprocal movements may be applied to the apparatus or vibration at up to ultrasonic frequencies may be used to assist the passage of the blades through the sludge.

The housing52comprises a collection chamber54surrounded by side walls55,56and end walls57but generally open in the forward direction. The collection chamber54tapers towards a port58at the rear end, from which water and fluidized sludge can be removed by a pump (not shown) for further processing externally to the apparatus. A grid (not shown) of suitable spacing may be provided to prevent objects greater than a certain size from entering the collection chamber54through the opening.

A central boss60extends across the collection chamber54between the end walls57. A blade62projects forwards from the boss60through the opening of the collection chamber54. The blade62may be integral with the boss60or it may be a separate component mounted on the boss60, which can optionally be replaced when it has become worn.

As the apparatus moves forwards, the forward blade62slices into the bulk sludge, deflecting it to each side of the blade62and into the opening of the collection chamber54. The front edges of the side walls55,56serve as cutting blades64,65that also penetrate the sludge and guide parts of it into the chamber54. If the sludge is sufficiently soft, the cutting blades64,65need not be particularly sharp. As shown inFIG. 6, it is preferred that the cutting blades64,65should converge slightly in the forward direction to promote a good seal against the undisturbed sludge. This helps to prevent the escape of fluidized sludge that is circulating in the collection chamber54.

An inlet (not shown) delivers water at high pressure from an external source to a manifold66that extends along the central boss60. A first set of nozzles68is arranged along one side of the manifold66to direct a first row of jets69of the pressurized water onto the surface of one cutting blade64(i.e. the side wall55). A second set of nozzles70is arranged along the other side of the manifold66to direct a second row of jets71of the pressurized water onto the surface of the opposite cutting blade65(i.e. the other side wall56). The apparatus is therefore mirror-symmetrical about its centre line, although it is possible for the symmetry to be slightly broken by alternating the first and second nozzles68,70along the length of the manifold8.

As in the first embodiment, the jets69,71preferably fan out from the nozzles68,70to form an unbroken line where they impinge on the side walls55,56. Again, as in the first embodiment, the angle of incidence of the jets69,71on the side walls55,56may vary substantially, provided that it has a component of motion in the rearward direction relative to the surface of the side walls in order that the jets rebounding from the side walls should convey the fluidized sludge towards the rear of the collection chamber54.