A sludge control system

Abstract

A sludge control system comprises a fluid storage tank (107) configured to retain a volume of high-density fluid, the fluid storage tank (107) having walls (128) and a base (120) and comprising at least one opening (127) configured as a tank inlet and / or tank outlet, the base (120) of the tank (107) sloped.

Description

[0001] A SLUDGE CONTROL SYSTEM

[0002] TECHNICAL FIELD

[0003] The present invention relates to a sludge control system for a high-density pumped energy storage system. More particularly, the present invention relates to a sludge control system for a high-density pumped energy storage system that uses a suspended solid-in liquid base. The present invention also relates to a sludge control method for a high-density pumped energy storage system. More particularly, the present invention relates to a power generation method using a suspended solid-in-liquid base.

[0004] BACKGROUND

[0005] Turbines are a reliable and efficient way to generate electricity, and are used extensively in hydro-electric projects or systems, with the turbine unit or units forming one part of the overall system. In a hydroelectricity generating system a fluid such as water flows under gravity from one part of the system to another and then into a turbine unit. The fluid flow over the blades of the turbine in the turbine unit causes the turbine blades to rotate, spinning the turbine. The spinning of the turbine produces power.

[0006] However, renewable energy sources such as wind and solar have highly variable power outputs. On-grid energy storage therefore plays a crucial role in smoothing out the electricity supply from these sources and ensuring that the supply of power matches demand. Energy storage at grid scale is well established in the form of Pumped Hydro Storage (PHS) systems. In such systems, during times of low on-grid electricity demand, water is typically pumped from a lower-level reservoir to an upper-level reservoir, thereby gaining potential energy. The water is then stored in the upper-level reservoir until times of high on-grid electricity demand. At such times, the water is allowed to flow from the upper reservoir back to the lower reservoir through a penstock. The water turns a turbine located in the penstock to generate electricity that is then sent to the grid to help meet the high electricity demand.

[0007] Although water is used almost exclusively in these types of systems, alternative fluids have also been investigated for use in systems similar to Pumped Hydro Systems. It has been found that the use of high-density fluids (fluids having a density greater than that of water at the same temperature and pressure) can be highly beneficial in systems that operate on a similar principle to Pumped Hydro Systems. For example, the use of high-density fluids in these types of systems can reduce the requirement for vertical elevation between the upper and lower-level reservoirs compared to conventional Pumped Hydro Systems (i.e. conventional systems that use water as the working fluid). High-density fluids for use with these types of system are usually made by suspending a finely-ground mineral in a liquid, such as for example Bentonite, lllemnite, Barite or Hematite. In order to produce a fluid where the solid will remain in suspension for extended periods, the quantity and particle size distribution of the mineral are carefully chosen, along with the amount of other additions to the fluid, such as for example viscosity modifiers and similar.

[0008] By choosing the composition carefully, it is possible to keep the solid in suspension for extended periods of time, and to keep it stable enough to meet day-to-day operational requirements. For example, care needs to be taken to ensure that the fluid has a viscosity low enough so that system performance is not negatively impacted. Furthermore, viscosity modifiers are expensive and their use can impact on operations and maintenance costs, and can also make the end-of-life of the system more complex.

[0009] In operation, the overall system will typically be cycled once or twice a day. This is generally sufficient to ensure that the fluid is sufficiently homogenised for maximum performance. However, if the system is not cycled regularly (for example during periods of extended shutdown for maintenance or similar) the mineral will start to settle out of the fluid, with the larger and heavier particles generally falling out of suspension first. The fluid will develop a vertical density gradient and become increasingly non-homogenous.

[0010] If the shut-down period is relatively short, homogeneity can be recovered by simple agitation, such as for example by running the system over 2-3 cycles. However, if sufficient settling occurs, the solid particles will accumulate at the bottom of the storage tank to form a sludge. With sufficient time, the sludge can ‘cake’, and once this happens, it is not possible to recover the particles into the fluid simply by cycling the system.

[0011] In this specification where reference has been made to patent specifications, other external documents, or other sources of information, this is generally for the purpose of providing a context for discussing the features of the invention. Unless specifically stated otherwise, reference to such external documents is not to be construed as an admission that such documents, or such sources of information, in any jurisdiction, are prior art, or form part of the common general knowledge in the art.

[0012] SUMMARY OF THE INVENTION

[0013] It is an object of the present invention to provide a sludge control system which goes some way to overcoming the abovementioned disadvantages or which at least provides the public or industry with a useful choice. It is a further object of the present invention to provide a sludge control method which goes some way to overcoming the abovementioned disadvantages or which at least provides the public or industry with a useful choice.

[0014] The term “comprising” as used in this specification and indicative independent claims means “consisting at least in part of”. When interpreting each statement in this specification and indicative independent claims that includes the term “comprising”, features other than that or those prefaced by the term may also be present. Related terms such as “comprise” and “comprises” are to be interpreted in the same manner.

[0015] As used herein the term “and / or” means “and” or “or”, or both.

[0016] As used herein “(s)” following a noun means the plural and / or singular forms of the noun.

[0017] Accordingly, in a first aspect the present invention may broadly be said to consist in a sludge control system comprising: a fluid storage tank configured to retain a volume of high-density fluid, the fluid storage tank having walls and a base, and comprising at least one opening configured as a tank inlet and / or tank outlet; the floor of the tank sloped.

[0018] In an embodiment, the floor is sloped at an angle of substantially ten degrees.

[0019] In an embodiment, the floor of the fluid storage tank sloped from a side of the fluid storage tank downwards towards an opposite side of the fluid storage tank, the fluid storage tank further comprising a front trough that extends substantially across the lower front of the sloped base.

[0020] In an embodiment, the lower surface of the front trough is configured as a triangle wave profile having at least one trough.

[0021] In an embodiment, the angle of the slope of the sides of the wave is substantially ten degrees.

[0022] In an embodiment, the wave height from apex to base is substantially 1 .4 metres.

[0023] In an embodiment, the wave is configured to have a plurality of crests and troughs, a tank outlet located at the lowest point of each of the troughs.

[0024] In an embodiment, the peak-to-peak dimension from one apex of the profile to the next is substantially 15.9 metres.

[0025] In an embodiment, the front trough has a width of substantially one metre.

[0026] In an embodiment, the front trough has a depth of substantially 6.2 metres.

[0027] In an embodiment, the tank further comprises at least one sludge pump inlet configured to allow sludge to be drained from the tank, the sludge pump inlet located substantially at the lowest part of the tank, and at least one sludge pump outlet configured to allow sludge to be added to the tank, the sludge pump outlet located above the sludge pump inlet.

[0028] In a second aspect the present invention may broadly be said to consist in a sludge control method for controlling sludge build-up in a sludge control system that comprises a fluid storage tank configured to retain a volume of high-density fluid, the fluid storage tank having walls and a floor, the floor of the fluid storage tank sloped, the fluid storage tank comprising at least one tank inlet and at least one tank outlet, the fluid storage tank configured so that at least one tank outlet is located at substantially the lowest point of the fluid storage tank and the tank inlet is located at or towards the top of the fluid storage tank, the sludge control system further comprising a sludge pump, the sludge pump connected to the tank outlet so as to receive sludge from the fluid storage tank via the tank outlet, and to pump this to the tank inlet, the method comprising the steps of: i) activating the sludge pump; ii) monitoring the current draw of the sludge pump when activated; ill) deactivating the sludge pump when the current draw decreases.

[0029] In an embodiment, the sludge control method comprises the further steps of: iv) recording the current draw at deactivation; v) activating the sludge pump at intervals and noting the current draw; vi) increasing the time intervals between activations until the current draw on activation is at the initial level in step ii).

[0030] With respect to the above description then, it is to be realised that the optimum dimensional relationships for the parts of the invention, to include variations in size, materials, shape, form, function and manner of operation, assembly and use, are deemed readily apparent and obvious to one skilled in the art, and all equivalent relationships to those illustrated in the drawings and described in the specification are intended to be encompassed by the present invention.

[0031] This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth. Therefore, the foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.

[0032] Throughout the description and claims of this specification, the words "comprise", "include", "have", and "contain" and variations of these words, for example "comprising" and "comprises", mean "including but not limited to", and do not exclude other components, items, integers or steps not explicitly disclosed also to be present. Moreover, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0033] BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Further aspects of the invention will become apparent from the following description which is given by way of example only and with reference to the accompanying drawings which show an embodiment of the device by way of example, and in which:

[0035] Figure 1 shows a schematic illustration of an embodiment of an energy storage system according to an embodiment of the present invention, the system comprising an upper fluid storage unit, a conduit or penstock connected to the base of the storage unit and extending downwards from the storage unit so as to channel fluid from the upper storage unit under gravity, the lower end of the penstock dividing into two branches, a first branch connected to a turbine unit and a second branch connected to a pump, each branch connecting to a lower reservoir, the first branch configured to connect with the main body of the penstock, the turbine, and the lower reservoir so that in use fluid flows under gravity from the storage unit through the turbine unit and into the lower reservoir, the second branch configured to connect with the main body of the penstock, the pump, and the lower reservoir so that in use the pump can pump fluid from the lower reservoir back up the penstock to the reservoir.

[0036] Figure 2 shows a perspective view of the lower reservoir of the system of figure 1 , the lower reservoir having a base or floor sloped and shaped so that settling solids from a high-density fluid within the tank flow forwards down the sloped base or floor.

[0037] Figure 3 shows a top view of the lower reservoir of figures 1 and 2, showing detail of a front trough located directly adjacent to the front of the base, and a side depression located at the right-hand side (right-hand when viewed from the front) of the lower reservoir. Figure 4 shows a front view of the lower reservoir of figures 1 to 3, showing detail of the profile of the lower surface of the front trough.

[0038] Figure 5 shows a side view of the interior of the lower reservoir of figures 1 to 3, looking from the right-hand side towards the left-hand side.

[0039] Figure 6 shows a sludge pump configured so that the sludge that collects in the lower reservoir can be pumped out of the lower reservoir through an inlet or inlets, into and through the sludge pump, and then back into the top of the lower reservoir, helping to remix and re-homogenise the high density fluid.

[0040] DETAILED DESCRIPTION

[0041] The following detailed description illustrates embodiments of the present disclosure and ways in which they can be implemented. Those skilled in the art will recognize that other embodiments for carrying out or practising the present invention are also possible.

[0042] General Overview

[0043] A schematic illustration of a power generation system 100 according to an embodiment of the invention is shown in figure 1. The system 100 comprises main parts as follows:

[0044] 1. An upper fluid storage unit 104;

[0045] 2. A conduit or penstock 114 fluidically connected to the upper fluid storage unit 104, the conduit 114 extending generally downwards from the storage unit 104 so as to in use channel fluid from the upper storage unit 104, the lower end of the penstock 114 branching into two parallel branches 114a, and 114b;

[0046] 3. A lower reservoir 107, each of the two parallel branches of the penstock 114 fluidically connected to the lower reservoir 107;

[0047] 4. A turbine unit 102 located in / on the first branch 114a of the penstock so as to receive fluid from the upper storage unit 104 via the first branch 114a, the fluid passing through the turbine unit 102 to drive the turbine unit, and then into the lower reservoir.

[0048] 5. A pump unit 105 located in / on the second branch 114b of the penstock and adapted to pump fluid from the lower reservoir 107, up the penstock 114, to the upper storage unit 104;

[0049] 6. Combination flow and safety valves 118.

[0050] These parts and their inter-relationship are briefly described below. Fluid Storage Tank

[0051] The upper fluid storage unit 104 comprises an enclosed fluid storage container or tank 104 that is configured to in use hold / contain / store a high-density fluid. The tank is fully enclosed to prevent leaking of the fluid into the environment, and to prevent the fluid becoming contaminated - e.g. by absorbing rainwater or similar. However, the tank is not sealed, so the pressure remains the same as the surrounding atmosphere in use. The tank 104 has a fluid outlet 110. In the embodiment shown in figure 1 , the fluid outlet is in the wall of the tank close to the base. In variations, the fluid outlet could also be located in the base or floor of the tank 104.

[0052] As shown in figure 1 , the tank 104 is located above, and spaced apart from, the turbine unit 102 and lower reservoir 107, so that in use fluid 106 stored in the tank can flow downwards to the turbine unit 102 and lower reservoir 107 under gravity.

[0053] Conduit / Penstock

[0054] The conduit or penstock 114 comprises a tube or series of tubes or similar with an upper end / ends that is / are fluidically connected to the fluid outlet 110 of the upper fluid storage unit 104. The main body of the penstock 114 extends downwards from the upper storage unit 104 so that in use fluid from the upper storage unit 104 is channelled along the penstock 114 under gravity.

[0055] As shown in figure 1 , the lower end of the penstock 114 branches into two parallel branches 114a and 114b, each of which connects at their lower or outer ends to the lower reservoir 107.

[0056] A turbine unit 102 is located in / on the first branch 114a of the penstock so as to receive fluid from the upper storage unit 104 via the penstock, and a pump unit 105 is located in / on the second branch of the penstock.

[0057] Lower Reservoir

[0058] The lower reservoir 107 acts to receive and store fluid that enters the reservoir 107 from the first branch 114a. The lower reservoir is fully enclosed to prevent leaking of the fluid into the environment, and to prevent the fluid becoming contaminated - e.g. by absorbing rainwater or similar. As for the upper fluid storage unit 104, lower reservoir 107 is not sealed, so the pressure remains the same as the surrounding atmosphere in use.

[0059] The lower reservoir 107 and the branch 114b are mutually configured so that fluid in the lower reservoir 107 can flow out of the lower reservoir 107 and into the branch 114b in use.

[0060] The lower reservoir 107 is further configured to assist with preventing the finely-ground mineral solid settling out of the fluid during periods of extended non-use of the overall system, as is detailed in the ‘sludge control’ section below.

[0061] Turbine Unit

[0062] The turbine unit 102 is configured so as to in use receive a flow of high-density fluid from the conduit or penstock 114 along the passage 114a. This fluid flow passes along the branch 114a and across the blades of the turbine within the turbine unit 102, causing the turbine to spin so as to generate power. On exiting the turbine unit 102, the fluid passes along the rest of the passage 114a and into the lower reservoir 107.

[0063] Pump Unit

[0064] The pump unit 105 is located in / on the second branch 114b of the penstock, and is adapted to in use pump fluid from or out of the lower reservoir 107, along the second branch 114b, up the penstock 114, and to the upper storage unit 104.

[0065] Use

[0066] In use, in order to commence a power generation cycle, valves 118 are opened so that fluid flows from the tank 104 along the conduit 114 to the turbine unit 102, to cause the turbine to spin and generate power.

[0067] At times of low demand, the turbine unit 102 is switched off and the pump unit 105 is switched on so that fluid is pumped out of the lower reservoir 107, along the sub-branch 114b, through the pump, and up along the main body of the penstock 114 to the upper storage unit 104.

[0068] Sludge Control

[0069] The lower reservoir 107 is configured to assist with preventing the mineral settling out of the fluid during periods of extended non-use of the overall system. It has been observed that as the solid settles out of the liquid, it continues to flow (albeit extremely slowly) towards the lowest available point. The lower reservoir 107 has been configured so as to take advantage of this. In this embodiment, the lower reservoir 107 is substantially cuboid with side walls 128 and a base or floor 120. The lower reservoir 107 has a length and width designated by numerals 1 and 2 as shown in figure 3. In the preferred embodiment, the length 1 is substantially sixty metres, and the width 2 is substantially eighteen metres. However, it should be noted that these dimensions are for this embodiment only, and the specific dimensions will vary for each individual project.

[0070] As shown in figure 5, the main base or floor 120 of the lower reservoir 107 is arranged at an angle (generally marked as ‘angle 10’ in figure 5), the floor 120 raised at the rear of the reservoir 107 and sloping downwards towards the front of the reservoir. Angle 10 is in the preferred embodiment substantially 10 degrees from the horizontal. Again, the specific angle can be varied as required for different projects. It should also be noted that where ‘base’ or ‘floor’ 120 is referred to in this specification, this refers to the internal or interior portion or lower face of the reservoir.

[0071] A narrow front trough 121 extends across the front of the reservoir, in front of the sloped floor 120. The front trough 121 has a width generally designated by numeral 3. The width 3 in the preferred embodiment is substantially one metre. As shown in figure 4, the front trough has a depth generally designated by numeral 9. In the preferred embodiment, the depth is substantially 6.2 metres.

[0072] A side depression 129 is located at one side of the lower reservoir 107, having a substantially horizontal surface located below the floor 120. The width of the side depression is generally designated by numeral 4 in figure 3, and in the preferred embodiment is substantially five metres.

[0073] In use, settling solids will flow down the slope of the floor 120 and into the front trough 121 .

[0074] Optionally, surface of the sloped lower reservoir 107 is smooth, namely surface finished. Herein, surface finish relates to minimizing irregularities and modifying the surface roughness via polishing, low-friction coating, or a hydrophobic coating. Beneficially, the angle and the smoothness of the surface finish works together to get the desired result, i.e., preventing the mineral settling out of the fluid during periods of extended non-use of the overall system. By extension, the required slope angle will depend on the surface characteristics of fabrication material and fabrication / construction techniques used to build the sloped floor 120 of the lower reservoir 107.

[0075] When viewed from the front, the lower surface of the front trough 121 has a triangle wave profile as shown in figure 4, the triangle wave profile extending from left to right across the front trough 121 when viewed from the front. Numeral 7 denotes the peak-to-peak dimension from one apex of the profile to the next. In the preferred embodiment, dimension 7 is substantially 15.9 metres.

[0076] The angle of the slope of the sides of the wave is designated by numeral 8, and is in the preferred embodiment substantially ten degrees. The wave height from apex to base is generally designated by numeral 6, and in the preferred embodiment is 1.4 metres.

[0077] This compound shape helps to ensure that there is a slope at all parts of the lower reservoir 7 and that multiple pumping points are available, including a pump / turbine inlet / outlet. In use, sludge flows to the lowest part of the trough between the wave shapes.

[0078] The overall design objective is to maintain the bottom slope everywhere in the lower reservoir 107 except for the side depression 129 (however, this can be sloped in variations if absolutely required). Sloping the reservoir 107 at all locations can be achieved more easily with tanks that have a high aspect ratio (ratio of height relative to diameter / length). The volume of the reservoir 107 is relatively large, so the aspect ratio is low (that is, the tank is long / wide in comparison to its depth). In a practical sense, it is difficult to maintain a consistent slope (i.e., of ten degrees in the preferred embodiment) across the entire base in a wide / long slab, and this can result in localised low points or “dead space” - that is, areas where the fluid settles rather than actively moving around the system. Using a front trough 121 with a triangle wave profile assists with minimising the dead volume to less than 5%. In the preferred embodiment, sludge pump inlets 123 are located at the lowest part of the trough - the base point between each of the waves. It should be noted that ‘inlet’ as used here (‘sludge pump inlet’) indicates flow out of the lower reservoir 107, and into a connected sludge pump. In contrast, where ‘tank inlet’ and ‘tank outlet’ are used in this specification, these indicate flow into the lower reservoir (‘tank inlet’) or out of the lower reservoir (‘tank outlet’).

[0079] The connected sludge pumps 124 are configured so that the sludge that collects at the base points can be pumped out of the lower reservoir 107 through the sludge pump inlets 123, into and through the sludge pump 124, and then back into the lower reservoir 107 at a point or points at or close to the top of the lower reservoir 107. In other variations, there are additional draw points for the sludge inlet. For example, a first draw point may be arranged for drawing from the lowest (densest) point in the tank and another less dense from higher up. Beneficially, with such additional draw points, it is easier to get better mixing of the sludge and avoid getting dense sludge into the fluid outlet. This helps to remix the high- density fluid and re-homogenise this. Moreover, such additional draw points help in avoiding the risk of a big chunk of solid from sinking to the bottom too quickly rather than mixing into the sludge. A sludge pump 124 is shown in figure 6, configured for this arrangement. The fluid / sludge is pumped from the lower reservoir 107 into inlet 123, through the sludge pump 124, and then back into the lower reservoir 107 via outlet 125. In the variation shown, the fluid is pumped into the tank below the surface of the fluid already in the tank (designated by numeral 126). The sludge / fluid could alternatively be pumped so as to be sprayed or sprinkled onto the top of the fluid in the lower reservoir 107.

[0080] In a variation, alternatively, the sludge pump 124 can be operated in a reverse direction. In other words, in the reverse direction, the sludge pumps 124 are configured to pump the fluid / sludge is pumped from the lower reservoir 107 into the outlet 125, through the sludge pump 124, and then back into the lower reservoir 107 via the inlet 123. Beneficially, such configuration helps clear any settled accumulation in the inlet 123. Moreover, such configuration may also be used to introduce low-density fluid / sludge from higher up into the lowest level. As a result, due to buoyancy the low-density fluid / sludge will then rise in the lower reservoir and along the way help mix the fluid and sludge.

[0081] To prevent stratification, or settling of the sludge and ‘caking’, the sludge pump is configured so as to circulate the entirety of the lower reservoir at least once a week. At intermittent operation, this can be achieved by operating the sludge pump for approximately ten minutes every three hours.

[0082] In the preferred embodiment, the sludge pump or pumps 124 are separate to the main pump unit 105 which has an inlet 127 (to receive fluid from the lower reservoir into the pump 105) on the side wall 128 at the right-hand side where the side depression 129 is located, the inlet 127 corresponding to a tank outlet for the lower reservoir.

[0083] It should be noted that this arrangement helps to ensure that continual operation of the sludge pump 124 is not necessary. The sludge pump 124 is activated at intervals and operated until such time as the current draw (and thus by extension the pump torque) decreases. As sludge pump torque is directly related to the thickness of the accumulated sludge, if the torque demand decreases, this indicates that the settled layer of sludge has been completely collected and smooth mixed fluid is now entering the pump instead. In this way, the pump duty cycle can be optimised so that operation only takes place when needed. That is, once the pump is activated, the current draw can be monitored, and when this decreases the pump can be de-activated. The control system of the power generation system can be configured so as to on activation and de-activation, and use these to alter the time intervals between activation cycles, so that the sludge pump(s) will only be run when there is a particular requirement to do so (e.g. the current draw may reach a maximum level if the sludge pump(s) are not cycled after e.g. one week, and the controller will schedule a full cycle to run every week). In a variation, the outlet 125 to the lower reservoir 107 may comprise jets or other dedicated distribution systems aimed at breaking up any settled accumulation in the outlet 125, and redistributing into the lower reservoir 107. Additionally, alternatively, the inlet 123 to the sludge pump 124 may comprise jets or other dedicated distribution systems aimed at breaking up any settled accumulation in the inlet 123, and redistributing into the lower reservoir 107 via the outlet 125. Beneficially, such configuration enables better mixing of the sludge and avoids dense sludge getting into the fluid outlet or the lower reservoir 107.

[0084] Optionally, the system 100 comprises at least one spray bar. It may be appreciated that the at least one spray bar can be placed in different positions (i.e., more spray bars, different locations) depending on the desired speed of filling the tank 104 and the fluid viscosity filling in the tank 104. Beneficially, the at least one spray bar ensures that the fluid / sludge flowing into the tank 104 is homogenized. In this regard, the term "spray bar” refers to a device or mechanism that is used to distribute a liquid, such as the HDF in the fluid storage tank, in a controlled and uniform manner by spraying it over a specific area or surface in the fluid storage tank. The at least one spray bar is equipped with orifices that are sized to ensure consistent pressure drop. Notably, the at least one spray bar spans the width of the fluid storage tank and is placed at various locations along the length of the fluid storage tank. The at least one spray bar is arranged within the fluid storage tank to distribute the HDF evenly throughout the tank, it ensures that settled particles in the HDF are effectively reagitated with minimal energy consumption.

[0085] In a variation, the system 100 comprises a regulator unit that maintains the density of the fluid passing through the sludge pump 124. In this regard, the regulator unit is configured to identify when the density of the fluid passing through the sludge pump 124 drops below a threshold. The regulator unit measures the density using one of: current draw of the motor; a sensor in the fluid measuring the density either directly or indirectly; RPM measurement, power measurement, and reference against the pump’s performance curves.

Claims

CLAIMS1 . A sludge control system, comprising: a fluid storage tank configured to retain a volume of high-density fluid, the fluid storage tank having walls and a base, and comprising at least one opening configured as a tank inlet and / or tank outlet ; characterised in that the floor of the tank is sloped.

2. A sludge control system as claimed in claim 1 wherein the floor is sloped at an angle of substantially ten degrees.

3. A sludge control system as claimed in claim 1 or claim 2 wherein the floor of the fluid storage tank sloped from a side of the fluid storage tank downwards towards an opposite side of the fluid storage tank, the fluid storage tank further comprising a front trough that extends substantially across the lower front of the sloped base.

4. A sludge control system as claimed in claim 3 wherein the lower surface of the front trough is configured as a triangle wave profile having at least one trough.

5. A sludge control system as claimed in claim 4 wherein the angle of the slope of the sides of the wave is substantially ten degrees.

6. A sludge control system as claimed in claim 4 or claim 5 wherein the wave height from apex to base is substantially 1.4 metres.

7. A sludge control system as claimed in any one of claims 4 to 6 wherein the wave is configured to have a plurality of crests and troughs, a tank outlet located at the lowest point of each of the troughs.

8. A sludge control system as claimed in any one of claims 4 to 7 wherein the peak-to- peak dimension from one apex of the profile to the next is substantially 15.9 metres.

9. A sludge control system as claimed in any one of claims 3 to 8 wherein the front trough has a width of substantially one metre.

10. A sludge control system as claimed in any one of claims 3 to 9 wherein the front trough has a depth of substantially 6.2 metres.11 . A sludge control system as claimed in any one of claims 1 to 10 wherein the tank further comprises at least one sludge pump inlet configured to allow sludge to be drained from the tank, the sludge pump inlet located substantially at the lowest part of the tank, and at leastone sludge pump outlet configured to allow sludge to be added to the tank, the sludge pump outlet located above the sludge pump inlet.

12. A sludge control method for controlling sludge build-up in a sludge control system that comprises a fluid storage tank configured to retain a volume of high-density fluid, the fluid storage tank having walls and a floor, the floor of the fluid storage tank sloped, the fluid storage tank comprising at least one tank inlet and at least one tank outlet, the fluid storage tank configured so that at least one tank outlet is located at substantially the lowest point of the fluid storage tank and the tank inlet is located at or towards the top of the fluid storage tank, the sludge control system further comprising a sludge pump, the sludge pump connected to the tank outlet so as to receive sludge from the fluid storage tank via the tank outlet, and to pump this to the tank inlet, the method comprising the steps of: i) activating the sludge pump; ii) monitoring the current draw of the sludge pump when activated; ill) deactivating the sludge pump when the current draw decreases.

13. A sludge control method as claimed in claim 12 comprising the further steps of: iv) recording the current draw at deactivation; v) activating the sludge pump at intervals and noting the current draw; vi) increasing the time intervals between activations until the current draw on activation is at the initial level in step ii).

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