CA2976961A1

CA2976961A1 - Dewatering tank with turbulent flow

Info

Publication number: CA2976961A1
Application number: CA2976961A
Authority: CA
Inventors: Paul Wayne Jeffery
Original assignee: Challenge Pumps Pty Ltd
Current assignee: Challenge Pumps Pty Ltd
Priority date: 2015-02-23
Filing date: 2016-02-12
Publication date: 2016-09-01
Also published as: AU2016224126B2; NZ733878A; ZA201705377B; WO2016134404A1; AU2018101296A4; AU2016224126A1

Abstract

The present disclosure provides a dewatering tank that is configured to retain fines in suspension. The tank comprises a floor portion that has a downward inclination. The tank further comprises a sump that is located such that the floor portion inclines towards the sump. The tank has an inlet for admission of water containing the fines and an outlet that is adapted for connection with a pump to discharge the water containing the fines from the tank and that is located in the sump. The tank is arranged such that the water flows from the inlet to the outlet. The tank comprises a wall or cover portion that is positioned to divert a flow of the water away from a direct flow direction from the inlet to the outlet in a manner such that turbulences are induced and fines that may otherwise settle in the sump and on the floor portion are suspended.

Description

DEWATERING TANK
Technical Field A dewatering tank is disclosed. In particular, although not exclusively, a dewatering tank is disclosed for receiving water containing particulates produced from one or more dewatering processes, wherein the dewatering tank is configured to retain particulates, such as fines, in suspension.
Background Dewatering is the removal or drainage of groundwater or surface water from surface or sub-surface excavation, earthworks, caissons or mine shafts. Typically, this frequently involves the use of submersible pumps, centrifugal pumps, positive displacement pumps, eductors and, less frequently, the application of vacuum to remove and transport the water via conduits to one or more storage locations.
In sub-surface operations, there may be several dewatering pumps operating at appropriate locations underground interconnected by a network of conduits.
Prior to pumping the water above ground, it may be more efficient and productive for water from one or more dewatering operations to be collected and stored in a dewatering tank. The pump to transport the water to the surface can be operated intermittently when the dewatering tank reaches or approaches capacity, rather than on a continuous basis.
Water produced from de-watering processes typically contains particulates of varying size range. Generally, it is advisable to separate larger particulates (>2-3mm), such as gravel from the water to protect the pump from damage prior to pumping the water to the surface. However, the water may still contain a considerable amount of particulate material including fines which may settle in the dewatering tank over time (in between pumping operations).
The build-up of sedimentation in the dewatering tank may occur very quickly and it is not unusual for the dewatering tank to fill up with sedimentation within 24 hours, thereby reducing the capacity of the dewatering tank to store water.
Consequently, the dewatering tank may need to be manually flushed on a daily basis, an operation that takes about 30-60 minutes. In some underground operations, there may be up to dewatering tanks which need to be manually flushed on a daily basis.
Consequently, a considerable amount of labour is consumed by this particular task. It would be beneficial to reduce the frequency at which a dewatering tank needs to be manually flushed.
Furthermore, the build-up of sedimentation in the dewatering tank may also result in slug flow of fines into the pump, causing pump blockages and/or high wear of the pump internals.
The holding capacity of most dewatering tanks is between 3 kilolitres (kL) to 9 kL. The magnitude of the problems discussed above appears to increase as the holding

2 capacity of the dewatering tank increases.
The above references to background art do not constitute an admission that the art forms a part of the common general knowledge of a person of ordinary skill in the art.
The above references are also not intended to limit the application of the tank as disclosed herein.
Summary of the Invention Generally, a dewatering tank is disclosed. In particular, although not exclusively, a dewatering tank is disclosed for receiving water containing particulates produced from one or more dewatering processes, wherein the dewatering tank is configured to retain particulates, such as fines, in suspension. In this way, the water containing particulates may be discharged from the dewatering tank and pumped to another location, thereby reducing the build-up of particulate sedimentation in the dewatering tank and consequently reducing maintenance and down time associated with manually flushing particulate sedimentation from the dewatering tank.
The present invention provides in a first aspect a dewatering tank configured to retain fines in suspension, the tank comprising a floor portion that has a downward inclination, the tank further comprising a sump that is located such that the floor portion inclines towards the sump, the tank having an inlet for admission of water containing the fines and an outlet that is adapted for connection with a pump to discharge the water containing the fines from the tank and that is located in the sump, wherein the tank is arranged such that the water flows from the inlet to the outlet and wherein the tank comprises a wall or cover portion that is positioned to divert a flow of the water in a manner such that turbulences are induced and fines that may otherwise settle in the sump and on the floor portion are suspended.
The sump may be partially covered by a cover portion that constricts flow of water into the sump and is configured such that the water containing the fines changes direction when flowing into the sump whereby turbulences are induced.
The outlet may be a single outlet through which in use the water with suspended fines flows.
The cover portion of the sump may be a baffle that is spaced apart from a wall portion of the tank such that a gap is formed for inflow of the water into the sump.
Alternatively, the cover portion of the sump may be a baffle that comprises one or more through holes, gaps or perforations that are located at or near an edge or end-portion of the baffle for inflow of the water into the sump.
The dewatering tank may comprise at least one member that is shaped and positioned such that turbulences are induced as water flows along the floor portion that extends in a downward inclination. The at least one member may be a screen member extending between wall portions of the tank.
In one embodiment the at least one member is a substantially vertical baffle.
The at least one substantially vertical baffle may take the form of a plate member or a screen

3 member extending between the front and rear longitudinal walls of the tank.
In one embodiment, the tank comprises a first wall portion and an opposite second wall portion and the sump is positioned at or near the second wall portion. The floor portion that has a downward inclination may extend substantially from the first wall portion along at least the majority of a length of the tank towards the sump.
In an alternative embodiment, the tank comprises a first wall portion and the floor portion that extends in a downward inclination is one of at least two floor portions that each extend along a portion of a length of the tank in a downward inclination.
In this embodiment the sump is positioned between two adjacent floor portions that incline towards the sump with the outlet.
In some embodiments, the at least one substantially vertical baffle is spaced above the floor of the tank to induce turbulence in the flow of water passing thereunder Can underflow). Alternatively, the at least one substantially vertical baffle may extend from the floor and be spaced below a water line in the tank to induce turbulence in the flow of water passing thereover Can overflow).
In one embodiment, the tank comprises a plurality of substantially vertical baffles spaced relative to the floor of the tank or the water line in the tank in an arrangement whereby the vertical baffles direct the flow of water in alternate underflows and overflows.
In one embodiment, the at least one substantially vertical baffle is disposed proximal to the sump in the floor of the tank.
In one embodiment, the at least one substantially vertical baffle comprises an opening therein to define a constricted flowpath therethrough. Alternatively, or additionally, the at least one substantially vertical baffle may be configured to direct the flow of water through the constricted flowpath. For example, the at least one substantially vertical baffle may have a generally truncated V-shaped cross section, wherein the opening is provided in an apex of the truncated V-shaped cross section.
In one embodiment, the sump comprises a stepped floor portion. In an alternative form the sump comprises a steeply inclined floor portion.
The present invention provides in a second aspect a dewatering tank configured to retain fines in suspension, the tank comprising a floor portion that has a downward inclination, the tank further comprising a sump that is located such that the floor portion inclines towards the sump, the tank having an inlet for admission of water containing the fines and an outlet that is adapted for connection with a pump to discharge the water containing the fines from the tank and that is located in the sump, wherein the tank is arranged such that the water flows from the inlet to the outlet and wherein sump is at least partially covered by a cover portion that constricts flow of water into the sump and is configured such that the water containing the fines changes direction when flowing into the sump whereby turbulences are induced and fines that may otherwise settle in the sump and on the floor portion are suspended.

4 PCT/AU2016/000041 The present invention provides in a third aspect a dewatering system comprising:
the dewatering tank of the first or second aspect of the present invention;
and a dewatering pump positioned at the outlet in the sump of the dewatering tank;
wherein a cover portion partially covers the sump at a position over the outlet of the dewatering tank such that, when the dewatering pump is not in use and the water is stationary, fines settle on the cover portion whereby the cover portion reduces the likelihood of blockage of the pump by the fines.
Description of the Figures Notwithstanding any other forms which may fall within the scope of the dewatering tank as set forth in the Summary, specific embodiments will now be described, by way of example only, with reference to the accompanying drawings in which:
Figure 1 is a schematic representation of a perspective view of a dewatering tank in accordance with the disclosure;
Figure 2 is a perspective view of the dewatering tank shown in Figure 1 indicating internal detail in phantom;
Figure 3 is a plan view of the dewatering tank shown in the preceding Figures;
Figure 4 is a front view of the dewatering tank shown in the preceding Figures indicating internal detail in phantom;
Figures 5a and 5b are respective opposing side views of the dewatering tank shown in the preceding Figures; and, Figure 6 is a perspective view of the dewatering tank, as shown in the preceding Figures, shown in use with a dewatering pump and gravel screen.
Detailed Description of Specific Embodiments of the Invention An embodiment of a dewatering tank configured to hold water containing particulates and retain fines in suspension will now be described by way of example only.
Referring to the figures, there is shown a dewatering tank. The dewatering tank includes a generally rectangular-box shaped tank 10 having a front longitudinal wall 12, a rear longitudinal wall panel 14, a pair of opposing side walls 16a, 16b interconnecting the front and rear longitudinal walls 12, 14 and a floor 18 that slopes downwardly from opposing side wall 16a to opposing side wall 16b.
The tank 10 may be open, as shown in the Figures. Alternatively, the tank 10 may be partially or fully closed, for example with a cover or screen as will be described later.
The tank 10 has an inlet 20 for admission of water containing particulates and an outlet 22 adapted for connection with a pump 100 (as shown in Figure 6) to discharge said water from the tank 10. Typically, the pump 100 may be a positive displacement pump, such as a helical rotor pump, that pumps the water containing fine particulates from outlet 22 of the tank at a flow rate of about 10-40 litres/second (L/s).
The floor 18 may be provided with a sump 24 proximal to opposing side wall 16b. In the embodiment shown in the Figures, the sump 24 comprises a stepped floor portion.
However, it will be appreciated that the sump 24 may take alternative forms.
For example, in an alternative embodiment, the sump 24 may comprise a steeply inclined floor portion.

5 A horizontal baffle 26 is arranged above the sump 24 and spaced apart from the front and rear longitudinal walls 12, 14 of the tank 10 to define two openings 25 into which a constricted flow of water may enter the sump 24. The horizontal baffle 26 may take the form of a plate member. The horizontal baffle 26 may be removable from the tank 10 to enable access to the sump 24 for maintenance and cleaning purposes.
The arrangement of the horizontal baffle 26 above the sump 24 induces turbulence in the flow of water into the sump 24. Advantageously, in the event that some settling of fine particulates in the sump 24 occurs over time, it is thought that the turbulent water flow into the sump 24 will re-suspend at least some of the fine particulates in the water.
In this way, the build-up of fine particulate sedimentation in the tank 10 will, at least in part, be reduced.
Generally, the inlet 20 is located or defined by an opening in the tank 10 proximal to the opposing side wall 16a of the tank 10. It will be appreciated that the inlet 20 is positioned proximal to opposing side wall 16a so that water admitted into the tank 10 through the inlet 20 will flow under gravity along the downwardly inclined floor 18 towards the outlet 22. The outlet 22 may be configured in a lower end 28 of the opposing side wall 16b adjacent to the sump 24.
The inlet 20 may be provided with a screen 200 (as shown in Figure 6) to prevent larger size particulates (<2-3 mm), such as gravel, from entering the tank 10.
A chute for gravel having a discharge port 32 may be provided on the front longitudinal wall 12 of the tank 10 to collect and discharge the gravel separated by the screen 200.
30 Alternatively, the chute 30 may be provided on the rear longitudinal wall 14 of the tank 10.
The tank 10 also includes vertical baffles 34a, 34b configured to induce turbulence in the flow of water to the outlet 22. The vertical baffles 34a, 34b may be positioned and/or configured to constrain the flow of water in the tank 10.
Vertical baffle 34a may comprise a plate member 36 extending between the front and rear longitudinal walls 12, 14 having an upper edge 38 and a lower edge 40.
The plate member 36 may be spaced apart from and disposed in parallel alignment with the opposing side walls 16a, 16b.
The plate member 36 may be positioned in the tank 10 so that the lower edge 40 is spaced above the floor 18 of the tank 10. In this orientation, the plate member 36 defines the inlet 20 of the tank 10 as comprising the space between the opposing side wall 16a and the plate member 36. Furthermore, in this orientation, the plate member 36 constrains the flow of water to pass through an opening defined by the opposing side panels 16a, 16b, the floor 18 and the lower edge 40 of the plate member 36, thereby creating an `underflow' and turbulence in the water. The inventor opines that

6 creating the underflow assists in scouring sedimentation of fine particulates from the floor 18 and re-suspending them in the water flow.
In an alternative embodiment, the vertical baffle 34a may take the form of a screen member. In this particular embodiment, the screen member may advantageously retain coarse particulates, preventing them from flowing toward the sump 24 and outlet 22 of the tank 10.
Alternatively, or additionally, the plate member 36 may be positioned in the tank 10 so that the upper edge 38 thereof is spaced below the water line of the tank 10 and the lower edge 40 abuts, or is integral with, the floor 18 of the tank 10. In this arrangement, the plate member 36 constrains the flow of water to pass through a limited flow path defined by the opposing side walls 16a, 16b, and the upper edge 38 of the plate member 36, thereby creating an 'overflow' and turbulence in the water.
In one embodiment, a plurality of plate members 36 may be spaced relative to the floor of the tank or the water line in the tank as described above in an arrangement as whereby the plate members 36 direct the flow in alternate underflows and the overflows.
Vertical baffle 34b includes a pair of plate members 42a, 42b, plate members 42a, 42b projecting inwardly, respectively, from the front and rear longitudinal walls 12, 14 at an angle towards one another. Respective lower edges 44 of the plate members 42a, 42b abut, or are integral with, the floor 18 of the tank 10. Respective upper edges 46 of the plate members 42a, 42b may be spaced below the water line.
Vertical baffle 34b further includes a plate member 48 interconnecting the pair of plate members 42a, 42b, thereby defining a truncated V-shaped cross-section (as shown in Figure 3). The plate member 48 is disposed in parallel alignment with the opposing side walls 16a, 16b. An upper edge 50 thereof is flush with the upper edges 46 of the plate members 42a, 42b. A lower edge 52 thereof is spaced above the floor 18 of the tank 10, thereby defining an opening 53 in the vertical baffle 34b.
In this particular form, the baffle 34b thus defines both an overflow above the upper edges 46, 50 of the plate members 42a, 42b, 48 and an underflow through the opening in the vertical baffle 34b. Furthermore, the angled orientation of plate members 42a, 42b defines a channel for water to flow through the opening in the vertical baffle 34b which also reduces sedimentation of fine particles on the floor 18 proximal to the vertical baffle 34b.
In the embodiment shown in the Figures, the vertical baffle 34b is disposed adjacent the sump 24. In this particular arrangement, the overflow and underflow created by this particular baffle 34b assists in scouring sedimentation of fine particulates which may have settled on the horizontal baffle 26 and/or in the sump 24 and re-suspending them in the water flow.
It will be appreciated that alternative baffle designs may be used to restrain flow within the tank 10 and induce turbulence in the water sufficient to retain, at least in part, fine particulates in suspension.

7 The floor 18 of the tank 10 may be further provided with one or more drain holes 52 to drain the tank 10 for maintenance and cleaning purposes. The one or more drain holes 52 may be disposed in the floor 18 and/or the sump 24.
The tank 10 may also be provided with an overflow pipe 54 having an upper end and a lower end 58 passing through the floor 18 of the tank 10. The overflow pipe 54 is arranged, in use, to drain excess water from the tank 10 through the floor 18 when the water line in the tank 10 rises above the upper end 56 of the overflow pipe 54. In the embodiment shown in the Figures, the overflow pipe 54 is spaced approximately midway along the floor 18.
The tank 10 may be supported on a plurality of leg members 60 disposed at opposing side walls 16a, 16b thereof. The leg members 60 may be interconnected by reinforcing members 62 in the form of braces or struts. In this particular embodiment, each leg member 60 may be provided with a foot plate 64 for engaging the ground.
The tank 10 may also be provided with a plurality of pad-eyes 66 as lifting points to enable the tank 10 to be lifted and transported to a desired location.
Embodiments of the dewatering tank 10 may be fabricated from metal such as aluminium or steel, or alloys thereof, and may comprise polymeric components such as injection moulded plastic materials. The capacity of the dewatering tank 10 may generally vary from 2 kilolitres (kL) to 10 kL, although it will be appreciated that some embodiments of the dewatering tank 10 may be configured to have a capacity outside of this range.
In one embodiment, the inlet 20 may be adapted for admission of water containing particulates from one or more conduits for transporting water produced from dewatering processes used underground in mines or civil engineering works. The screen 200 covering the inlet 20 separates larger particulates, such as gravel, which collect in the gravel chute 30 and are discharged through discharge port 32 into an underlying bin or skip.
The screened water entering the tank 10 through inlet 20 still contains particulates including fine particulates (<2-3 mm). The screened water falls under gravity onto the inclined floor 18 of the tank 10 proximal to the opposing side wall 16a and flows along the downwardly inclined floor 18 towards the outlet 22 located in the opposing side wall 16b.
The flow of water within the tank 10 is first constrained to an underflow of water by vertical baffle 34a and subsequently to both an underflow and an overflow of water by baffle 34b proximal to the sump 24 in the floor 18 of the tank 10, thereby inducing turbulence in the water. Any fine particulate which may have settled or begun to settle on the floor 18, on an upper surface of the horizontal baffle 26, or the sump 24 in the floor of the tank 10 may be disturbed by the turbulence and re-suspended in the water.
Additionally, the flow of water into the sump 24 is restricted by the pair of openings 25

8 defined by the horizontal baffle 26 and the front and rear longitudinal walls 12, 14. The horizontal baffle 26 significantly changes the direction of the flow of water from a longitudinal flow path relative to the floor 18 to a substantially perpendicular flow path with respect to the front and rear longitudinal walls 12, 14 of the tank 10.
Furthermore, the provision of the two openings 25 defined by the horizontal baffle 26 above the sump 24 effectively 'splits' the flow of water into the sump 24 into a pair of opposing streams. The two streams generally increase turbulence in the sump 24 and provide a scouring action to re-suspend particulate fines which may have settled in the sump 24.
The pump 100 attached to the outlet 22 of the tank 10 may be used continuously to withdraw water containing fine particulates from the tank 10. Alternatively, the pump 100 may be used intermittently, for example, only when the tank 10 has reached, or is approaching, capacity. Pumping water from the tank 10 will also create a flow of water toward the outlet 22 but the flow will be interrupted and constrained by the vertical baffles 34a, 34b and the horizontal baffle 26 as described above, thereby inducing turbulence in the flow. Therefore, any fine particulate which may have settled or begun to settle on the floor 18, the horizontal baffle 26 or the sump 24 may be disturbed by the turbulence and re-suspended in the water prior to discharge from the outlet 22.
it will be appreciated from the above description that sedimentation of fine particulates in the dewatering tank 10 is likely to be reduced. Consequently, the inventor anticipates that the frequency with which the dewatering tank 10 must be flushed manually to remove build-up of settled fine particles is likely to be substantially reduced, potentially to the extent where manual flushing could become redundant.
Numerous variations and modifications will suggest themselves to persons skilled in the relevant art, in addition to those already described, without departing from the disclosure. All such variations and modifications are to be considered within the scope of the disclosure. For example, it will be appreciated that the tank may not have a generally rectangular shape, but may have any suitable shape, such as an oval or round shape. Further, the inclined floor portion of the tank may be one of two inclined floor portions and the outlet may for example be positioned between the two inclined floor portions.
In the claims which follow, and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word "comprise" and variations such as "comprises" or "comprising" are used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the apparatus and method as disclosed herein.

Claims

9

1. A dewatering tank configured to retain fines in suspension, the tank comprising a floor portion that has a downward inclination, the tank further comprising a sump that is located such that the floor portion inclines towards the sump, the tank having an inlet for admission of water containing the fines and an outlet that is adapted for connection with a pump to discharge the water containing the fines from the tank and that is located in the sump, wherein the tank is arranged such that the water flows from the inlet to the outlet and wherein the tank comprises a wall or cover portion that is positioned to divert a flow of the water in a manner such that turbulences are induced and fines that may otherwise settle in the sump and on the floor portion are suspended and water with the fines flows out of the outlet.

2. The dewatering tank of claim 1 wherein the sump is partially covered by a cover portion that constricts flow of water into the sump and is configured such that the water containing the fines changes direction when flowing into the sump whereby turbulences are induced.

3. The dewatering tank of any one of the preceding claims wherein the outlet is a single outlet through which in use the water with suspended fines flows.

4. The dewatering tank of claim 2 or claim 3 when dependent on claim 2 wherein the cover portion of the sump is a baffle that is spaced apart from a wall portion of the tank such that a gap is formed for inflow of the water into the sump.

5. The dewatering tank of claim 2 or claim 3 or 4 when dependent on claim 2 wherein the cover portion of the sump is a baffle that comprises a group of through holes or perforations that are located at or near an edge or end-portion of the baffle for inflow of the water into the sump.

6. The dewatering tank of claim 4 or 5 wherein the baffle is in use positioned substantially horizontal.

7. The dewatering tank of any one of the preceding claims wherein the tank comprises a first wall portion and an opposite second wall portion and wherein the sump is positioned at or near the second wall portion.

8. The dewatering tank of claim 7 wherein the floor portion that has a downward inclination that extends substantially from the first wall portion along at least the majority of a length of the tank towards the sump.

9. The dewatering tank of any one of the preceding claims wherein the tank comprises a first wall portion and wherein the floor portion that extends in a downward inclination is one of at least two floor portions that each extend along a portion of a length of the tank in a downward inclination and wherein the sump is positioned between two adjacent floor portions that incline towards the sump.

10. The dewatering tank of any one of claims 7 to 9 wherein the inlet is positioned at or near the first wall portion.

11. The dewatering tank of any one of the preceding claims comprising at least one member that is shaped and positioned such that turbulences are induced as water flows along the floor portion that extends in a downward inclination.

12. The dewatering tank of claim 11, wherein the at least one member is a screen member extending between wall portions of the tank.

13. The dewatering tank of claim 11 or 12 wherein the at least one member is a substantially vertical baffle.

14. The dewatering tank claim 13, wherein the substantially vertical baffle is spaced above the floor portion of the tank to induce turbulence in the flow of water passing thereunder ('an underflow').

15. The dewatering tank of claim 13, wherein the substantially vertical baffle extends from the floor and is in use spaced below a water line in the tank to induce turbulence in the flow of water passing thereover ('an overflow').

16. The dewatering tank according to any one of claims 13 to 15, wherein the substantially vertical baffle is disposed proximal to the sump at the floor portion of the tank.

17. The dewatering tank of claim 14, wherein the substantially vertical baffle is one of a first plurality of substantially vertical baffles and wherein the tank also comprises a second plurality of substantially vertical baffles that extend from the floor and are in use spaced below a water line in the tank to induce turbulence in the flow of water passing thereover ('an overflow'), wherein the tank has a plurality of substantially vertical baffles spaced relative to the floor of the tank or the water line in the tank in an arrangement whereby the substantially vertical baffles direct the flow of water in alternate underflows and overflows.

18. The dewatering tank according to any one of claims 13 to 16, wherein the or each substantially vertical baffle has an opening therein to define a constricted flowpath therethrough.

19. The dewatering tank according to claim 18, wherein the or each substantially vertical baffle has a generally truncated V-shaped cross section, wherein the opening is provided in an apex of the truncated V-shaped cross section.

20. The dewatering tank according to any one of the preceding claims, wherein the sump comprises a stepped floor portion.

21. The dewatering tank according to any one of claims 1 to 19, wherein the sump comprises a steeply inclined floor portion.

22. A dewatering tank configured to retain fines in suspension, the tank comprising a floor portion that has a downward inclination, the tank further comprising a sump that is located such that the floor portion inclines towards the sump, the tank having an inlet for admission of water containing the fines and an outlet that is adapted for connection with a pump to discharge the water containing the fines from the tank and that is located in the sump, wherein the tank is arranged such that the water flows from the inlet to the outlet and wherein the sump is partially covered by a cover portion that constricts flow of water into the sump and is configured such that the water containing the fines changes direction when flowing into the sump whereby turbulences are induced and fines that may otherwise settle in the sump and on the floor portion are suspended.

23. A dewatering system comprising:
the dewatering tank of claim 2 or claim 22 or any one of claims 3 to 21 when dependent on claim 2; and a dewatering pump positioned at the outlet in the sump of the dewatering tank;
wherein a cover portion partially covers the sump at a position over the outlet of the dewatering tank such that, when the dewatering pump is not in use and the water is stationary, fines settle on the cover portion whereby the cover portion reduces the likelihood of blockage of the pump by the fines.