GB2528597A

GB2528597A - Settlement tank

Info

Publication number: GB2528597A
Application number: GB1517789.2A
Authority: GB
Inventors: David Middleton
Original assignee: A&j Water Treat Ltd
Current assignee: A&j Water Treat Ltd
Priority date: 2013-05-09
Filing date: 2013-05-09
Publication date: 2016-01-27
Anticipated expiration: 2033-05-09
Also published as: EP2994210A1; GB2528597B; GB2513889A; WO2014181124A1; GB2513889B; GB201308371D0; GB201517789D0

Abstract

A settlement tank 110 for use in a water treatment process comprises a perimeter wall 112 and base 114 defining a fluid holding region 113 for holding fluid such as waste water in the settlement tank. The settlement tank also comprises a launder trough 174 via which fluid from the fluid holding region can exit the settlement tank. The launder trough is constructed and arranged to define a walk way 172 along which a person can walk. The launder trough may comprise a barrier wall with a plurality of holes (178, 180, Fig 4) below the operational fluid and scum level and through which fluid can flow to the launder trough. Advantageously, the settlement tank does not require a moveable platform or scaffold for weir or launder trough maintenance and as such improves safety and reduces maintenance time.

Description

Settlement tank

FIELD OF THE INVENTION

The present invention relates to a settlement tank, in particular a settlement tarik for use in the primary or final stages of water treatment processes.

BACKGROUND OF THE INVENTION

Settlement tanks are commonly used in the primary and final stages of water treatment processes. During treatment, water in a settlement tank is substantially calm and non-agitated. This allows solids and heavy matter in the water to settle to the bottom of the settlement tank due to gravitational forces. Clean water can then be removed from an upper region of the settlement tank, e.g. over a weir at or near the periphery of the tank.

The solids that settle within the tank create a blanket of material on the base of the tank, commonly referred to as sludge'. The sludge needs to be removed constantly or at regular intervals, otherwise unwanted anaerobic reactions occur, resulting in failure of the settlement process. Such sludge is conventionally removed from the base of the tank using a scraper assembly.

A conventional scraper assembly includes a scraper array arranged to scrape the base of the settlement tank. The scraper is affixed to a bridge that extends between the centre of the tank and the perimeter of the tank. The bridge is rotated about the centre of the tank, causing the scraper to move in a circular path and move sludge along the base of the tank. The base of the tank is typically inclined towards the centre, and the movement and shape of the scraper array urges the sludge to move towards a central hopper, where the sludge is pumped out through an outlet of the settlement tank. The exiting sludge is sent to a pumping station, where it may be recycled or treated and used in other applications.

Conventional scraper assemblies and their associated bridges are often increased in size on larger settlement tanks to improve scraping efficiency, for example, from a single radial span, to three-quarter diameter and full tank diameter scraper bridges.

Conventional scraper assemblies are costly to build and therefore add to the costs of installation of a settlement tank, As mentioned above, "clean water" may flow from the tank over a weir at (or near) the periphery of the tank. Water flowing over the weir flows to a launder trough and away from the settlement tank, During the settlement process, some solids may form a scum at the surface of the water within the tank, and it is known to include a scum board arrangement for preventing scum from flowing over the weir, A conventional scum board is located immediately adjacent the weir. The amount of scum within the tank will increase over time, and so the scum needs to be removed at regular intervals. Accordingly, it is known to provide the tank with a scum removal device affixed to an outer IS circumferential wall of the settlement tank or to a bridge of a scraper assembly to remove the scum from the surface of the settlement tank. Blades are affixed to the the bridge of the scraper assembly and/or to the outer circumferential wall of the settlement tank to guide the scum towards the removal device.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a settlement tank comprising: a substantially circular perimeter wall; a substantially frusto-conical base surounded by the perimeter wall, the perimeter wall and the base defining a fluid holding region for holding fluid in the settlement tank at an operational fluid level; and a scraper assembly comprising: a scraper for scraping sludge from the base; a support connected to a central region of the base, the support comprising a mount rotatably mounting one end of the scraper to the support; wherein the mount is positioned below the operational fluid level so as to be fully snbmerged during operation of the settlement tank, The configuration of the scraper assembly means that the scraper assembly is bridgeless. A bridgeless scraper assembly results in reduced material costs for constmction, and simplified installation and maintenance.

The settlement tank may comprise a weir, and the mount may be positioned below a level coincident with a weir; and/or the settlement tank may comprise a fluid supply pipe for delivering fluid to the fluid holding region, and the mount may be positioned below a level coincident with an outlet end of the fluid supply pipe; and/or the mount may be positioned below a level coincident with the height of the base at the outer perimeter thereof The scraper may be angled to be substantially parallel to the base. in exemplary embodiments, the mount may be positioned below a level coincident with a highest point of the scraper.

The scraper may be arranged so that in use the scraper extends from a position proximal to a centre of settlement tank, to a position proximal to a perimeter of a IS settlement tank, The scraper may be mounted to the rotatable mount via an ann that is fixed to the scraper array and the mount. The arm may be positioned b&ow the operational fluid level. The arm may be arranged such that the highest point on the arm may be below the highest point on the scraper.

The arm may be fixed to the scraper at a position spaced from the mount. The arm may extend to a position on the scraper at one quarter to one haff the length of the scraper, for example the arm may extend to a position on the scraper at one third of the length of the scraper, The connection between the mount and the scraper may be reinforced by one or more stmts extending between the scraper and the arm.

The arm may extend substantially horizontally from the mount to the scraper.

The mount may comprise a bearing (eg. a slew bearing) to rotatably mount the scraper array to the support.

The settlement tank may comprise a perimeter support mounted to the scraper and moveably connected to the perimeter wall. Provision of a perimeter support provides additional support for the scraper assembly. Further, the perimeter support also permits rotation of the scraper assembly about the rotatable mount to be controlled from the perimeter of the settlement tank.

The perimeter support may be mounted to the scraper at a position proximal to the perimeter wall.

The perimeter support may be mounted to the scraper at a position proximal to the perimeter wall, and wherein the connection between the perimeter support and the scraper may be reinforced by one or more struts connected between the perimeter support and the scraper. For example, the strut may be angled to the perimeter support in a region of connection.

The perimeter support may comprise a portion that extends over the perimeter wall.

The settlement tank may comprise a track around (e.g. positioned on and around or positioned near and around), the perimeter wall and the perimeter support may be moveably connected to the track.

The settlement tank may comprise a drive mechanism for moving the perimeter support along the track, The drive mechanism may be positioned outside the perimeter wall. The drive mechanism may comprise a drive to provide motive power to move the perimeter support along the track.

The drive may be configured to be substantially static during operation with respect to the perimeter wall.

The drive mechanism may indude a cable or chain for pulling the perimeter support along the track, The drive mechanism may comprise one or more wheels or a train arrangement, arid a drive is configured to directly move the perimeter support along the track, The central support may comprise a plurality of legs extending from a mounting region, The mount may be connected to the mounting region and the plurality of legs may be connected to a base of a settlement tank. For example, the support may comprise three legs.

The settlement tank may comprise a fluid inlet pipe. The support may be positioned substantially coaxially with a fluid inlet pipe.

The scraper may be formed from an array of connected scraper units, each of which may include a component for scraping the base of the settlement tank.

The settlement tanlc may comprise: an outlet formed in the perimeter wall through which fluid can flow from the fluid holding region; and an energy recovery device for harnessing energy from fluid flow through the outlet from the fluid holding region.

The settlement tank may comprise a weir positioned radially inwardly from the perimeter wall for controlling flow rate of fluid to the outlet.

The outlet may be positioned at a level above the base of the settlement tank, and fluid flow through the outlet from the fluid holding region may be directed to a flow path at a level lower than the outlet.

The energy recovery device may comprise a water turbine, or a siphonically-driven air turbine, or any other suitable energy recovery device. For example, the energy recovery device may comprise a pelton wheel.

The settlement tank may comprise a drive connection between the energy recovery device and the scraper assembly such that the energy harnessed from fluid flow from the fluid holding region can be used to move the scraper assembly.

The drive connection may include a series of gears, one or more pulleys, a cable or chain and/or a pinion and track arrangement, e.g. to convert motion of the energy recovery device into motion of the scraper assembly.

A second aspect of the invention provides a scraper assembly for removing sludge from a settlement tank, the scraper assembly comprising: a scraper for scraping sludge from a frustoconical base of a settlement tank; and a support connectable to a base of a settlement tank, the support comprising a mount rotatably mounting one end of the scraper array to the support.

The scraper assembly may comprise a perimeter support movably connectable to a perimeter of the settlement tank. In use, the scraper may be angled downwardly in a direction from the perimeter support to the mount. The mount may be positioned lower than a highest point of the perimeter support, In an exemplary embodiment, the mount may be positioned lower than the highest position of the scraper.

The scraper assembly of the second aspect may have one or more features of the scraper assembly of the settlement tank of the first aspect.

A settlement tank comprising: a substantially circular perimeter wall; a substantially frusto-conical base surrounded by the perimeter wall, the perimeter wall and the base defining a fluid holding region for holding fluid in the settlement tank at an operational fluid level; a scraper assembly for scraping sludge from the base, the scraper assembly being rotataNy mounted with the fluid holding region so as to rotate substantially about the centre of the fluid holding region; and a drive mechanism for rotating the scraper assembly about the centre of the fluid holding region; wherein the drive mechanism is positioned outside the perimeter wall.

Provision of a drive mechanism outside the perimeter results in more straight forward installation of the scraper assembly and simplified maintenance procedures.

The entirety of the drive mechanism may be positioned outside the perimeter wall.

The drive mechanism may comprise a drive to provide motive power to the drive mechanism, The drive may be configured to be static, during operation, with respect to the perimeter wall.

Provision of a static drive eliminates the need for an electrical slip ring device at the centre of the tank, thus negating the need for a bridge to provide access and a cable path to the electrical slip ring device, as is required with settlement tanks of the prior art.

The drive mechanism may include a cable or chain for pulling one end of the scraper assembly around the perimeter wall so as to rotate the scraper assembly about the centre of the fluid holding region.

The drive mechanism may include a track around the outside of the perimeter wall along which one end of the scraper assembly can be moved.

The drive mechanism may include an energy recovery device for harnessing energy from fluid flow from the fluid holding region.

The settlement tank may comprise a mount positioned below an operational fluid level so as to be fully submerged during operation of the settlement tank.

The scraper array may be a scraper array of the settlement tank of the first aspect, or may be a scraper array of the second aspect.

The settlement tank may have one or more of the optional features of the settlement tank of the first aspect.

A fourth aspect of the invention provides a settlement tank comprising: a perimeter wall and a base that define a fluid holding region; an outlet formed in the perimeter wall through which fluid can flow from the fluid holding region; and an energy recovery device for harnessing energy from fluid flow through the outlet from the fluid holding region.

The settlement tank may comprise a scraper assembly. The provision of an energy recovery device advantageously means that the scraper assembly can be self-powered.

The self powered nature of the scraper assembly means that the scraper assembly is energy efficient, which also means that the scraper assembly has advantageous applications in developing countries.

The outlet may be positioned at a level above the base of the settlement tank, and fluid flow through the outlet from the holding region may be directed to a flow path at a level lower than the outlet.

The energy recovery device may comprise a water turbine, siphonically driven air turbine, or any other suitable energy recovery device. The energy recovery device may comprise a pelton wheel.

The settlement tank may comprise a scraper assembly, and a drive mechanism may operationally connect the energy recovery device to the scraper assembly.

The drive mechanism may include a series of gears, one or more pulleys, and/or a pinion and track arrangement.

The drive mechanism may comprise a generator for providing electrical energy to an electrical drive configured to move the scraper assembly.

The settlement tank of the fourth aspect may comprise a scraper assembly of the settlement tank of the first aspect, or a scraper assembly of the second aspect.

The settlement tank of the fourth aspect may include one or more features of the settlement tank of the first and/or third aspects.

A fifth aspect of the invention provides a settlement tank comprising: a perimeter wall and base defining a fluid holding region; a launder trough via which fluid from the fluid holding region can exit the settlement tank; and a walk way along which a person can walk; wherein the launder trough is constructed and arranged to define the walk way.

Advantageously, the settlement tank does not require the moveable platform or scaffold (e.g. for weir or launder trough maintenance) associated with the prior art, and as such improves safety and reduces maintenance time.

The settlement tank may comprise a weir for controlling flow of fluid from the fluid holding region to the launder trough.

The launder trough may be defined by a platform that forms a surface along which a person can walk, an outer circumferential wall on one side of the platform, and an inner circumferential wall on the other side of the platform, the inner circumferential wall being positioned adjacent the fluid holding region. In such embodiments, the inner and outer circumferential walls together form edge protection for an occupant on the walkway.

The inner circumferentia' wall may extend to a height above an operational level of fluid and any scum formed on the fluid during use.

S The inner and/or outer circumferential wall may extend to a height greater than or equal to 1100 mm from the platform.

The inner circumferential wall may comprise a plurality of holes through which fluid can flow from the fluid holding region to the launder trough (e.g. via a weir).

The plurality of holes may be positioned b&ow the operational level of fluid.

The settlement tank may comprise a weir plate mounted to the inner circumferential wall and angled and/or offset thereto, such that, in use, fluid flows from the plurality of holes over the weir plate to the launder trough. The weir plate controls flow of fluid from the fluid holding region to the launder trough.

The weir plate may be adjustably mounted to the inner circumferential wall such that positioning of the weir plate relative to the inner circumferential wall can be selectively varied, For example, the plate may be adjustably mounted to the inner circumferential wall to create an increased or decreased barrier to flow from the plurality of holes to the launder trough.

The weir plate may comprise a plurality of V-shaped notches, For example, an edge of the weir plate over which, in use, fluid flows (e.g. an upper or top edge of the weir plate) may comprise a plurality of V-shaped notches.

The weir plate may be reinforced using one or more reinforcement struts.

The weir plate may be made from glass reinforced plastic. Alternatively, the weir plate may be made from stainless steel, or any other suitable material.

The settlement tank may comprise a hand rail connected to the inner circumferential wall and/or the outer circumferential wall, The settlement tank may comprise a non-slip flooring along the walk way.

The walk way may have a width equal to or greater than 600 mm. In embodiments including a weir plate, the width of the walk way is measured between the outer circumferential wall and the weir plate. For example, the width of the walk way is the clear distance along which a person can move without obstruction.

The launder trough and/or the weir may be positioned inwardly from the perimeter wall.

The outer circumferential wall may be substantially aligned with the perimeter wall e.g. an extension thereof In alternative embodiments, it may be the inner circumferential wall which is an extension of the perimeter wall, with the launder trough and outer circumferential wall sited radially outside of the fluid holding region of the tank, The form and function of each component would remain unaffected.

The perimeter wall and base may define a circular fluid holding region.

The walkway may be made from concrete. For example, the walkway may be made from cast concrete, prefabricated concrete sections, steel, GRP or any other structural material.

A sixth aspect of the invention provides a settlement tank comprising: a perimeter wall and base defining a fluid holding region; and a launder trough via which fluid from the fluid holding region can exit the settlement tank; wherein, the launder trough comprises a barrier wall adjacent the fluid holding region, and the barrier wall extends to a height above an operational fluid and scum level, and a plurality of holes are provided in the barrier wall below the operational fluid and scum level through which fluid can flow to the launder trough, Advantageously, configuring the barrier wall to perform the function of retaining scum simplifies the design of the settlement tank, and simplifies maintenance of the settlement tank.

The settlement tank may comprise a weir for controlling flow of fluid from the fluid holding region to the launder trough. 1]

In exemplary embodiments, the launder trough comprises an inner circumferential wall, an outer circumferential wall and a trough base extending therebetween, and the barrier wall is formed by the inner circumferential wail.

The settlement tank may comprise a wier plate. The weir plate may comprise a plurality of V-shaped notches. The weir plate may be positioned between the fluid holding region and the launder trough.

The weir plate may be connected to the barrier wall on a side of the barrier wall nearest the launder trough.

The weir plate may be angled to and/or offset from the barrier wall such that fluid flow from the holding region flows over the plate to enter the launder trough.

The weir plate may be adjustably mounted to the barrier wall such that positioning of the weir plate relative to the barrier wall can be selectively varied. For example, the plate may be adjustably mounted to the inner circumferential wall to create an increased or decreased barrier to flow from the plurality of holes to the launder trough.

The weir plate may be reinforced using one or more reinforcement struts.

The launder trough may define a walk way along which a person can walk, The perimeter wall and base may define a circular fluid holding region.

A seventh aspect of the invention provides a settlement tank of the first aspect, third aspect, fourth aspect, fifth aspect and/or sixth aspect, in any combination thereof

BRIEF DESCRIPTTON OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows a cross section through a settlement tank with a scraper assembly; Figure 2 shows a cross section through an alternative settlement tank; Figure 3 shows a plan view of the settlement tank of Figure 2; Figure 4 shows a detailed view of a launder trough, weir and walkway of the settlement tank of Figure 2; and Figure 5 shows a plate of the weir of Figure 4.

DETAILED DESCRIPTION OF EMBODIMENT(S)

Referring to Figure 1, a settlement tank is indicated generally at 10. The settlement tank 10 is circular in cross section. A perimeter wail 12 defines the circular extent of the settlement tank. The settlement tank 10 includes a base 14 that is generally frustoconical in shape, i.e. the base includes a circumferential wall sloping downwardly. A central region of the base 14 is stepped so as to form a recessed hopper 15 having a sloped circumferential wall 17. The perimeter wall 12 and the base 14 define a fluid holding region 13 in which fluid, in particular waste water, can be held in the settlement tank at an operational fluid level 22.

The base 14 of the settlement tank includes an outlet for sludge to exit the settlement tank, usually by pumped extraction. The outlet is not shown in the figures, but arrow 16 indicates the general direction in which sludge flows from the settlement tank, In the present embodiment, the outlet is a substantially horizontal outlet, but in alternative embodiments the outlet may be arranged to direct sludge in any suitable direction. The frustoconical shaped base encourages sludge to flow from the settlement tank to the hopper 15 arid towards the outlet.

A fluid inlet pipe 18 is positioned centrally in the settlement tank 10. The fluid inlet pipe extends from the hopper 15 of the base 14 upwardly to a level below the operational fluid level 22. Water enters the settlement tank via the water inlet 18, as indicated by arrow 20 in Figure 1, Fluid (substantially free from sludge and scum) exits the settlement tank through an outlet located in the perimeter wall 12. Fluid exits the settlement tank via a launder trough 23. A V-notch weir 24 is positioned near the perimeter wall to control the flow of fluid exiting the settlement tank, In the present embodiment, the weir extends upwardly from an inner circumferential wall of the launder trough, such that fluid must flow over the weir to enter the launder trough and exit the settlement tank.

The settlement tank 10 further includes a scraper assembly 26 for scraping sludge from the base 14 of the settlement tank.

The scraper assembly includes a scraper array 28 (e.g. multiple scraper elements arranged together) substantially aligned with the base 14 and extending generally, in the present embodiment, from the edge of the hopper 15 to the perimeter wall 12. The scraper array 28 is of a similar configuration to scraper arrays of the prior art, Accordingly, the skilled person will be familiar with the features of a scraper array and as such the scraper array 28 will not be described in further detail here, In alternative embodiments a single radially extending scraper body may be used instead of an array of scraper elements. In further alternative embodiments, the scraper array may span in both directions from the centre of the settlement tank. For example, the scraper array may span a full diameter of the settlement tank or, may partially span the tank radius on one side and e.g. have a frill length of three quarters of the tank diameter, The scraper assembly includes a central support 30 positioned in a central region of the settlement tank 10, The central support 30 includes a frame 38 that is connected to the wall 17 of the hopper 5 of the base 14, The frame 38 includes legs 29 and a top face 41. A slew bearing 40 is provided on the top face 41. The slew bearing 40 rotatably mounts the scraper array to the central support 30. In alternative embodiments alternative bearings or rotatable mounts may be used.

The frame 38 and the slew bearing 40 are arranged to circumferentially surround the fluid inlet pipe 18 and be substantially coaxial therewith. The frame 38 supports the slew bearing 40 above the hopper 15. As can be seen in Figure 1, the operational fluid level 22 is substantially higher than the slew bearing 40, such that the slew bearing is fully submerged in fluid during normal operational use of the settlement tank 10. Tn alternative embodiments, the frame may be modified to alter the position of the slew bearing, but in all embodiments it is envisaged that the slew bearing will be fully submerged during normal operation.

The scraper array 28 is connected to an outerjournal of the slew bearing 40 via an arm 32. In the present embodiment, the arm 32 is arranged to be substantially horizontal.

The arm 32 connects to the scraper array 28 at a position approximately one quarter to one third of the length of the scraper array, but in alternative embodiments the arm 32 may mount to the scraper array at any suitable position.

In the present embodiment the connection between the slew bearing 40 and the scraper array is reinforced by a stmt 46 that is connected between the arm 32 and the scraper array 28, but in alternative embodiments alternative structural tying arrangements may be used between the slew bearing and the scraper array.

A perimeter support 34 is mounted to the scraper array at a position near the perimeter wall 12 of the settlement tank 10. The perimeter support 34 extends substantially vertically upwards from the scraper array 28. An angled portion 35 is provided at an upper end of the perimeter support 34 arid extends to, and over, the perimeter wall 12.

In some embodiments, wheels, bearings or a running surface may be applied between the angled portion 35 and the top face of the outer wall 12 in order to reduce the vertical loading on other components caused by the self-weight of the scraper assembly, The perimeter support 34 is connected to the scraper array 28 at an end near the perimeter wall 12, but sufficiently spaced radially inward from the perimeter wall 12 to avoid the weir 24 and other in-tank features as may be required (scum removal equipment for instance).

Connection of the perimeter support to the scraper array 28 is reinforced by a strut 48 that connects between the perimeter support 34 and the scraper array 28. The strut 48 is angled to the perimeter support 34 so as to connect to the scraper array at a position radially inward from the position of connection of the perimeter support 34. In the present embodiment, only one strut 48 is provided, but in alternative embodiments a plurality of struts may be provided.

A drive mechanism 36 is provided on an outerside of the perimeter wall. The drive mechanism 36 moves the perimeter support 34 around the perimeter wall 12, so as to permit rotation of the scraper array 28 about the slew bearing 40.

A track 50 is provided around the perimeter wall 12 along which the perimeter support 34 can move. In the present embodiment, the drive mechanism 36 includes a drive that is stationary with respect to the perimeter wall 12 during rotation of the scraper assembly 26. The perimeter support 34 is connected to a cable or chain arrangement, and one or more cable pulleys are provided to pull the cable and therefore the perimeter support 34 around the perimeter wall 12. The stationary drive turns the pulleys to move the cable. In some embodiments a gear arrangement may be used between the drive and the pulley. In alternative embodiments, the drive may be configured to directly move the perimeter support 34 along the track 50, for example using wheels or a train arrangement.

An outlet is provided in the perimeter wall 12, in the form of a hole or opening 56 through the perimeter wail 12. A pipe or channel 54 of similar dimensions to the hole 46 extends from the hole 46 to direct fluid away from the perimeter wall 12. An energy recovery device 52 is positioned in or near the outlet (e.g. in the hole 56, the pipe 54, or the joint therebetween).

In the present embodiment, the energy recovery device is a pelton wheel, but any suitable energy recovery device may be used, for example a siphonic arrangement, Fluid flow from the fluid holding region 13 through the hole 56 to the pipe 54 rotates the pelton wheel. In the present embodiment, the pelton wheel is mechanically connected to the drive mechanism 36, such that rotation of the pelton wheel pulls the cable around the perimeter of the settlement tank, In an alternative embodiment, the pelton wheel is connected to a generator via a gearbox to generate electrical energy to drive the perimeter support 34 around the track 50 by electrically driven means.

As will be appreciated by the person skilled in the art, gravitational potential energy of the fluid flowing through the hole 56 is used to rotate the pelton wheel, In use, fluid from which sediment has settled flows over the weir 24 to the hole 56 in the perimeter wall 12, The fluid then flows to the pipe 54 via the energy recovery device 52, in this case a pelton wheel. Rotation of the pelton wheel mechanically rotates the pulley system to move the cable to pull the perimeter support 34 around the track 50.

Movement of the perimeter support 34 around the track 50 rotates the scraper array 28 about the mount 30 due to connection of the scraper array to the perimeter support 34 and the slew bearing 40.

As the scraper array 28 rotates in the settlement tank 10, sludge from the base 14 of the settlement tank is scraped and encouraged to be moved to the hopper 15, and out of the settlement tank via a sludge removal system.

Advantageously, the described scraper assembly does not require a bridge associated with scraper assemblies of the prior art. This means that manufacture of the scraper assembly is simplified and cost of manufacture is reduced because less raw materials are required to manufacture the scraper assembly.

In the embodiments where the drive is stationary with respect to the settlement tank, the energy required to rotate the scraper array is reduced. Further, positioning the drive outside the perimeter wall, particularly when the drive is stationary, results in more straight forward installation of the scraper assembly and simplified maintenance IS procedures.

The provision of an energy recovery device advantageously means that the scraper assembly is self powered. The self powered nature of the scraper assembly means that the scraper assembly is energy efficient, which also means that the scraper assembly has advantageous applications in developing countries.

Now referring to Figures 2 to 4, an alternative settlement tank is indicated generally at 110. The settlement tank 110 includes a perimeter wall 112 and a base 114 defining a fluid holding region 113. Fluid fills the fluid holding region 113 to an operational fluid level 122. In the present embodiment the settlement tank is a circular settlement tank, but in alternative embodiments the settlement tank may be a square or rectangular settlement tank, In Figure 2 the settlement tank is shown partially sunk into the ground (ground level is indicated at 170 in Figure 2), but the settlement tank may be positioned entirely above or entirely below ground level.

The settlement tank 110 includes a launder trough 174 and a weir 182 extending around the perimeter of the settlement tank 110. The launder trough 174 and weir 182 are positioned radially inward from the perimeter wall 112. The launder trough and weir define a walkway 172 extending around the perimeter of the settlement tank 110.

As shown in detail in Figure 4, the launder trough 174 is defined by a platform 175, an outer circumferential wall 173, and an inner circumferential wall 178, The outer circumferential wall 173 is an extension of the perimeter wall 112. In the present embodiment the platform 175, outer circumferential wall 173 and inner circumferential wall 178 are made from concrete which may be in pre-case sections or cast on-site. In alternative embodiments such circumferential structural components may be made of GRP, steel or other structural material, In use, scum forms on top of the fluid in the settlement tank.

A plurality of holes 180 are formed through the inner circumferential wall 178 of the launder trough. The holes 180 are positioned to be below the fluid level 122 (and also below the scum formed on top of the fluid). The holes 180 permit fluid flow from the fluid holding region 113 of the settlement tank to the launder trough 174.

In the present embodiment the weir 182 is defined by a plate 183 angled to the wall 178. However, in alternative embodiments the weir may be formed by a stepped shape having a substantially vertical section parallel to, but offset from, the inner circumferential wall. The plate 183 is adjustably connected to the wall 178 to vary the height of the weir 182 so as to control fluid flow from the fluid holding region 113 to the launder trough 174, Referring to Figure 5, the plate 183 includes a series of V-shaped notches in an upper portion of the plate 183. In the present embodiment the plate 183 is made from glass reinforced plastic, but in alternative embodiments the plate 183 may be made from stainless steel or any other suitable material.

The platform 175 of the launder trough provides a surface along which a person can walk, and the inner circumferential wall 178 and the outer circumferential wall 173 define a safety bather for the walk way 172. In the present embodiment, to meet safety requirements in the UK water treatment industry, the outer circumferential wall 173 and the inner circumferential wall 178 extend to a height above the platform of 1100 mm or more, and the walkway 172 has a width equal to or greater than 600mm, In alternative embodiments, the wall may extend to a lower height but a handrail may be provided at the height required to meet safety requirements. A non-slip surface is applied to the walk way (i.e. along the platform 175). A hand rail maybe provided on the outer circumferential wall and/or on the inner circumferential wall.

In use, fluid from the fluid holding region 113 flows to the launder trough through the plurality of holes 180 formed in the inner circumferential wall 178. The holes are formed below the operational level of fluid level 122 and the inner circumferential wall 178 extends above the operational fluid level and scum level, This means that the inner circumferential wall 178 performs the fbnction of a scum board by substantially preventing flow of scum to weir 182 and to the launder trough 174.

Fluid flow from the holes 180 flows over the plate 183. The plate 183 controls the flow rate of fluid to the launder trough 174. Fluid then flows from the launder trough 174 to, in the present embodiment, subsequent water treatment stages or to outfall.

When maintenance work is required, for example to remove debris from a plate 183 IS or undertake an inspection of the weirs, fluid is drained from the fluid holding region 113 to a level below the V-shaped notches 184 of the plate 183. This prevents flow of fluid from the fluid holding region 113 to the launder trough 174. Water is then drained from the launder trough 174.

A person can then walk along the walk way 172 to carry out maintenance work,, Good access is permitted for such work due to the walk way extending frilly around the perimeter of the settlement tank, If the plate 183 needs adjusting or otherwise maintaining, the fluid in the fluid holding region 113 is drained to a level below the holes 180 in the inner circumferential wall 178. The plate 183 can then be adjusted or removed as required.

Fluid can then be received in the fluid holding region 113 to fill the settlement tank with fluid to the fluid operational level 122, re-instigating flow over the plate 183.

The settlement tank 110 is advantageous over settlement tanks of the prior art because it removes the need for draining the tank and using a moveable platform or scaffold in the fluid holding region 113 for maintenance of the weir, as is done in the prior art, and as such improves safety and reduces maintenance time. Further, the settlement tank 110 eases inspection of the weir and launder trough, which permits more regular

inspection than settlement tanks of the prior art.

Further, the inner wall 178 performs the dual function of the scum board arid edge protection, which simplifies the design and simplifies maintenance of the settlement tank.

Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.

For example, it will be appreciated by the person skilled in the art that the scraper assembly 26 of the settlement tank 10 of Figure 1, may be used in the settlement tank of Figure 2.

Claims

Claims A settlement tank comprising: a perimeter wall and base defining a fluid holding region; a launder trough via which fluid from the fluid holding region can exit S the settlement tank; and a wa& way along which a person can walic; wherein the launder trough is constmcted and arranged to define the walk way.
2. The settlement tank according to claim 1, wherein the launder trough is defined by a platform that forms a surface along which a person can walk, an outer circumferential wall on one side of the platform, and an inner circumferential wall on the other side of the platform, the inner circumferential wall being positioned adjacent the fluid holding region.
3. The settlement tank according to claim 2, wherein the inner circumferential wall extends to a height above an operational level of fluid and any scum formed on the fluid during use.
4, The settlement tank according to claim 3, wherein the inner and/or outer circumferential wall extend to a height greater than or equal to 1100 mm from the platform.
5. The settlement tank according to any one of claims 3 or 4, wherein the inner circumferential wall comprises a plurality of holes through which fluid can flow from the fluid holding region to the launder trough.
6. The settlement tank according to claim 5, comprising a weir plate mounted to the inner circumferential wall and mgled and/or offset thereto, such that, in use, fluid flows from the plurality of holes over the weir plate to the launder trough. 2]
7. The settlement tank according to claim 6, wherein the weir plate is adjustably mounted to the inner circumferential wall such that positioning of the weir plate relative to the inner circumferential wall can be selectively varied.
8. The settlement tank according to claim 6 or 7, wherein the weir plate comprises a plurality of V-shaped notches.
9. The settlement tank according to any one of claims 6 to 8, wherein the weir plate is reinforced using one or more reinforcement struts.
10. The settlement tank according to any one of claims 6 to 9, wherein the weir plate is made from glass reinforced plastic.
Ii. The settlement tank according to any one of claims 2 to 10 comprising a hand rail connected to the inner circumferential wall and/or the outer circumferential wall.
]2. The settlement tank according to any one of claims] to 11 comprising non-slip flooring along the walk way.
13. The settlement tank according to any one of claims ito 12, wherein the width of the walk way is equal to or greater than 600 mm,
14. The settlement tank according to any one of claims i to 13, wherein the launder trough is positioned inwardly from the perimeter wall,
]5. The settlement tank according to any one of claims] to 14, wherein the perimeter wall and base define a circular fluid holding region.
16. The settlement tank according to any one of claims i to 15, wherein the walkway is made from concrete.
17. A settlement tank according claim 1 or 2: wherein, the launder trough comprises a barrier wall adjacent the fluid holding region, and the barrier wall extends to a height above an operational fluid and scum level, and a plurality of holes are provided in the barrier wall below the operational fluid and scum level through which fluid can flow to the launder trough.
18. The settlement tank according to claim 17, wherein a wier plate comprising a plurality of V-shaped notches is positioned between the fluid holding region and the launder trough.
19. The settlement tank according to claim 18, wherein the weir plate is connected to the barrier wall on a side of the barrier wall nearest the launder trough.
20. The settlement tank according to claim 19, wherein the weir plate is angled to or offset from the barrier wall such that fluid flow from the holding region tO flows over the plate to enter the launder trough.
21. The settlement tank according to claim 20, wherein the weir plate is adjustably mounted to the barrier wall such that positioning of the weir plate relative to the barrier wall can be selectively varied.
22. The settlement tank according to any one of claims 18 to 21, wherein the plate tS is reinforced using one or more reinforcement stmts,
23. The settlement tank according to any one of claims 18 to 22, wherein the plate is made from glass reinforced plastic.
24. The settlement tank according to any one of claims 17 to 23, wherein the perimeter wall and base define a circular fluid holding region.