WO2000065163A1

WO2000065163A1 - Method and apparatus for removing particulate solids from a fluid stream

Info

Publication number: WO2000065163A1
Application number: PCT/GB2000/000351
Authority: WO
Inventors: Richard Chilton; Jonathan Rains
Original assignee: United Utilities Plc
Priority date: 1999-04-22
Filing date: 2000-02-07
Publication date: 2000-11-02
Also published as: GB9909108D0; AU2309400A

Abstract

Apparatus for removing particulate solids from a bulk flow of liquid comprises a substantially straight channel (1) for receiving the flow and provided with means for controlling the flow such that solid particles above a predetermined size will tend to settle to the bed of the channel (1) and be transported along the channel bed by shear force exerted by the bulk flow. At least one solid particle removal port (3) is provided at the bed of the channel extending across substantially the full width of the channel bed. In operation solid particles are delivered to the port (3) by said shear force.

Description

METHOD AND APPARATUS FOR REMOVING PARTICULATE SOLIDS FROM A FLUID STREAM

The present invention relates to a method and apparatus for removing particulate solids from a fluid stream. Particularly, but not exclusively, the invention relates to the removal of grit from sewage.

Sewage must be treated in different ways to remove the different forms of pollutant typically present. For instance, organic sludge is conventionally removed by settlement in large sedimentation tanks. It would be possible to rely on this sedimentation process to remove grit (which typically enters a sewage system from roads and soil) from the sewage but this presents several problems. Most significantly, the presence of relatively heavy grit in the organic sludge tends to bind the sludge by a process of accretion greatly increasing the solidity of the sludge thereby making it much more difficult to remove and treat. It is therefore conventional in sewage treatment plants to remove as much grit as possible from the sewage upstream of the sedimentation tanks.

There are a number of known methods and apparatus for the removal of grit from sewage which take advantage of the relatively high specific gravity of the grit (which is higher than that of water and of, for instance, organic sludge which has a specific gravity about the same as that of water).

In relatively fast and turbulent sewage flow conditions the majority of grit will be transported in suspension. However, under slower, less turbulent flow conditions grit will tend to settle out at the bottom of the flow channel and form dunes so that grit is transported slowly along the bottom of the channel by the process of saltation. At even lower flow rates saltation transport will stop and the grit dunes will simply build up. Some small grit particles will still be transported in suspension, but generally the slower the flow the more grit will settle.

Taking advantage of the above transport principles, it is known to remove grit from sewage by flowing the sewage along generally straight channels in which the flow velocity is controlled at about 0.3 meters per second at which the majority of grit particles of about 0.3mm diameter or bigger settle out and collect at the bottom of the channel. Such channels are known as constant velocity channels and the velocity within such channels is typically controlled by the design of the channel profile in conjunction with a downstream flume. At flow speeds of approximately 0J meters per second the grit will settle but will not be transported by saltation. This therefore ensures that the grit is not simply transported through the channel but rather collects. Slowing the flow rate yet further would have the effect of allowing even smaller grit particles to settle out but would have a disadvantage that some organic sludge might also settle. This would pose a grit washing and disposal problem.

The grit may then be removed from the channel by any appropriate mechanism. For instance, in one known apparatus the channel comprises an elongate tank which is long enough for the flow to straighten and the grit to settle on the base of the tank. The grit is then removed by suction of a pump mounted on a bridge to traverse the length of the tank.

A further example of a grit removal apparatus utilising the above principle is the apparatus known as a detritor in which sewage is passed from a straight channel into a much wider channel including a basin to reduce the velocity of the water to approximately 0.3 meters per second so that the grit settles out into the basin (and is not transported by saltation). A scrapper member incorporating three scrapper arms is rotatably mounted on a member which is suspended from a bridge across the basin. The scrapper is submerged in the flow and rotates to transport settled grit towards a hopper mounted at the side of the basin. A problem with this design is that the required uniform constant velocity distribution is not always achieved so that grit which has settled out of the sewage may often be swept through the channel bypassing the scrapper and collection hopper. Indeed, as little as 30% of the incoming grit may be retained by such an apparatus.

A relatively recent method of grit removal proposed to overcome the problems associated with apparatus such as those mentioned above is described in International patent specification number WO91/14053. This comprises passing the sewage through an initial straight flow channel to stabilise the flow and then subsequently passing the sewage around a bend (having an angle of at least 10°) such that grit collects at, and is removed via, a port situated at the inside of the bend. The straight portion of the channel is designed so that the sewage flow is controlled to a stable flow at a speed of around 0.6 to 1J meters per second so that the grit settles to the bottom and is transported by saltation towards the bend. In the region of the bend a rotation of the fluid flow is set up (by the same mechanism which in nature causes rivers to take a meandering path across planes) the strength of which is a function of the curvature radius relative to the width of the channel, the velocity of the flow and the angle through which the flow is turned. The result is that grit is moved to the bottom of the channel on the inside of the curve whilst lighter material is moved to the top of the sewage surface on the outside of the curve. Thus, downstream of the bend, grit travels along the bottom of the channel close to the inside wall of the bend and can thus be removed simply by locating a port on the inside edge at the bottom of the channel. One of the aims of providing the bend so that grit is directed to a particular part of the channel is so that the bulk flow through the port is minimised to reduce the proportion of organic solids collected. A disadvantage with this method of removing grit from sewage is the relative complication of the channel construction. The angle of the bend and the position of the port have to be calculated and arranged very carefully to optimise performance.

It is an object of the present invention to provide an improved grit removal method and apparatus.

According to a first aspect of the present invention there is provided an apparatus for removing particulate solids from a bulk flow of liquid comprising a substantially straight channel for receiving said flow, means for controlling the flow through the channel such that solid particles above a predetermined size will tend to settle to the bed of the channel and be transported along the channel bed by shear force exerted by the bulk flow, wherein at least one solid particle removal port is provided at the bed of the substantially straight channel the port or ports extending across substantially the full width of the channel bed, such that in operation solid particles are delivered to the or each port by said shear force.

According to a second aspect of the present invention there is provided a method for removing particulate solids from a bulk flow of liquid comprising flowing the liquid through a straight channel which is provided with a solids removal port at the channel bed and extended across substantially the whole width of the channel bed, controlling the flow through the channel such that solid particles above a predetermined size will tend to settle to the bed of the channel upstream of the port and be transported along the channel bed to the port by shear force imparted by the liquid flow, such that solid particles entering the port is thereby removed from the channel.

The present invention provides advantages over existing methods and apparatus such as the constant velocity channels mentioned above in that the shear force produced by the liquid flow effectively transports the solid particles along the channel to a removal point (or a plurality of removal points). In this respect the invention is similar to the curved channel method and apparatus discussed above but has the further advantage that it is relatively simple in construction and thus it is a relatively straight forward matter to modify existing constant velocity channels to operate in accordance with the present invention. Furthermore, the provision of a removal port (or ports) which extends at least substantially across the full width of the channel ensures that no solid particles will be carried past the or each port (except for very fine particles which remains in suspension and which do not in any event need to be removed from the sewage at this stage).

A specific embodiment of the present invention will now be described, by way of example only , with reference to the accompanying drawing which is a schematic illustration of apparatus for removing grit from sewage in accordance with the present invention.

The illustrated grit removal apparatus comprises a straight flow channel 1 of constant rectangular cross-section. The direction of the flow of sewage through the channel is indicated by the arrows A and B. At its downstream end the channel 1 is provided with a flume 2 which effectively controls the depth of sewage flow within the channel as a function of the flow rate through the channel. A rectangular grit removal port 3 is formed in the bed of the channel 1 immediately upstream of the flume 2 and extending across the complete width of the channel bed. The port 3 opens into a grit collection hopper 4 located beneath. The hopper 4 has an outlet 5 from which grit may be removed. For instance, grit may be pumped from the hopper outlet 5 in any direction and the pump rate may vary relative to the bulk flow rate through the channel.

The channel cross section and the flume 2 are designed so that the variation of bulk flow velocity through the channel will remain within predetermined limits as the rate and depth of the flow varies. That is, the velocity is controlled such that grit particles above a predetermined size (about 0.3mm) which enter the channel 1 will settle to the bottom of the channel 1 upstream of the port 3 and will be transported along the channel bed towards the port 3 by the shear force imparted by the bulk flow. In other words, the bulk flow velocity must be maintained low enough that grit particles of the targeted size entering the channel 1 at the surface of the sewage flow will settle to the channel bed before reaching the port 3, but must be maintained high enough that the shear forces at the channel bed are sufficient to transport the grit particles along the channel bed to the port 3, at least under normal flow conditions. Typically, this means controlling the bulk flow velocity in the range of approximately 0.4 to 1.5 meters per second to allow grit particles of about OJrnm diameter or above to settle.

Thus, in operation, grit will settle to the bed of the channel 1 and be transported along the channel bed to the port 3 and into the hopper 4 from which it may then be removed. Variations in the bulk flow velocity and depth of the flow within the parameters determined by the design of the flume and the channel should have no significant effect on the grit removal process.

In practical installations it may be preferable to provide a plurality of channels as described above operating in parallel. The incoming sewage flow could then be diverted to more or less channels as required to accommodate unusual variations in the sewage supply. For instance, if the flow rates drop the sewage could be diverted through fewer channels to ensure that an appropriate depth and flow rate is maintained.

The grit removal method and apparatus according to the present invention thus has advantages over known constant velocity channel grit removal methods mentioned above in that no separate means is required to remove the grit from the channel bed. Furthermore, the apparatus has an advantage over the curved channel grit removal methods mentioned above in that the construction is simpler. For instance existing constant velocity channels could readily be modified to operate in accordance with the present invention. In addition, because the port 3 extends across the full width of the channel there is no possibility of grit travelling along the channel bed bypassing the port 3. It will be appreciated that the basic design requirement of a grit removal apparatus in accordance with the present invention is that there is a port extending across substantially the whole width of the channel bed and that the channel is designed to control flow such that at least a substantially portion of grit particles above a given size will drop to the bed of the channel upstream of the grit removal port and such that the flow is sufficient, at least under normal flow conditions, to transport the settled out grit to the port by saltation. Should the flow rate change to such an extent that the shear force at the bed of the channel is insufficient to transport grit particles along the bed this would have no detrimental effect since the grit would simply remain in place until normal flow conditions are resumed.

Beyond the above mentioned design criteria it will be appreciated that many modifications may be made to the illustrated apparatus. For instance, the flume and channel could be designed to maintain a constant bulk flow velocity within the channel, for example, by constructing the channel with a trapezoid cross-section such that flow velocity is not affected by changes in depth. The shear force at the channel bed would still vary with flow depth but the velocity could be determined to ensure that under normal flow conditions the shear force would be sufficient to transport grit to the port by saltation. A velocity of the order of 0.6 meters per second would typically be appropriate. Alternatively, means other than a flume and trapezoid section channel could be used to control the flow velocity within the channel. The channel could also be constructed with a cross-section which varies along its length. Similarly the flume could be constructed with a cross-section which varies along its depth.

It will also be appreciated that the dimensions and configuration of the grit removal port and its position within the channel port may be varied. For instance, the port could be provided at an angle to the channel bed rather than provided as a simple aperture in the bed of the channel. For example, the port may be provided as a "letterbox" slot which effectively trims off a bottom portion of the fluid flow. As further alternative a plurality of ports could be provided. For instance, the simple single slot illustrated could be replaced by a plurality of shorter slots which collectively extend across the width of the channel bed. As a further refinement, a plurality of ports could be spaced along the length of the channel to provide for particle size sorting, since larger particles would tend to settle at the channel bed further up-stream than smaller particles.

The design of the hopper may vary considerably as may the mechanisms used for removing grit from the hopper. For instance, the flow of grit through the port could be pump assisted. The hopper could be replaced by a grit transport channel transporting the grit to further treatment facilities.

Many other possible modifications to the detail of the apparatus will be apparent to the appropriately skilled person.

Claims

1. Apparatus for removing particulate solids from a bulk flow of liquid comprising a substantially straight channel for receiving said flow, means for controlling the flow through the channel such that solid particles above a predetermined size will tend to settle to the bed of the channel and be transported along the channel bed by shear force exerted by the bulk flow, wherein at least one solid particle removal port is provided at the bed of the substantially straight channel the port or ports extending across substantially the full width of the channel bed, such that in operation solid particles are delivered to the or each port by said shear force.

2. Apparatus according to claim 1, wherein the means for controlling the flow controls the velocity of the bulk flow between predetermined limits within which shear force at the bed of the channel is sufficient to produce said transport.

3. Apparatus according to claim 1 or claim 2, wherein said means for controlling the flow controls the flow such that shear forces produced at the channel bed under at least some flow conditions are sufficient to transport the solid particles along the channel bed towards said port.

4. Apparatus according to any preceding claim, wherein said means for controlling the flow constrains the flow to a predetermined constant velocity.

5. Apparatus according to any preceding claim wherein the channel has a substantially constant cross-section upstream of said port.

6. Apparatus according to any preceding claim, wherein said means to control the fluid flow includes a flume which varies the depth of flow within the channel as a function of the flow rate through the channel.

7. Apparatus according to any preceding claim, wherein said means for controlling the fluid flow comprises at least in part the design of said channel itself.

8. Apparatus according to claim 7, wherein said channel has a cross-section designed to maintain the required flow velocity and/or shear stresses as the flow rate varies.

9. Apparatus according to any preceding claim, wherein the velocity of the flow upstream of the port is controlled between about 0.4 and 1.5 meters per second.

10. Apparatus according to any preceding claim, wherein the flow upstream of the port is constrained to a constant velocity of approximately 0.6 meters per second.

11. Apparatus according to any preceding claim, wherein the or each port is a simple aperture formed in the bed of the channel.

12. Apparatus according to claim 11, wherein the or each aperture is substantially rectangular.

13. Apparatus according to claim 10 or claim 11, wherein the or each aperture lies substantially in the plane of the channel bed.

14. Apparatus according to any preceding claim, comprising a plurality of said ports are provided which collectively extend across substantially the full width of the channel bed.

15. Apparatus according to any preceding claim, wherein a plurality of said ports are provided, at least two of which are spaced apart in the direction of the fluid flow.

16. Apparatus according to any preceding claim, wherein a grit hopper or the like is coupled to the port to receive grit removed from the channel via said port.

17. Apparatus according to any one of claims 1 to 10, wherein at least one of the or each ports lies in a plane extending at an angle to the channel bed.

18. Apparatus according to claim 17, wherein solid particles are pumped from the hopper.

19. Apparatus according to claim 13, wherein solid particles are removed from the hopper by a mechanical device such as a screw or conveyor.

20. A method for removing particulate solids from a bulk flow of liquid comprising flowing the liquid through a straight channel which is provided with a solids^ removal port at the channel bed and extended across substantially the whole width of the channel bed, controlling the flow through the channel such that solid particles above a predetermined size will tend to settle to the bed of the channel upstream of the port and be transported along the channel bed to the port by shear force imparted by the liquid flow, such that solid particles entering the port is thereby removed from the channel.

21. Apparatus for removing particulate solids from a bulk flow of liquid, substantially as hereinbefore described with reference to the accompanying drawings.

22. A method of removing particulate solids from a bulk flow of liquid, substantially as hereinbefore described with reference to the accompanying drawings.