GB2280124A

GB2280124A - Granular bed filter

Info

Publication number: GB2280124A
Application number: GB9315076A
Authority: GB
Inventors: David Gordon Stevenson; Ian Alexander Crossley
Original assignee: PWT Projects Ltd
Current assignee: PWT Projects Ltd
Priority date: 1993-07-21
Filing date: 1993-07-21
Publication date: 1995-01-25
Also published as: GB9315076D0; JPH07163808A

Abstract

A filter bed comprises a layer of active carbon 3 on top of a layer of sand 4 and is arranged for downward filtering flow and upward flow cleaning by backwash. The enclosing housing includes an outlet (10) for discharging the carbon in a fluidised state while the sand, also fluidised, is retained in the housing by a weir 17. Residual spent carbon may be flushed from the housing by feeding a small amount of fresh carbon via inlet 11. In Fig. 3 a plurality of weirs divide the sand layer and give its upper surface a stepped form. <IMAGE>

Description

Improved Filter for use with Friable Granular Materials Filters using granular materials such as activated carbon to absorb impurities from water must be provided with facilities to enable the absorbent material to be removed when it is exhausted. Also the material must be backwashed at regular intervals to remove any particles that may be filtered out. In normal circumstances the flow is downwards and the material at the top of the bed is the most highly saturated at the end of its life.

Because removal is normally via the base of the filter, any residue tends to comprise this highly saturated material which may in fact release some of the adsorbate if allowed to come into contact with purified water. It is therefore important that as much as possible if not all of the charge is removed before new material is added. A parallel patent application (GB 9217097.6) describes one means of removing such material in an efficient manner, and transferring it to an intermediate silo.

An additional problem with friable absorbents such as activated carbon and even non absorbent friable materials, is that fragments and dust tend to break off the grains, both during handling and also during backwashing, and possibly also during service as the material is compressed by the flow and the consequent pressure loss. Such fragments are undesirable in a drinking water supply or in industrial process water.

They may however be retained within the filter by a supporting layer of denser abrasion-resistant material such as sand. However the sand must be washed regularly to remove the fragments but it must not interfere with the efficient removal of the absorbent material when this is due.

The present invention therefore comprises means for preventing fragments of the friable material from entering the supply, which involves the use of an abrasion-resistant material denser than the absorbent or friable material, and of such a size that both materials can be fluidised at the same flow rate, with expansion of the denser material high enough to achieve satisfactory washing but without leading to loss of material. Substantially all of the sand in this case is retained within the filter while substantially all of the exhausted absorbent material is removed.

In order to achieve the desired separation of the denser material from the lighter, it is necessary for the filter to have an internal arrangement whereby the lighter material can be selectively removed under fluidisation.

According to the invention, this is achieved by providing a filter vessel for use with two filter materials of different density, comprising a container provided with a floor having inlets for backwash liquor and/or air, a high inlet for liquor to be treated, a sub-floor outlet for filtered liquor and an outlet for filter material situated above the floor, in which the floor is provided along at least one edge with a retaining weir adapted to define an area to contain denser material and being of a height no higher than the fluidised height of the denser material to be contained therein, the outlet for the less-dense filter material being located outside the defined area at a level no higher than the weir.

In general, the weir may be located inside the periphery of the floor, to leave a relatively narrow channel to one side which will be filled with the less dense material during use, so the floor area of the less-dense material is greater than that of the denser material bounded by the weir. In an alternative embodiment, the weir may be formed by the provision of a slot along one or more of the side walls of the filter container at the same height above the floor, the slot communicating with a weir box adjacent the outside of the wall and containing the outlet for the less-dense material.

In a particularly preferred embodiment, the floor may be provided with one or more intermediate weirs, preferably forming a cascade from an opposite side down to the level of the retaining weir.

In another particularly preferred embodiment, the container is provided at a high level, with an inlet for clean less-dense material closely adjacent a side wall directly opposite the retaining weir, the inlet being arranged to direct the material towards the wall so that it can flow down the wall and across the floor towards the retaining weir, thereby displacing any residual dirty material left on top of the denser material after the container has been emptied.

In practice, the selection of the two materials should be such that when the less-dense material is suitably fluidised, the denser material is also fluidised, but with much less expansion. For example the less dense material can comprise a coal-based activated carbon with a size range of 0.5 to 1.4 mm and the denser material can comprise a 0.5 to 1.0 mm sand.

Under backwash conditions of say 25 m/hr (rate of water rise) the carbon will be expanded about 25 to 40% by volume, while the sand will be expanded only about 10%.

A finer sand, e.g. 0.425-0.85 mm could also be used, but would expand more.

The invention will now be more particularly described with reference to the accompanying drawings, in which: Fig. 1 represents a general sectional view of a filter comprising one embodiment of the present invention; and Fig. 2 and Fig. 3 represent similar sectional views of two other embodiments.

With reference to Fig.l, a tank or vessel 1, is provided with a conventional filter floor 2, and is filled with a friable material comprising activated carbon or any other appropriate absorbent 3, supported on a layer of sand 4 or another material which is denser than the friable material. In normal service, the feed enters at a high level, from a feed manifold or channel 5, and the filtrate emerges at low level from the floor through an outlet pipe 6. The level in the filter is controlled by means well known to water filtration engineers.

When the filter is to be backwashed to remove particulate dirt, the backwash water is admitted though an inlet 7, to the floor 2, and the used wash water overspills a weir and is discharged through a pipe 8.

Air may be applied if desired, usually prior to the backwash, by means not shown in the figure.

The layer of sand intercepts any particles shed from the friable material during use, and such particles are washed out through the upper layer 3 and over the weir.

When the time eventually comes for the friable material to be removed and replaced, it is discharged from the base of the layer, through a side box 9 and out via valve 10 to a discharge pipe. The box 9 runs along the entire length of the side, with a full length slot aperture. An upstanding edge forms a weir 17 to retain the sand. The floor of the box slopes towards the discharge pipe. Thus at the end of the discharge phase, the box 9 is empty and the sand is retained in the same manner as in a simple sand filter with a side weir washout trough. When charged with the absorbent, the side box 9 is full of absorbent which is idle. However the amount is very small.

A small amount of spent absorbent inevitably remains on the sand layer. To remove this, virgin or regenerated absorbent is supplied at the back wall of the filter through a high level manifold 11. The virgin material flushes the spent material forward (to the right, in Fig. 1) and out. (A small deflector in the form of a ledge 12 is included to prevent the mixture cascading down the back wall from gouging the sand layer.) This flushing material is supplied for example from a small hopper 13, with appropriate water connections from the conveying water system or from a local bulk silo of virgin material, or indeed direct from a road tanker bringing in virgin or regenerated material. The quantity of material required for displacement is small (for example a few percent of the charge) and corresponds to a small part of the next charge. Any excess introduced in order to be certain that all of the spent material has been removed will be recycled through the regeneration system.

Fig.2 shows a variant on the system for use where the external box is inconvenient. It is possible to provide a trough 14 in the floor, alongside the weir 17, to conduct the friable material to the outlet valve 10.

This will leave a dead zone at the side of the filter which may or may not be acceptable. As an optional refinement this trough may be formed as part of the floor but isolated hydraulically from the rest of the floor. The lower layer of sand or other dense material in this case is excluded from the trough by the low weir wall 17. The outlet pipe 6 serves the main area of the floor but not the small section 14. Likewise the backwash pipe 7 in this case has separate branches 18, 19 with valves to maintain the isolation.

In service, only the main section of the floor is in use. At the surface of the absorbent or friable material all the material will be in operation, but flow will taper at the base, away from the small (trough) section. The whole floor is supplied with backwash water using both branches of the inlet 7, and during transfer the material in the trough can be fluidised and will remain mobile.

To discharge the absorbent, the main part is allowed to flow through the outlet 10, with intermittent fluidisation as required. As in the previous variant, a small amount of absorbent will be left on the sand layer after final fluidisation and this may be flushed off with virgin or regenerated material introduced through the manifold 11, as before.

The third variant shown in Fig. 3 uses a similar procedure for the discharge of the main part of the absorbent charge but the sand or other material used for the lower layer is recirculated across the filter instead of using virgin absorbent. To improve the flow pattern and ensure uniform movement a set of submerged weirs 15 is included to provide a terraced sand layer so that the sand overspills the weirs carrying the friable or absorbent material with it.

The recirculation of sand may either involve an external sand/absorbent separation device, from which the sand is returned to the back end of the filter, or the sand may be recirculated within the filter, for example by an air lift device 16 or a set of such devices.

The filter floor under the sand layer may itself be terraced, or sloping or indeed flat. In the latter case the nozzles of the floor must be able to control the backwash flow into the differing depths of sand.

The stepped sand layer shown in Fig.3 may also be used without movement of the sand as it will also regulate the cross flow pattern of flushing material.

Claims

1 A filter vessel for use with two filter materials of different density, comprising a container provided with a floor having inlets for backwash liquor and/or air, a high inlet for liquor to be treated, a sub-floor outlet for filtered liquor and an outlet for filter material situated above the floor, in which the floor is provided along at least one edge with a retaining weir adapted to define an area to contain denser material and being of a height no higher than the fluidised height of the denser material to be contained therein, the outlet for the less-dense filter material being located outside the defined area at a level no higher than the weir.

2 A filter vessel according to Claim 1, in which the weir is located inside the periphery of the floor to leave a relatively narrow channel to one side.

3 A filter vessel according to Claim 1, in which the weir is formed by the provision of a slot along one or more of the side walls of the filter container at the same height above the floor, the slot communicating with a weir box adjacent the outside of the wall and containing the outlet for the less-dense material.

4 A filter vessel according to Claim 1 or Claim 2, in which the floor is provided with one or more intermediate weirs.

5 A filter vessel according to Claim 1 or Claim 2, in which the intermediate weirs form a cascade from an opposite side down to the level of the retaining weir.

6 A filter vessel according to any of Claims 1 to 5, the container is provided at a high level, with an inlet for clean less-dense material closely adjacent a side wall directly opposite the retaining weir, the inlet being arranged to direct the material towards the wall so that it can flow down the wall and across the floor towards the retaining weir, thereby displacing any residual dirty material left on top of the denser material after the container has been emptied.

Amendments to the claims have been filed as follows 1 A filter vessel for use with two filter materials of different density, comprising a container provided with a floor having inlets for backwash liquor and/or air, a high inlet for liquor to be treated, a sub-floor outlet for filtered liquor and an outlet for filter material situated above the floor, in which the floor is provided along at least one edge with a retaining weir adapted to define an area to contain denser material and being of a height no higher than the fluidised height of the denser material to be contained therein, the outlet for the less-dense filter material being located outside the defined area at a level no higher than the weir.

6 A filter vessel according to any of Claims 1 to 5, in which the container is provided at a high level, with an inlet for clean less-dense material closely adjacent a side wall directly opposite the retaining weir, the inlet being arranged to direct the material towards the wall so that it can flow down the wall and across the floor towards the retaining weir, thereby displacing any residual dirty material left on top of the denser material after the container has been emptied.

7 A filter containing two filter materials of different density, comprising a container provided with a floor having inlets for backwash liquor and/or air, a high inlet for liquor to be treated, a sub-floor outlet for filtered liquor and an outlet for filter material situated above the floor, in which the floor is provided along at least one edge with a retaining weir adapted to define an area containing denser material and being of a height no higher than the fluidised height of the denser material contained therein, the less-dense material being located above the denser material, the outlet for the less-dense filter material being located outside the defined area at a level no higher than the weir.

8 A filter according to Claim 7, in which the weir is located inside the periphery of the floor to leave a relatively narrow channel to one side.

9 A filter according to Claim 7, in which the weir is formed by the provision of a slot along one or more of the side walls of the filter container at the same height above the floor, the slot communicating with a weir box adjacent the outside of the wall and containing the outlet for the less-dense material.

10 A filter according to Claim 7 or Claim 8, in which the floor is provided with one or more intermediate weirs.

11 A filter according to Claim 7 or Claim 8, in which the intermediate weirs form a cascade from an opposite side down to the level of the retaining weir.

12 A filter according to any of Claims 7 to 11, in which the container is provided at a high level, with an inlet for clean less-dense material closely adjacent a side wall directly opposite the retaining weir, the inlet being arranged to direct the material towards the wall so that it can flow down the wall and across the floor towards the retaining weir, thereby displacing any residual dirty material left on top of the denser material after the container has been emptied.