GB2318070A

GB2318070A - Separating floating solids from a liquid

Info

Publication number: GB2318070A
Application number: GB9621337A
Authority: GB
Inventors: Michael David Whittam
Original assignee: United Utilities PLC
Current assignee: United Utilities PLC
Priority date: 1996-10-12
Filing date: 1996-10-12
Publication date: 1998-04-15
Also published as: GB9621337D0

Abstract

Apparatus for separating floating solids from a volume of liquid contained within a container (1) an edge of which forms a weir (3) comprises means (7,8) for sweeping surface floating solids over the weir (3). Operation of the sweeping means (8) is confined to an area of the liquid surface adjacent to the weir (3). Surface floating solids are continuously drawn into the 'operating' area by tension in the liquid surface.

Description

APPARATUS AND METHOD FOR SEPARATING SOLIDS FROM A LIQUID The present invention relates to an apparatus and method for separating solids from the surface of a liquid. Particularly, but not exclusively, the invention provides apparatus for removing solids from water in the treatment of potable water.

Conventional potable water treatment essentially comprises a first stage for the removal of larger particles of sand and silt, followed by a second fine filtration stage.

In the past, a flocculant assisted gravity settlement system has been widely used for the initial stage of removing the larger particles of sand/silt. More recently, the conventional settlement process has been replaced by a process of dissolved air flotation (DAF) in which rnicro-bubbles introduced into the water attach themselves to chemically-coagulated particles which are then carried to the surface. A scraper mechanism is then used to skim the surface and push the sludge of foamed particles over a weir into a collection trough.

In known DAF plants, the sludge removal apparatus comprises a number of brush members which extend across the width of a tank in which the air flotation is performed. The brush members are dragged across the surface of the tank of water by a system of chainwheels and chains. That is, each end of each brush member is fixed to a chain which runs along a respective side of the tank. The chains are endless chains which pass around motor driven chainwheels at each end of the tank. The system operates by dragging the brush members across the surface of the water from one end of the tank to the other to scrape surface sludge over a weir. As a brush reaches the weir it is lifted free of the surface (as the chains pass around chainwheels situated at the weir end of the tank) returned to the opposite end of the tank, and then re-introduced into the surface of the liquid (as the chains round further chainwheels) and once again dragged across the surface of the tank. Thus, whilst the apparatus is activated there is a continuous scraping operation which scrapes sludge from the surface of the liquid over a weir and into a sludge collection chamber from where the sludge can be drained to suitable holding tanks.

A problem with such systems is that they can require frequent, and expensive, maintenance. For instance the chains need to be tensioned on a regular basis.

Moreover, with continuous use various components can fail so that it is periodically necessary to replace, for example, the chains, chainwheels and in some cases brushes.

This can be a costly procedure both in terms of the cost of the replacement parts and the man hours necessary to carry out the replacement (during which time the plant will be out of service).

It is an object of the present invention to obviate or mitigate the above disadvantages.

According to a first aspect of the present invention there is provided apparatus for separating floating solids from a volume of liquid contained within a container an edge of which forms a weir, the apparatus comprising means for sweeping surface floating solids over the weir, wherein operation of said sweeping means is confined to an area of the liquid surface adjacent to said weir.

Although the sweeping operation is confined to an area of the liquid surface adjacent to the weir, it has been found that tension in the surface of the liquid and/or cohesion between particles floating on the surface of the liquid is sufficient to continuously draw surface floating solids into the "sweeping area" as solids are swept over the weir. Indeed, entire the surface may be mobilised towards the weir. Thus, the present invention dispenses with the need to provide apparatus which sweeps scrapers across the entire surface of the volume of liquid from which solids are to be removed. The apparatus can thus be compact, and simpler and cheaper to install and maintain than known systems described above.

The sweeping means preferably extends along substantially the entire length of the weir. Alternatively, the sweeping means could operate over only a portion of the length of the weir, the surface tension still being sufficient to ensure that surface solids are drawn into the area of operation of the sweeping means.

The sweeping means could take a variety of forms. For instance the sweeping means might comprise means for blowing air (or some other gas), or water jets, at the surface of the liquid to push solids floating on the surface over the edge of the weir.

However, in preferred embodiments of the invention the sweeping means comprises at least one sweeping member which repeatedly dips into the surface of the liquid to push solids over the weir.

For instance, the or each sweeping member may be repeatedly dipped into the surface of the liquid at a first location in said area adjacent to the weir, moved within the surface towards the weir to push solids over the weir, and withdrawn from the surface of the liquid and returned to said first location for re-introduction into the surface of the liquid to repeat the cycle.

The sweeping means may thus effectively function as a paddle wheel.

The or each sweeping member may take a variety of forms and is preferably an elongate member which extends along substantially the entire length of the sweeping means.

For instance, in one preferred embodiment of the invention the sweeping means comprises a rotating shaft extending above the weir along its length, and at least one sweeping member mounted to the shaft for rotation about the shaft as the shaft itself rotates. Preferably two sweeping members are mounted on diametrically opposed sides of the shaft.

Possible alternative arrangements include a single sweeping member which 'spirals' about a rotating shaft, or replacing the or each elongate sweeping member with a plurality of smaller sweeping members.

The or each sweeping member preferably comprises a brush but it will be appreciated that other arrangements, such as plain bars or rubber strips, might also be used.

According to a second aspect of the present invention there is provided apparatus for separating floating solids from a volume of liquid, comprising a container for holding said volume of liquid, a weir defined by one edge of the container, and means for sweeping surface floating solids over the weir, wherein operation of said sweeping means is confined to an area of the liquid surface adjacent to said weir.

Preferably the weir has a surface portion lying beneath the surface of the liquid and a raised edge extending along its length which rises above the surface level of the liquid, wherein said sweeping means sweeps surface floating solids across said surface of the weir and over the weir edge. For instance the weir may be what is conventionally termed a "beach" weir.

The sweeping means may comprise at least one sweeping member which repeatably dips into the surface of the liquid into contact with said surface of the weir and then sweeps across the said surface of the weir to push solids over the weir edge.

The sweeping means may, for example, be as described above in relation to the first aspect of the invention According to a third aspect of the present invention there is provided a method of separating floating solids from a volume of liquid contained within a container an edge of which forms a weir, the method comprising sweeping surface floating solids over the weir from a region confined to an area of the liquid surface adjacent to said weir, surface floating solids being continually drawn into said region by tension in the surface of the liquid.

A specific embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a part-sectioned perspective view of apparatus in accordance with the present invention; and Fig. 2 is a sectional end view illustrating a detail of the apparatus of Fig. 1.

Referring to Fig. 1, this illustrates apparatus in accordance with the present invention fitted to an otherwise conventional dissolved air flotation tank (only one end of which is shown) used in the treatment of potable water. The tank comprises a flotation chamber 1 defined between tank side walls 2 and a beach weir 3. The surface level of water within the chamber 1 is indicated by the chain - dot lines 4. A sludge collection trough 5 is defined at one end of the tank between the weir 3 and tank end wall 6.

A shaft 7 (only a short length of which is shown in Fig. 1) extends across the width of the tank above the "plateau" of the beach weir 3. The shaft 7 is mounted for rotation between the side walls 2.

Two elongate brush members 8 are mounted to the shaft 7 so that they extend across the width of the chamber 1 parallel to the shaft 7. The brush members 8 are mounted to the shaft 7 by way of mounting brackets 9 which are spaced apart along the length of the shaft 7. Only two mounting brackets 9 are shown in Fig. 1 but it will be appreciated that any number may be provided as necessary to adequately support the brush members 8 across the width of the tank.

The construction of the brush members 8 and mounting brackets 9 is best seen from Fig. 2 which is a cross sectional view looking in the direction of arrow A on Fig.

1. Each brush member 8 comprises an angled support bar 8a which supports a brush 8b such that each brush extends in a radial direction away from the shaft 7. The brush members 8 are secured to the shaft 7 by a two part mounting bracket 9, each part of which includes a hemi-cylindrical proportion 9a and a radial support arm 9b to which the support bars 8a of the brush members 8 are bolted. The hemi-cylindrical portion 9a are fitted around the shaft 7 and bolted together by bolts 10.

In operation, the air flotation is performed in a conventional manner so that a sludge comprising foamed particles of sand and silt floats to the surface of the water within the flotation chamber 1 forming a surface foam or sludge 4a. However, the shaft 7 and brush members 8 replace the scraper mechanisms conventionally used.

The shaft 7 is rotated about its axis (for instance by a motor and gear system positioned outside the tank and not illustrated) in the direction as shown by the arrows in both Figs. 1 and Fig. 2. The brush members 8 are thus in turn swept across the beach weir 3 so as to push surface sludge 4a over the lip 3a of the weir and into the sludge collection trough 5. This also mobilises the surface floating sludge 4a, so that sludge 4a is continually drawn across the surface of the water towards the weir 3 from where it is swept into the trough 5.

It has been found, in practice, that this system is as effective at removing sludge from the surface of water within the tank 1 as a conventional system which sweeps brushes across the entire surface. The performance is thus comparable with conventional systems but the number of components and moving parts required is substantially reduced. This is turn reduces not only the cost of the overall system but also the cost of installation, maintenance and, when necessary, replacement of component parts. Installation and repair time is also greatly reduced.

Moreover, the apparatus described above can readily be retrofitted to existing DAF plants simply by fitting the brush members 8 to the existing primary drive shaft of conventional systems and dispensing with the chain/chainwheel system and additional brushes incorporated in such systems. There is no need to change the drive mechanism itself, since the apparatus described above need only be rotated at the same speed as the drive shafts in conventional systems to give compatible performance.

It will be appreciated that many modifications could be made to the detail of the apparatus described above. For instance, the exact structure of the brush members and their manner of mounting to the shaft 7 could be varied considerably. Similarly, details of the tank and weir etc. may also vary. For instance, rather than a 'flat top the weir top could have an arcuate profile corresponding to the swept area of the brush members 8 as they rotate, so that the ends of the brushes remain in contact with the weir as they pass over the weir.

Moreover, although operation is most effective when a beach weir is used (so that the brushes are drawn across the surface of the "plateau" of the weir) other forms of weir could be used. For instance, the weir 3 could be replaced by a simple wall over which the sludge is swept.

Other possible modifications to the detailed structure of the apparatus described above will be readily apparent to the appropriately skilled person.

Claims

1. Apparatus for separating floating solids from a volume of liquid contained within a container an edge of which forms a weir, the apparatus comprising means for sweeping surface floating solids over the weir, wherein operation of said sweeping means is confined to an area of the liquid surface adjacent to said weir.

2. Apparatus according to claim 1, wherein said sweeping means extends along substantially the entire length of the weir.

3. Apparatus according to claim 1 or claim 2, wherein said sweeping means comprises at least one sweeping member which repeatedly dips into the surface of the liquid to push solids over the weir.

4. Apparatus according to claim 3, wherein the or each sweeping member is repeatedly dipped into the surface of the liquid at a first location in said area adjacent to the weir, moved within the surface towards the weir to push solids over the weir, and withdrawn from the surface of the liquid and returned to said first location for reintroduction into the surface of the liquid to repeat the cycle.

5. Apparatus according to claim 3 or claim 4, wherein the sweeping member effectively functions as a paddle wheel.

6. Apparatus according to any one of claims 3 to 5, wherein the or each sweeping member is an elongate member which extends along substantially the entire length of the sweeping means.

7. Apparatus according to any one of claims 3 to 6, wherein the sweeping means comprises a rotating shaft extending above the weir along its length, and at least one sweeping member mounted to the shaft for rotation about the shaft as the shaft itself rotates.

8. Apparatus according to claim 7, comprising two sweeping members mounted on diametrically opposed sides of the shaft.

9. Apparatus according to any one of claims 3 to 8, wherein the or each sweeping member comprises a brush.

10. Apparatus for separating floating solids from a volume of liquid; comprising a container for holding said volume of liquid, a weir defined by one edge of the container, and means for sweeping surface floating solids over the weir, wherein operation of said sweeping means is confined to an area of the liquid surface adjacent to said weir.

11. Apparatus according to claim 10, wherein the weir has a surface portion lying beneath the surface of the liquid and a raised edge extending along its length which rises above the surface level of the liquid, wherein said sweeping means sweeps surface floating solids across said surface of the weir and over the weir edge.

12. Apparatus according to claim 11, wherein said sweeping means comprises at least one sweeping member which repeatably dips into the surface of the liquid into contact with said surface of the weir and then sweeps across the said surface of the weir to push solids over the weir edge.

13. Apparatus according to any one of claims 10 to 12, comprising sweeping means according to any one of claims 1 to 9.

14. A water treatment plant comprising apparatus according to any preceding claim.

15. A method of separating floating solids from a volume of liquid contained within a container an edge of which forms a weir, the method comprising sweeping surface floating solids over the weir from a region confined to an area of the liquid surface adjacent to said weir, surface floating solids being continually drawn into said region by tension in the surface of the liquid.

16. Apparatus for separating floating solids from a volume of liquid, substantially as hereinbefore described, with reference to the accompanying drawings.

17. A method for separating floating solids from a volume of liquid, substantially as hereinbefore described, with reference to the accompanying drawings.