GB1594358A - Clarifier - Google Patents

Description

(54) IMPROVED CLARIFIER (71) We, PATERSON CANDY INTERNA TIONAL LIMITED, of 21, The Mall, Ealing, London W5 2PU, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a clarifier.

Sludge blanket clarifiers have been used for many years to purify water for drinking or industrial purposes. Such a clarifier comprises essentially a tank, a raw water distribution system, a clarified water decanting system and a sludge concentrating system.

The distribution system directs the incoming water in single, or multiple, jets downwards towards the bottom of the tank. These jets prevent the suspended floc blanket from settling on the floor and at the same time provide additional energy to mix the incoming water with the floc blanket. The raw water is dosed with a coagulant, such as aluminium sulphate which hydrolyses to form a microfloc in the incoming pipework and channels. This microfloc is then filtered out as it rises through the floc blanket. The latter is in essence a fluidised bed of floc particles. The total solids inventory in this floc blanket tends to increase as it collects suspended solids from the incoming water.

It is necessary, furthermore, to maintain a definite space below the decanting system so that surges in the level of the blanket, for instance when the flow rate is changed, do not lead to suspended matter passing over into decanting troughs. It is necessary, therefore, to provide sludge concentrators in the form of pyramidal or conical hoppers devoid of mechanical aids. The rim of these concentrators is set close to the intended upper limit of the floc blanket so that excess suspended matter spills over into the concentrators and is drawn off. On the other hand, if the bleed from the concentrators is excessive only clear water will be discharged. It is then not possible to draw off excess blanket and so denude the process of the material on which the filtering action depends.

In a normal situation the sludge bleed from a typical water might be per cent of the nett throughput and the concentration of solids bled off between 0.2 and 1.5 per cent by weight, at which concentration the resultant sludge is full flowing and does not 'hang up' on the hopper walls.

In a complete treatment plant, particularly where land is expensive, it is common to treat the sludge with a polyelectrolyte and feed it forward to sludge thickeners which comprise a conical or flat bottomed circular basin having a slowly rotating scraper system which draws the sludge towards a central discharge point and, furthermore, provides a slow stirring action which enables the sludge to consolidate. The thickened sludge may be passed forward to dewatering equipment, such as a filter press. The concentration of sludge passing forward to the filter press might be 2 to 10 per cent by weight, and thus a considerable volume of clarified water is decanted from the sludge thickener and must be returned to the inlet end of the plant.

Normally the achievement of the above concentrations is dependent upon the presence of a polyelectrolyte. Considerable care has to be exercised in transferring the sludge from the clarifier to the thickener because floc damage may occur if local velocities are too high, or if pumps are used to transfer sludge from sumps through a mixing system and on to the thickener.

It is known to incorporate a thickener underneath a set of inclined plates in a tank in which the sludge flow pattern is essentially of the single path variety, where floc is generated mechanically and is separated and thickened, without the intervention of a sludge blanket. Furthermore, a clarifier is known, which is a solids recirculation device, and uses a scraper to convey the sludge, settling from the reaction zone onto the floor, back towards the centre. There is a central discharge well in the base of the tank which is usually provided with scrapers and is described as a thickening zone. However, in both of these clarifiers sludge will enter the thickening zone preferentially.

According to one aspect of the invention there is provided a clarifier comprising a tank having an inlet for raw water, decanting means arranged to decant off clarified water above a suspended floc blanket, in use. in the tank, and a thickener in the tank. the thickener having a vertical side wall (or walls) which acts as a weir over which excess floc is decanted. in use, into the thickener, a scraper which is rotatable about a vertical axis and which extends to or almost to the vertical side wall (or walls) and a sludge outlet.

According to a further aspect of the present invention, there is provided a method of clarifying water comprising feeding raw water into a tank to form a floc blanket therein, decanting off clarified water above the floc blanket, decanting off excess floc into a thickener in the tank. the thickener having a vertical side wall (or walls) and a scraper which is rotatable about a vertical axis and which extends to or almost to the vertical side wall (or walls). forming a sludge in the thickener, and conveying the sludge to a sludge outlet of the thickener.

Thus, the usual sludge concentrators, sludge bleed valves, sludge sumps, transfer pumps, etc., are entirely eliminated together with the decanted water return system and, moreover, the entire concrete involved in the usual external sludge thickener is eliminated apart from the walls of the weir which define the area of the thickener within the clarifier.

Apart from the savings in cost, it has been discovered that the particular arrangement according to the invention provides significant process advantages in that the concentration of sludge collected in the thickener is significantly greater (perhaps 2 to 4 times) than the concentration normally bled off from a clarifier and indeed comparable to the sludge normally obtained from a sludge thickener but without necessarily the aid of polyelectrolytes. It is believed that this additional benefit is obtained by virtue of the gentle manner in which sludge is transferred from the sludge blanket into the thickener there being no significant shear in valves or pumps. Furthermore, it is evident that the thickener provides a considerable storage volume for concentrated sludge which at values of several per cent enables several days worth of sludge production to be held within the clarifier so that problems of blanket control and bleed are avoided.

Because of the enhanced thickening action which is obtained when the floc is transferred to the thickener, in the above described manner, polyelectrolytes are less essential. In many cases polyelectrolyte addition to the raw water will provide a floc which consolidates very effectively within the thickener.

However, if preferred, polyelectrolyte may be added separately to the thickened sludge.

The thickener may have a flat or conical bottom, and may sit on the floor of the tank or may be suspended.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawing which shows schematically a clarifier in section.

In the drawing a clarifier comprises a tank 1 having a plurality of raw-water distributors 2 supplied by raw-water channels 3. In use a suspended floc blanket is formed in the tank I and has an upper interface 4. The raw water is filtered by the floc blanket and clarified water spills over into decanting troughs 5 arranged adjacent the raw-water channels 3.

A thickener 6 is arranged in the tank I and as shown in the drawing comprises a circular vertical side wall 7. As can be seen from the drawing, the wall 7 extends upwardly to the blanket interface 4 and acts as a weir over which excess floc is decanted into the thickener 6. The decanted floc sinks downwards and forms a sludge 8. which is stirred and moved slowly to a sludge outlet 9 by a rotating scraper 10 which extends to or almost to the vertical side wall 7 as shown.

The scraper 10 is driven by a motor II and is of lattice-like structure.

If desired, the scraper 10 may be provided with a distribution pipe (not shown) for adding polyelectrolyte to the sludge.

WHAT WE CLAIM IS: 1. A clarifier comprising a tank having an inlet for raw water, decanting means arranged to decant off clarified water above a suspended floc blanket, in used, in the tank, and a thickener in the tank, the thickener having a vertical side wall (or walls) which acts as a weir over which excess floc is decanted, in use, into the thickener, a scraper which is rotatable about a vertical axis and which extends to or almost to the vertical side wall (or walls) and a sludge outlet.

2. A clarifier as claimed in Claim 1, wherein the thickener is flat-bottomed.

3. A clarifier as claimed in Claim 1, wherein the thickener has a conical bottom.

4. A clarifier as claimed in any of claims 1--3, wherein the thickener is located on the floor of the tank.

5. A clarifier as claimed in any one of claims 1--3, wherein the thickener is suspended in the tank.

6. A clarifier as claimed in any one of the preceding claims, wherein the scraper is of lattice-like structure.

7. A method of clarifying water comprising feeding raw water into a tank to form a floc blanket therein, decanting off clarified water above the floc blanket, decanting off excess floc into a thickener in the tank, the thickener having a vertical side wall (or walls) and a scraper which is rotatable about a vertical axis and which extends to or almost to the vertical side wall (or walls), forming a

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. having an inlet for raw water, decanting means arranged to decant off clarified water above a suspended floc blanket, in use. in the tank, and a thickener in the tank. the thickener having a vertical side wall (or walls) which acts as a weir over which excess floc is decanted. in use, into the thickener, a scraper which is rotatable about a vertical axis and which extends to or almost to the vertical side wall (or walls) and a sludge outlet. According to a further aspect of the present invention, there is provided a method of clarifying water comprising feeding raw water into a tank to form a floc blanket therein, decanting off clarified water above the floc blanket, decanting off excess floc into a thickener in the tank. the thickener having a vertical side wall (or walls) and a scraper which is rotatable about a vertical axis and which extends to or almost to the vertical side wall (or walls). forming a sludge in the thickener, and conveying the sludge to a sludge outlet of the thickener. Thus, the usual sludge concentrators, sludge bleed valves, sludge sumps, transfer pumps, etc., are entirely eliminated together with the decanted water return system and, moreover, the entire concrete involved in the usual external sludge thickener is eliminated apart from the walls of the weir which define the area of the thickener within the clarifier. Apart from the savings in cost, it has been discovered that the particular arrangement according to the invention provides significant process advantages in that the concentration of sludge collected in the thickener is significantly greater (perhaps 2 to 4 times) than the concentration normally bled off from a clarifier and indeed comparable to the sludge normally obtained from a sludge thickener but without necessarily the aid of polyelectrolytes. It is believed that this additional benefit is obtained by virtue of the gentle manner in which sludge is transferred from the sludge blanket into the thickener there being no significant shear in valves or pumps. Furthermore, it is evident that the thickener provides a considerable storage volume for concentrated sludge which at values of several per cent enables several days worth of sludge production to be held within the clarifier so that problems of blanket control and bleed are avoided. Because of the enhanced thickening action which is obtained when the floc is transferred to the thickener, in the above described manner, polyelectrolytes are less essential. In many cases polyelectrolyte addition to the raw water will provide a floc which consolidates very effectively within the thickener. However, if preferred, polyelectrolyte may be added separately to the thickened sludge. The thickener may have a flat or conical bottom, and may sit on the floor of the tank or may be suspended. An embodiment of the invention will now be described by way of example with reference to the accompanying drawing which shows schematically a clarifier in section. In the drawing a clarifier comprises a tank 1 having a plurality of raw-water distributors 2 supplied by raw-water channels 3. In use a suspended floc blanket is formed in the tank I and has an upper interface 4. The raw water is filtered by the floc blanket and clarified water spills over into decanting troughs 5 arranged adjacent the raw-water channels 3. A thickener 6 is arranged in the tank I and as shown in the drawing comprises a circular vertical side wall 7. As can be seen from the drawing, the wall 7 extends upwardly to the blanket interface 4 and acts as a weir over which excess floc is decanted into the thickener 6. The decanted floc sinks downwards and forms a sludge 8. which is stirred and moved slowly to a sludge outlet 9 by a rotating scraper 10 which extends to or almost to the vertical side wall 7 as shown. The scraper 10 is driven by a motor II and is of lattice-like structure. If desired, the scraper 10 may be provided with a distribution pipe (not shown) for adding polyelectrolyte to the sludge. WHAT WE CLAIM IS:

1. A clarifier comprising a tank having an inlet for raw water, decanting means arranged to decant off clarified water above a suspended floc blanket, in used, in the tank, and a thickener in the tank, the thickener having a vertical side wall (or walls) which acts as a weir over which excess floc is decanted, in use, into the thickener, a scraper which is rotatable about a vertical axis and which extends to or almost to the vertical side wall (or walls) and a sludge outlet.

2. A clarifier as claimed in Claim 1, wherein the thickener is flat-bottomed.

3. A clarifier as claimed in Claim 1, wherein the thickener has a conical bottom.

4. A clarifier as claimed in any of claims 1--3, wherein the thickener is located on the floor of the tank.

5. A clarifier as claimed in any one of claims 1--3, wherein the thickener is suspended in the tank.

6. A clarifier as claimed in any one of the preceding claims, wherein the scraper is of lattice-like structure.

7. A method of clarifying water comprising feeding raw water into a tank to form a floc blanket therein, decanting off clarified water above the floc blanket, decanting off excess floc into a thickener in the tank, the thickener having a vertical side wall (or walls) and a scraper which is rotatable about a vertical axis and which extends to or almost to the vertical side wall (or walls), forming a

sludge in the thickener, and conveying the sludge to a sludge outlet of the thickener.

8. A clarifier substantially as herein described with reference to and as shown in the accompanying drawings.

9. A method as claimed in claim 7, substantially as herein described.