WO1992002286A1

WO1992002286A1 - Method of treating liquids and apparatus therefor

Info

Publication number: WO1992002286A1
Application number: PCT/GB1991/001313
Authority: WO
Inventors: Peter Wraith; Kenneth John Hodkinson
Original assignee: Welsh Water Enterprises Ltd
Current assignee: Welsh Water Enterprises Ltd
Priority date: 1990-08-06
Filing date: 1991-08-01
Publication date: 1992-02-20
Anticipated expiration: 1993-02-06
Also published as: GB2263073B; GB2263073A; AU8294791A; GB9301820D0; GB9017172D0; IE69318B1; IE912776A1

Abstract

A method of treating liquids comprises passing the liquid upwardly through a vessel (7) having a permeable wall (9, 10) at each end and containing a buoyant granular filtration medium (8). During filtration, the filtration medium (8) is closely packed against the upper wall (9) of the vessel (7). The filtration medium (8) is cleaned intermittently by draining the liquid from the vessel (7) and back-washing the filtration medium (8) whilst the filtration medium is lying loosely on the bottom (10) of the vessel (7).

Description

Method of treating liquids and apparatus therefor

The present invention relates to a method of treating liquids and an apparatus therefor.

In a known method of treating liquids, the liquid is passed upwardly through a buoyant granular filtration medium. The filtration medium is contained in a vessel having a liquid inlet at its lower end and a perforated plate at its upper end which allows the liquid to flow through whilst retaining the filtration medium. The buoyancy of the filtration medium causes it to pack closely against the perforated plate, so that it forms an effective filter in which solid matter suspended in the liquid is trapped.

From time to time it is necessary to clean the filtration medium, and this is done by back-washing. Clean liquid is introduced into the top of the vessel and flows downwardly through the filtration medium to an outlet in the bottom of the tank. At the same time the granules are agitated by a stirrer device and this movement of the granules dislodges the matter adhering to them. The matter removed from the granules is carried away by the back- washing fluid. This cleaning process is not, however, very efficient, and requires the consumption of a large quntity of liquid. According to the present invention, there is provided a method of treating liquids by passing the liquid through a vessel having a permeable wall ab each end and containing a buoyant granular filtration medium so that, during filtration, the filtration medium is closely packed against the upper wall of the vessel, wherein the filtra¬ tion medium is cleaned intermittently by draining the liquid from the vessel and back-washing the filtration medium whilst the filtration medium is lying loosely on the bottom of the vessel. Draining the liquid from the vessel before back- washing the granular filtration medium makes the back- washing process much more efficient. Since the granules are no longer floating in the liquid but are lying on the lower wall of the vessel, the collisions between them, and with the lower wall, as they are washed are considerably more violent than in the known method and the adhering matter is therefore removed more quickly. The back-washing liquid also has a percussive effect on the granules, rather than simply flowing over them, and so helps to remove the accumulated matter. Further, since the back-washing liquid can be delivered as a spray rather than as a continuous flow, only a relatively small quantity of washing liquid is require .

Preferably, the vessel is located in a liquid tank and, during back-washing, the liquid is drained from the vessel by lowering the liquid level in the tank. Alterna¬ tively, the vessel is located in a liquid tank and, during back-washing, the liquid is drained from the vessel by raising the vessel out of "t e tank.

The liquid tank is preferably a settlement tank.

The filtration medium may be back-washed by being sprayed with clean liquid.

Advantageously, the vessel has permeable upper and lower walls and, during filtration, the fluid flows upwardly through the vessel.

During filtration, the liquid may be caused to flow upwards through the vessel by gravity.

The present inventon further provides a method of treating sewage effluent, the method being as described in any one of the preceding paragraphs.

Advantagec , the effluent is treated biologically prior to passing through the vessel.

The present invention yet further provides an appar¬ atus for treating liquids, the apparatus comprising a vessel having a permeable wall at each end and containing a buoyant granular filtration medium, the arrangement being such that when liquid flows through the vessel, the filtration medium is packed closely against the upper wall of the vessel and, when the liquid is drained from the vesse^"" the filtration medium lies loosely on the bottom wall oi essel. Advantageously, the vessel is located in a liquid tank having a liquid inlet and a liquid outlet, the arrangement being such that when liquid flows from the inlet to the outlet, it flows through the vessel.

The apparatus preferably includes means for back- washing the filtration medium when the medium is lying loosely on the bottom wall of the vessel. The back-washing means preferably comprises a spraying device arranged above the filtering apparatus. Alternatively, the back-washing means may include a sparge pipe.

Advantageously, the tank is divided into two portions, the vessel occupying the portion of the tank adjacent the outlet.

The vessel may have permeable upper and lower walls, the arrangement being such that, during filtration, liquid flows upwardly through the vessel.

The tank may include means for draining liquid from the tank.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, of which:

Figure 1 is a side view, in cross-section, of a liquid treatment apparatus, and

Figure 2 is a plan view of the apparatus.

The liquid treatment apparatus comprises a rectangular clarifier tank 1 of conventional design, having a liquid inlet weir 2 at one end and a liquid outlet weir 3, at a slightly lower level, at the other end. The tank has a sloping bottom 4, leading to a sludge outlet 5, which is provided with a valve (not shown). Liquid enters the tank by flowing over the inlet weir 2, and leaves via the outlet weir 3. The tank is of sufficient size to ensure that the liquid stands for a substantial time before leaving the tank, thereby allowing much of the solid matter suspended in the liquid to fall to the botttom of the tank as sludge.

Located within the tank is a filter apparatus compris- ing an impermeable barrier 6 that extends across the width of the tank dividing it into two portions, and a pair of clarifier units, indicated generally by the reference numeral 7, that occupy the portion of the tank between the barrier 6 and the outlet weir 3. The barrier 6 extends upwardly slightly above the height of the outlet weir 3, and downwards below the bottom of the clarifier units, so that liquid flowing from the inlet weir to the outlet weir has to flow underneath the barrier 6 and then upwards through the clarifier units 7. Each clarifier unit 7 consists of a vessel containing a buoyant granular filtration medium 8 such as, for example, a multitude of polystyrene beads. The granules of the filtration medium may be of various different sizes: for example, the medium may consist of a mixture of beads having diameters in the range 5mm to 20mm. The upper and lower walls 9, 10 of the vessel are permeable to liquids, but impermeable to the filtraton medium: they may, for example, be perforated plates or meshes, the apertures of which are smaller than the size of the beads of the filtraton medium.

The upper walls 9 of the clarifier units 7 are located slightly below the level of the outlet weir 3 so that, when the tank 1 is full of liquid, the clarifier units are completely submerged in the liquid. Since the beads 8 are buoyant, when the tank is full of liquid, they rise to the top of the vessels and pack closely against the upper walls 9, forming a dense filtration medium.

In use, the liquid to be treated flows into the tank 1 over the inlet weir 2, and flows slowly towards the outlet weir 3. During this time, larger particles suspended in the liquid fall to the bottom of the tank 1 as sludge. As the liquid moves through the tank towards the outlet weir 3, as indicated by the arrows 11, it flows downwards underneath the impermeable barrier 6 and then upwards through the clarifier units 7. In passing through the closely-packed filtration medium 8, any solid particles remaining in the liquid are attracted to the medium and are trapped by it, thereby clarifying the liquid. The clarified liquid then flows out and over the top of the clarifier units to the outlet weir 3.

Eventually, after the apparatus has been in use for a considerable period of time, it will be necessary to clean the trapped matter from the filtration medium. First, the valve at the sludge outlet 5 is opened, allowing the liquid an^1,1 any sludge that has accumulated in the bottom of the tank 1 to flow out. Draining the liquid from the tank causes the beads 1 to fall to the bottom of the vessel, onto the lower permeable walls 10. The beads are then back-washed by spraying them with clean liquid, such as clean water, either from a fixed spray system mounted above the clarifier units, or by means of a hose. Alternatively, an internal sparge pipe π y be provided at the bottom of the vessel to wash the beads when they are lying loosely around it. If necessary, a pump can be provided to increase the back-washing pressure. The matter that is washed off the beads falls into the tank with the washing liquid, and out through the sludge outlet.

Since, during the washing process, the beads lie loosely on the lower wall of the vessel and can move about under the influence of the spray, the backwashing process is very effective, removing the trapped matter very quickly and easily. Further, the fact that the clarifier units can be exposed for cleaning, simply by reducing the liquid level in the tank, means that they can be washed in situ. without having to remove the units from the tank. Alternatively, however, if it is desirable to clean the clarifier units without draining the tank, they can be raised out of the liquid, so that the liquid level is below the lower wall of the clarifier vessels.

Various modifications of the apparatus are, of course, possible: for example, the tank 1 can be circular instead of rectangular, with the liquid flowing radially outwards from a central inlet to a peripheral outlet weir. The clarifier units can installed as inserts or retrofits on existing clarifier units, and can be installed either as single or multiple units. It is also possible for the vessels to be mounted at an angle or on their sides so that, during filtration, the liquid does not flow upwardly through the filtration medium. The filtration medium would still, however, be cleaned by the same method, that is, by draining the liquid from the vessels and backwashing the medium.

Although the apparatus is designed particularly for the clarification (or "polishing") of sewage effluents after they have been treated biologically and prior to their discharge into a river or the sea, it can also be used for filtering suspended solid matter from other liquids. If only a coarser degree of filtration is required, larger beads may be used for the filtration medium, thereby allowing liquids to flow through the apparatus at a greater rate.

Claims

Claims :

1. A method of treating liquids by passing the liquid upwardly through a vessel having a permeable wall at each end and containing a buoyant granular filtration medium so that, during filtration, the filtration medium is closely packed against the upper wall of the vessel, wherein the filtration medium is cleaned intermittently by draining the liquid from the vessel and back-washing the filtration medium whilst the filtration medium is lying loosely on the bottom of the vessel.

2. A method according to claim 1, wherein the vessel is located in a liquid tank and, during back-washing, the liquid is drained from the vessel by lowering the liquid level in the tank.

3. A method according to claim 1, wherein the vessel is located in a liquid tank and, during back-washing, the liquid is drained from the vessel by raising the vessel out of the tank.

4. A method according to claim 2 or claim 3, wherein the liquid tank is a settlement tank.

5. A method according to any preceding claim, wherein the filtration medium is back-washed by being sprayed from with clean liquid.

6. A method according to any preceding claim, wherein the vessel has permeable upper and lower walls and, during filtration, the fluid flows upwardly through the vessel.

7 . A method according to claim 6, in which, during filtration, the liquid is caused to flow upwards through the vessel by gravity.

8. A method of treating a liquid, the method being substantially as described herein with reference to, and as shown in, the accompanying drawings.

9. A method of treating sewage effluent, the method being as claimed in any one of the preceding claims.

10. A method according to claim 9, wherein the effluent is treated biologically prior to passing through the vessel.

11. An apparatus for treati liquids, the apparatus comprising a vessel having a permeable wall at each end and containing a buoyant granular filtration medium, the arrangement being such that when liquid flows through the vessel, the filtration medium is packed closely against the upper wall of the vessel and, when the liquid is drained from the vessel, the filtration medium lies loosely on the bottom wall of vessel.

12. An apparatus according to claim 11, in which the vessel is located in a liquid tank having a liquid inlet and a liquid outlet, the arrangement being such that w en liquid flows from the inlet to the outlet, it flows through the vessel.

13. An apparatus according to claim 11 or claim 12, including means for back-washing the filtration medium when the medium is lying loosely on the bottom wall of the vessel.

14. An apparatus according to claim 13, wherein the back- washing means comprises a spraying device arranged above the filtering apparatus.

15. An apparatus according to claim 13, wherein the back- washing means includes an internal sparge pipe.

16. An apparatus according to any one of claims 11 to 15, wherein the tank is divided into two portions, the vessel occupying the portion of the tank adjacent the outlet.

17. An apparatus according to any one of claims 11 to 16, in which the vessel has permeable upper and lower walls, the arrangement being such that, during filtration, liquid flows upwardly through the vessel.

18. An apparatus according to any one of claims 11 to 17, wherein the tank includes means for draining liquid from the tank.

19. An apparatus for treating liquids, the apparatus being substantially as described hereinwith reference to, and as illustrated by, the accompanying drawings.