NZ330480A - Device, for admitting backwash fluid to a filter bed, comprising a member having a plurality of elongate orifices - Google Patents

Description

New Zealand No. 330480 International No. PCT/ TO BE ENTERED AFTER ACCEPTANCE AND PUBLICATION Priority dates: 13.05.1998; Complete Specification Filed: 21.05.1998 Classification:^) B01 D24/00.26; B01D101:04 Publication date: 24 September 1998 Journal No.: 1432 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION Title of Invention: Filters Name, address and nationality of applicant(s) as in international application form: THAMES WATER UTILITIES LIMITED, A United Kingdom company of Nugent House, Vastern Road, Reading, Berks RG1 8DB, United Kingdom 330480 OF N2 —OfFicE] 2 1 MAY 1998 £§££iVED Patents Form No. 5 Our Ref: MH502238 NEW ZEALAND PATENTS ACT 1 953 COMPLETE SPECIFICATION FILTERS We, THAMES WATER UTILITIES LIMITED, a United Kingdom company of Nugent House, Vastern Road, Reading, Berks RG1 8DB, United Kingdom, United Kingdom 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: MH:SU:PT0542449 1 3304 1A FILTERS The invention relates to a filter, particularly to a slow sand filler for removing impurities from water in for example water treatment works.

Granular media filters as generally used in water treatment comprise a bed of sand or other granular media, either alone or in combination, contained within a tank or pressure vessel and supported on a porous floor system connected to an outlet.

Water to be filtered is usually fed in at the top, flows through the porous granular bed and out through the floor, or underdrain, systems. The latter must be able to support the dead weight of the medium as well as the pressure loss resulting form the flow and also it must be porous to permit the water to pass while retaining the granular medium in position without passing to the outlet.

In addition, in most types of filter accumulated dirt is removed by passing water in the reverse direction at a higher rate than the forward flow. The distribution of this backwash water is in fact a more critical design feature than the forward flow.

There must be a minimum pressure loss at the point of discharge into the bed to achieve the desired evenness of flow across the bed. There are two factors that have to be considered, firstly the uniformity of flow into an empty filter through the ducts or pipes of the floor or underdrain, and secondly the control of the flow into the bed itself which has an unstable characteristic and can break down from even fluidisation into a situation known as spouting or 33 0 2 boiling.

Indeed the rapid sand filtration process for the purification of water was invented early this century and is still used in a broadly similar form. Water treated with chemicals to collect contaminants into tiny particles is passed down through a bed of sand and the contaminants are retained by the sand allowing clean water to be collected in an underdrain system beneath the sand.

A variety of methods have been used to avoid particles of sand being carried down into the underdrain system, ranging from layers of gravel of decreasing size above the holes into the underdrains to the underdrains fitted with nozzles. A further general type of underdrain system comprises a plenum floor of porous material which allows water to flow through the pores.

It is common practice in washing filters to use gas, usually air to assist washing either before water or simultaneously with it. The same underdrain system should therefore be capable of distributing this air uniformly in the same way as with water.

Sand filters are cleaned of accumulated contaminant particles on a batch basis, using a backwashing process. The backwashing process is primarily a reverse flow of water up through the sand which carries the accumulated contaminants away to waste. In many filters, this process is improved by a flow of air up through the bed of sand which further agitates the sand grains and facilitates the removal of the contaminants.

In some designs of filters, the air and the backwash water are introduced 330480 3 concurrently through individual nozzles for distributing the upwards flows of water and air respectively. In the majority of existing filters, the air flow precedes the water flow, with the air bubbles serving to loosen adhering contaminants for the water flow to carry away.

Some modern systems utilise air and water distributed concurrently into the base of the sand bed, providing a combined air and water backwash. This is more effective than the separate air and water flows but requires special provisions to maintain the uniform distribution of air and water per unit area of filter floor. In some systems the air and water are combined in special nozzles below the sand. In other systems, the air and water are distributed separately and allowed to mingle close to the bottom of the sand bed so that virtually all of the sand bed is subject to a mixture of rising air and water.

Where a separate air distribution system is used, then a key factor is that the minimum aperture size through which the air is introduced must be a small proportion of the minimum selected sand grain size to prevent ingress of the sand.

The second requirement for the means of introducing air is that the amount of air introduced to the bottom of the filter must be almost constant per unit area of filter floor, so that a similar amount of air rises up through each portion of the sand in the filter. Were this not the case, then it would be necessary to introduce excessive air into some parts of the filter to ensure an adequate flow to those parts receiving the least. If sufficient air is not supplied, then the sand would become clogged in those parts receiving inadequate air and the clogging would tend to propagate further into the sand bed, leading to failure of the process. 330480 4 In many designs the air and water are fed through the same ducts or pipes but the rates are then limited otherwise maldistribution occurs. A common alternative is the use of suspended floors with a plenum chamber below. The depth of the latter guarantees low velocities and stable uniform distribution but with the penalty of additional tank depth and often additional excavation.

To provide the necessary headlosses for distribution and to retain the medium nozzle strainer devices are extensively used. These add to the cost of the underdrain and also can be damaged, in some cases allowing the medium into the plenum or lateral pipes below.

Another prior system involves perforated lateral pipes which are buried in graded gravel of decreasing size from bottom to top. Hitherto it has not been possible to place the working media around the lateral without using gravel, in an economical manner, because of cost limitations and the difficulty of forming fine orifices in long lengths of pipe.

It is possible to operate with air and water distributed in sequence from the same lateral pipe, but difficult with lir and water simultaneously over the lengths required for large filters as used in public water supply.

It is accordingly an object of the invention to seek to mitigate these disadvantages.

A device, system and slow sand filter embodying the invention are hereinafter described by way of example, with reference to the accompanying drawings.

Fig. 1 is a schematic perspective view of a slow sand filter according to the 330480 invention; Fig. 2 is an elevational view of part of a member according to the invention, to a larger scale than Fig. 1; Fig. 3 is an enlarged view of detail 'A' of Fig. 2; and Fig. 4 is an enlarged view of detail 'B' of Fig. 3.

Referrirg to the drawings there is shown in Fig. 1 a slow sand filter 1 which has a backwashing system 2 comprising a plurality of members in the form of slotted stainless steel tubes 3 extending laterally of the filter below the medium, sand in the embodiment, each being substantially parallel and each being connected to a manifold or air supply pipe 5 by a respective supply means or downpipe 6.

Each tube 3 has a plurality of longitudinally extending slots 7, through the circumference of the tubes, the slots being in the embodiment 0.25mm in thickness and being arranged in groups of three at different "levels", the two "upper" 7', 7" ones as viewed and as in use being angularly spaced by 120°, and the lower one 7'" being in use on the floor of the filter. The slots are preferably formed by a laser cutting device to provide uniformity of thickness and length, and with little or no swarf.

The filter includes water nozzles 8.

Thus, in the invention fine slit lateral pipes are used to perform all the necessary functions. The slots, or slits, have a width less then that of the 330480 6 finest fraction of the granular medium so that super-imposed gravel layers are no longer necessary. These lateral pipes are laid directly on the structural floor of the filter to distribute and collect water. They may also be used for the distribution of air and water in sequence. Where air is to be applied simultaneously separate systems of air and water distribution lateral pipes may be laid between each other so that alternate pipes admit water and air respectively during washing.

In an alternative separate air lateral systems are laid above or in between conventional lateral systems, or above gravel packing layers and in conjunction with nozzles which are then used for distribution of water only. One advantage of the separate lateral system is its suitability for retrofit conversions from separate sequential air and water to simultaneous air and water.

A particular feature of the fine slit or slotted pipe is the absence of separate components which as well as adding to cost can also become damaged. The length and width of the slits or slots and their spacing are as desired and the pressure loss is selected to achieve the intended accuracy of distribution. Because there are no intrusions into the pipe, as with many types of nozzle, the required accuracy can be achieved over longer lengths of pipe.

The avoidance of additional components and the labour involved in fitting them also reduces costs.

In one form pipes for use in filter underdrains may be slit with a fine slitting saw, but below 0.4mm these become rather fragile. Also sawing may create swarf which may block the slits unless considerable care is exercised. 330480 7 In a preferred form laser cuts are used to achieve finer slits or slots. In this case the cut has a fused edge and a stringy swarf is not produced.

It is also preferred that the slits or slots be longitudinal (in contrast to common drainage pipes which are transverse) as in this direction the bending strength of the pipe is not compromised by the slit to the same extent.

It is inevitable that air lateral pipes will fill with water while the filter is in service. Laterals for water likewise may occasionally receive some air (e.g. on start up). Slits are therefore arranged in more than one line round the circumference of the pipe to permit filling and emptying, and the pressure loss through the slits during backwashing is always well in excess of the pressure difference corresponding to the head of water of the diameter.

In addition to laser cutting, other techniques involving fusion of the pipe material may be used.

By way of example and without restricting the scope of the invention, pipes used for water distribution may have a diameter of 50 to 150mm depending on the length and specific flow rate required, in lengths of several metres.

For air, typical diameters are 20 to 30mm. Slits may be 0.15 - 0.3mm width and of lengths not exceeding the longitudinal pitch, but as required by the pressure loss calculations. Backwash water and air flow rates both range typically from 4 to 20 litres/m2/second. The spacing between lateral pipes and the pitch of the slits along the pipes is usually between 150mm and 250mm, but may be outside these limits. 330480 8 It is usual to connect lateral pipes of such sizes to a larger diameter "header" or feed pipe or pipes which penetrate the outer wall of the tank or vessel. Such headers can cast a shadow on the bed and cause the medium over them to be washed less efficiently. It is a further feature of the invention that such headers may also be cut in a similar way to distribute air, and so eliminate such shadows.

In the above it has been assumed that the slits are formed in a circular section pipe. Rectangular or square ducts may also be slit to allow air or water to be distributed. This may be preferred in the case of headers, which can then be fitted against the wall or bottom corner of the vessel.

Lastly for distribution and collection of water the fine slits or slots described above may be arranged as a matrix in flat or corrugated floor panels. In the latter case rows of slits may be provided at different heights in the side of the corrugations and air may be distributed from below into the inverted channels of the corrugated sheet and thence via the upper slits into the granular media above. The corrugations then act as a set of lateral pipes. However the pressure loss for air will be limited by the height of the corrugations. Also, the slits or slots may be formed in a rollable material, which when rolled into a tubular form provide a device embodying the invention.

Claims (22)

1. A device for admitting a backwash fluid to filter medium of a filter bed, comprising a member having a plurality of elongate blind through orifices adapted to allow passage therethrough of the fluid but not the media.

2. A device according to Claim 1, the elongate orifices being through orifices each comprising a slot. each

3. A device according to Claim 2, the slotslhaving a width of less than 0.5mm.

4. A device according to Claim 3, the width being between 0.15 -0.3mm.

5. A device according to Claim 4, each slot having a width of 0.25mm. one

6. A device according to any/of Claims 2 to 5, the member comprising a tube and the slots being directed longitudinally of the tube. one

7. A device according to any^of Claims 2 to 5, the member being a corrugated member, and the slots being in walls of the corrugation and directed longitudinally thereof.

8. A device according to Claim 6 or Claim 7, the slots being arranged in groups of a plurality of slots. 10

9. A device according to Claim 8, the slots being arranged in groups of three.

10. A device according to Claim 9, the length of each slot being not greater than the longitudinal pitch along the pipe.

11. A device according to Claim 9, the slots being staggered along the length of the member. one

12. A device according to anytaf Claims 2 to 11, the slots beir.g formed by a laser.

13. A device according to Claim 12, the orifices comprising of stainless steel. one

14. A device according to any^of Claims 2 to 12, the member including a series of nozzles.

15. A device for admitting a backwash fluid to a filter medium of a filter bed, substantially as hereinbefore described with reference to the accompanying drawings.

16. A system for backwashing a filter medium of a filter bed, comprising a plurality of orifices according to any previous claim, extending substantially parallel to one another and each being connected with a fluid supply means.

17. A system according to Claim 16, the fluid supply means comprising I 11 a supply pipe for each member and a common manifold to which each supply pipe is connected.

18. A system for backwashing a filter medium of a further filter bed, substantially as hereinbefore described, with reference to the accompanying drawings.

19. A slow sand filter, comprising a system according to any of Claims 16 to 18.

20. A slow sand filter according to Claim 18, the members extending laterally of the filter.

21. A slow sand filter according to Claim 20, the members being positioned at the base of the filter.

22. A slow sand filter, substantially as hereinbefore described, with reference to the accompanying drawings. END OF CLAIMS