Liquid purification or separation – Filter – With movable means to compress medium
Patent
1996-06-20
1997-11-25
Lithgow, Thomas M.
Liquid purification or separation
Filter
With movable means to compress medium
210335, 210489, 210446, 435 34, 435 39, 435246, 435261, 4352971, B01D 3900, B01D 2958, B01D 2978, G01N 33569
Patent
active
056908253
DESCRIPTION:
BRIEF SUMMARY
This application claims benefit of international application PCT/ GB94/02782 filed Dec. 21, 1994.
The present invention relates to methods and apparatus for filtration and has particular but not exclusive relevance to the filtration of micro-particles from large volumes of liquid in which they are present at high dilutions.
Filtration of large volumes of liquid to capture and recover small particles such as micro-organisms poses a number of problems. Membrane filters tend to clog easily and so are only suitable for relatively small volumes of liquid containing particulates. Recovery can be difficult due to physical, electrostatic, or chemical trapping of the micro-particles on the filter membrane. Depth filters are therefore used to filter large volumes, or smaller volumes in which the solids content is high. These filters clog less easily, but full recovery of captured particles is difficult because the particles are trapped deep within the filter matrix.
An example of this problem occurs in the water industry in which, typically, 1000 liters of drinking water are filtered to test for the presence of oocysts of the parasitic protozoan Cryptosporidium. After filtering, the oocysts must be recovered for identification and counting. Current practice is to use a wound cotton fibre filter for this purpose. In order to recover the oocysts, the filter element is cut up into pieces, and the pieces are further shredded and the fibres are teased apart before a complex washing procedure can take place. This involves pounding the fibres, for example in a stomacher homogeniser, in the presence of a succession of volumes of washing liquid, typically three 1 liter aliquots. This large volume, when mixed with a further 1 liter of water that remains in the filter housing, poses problems in further processing to isolate the oocysts, in terms of the number of operations required and undesirable losses on the sides of containers. The washing procedure itself is inherently unsatisfactory because freed oocysts may become re-trapped during stomaching. Also, the washing procedure is time consuming and difficult to standardise, and therefore prone to operator induced variability.
A further example of this type of problem, from the water industry again but on a very different scale, is the treatment of sewage or other effluent by filtration. This is typically done in part by filtration through beds of media such as sand. When the filter medium has become loaded with filtered out particles, it is back-washed with large quantities of water, which is then stored in settling tanks to drop the particles as sludge before being recycled through the filtration process. It would be highly desirable to reduce the quantity of back-washing water, thus reducing pumping requirements and the size of the settlement tanks.
It is known from U.S. Pat. No. 4,213,863 to use pieces of uncompressed foam as a filter medium and no regenerate the filter medium by compressing it to drive out liquid and a portion of the captured solids from the foam. Whilst the foam will be partially cleared of captured particulates by this method and will be capable of further use in filtration, much of the captured particulate manner will remain trapped. Furthermore, if the pore size of the foam is sufficiently small no trap micro-organisms effectively, the total pore volume before compression will be small and the regeneration method will be only poorly effective.
GB-A-2177316 teaches the use of a reticulated foam as a filter element. The foam is compressed to reduce its effective pore size and the compression is locked in by heating the foam so than in will retain its compressed volume once the compressing force is removed. The filter is a substitute for a pleated paper oil filter and it is not intended that the particulates captured by the filter can be efficiently recovered.
The present invention provides filter apparatus comprising as a filter medium an expansible, compressed, open-cell, solid foam.
The foam is compressed so as to reduce its effective pore size to a l
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patent: 1367325 (1921-02-01), Probst
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patent: 3935111 (1976-01-01), Bentley
patent: 4213863 (1980-07-01), Anderson
patent: 4302310 (1981-11-01), Watson
patent: 4303533 (1981-12-01), Fremont
patent: 4468324 (1984-08-01), de Lappe
patent: 4524139 (1985-06-01), Fuchs
Genera Technologies Limited
Lithgow Thomas M.
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