GB2210805A

GB2210805A - Filtering and filters

Info

Publication number: GB2210805A
Application number: GB8829037A
Authority: GB
Inventors: John Edward Cope
Original assignee: SEV TRENT WATER AUTHORITY
Current assignee: SEV TRENT WATER AUTHORITY
Priority date: 1987-12-15
Filing date: 1988-12-13
Publication date: 1989-06-21
Also published as: GB8829037D0; GB8729208D0

Abstract

To measure the solids content of a sample of water, the sample is filtered using a filter cup 9 having an integral filter paper 10 therein to which suction is applied via a Buchner flask, and the whole cup and filter paper unit is weighed before and after. The process can be automated, the cup 9 being easier for robot arms to handle than a separate wet filter paper. <IMAGE>

Description

FILTERING AND FILTERS FOR LIQUIDS This invention relates to filters for liquids, and a method of filtering a liquid to determine the mass of filterable material in the liquid.

One field in which it is important to determine the mass of filterable material held in a liquid is in determining water quality. The amount of suspended solids in water is checked by water authorities to ensure that the water is safe to use, and also to estimate charges for the discharge of effluents by industry. The tests which water authorities use have been standardised and are set out in the Department of the Environment guidelines on "Suspended, Settleable and Total Dissolved Solids in Waters and Effluents 1980".One of the tests basically comprises taking a sample of water up to a maximum of 250 ml, vacuum-filtering it through a pre-weighed filter paper of a specified type supported on a Hartley disc, rinsing the apparatus with water to ensure that all solid matter reaches the filter paper, removing the filter paper from the Hartley disc, drying the filter paper and re-weighing it; the difference between the initial and final weights indicating the purity of the water. Accompanying Figure 1 shows the apparatus used to perform hundreds of thousands of such tests each year. The test is difficult to automate.

According to a first aspect the present invention consists in a a method of indicating the amount of solid matter suspended in a liquid sample comprising the steps of taking a sample of known volume of liquid to be tested and putting it in a vessel of known weight incorporating a filter as a single unit with the vessel, filtering the sample through the filter, and weighing the vessel after filtering; the difference in weight of the vessel before and after filtering indicating the amount of solid matter in the sample.

The method avoids handling the wet or dry filter paper alone, as in the existing method of testing the purity of water, since the vessel and filter paper are a single unit. The method also eliminates the need to rinse the vessel with pure water to ensure that all of the solid matter reaches the filter paper. These features make the method simpler to automate, the first of them being of particular advantage since it avoids difficulties in handling wet filter papers with robot arms.

Preferably the vessel and incorporated filter are dried after filtering and before weighing. The filtration step is preferably assisted by a pressure difference over the filter, such as by applying suction beneath the vessel. This is conveniently provided by providing a collecting member beneath the vessel, sealing the member to the vessel, and applying suction at an aperture in the member. The collecting member is preferably a conventional Buchner Flask.

The apparatus used in the method can correspond very closely with the apparatus used with the existing method. The new test is thus cheap to introduce, and because the test conditions can be kept substantially the same as the existing test conditions results obtained from the two tests can be readily correlated or even equated.

According to a second aspect of the invention we provide as a single unit a vessel comprising a surrounding wall upstanding from a base incorporating a filter, the base having a perforated region and the filter overlying the perforated region and being sealed to the base along a continuous line surrounding the perforated region, or being integral with the base.

Preferably the perforated region is surrounded by a shoulder region of the base which engages in use against means for applying suction to the underside of the vessel. The perforated region of the base preferably depends from the shoulder. Alternatively the base may be flat. The material of the vessel is preferably thermally stable at temperatures up to at least 1050C, and generally to temperatures substantially higher.

In order to stay as close as possible to the existing standard test, the filter is preferably a Whatman GF/C glass fibre filter paper sealed to the base and the perforations of the base are preferably the same size and pattern as found on the recommended standard Hartley disc. The filter paper is preferably heat sealed to the base to prevent water and associated particulate matter leaking past the filter paper.

The vessel is preferably generally frusto-conical having a volume of at least 280 ml, and most preferably a volume of 310 ml. This enables it to accommodate a 250 ml sample.

We also provide according to a third aspect of the invention a stack of vessels described above, the vessels being inserted one inside another in the manner of paper cups, but such that the base of an upper vessel does not touch the filter of the lower vessel into which it is inserted.

It will also be appreciated that an assembly of a vessel as described above and an associated forced filtration means is envisaged.

According to a further aspect the invention consists in an automated testing device for indicating the amount of solid matter in a liquid sample, the device having automatic means for manipulating a vessel in accordance with the second aspect of the invention, automatic weighing means for weighing the vessel before and after filtration, automatic sample manipulation means, and control means which operates the device in accordance with the method of the first aspect of the invention.

Preferably the automatic testing device has force filtering means. The device may have drying means.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings of which: Figure l shows schematically known filter apparatus for filtering water to indicate its purity, and Figure 2 shows schematically filter apparatus according to the present invention for use in the method of the present invention.

Water is currently filtered to check its purity in the apparatus of Figure 1, which comprises a glass cylinder 1 and a Buchner Flask (the neck of which is shown at 2), clamped together around a Hartley disc 3 and a Whatman GF/C glass fibre filter paper 4 by means of clips 5 acting on flanges 6 and 7 provided on the cylinder and flask respectively. The cylinder 1 is open at both ends. The Hartley disc 3 has a pattern of holes 8 extending through it and serves to support the filter paper.

The water sample is poured into the cylinder 1, suction F applied to the Buchner Flask to force-filter the water through the filter paper 4. The cylinder is rinsed with pure water and the filter paper removed and dried and weighed as described earlier.

Figure 2 shows a vessel 9, incorporating a filter 10, and a neck 2 of a similar Buchner Flask to that of Figure 1. The vessel 9 has an internal volume of about 310 ml and comprises a frusto-conical peripheral wall 11 and a base 12. The wall 11 is about 80 mm high and has a diameter of about 80 mm at its upper end and 60 mm at its lower end.

The base 12 has a central perforated region 13 surrounded by and depending from an annular shoulder region 14. The perforated region 13 has a series of holes 15 through it of 1 mm diameter spaced 5 mm apart over a base area of radius 50 mm (in equivalent pattern to the conventional Hartley disc 4). The filter 10 is a Whatman GF/C glass fibre filter paper of approximately 60 mm diameter and is provided overlying the perforated region 13 and is heat sealed around its circumference to the base of the vessel. Seal line 16 between the filter paper and the base is shown in Figure 2 and surrounds all of the holes 14 so that water in the vessel 9 must pass through the filter 10 before it can leave the vessel.

The underside of the shoulder region 14 of the vessel 9 seats in use upon a seal 17 provided at the lip of the neck 2 of the Buchner flask. When suction F is applied in use to the flask the seal 17 seals the flask to the vessel at a line surrounding the perforated region 13. In use, the holes 15 are at a lower position than the seal 17. The material of the vessel is stable at temperatures up to at least 1050C.

The apparatus of Figure 2 is used as follows: the vessel 9 is weighed, a sample of up to 250 ml of water to be tested is poured into the vessel 9 (this is the same volume of sample as is currently used) and the sample force-filtered through the filter 10 by suction F applied to the Buchner Flask. The vessel 9 is then removed from the flask and dried at 1050C to constant weight and its final weight recorded. The difference between this final weight and the known starting weight of the flask represents the amount of solid matter suspended in the sample.

In addition to avoiding handling wet filter papers on their own, and having to rinse the vessel, our invention alleviates any potential inaccuracies in the test set out in the Department of the Environment guidelines caused by any slight seapage by capillary action of water and soluble components into the part of the filter paper 4 that is trapped between the flask and the disc 3. The soluable matter retained in the filter paper may not be completely removed by the rinising step of the existing method and can influence test results. We avoid this problem.

The vessel 9 is a "one-shot" disposable item and vessels are supplied and stored stacked one inside another to minimise storage space. EIowever, care is taken that in the stack of vessels the base of each vessel is spaced above the filter of the adjacent vessel into which it is inserted. Lugs or other such means may be provided for this.

We also envisage an automated testing device for carrying out the method of the first aspect of invention using the apparatus of the second aspect of the invention. The automated testing device would have automated means for manipulating a vessel 1, automated weighing means, automated sample manipulation means, and control means to operate the device in accordance with the method. The preferred device would also have force filtering means and drying means. An operable arrangement of these means to enable an automated testing device to be constructed is self-apparent to a skilled technician.

The vessel 9 may be too thin to support itself when vacuum is applied, for which purpose we may provide a mesh or other support beneath the vessel to support the base 12.

The invention has been described in terms of an existing test for water purity. However, it will be apparent that should the standard test be varied (for example by specifying a different filter paper, sample volume, drying temperature) then the materials used in the apparatus can be varied accordingly.

Although the invention has been described in relation to testing water, its applicability to determining the amount of suspended material in any liquid will be readily apparent.

Claims

I. A method of indicating the amount of solid matter suspended in a liquid sample comprising the steps of taking a sample of known volume of liquid to be tested and putting it in a vessel of known weight incorporating a filter as a single unit with the vessel, filtering the sample through the filter, and weighing the vessel after filtering; the difference in weight of the vessel before and after filtering indicating the amount of solid matter in the sample.
2. A method according to claim 1 which further comprises drying the vessel and incorporated filter before weighing the vessel after filtering.
3. A method according to claim 1 or claim 2, in which the filtration of the sample is assisted by a pressure difference over the filter.
4. A method according to claim 3, in which a collecting member is provided beneath the vessel, the vessel is sealed to the member, and suction is applied to an aperture in the member.
5. A vessel for use in a method according to any one of claims 1 to 4 comprising as a single unit a surrounding wall upstanding from a base incorporating a filter, the base having a perforated region and the filter overlying the perforated region and being sealed to the base along a continuous line surrounding the perforated region, or being integral with the base.
6. A vessel according to claim 5, in which the perforated region is surrounded by a shoulder region of the base which engages in use against means for applying suction to the underside of the vessel.
7. A vessel according to claim 6 in which the perforated region depends from the shoulder.
8. A vessel according to any one of claims 5 to 7, in which the material of the vessel is thermally stable at temperatures up to at least 1050C.
9. A vessel according to any one of claims 5 to 8 in which the filter comprises a filter paper sealed to the base.
10. A vessel according to claim 9, in which the filter comprises a Whatman GF/C glass fibre filter paper.
11. A vessel according to claim 9 or claim 10 in which the filter paper is heat sealed to the base.
12. A vessel according to any one of claims 5 to 11, in which the perforations of the base comprise a regular pattern of holes of substantially Imn diameter spaced substantially 5mm apart.
13. A vessel according to claim 12, in which the perforations extend over a substantially circular area of the base of radius substantially 50mm.
14. A vessel according to any one of claims 5 to 13 which has a volume of at least 280ml.
15. A vessel according to any one of claims 5 to 14 in which the surrounding wall is generally frusto-conical.
16. A stack of vessels comprising at least first and second vessels according to claim 15 with the first vessel inserted inside the second vessel with the exterior of its frusto-conical surrounding wall engaging the interior of the surrounding wall of the second vessel and its base being spaced from the base of the second vessel.
17. A filtering assembly comprising a vessel according to any one of claims 5 to 15 and force-filtration means sealingly engaged with the vessel.
18. An automated testing device for indicating the amount of solid matter in a liquid sample, the device having automatic means for manipulating a vessel which is in accordance with any one of claims 5 to 15, automatic weighing means for weighing the vessel before and after filtration, automatic sample manipulation means, and control means which operates the device in accordance with the method of any one of claims 1 to 4.
19. An automated testing device according to claim 18 in which the device has force filtering means.
20. An automated testing device according to claim 18 or claim 19 in which the device has drying means.
21. A method of determining the amount of solid matter suspended in a liquid sample substantially as described and illustrated herein with reference to Figure 2 of the accompanying drawings.
22. A vessel substantially as described and illustrated herein with reference to Figure 2 of the accompanying drawings.
23. An automated testing device substantially as described herein.