WO2001049612A1

WO2001049612A1 - Water purification filter

Info

Publication number: WO2001049612A1
Application number: PCT/GB2000/000007
Authority: WO
Inventors: Alan Kirkpatrick
Original assignee: Clear Water 42 Holding Asa
Priority date: 2000-01-07
Filing date: 2000-01-07
Publication date: 2001-07-12
Also published as: NO20023184L; EP1252101A1; NO325186B1; CN1424983A; NO20023184D0; IL150535A0; AU1989600A

Abstract

An activated carbon having an ash content as determined in accordance with ASTM 2866 of less than 1 % and which has a zinc adsorption capability such that a filter containing 1.5 g of the activated carbon will adsorb zinc contained in water at a concentration of from 3 to 25 mg/l so as to reduce the concentration of the zinc to below 3 mg/l is described. The activated carbon is suitable for use in water filters. Also described is a water purification system comprising an activated carbon filter and a redox alloy filter which is located upstream of the activated carbon filter so that in use water passes sequentially through the redox alloy filter and then through the activated carbon filter.

Description

WATER PURIFICATION FILTER

The present invention relates generally to water purification and to water purification systems. The invention also relates to novel materials that can be used in water purification systems. More particularly, the present invention relates to a novel form of activated carbon and to a filter containing that carbon which can be employed in a water purification system together with a redox alloy filter and/or a porous ceramic filter to produce water of high purity.

The use of activated carbon in water purification systems to remove organic contaminants and chlorine contained in the water is conventional. The activated carbon is typically granular, but it can also be powdered and cast into porous blocks or cylinders.

Water purification systems comprising porous ceramic filter elements are also known. The ceramic filter elements in these systems typically comprise pores having a size across their largest dimension in the range of from 0.7 to 1.0 micron (μm). The size of the pores is small enough to remove 99.0 to 99.9% of most pathogenic bacteria, but it is not small enough to guarantee protection from water borne diseases. Furthermore, although it is possible to make ceramic filter elements with smaller pore sizes, e.g. down to 0.2 micron, these elements tend to be too fragile to be safely used in water purification systems where a crack in the element could be fatal.

It is also known to use redox alloys of zinc and copper in the purification of water. These materials exhibit the ability to remove toxic metal ions and chlorine contaminating water by losing electrons to or gaining electrons from the contaminants. However, the alloys suffer from the disadvantage that they release zinc and copper into the water which are themselves detrimental to human health and, therefore, can only be used in small quantities.

Other known methods for sterilising water involve the use of chlorine, iodine, ozone and ultra violet radiation.

We have now developed a novel form of activated carbon and a novel water purification system which utilises me activated carbon.

According to a first aspect of the present invention, there is provided an activated carbon having an ash content as determined in accordance with ASTM 2866 of less than 1 % and which has a zinc adsorption capability such that a filter containing 1.5 g of the activated carbon will adsorb zinc contained in water at a concentration of from 3 to 25 mg/1 so as to reduce the concentration of the zinc to below 3 mg/1, e.g. below 1700 μg/1.

According to a second aspect of the present invention there is provided a filter comprising the activated carbon of the first aspect of the present invention. This filter is preferably a water filter and typically takes the form of a filter cartridge comprising a chamber which contains the activated carbon as a bed or layer.

The activated carbon of the present invention has an affinity for positively charged metal ions (cations), and especially zinc ions, which can have certain advantages when the carbon is used in a water purification system alongside a redox alloy.

The zinc adsorption capability of the activated carbon of the present invention is such that a filter containing 1.5 g of the activated carbon will adsorb zinc contained in water (as zinc ions) at a concentration of from 3 to 25 mg/1 and reduce the concentration of the zinc in the water to below 3 mg/1, e.g. below 1700 μg/1 . More particularly, the activated carbon of the present invention is able to treat water contenting from 3 to 25 mg/1, e.g. from 5 to 25 mg/1, of zinc so as to reduce the concentration of the zinc to 15 to 1700 μg/1, e.g. to 15 to 180 μg/1. Typically, the zinc adsorption capability of the activated carbon of the present invention is achieved at a water flow rate through the filter in the range of from 0.5 to 4.0 litres/minute.

As will be explained in more detail below, the zinc adsorption capability of the activated carbon of the present invention is particularly important when a filter containing that carbon is located downstream of a filter containing a redox alloy of zinc and copper. When the redox alloy and activated carbon filters are fed in sequence with water containing chlorine, the redox alloy filter will adsorb the chlorine and typically release zinc into the water at a concentration in the range of from 3 to 25 mg/1, e.g. from 5 to 25 mg/1. It has been found that a filter containing 1.5 g of the activated carbon of the present invention can substantially adsorb, e.g. about 80 mole % of, the zinc released into water by a filter containing 1 g of a redox alloy of zinc and copper when the redox alloy filter and the activated carbon filter are fed in sequence with water containing 0.1 to 10 mg/1 of chlorine at a flow rate of from 0.5 to 4 litres/minute. This adsorption can reduce the concentration of the zinc in the water to below 3 mg/1, e.g. below 1700 μg/I, and will preferably reduce the concentration to 15 to 1700 μg/1, and more preferably to 15 to 180 μg/1.

In a preferred embodiment, the activated carbon is in particulate form and more particularly is in platelet form. The platelets of activated carbon preferably have a mean thickness in the range of from 0.02 to 0.2 mm, a mean particle size across their largest dimension in the range of from 0.2 to 0.6 mm and an aspect ratio (by which is meant the ratio of the largest dimension to the thickness for the platelets) in the range of from 20:1 to 10:3. More preferably, the activated carbon platelets have a mean thickness in the range of from 0.05 to 0.1 mm; a mean particle size across their largest dimension in the range of from 0.25 to 0.55 mm, particularly in the range of from 0.32 to 0.52 mm; and an aspect ratio in the range of from 25:2 to 4: 1 , particularly in the range of from 10: 1 to 20:3.

The activated carbon of the present invention has an ash content less than 1 % by weight as determined in accordance with ASTM 2866. ASTM 2866 can be found in Coal ASTM Book of Standards.

The iodine number of the activated carbon is typically in the range of from 1000 to 1400 mg/g, preferably in the range of from 1100 to 1300 mg/g and particularly in the range of from 1250 to 1300 mg/g.

The activated carbon also preferably has a specific surface area as determined in accordance with the nitrogen BET isotherm method in the range of from 1000 to 1400 m²/g, more preferably in the range of from 1100 to 1300 m²/g and particularly in the range of from 1175 to 1200 m²/g; a pore volume as determined in accordance with ASTM 3838 in the range of from 0.5 to 0.7 ml/g, more preferably in the range of from 0.55 to 0.65 ml/g and particularly in the range of from 0.6 to 0.62 ml/g; a hardness as determined in accordance with ASTM 3802 in the range of from 90 to 100%, more preferably in the range of from 95 to 100 % and particularly in the range of from 98 to 100 % ; a K value gold loading as deteπnined in accordance with the AARL method in the range of from 15 to 30 mg Au/g, more preferably in the range of from 20 to 25 mg Au/g and particularly in the range of from 23 to 25 mg Au/g; and an R value gold kinetics as deteπnined in accordance with the AARL method in the range of from 45 to 75 % , more preferably in the range of from 50 to 70 % and particularly in the range of from 55 to 60 % .

The activated carbon of the present invention may, in principle, be derived from various sources, but it is preferably derived from coconut shells by burning the shells to produce carbon and then subjecting the resulting coconut shell carbon to an activation process in which it is superheated.

Thus, according to a third aspect of the present invention there is provided a process for preparing an activated carbon which process comprises treating carbon produced from the combustion of coconut shells to an activation process in which it is heated to a temperature in the range of from 1000 to 3500°C for a time in the range of from 10 to 50 minutes.

The activation process is typically conducted using a superheated gas, preferably superheated steam, and in a preferred embodiment is conducted at a temperature in the range of from 2000 to 3500°C, more preferably in the range of from 2800 to 3100°C and particularly in the range of from 2950 to 3000°C.

The duration of the activation process has also been found to be important and is preferably conducted for a time in the range of from 15 to 45 minutes, more preferably in the range of from 25 to 35 minutes and particularly in the range of from 28 to 32 minutes. Obviously, the duration of the activation process to which the coconut shell carbon is subjected will to some extent depend on the temperature employed in the process. However, the duration of the activation process is typically considerably longer than the duration of conventional activation processes.

The atmosphere in which the activation process is conducted is preferably oxygen free.

The activation process may be conducted in any suitable activation furnace or kiln of the type which is commercially used to prepare the known granular form of activated carbon.

The conversion of the activated carbon of the invention to a platelet form can be accomplished by the known industrial process of chellation.

The activated carbon filter of the second aspect of the present invention typically takes the form of a filter cartridge comprising a chamber which contains the activated carbon as a bed or layer. The filter cartridge is preferably cylindrical. The activated carbon may form a bed in the filter cartridge and the configuration of the cartridge may be such that in use water is caused to flow longitudinally through the bed of activated carbon as it passes from one end of the cartridge to the other. Alternatively, the filter cartridge may have a tubular configuration in which the activated carbon forms an annular layer between inner and outer sleeves of a porous, water permeable material so that in use water is caused to flow laterally through the activated carbon as it passes from the inside of the filter cartridge to the outside or vice versa. The filter cartridge may be provided with an inlet and outlet connector to which conduits for conveying water to and from the cartridge can be attached. Alternatively, the filter cartridge may be adapted to fit into a filter housing which is provided with the inlet and outlet connectors and is adapted to convey water to and from the filter cartridge containing the activated carbon.

The present invention also provides a water purification system which comprises an activated carbon filter of the second aspect of the present invention and a redox alloy filter which is located upstream of the activated carbon filter so that in use water passes sequentially through the redox alloy filter and then through the activated carbon filter.

The activated carbon filter and the redox alloy filter are connected in series, e.g. by an arrangement of conduits, so that the water exitmg the redox alloy filter is conveyed to the activated carbon filter.

The redox alloy is a granular material and is preferably an alloy of zinc and copper. Preferred redox alloys of zinc and copper are those comprising from 30 to 70 weight % of the zinc and from 70 to 30 weight % of the copper, more preferably from 40 to 60 weight % of the zinc and from 60 to 40 weight % of the copper and particularly about 50 weight % of both zinc and copper. An especially preferred redox alloy of zinc and copper is that sold under the trade mark KDF-55 (available from KDF Fluid Systems Inc).

The redox alloy filter typically forms part of a filter cartridge comprising a chamber which contains the redox alloy as a bed or layer. The filter cartridge is preferably cylindrical. The redox alloy may form a bed in the filter cartridge and the configuration of the cartridge may be such that in use water is caused to flow longitudinally through the bed of redox alloy as it passes from one end of the cartridge to the other. Alternatively, the filter cartridge may have a tubular configuration in which the redox alloy forms an annular layer between inner and outer sleeves of a porous, water permeable material so that in use water is caused to flow laterally through the redox alloy as it passes from the inside of the filter cartridge to the outside or vice versa. The filter cartridge may be provided with an inlet and outlet connector to which conduits for conveying water to and from the cartridge can be attached. Alternatively, the filter cartridge may be adapted to fit into a filter housing which is provided with the inlet and outlet connectors and is adapted to convey water to and from the filter cartridge containing the redox alloy.

We do not, of course,, exclude the possibility that the redox alloy filter and the activated carbon filter of the second aspect of the present invention may be incorporated into a single filter cartridge, and it is a simple matter to arrange the beds or layers of redox alloy and activated carbon so that the water being purified flows firstly through the redox alloy and then through the activated carbon. In the water purification system, the weight ratio of redox alloy to activated carbon in the redox alloy filter and activated carbon filter is preferably in the range of from 1:1 to 1:10, more preferably in the range of from 1:1 to 1 :2 and particularly in the range of from 1 : 1.4 to 1:1.6. In an especially preferred embodiment, for each 250 g of redox alloy which is used, 375 g of the activated carbon is used.

In conventional water purification systems, redox alloys, which are used to remove toxic metal ions and chlorine, can only be used in small quantities because they release zinc and copper into the water which are themselves detrimental to human health. In contrast, we have found that redox alloys can be used in much greater quantities in the water purification system of the present invention, therefore allowing for the removal of much greater quantities of toxic metal ions and chlorine, because the activated carbon filter of the second aspect of the present invention which follows the redox alloy filter can sequester or adsorb the metals, e.g. zinc and copper, which are released into the water by the redox alloy.

Drinking water will typically contain from 0.1 to 10 mg/1 of chlorine and when this is passed through a filter containing 1000 g of a redox alloy of zinc and copper at a rate of from 0.5 to 4 litres/minute, the chlorine will be substantially removed by the filter resulting in the release of zinc into the water in an amount of from 3 to 25 mg/1. The amount of zinc released into the water will, of course, increase as the amount of chlorine in the water supplied to the filter increases. Typically, if water containing chlorine in an amount of around 2 mg/1 (2 ppm) is passed through a filter containing 1000 g of a redox alloy of zinc and copper at a rate of from 0.5 to 4.0 litres/minute, then the water exiting the filter will contain from 5 to 8 mg/1 of zinc. However, if the chlorine content of the water entering the same filter is around 10 mg/1 (10 ppm), then the water exiting the filter will typically contain from 20 to 25 mg/1 of zinc.

The water purification system of the present invention preferably further comprises a porous ceramic filter element which is located downstream of the activated carbon filter and in fluid flow communication therewith so that in use water exiting the activated carbon filter is conveyed to the ceramic filter element for passage therethrough. When the water purification system also comprises a porous ceramic filter element, it will be appreciated that in use water will pass sequentially through the redox alloy filter, the activated carbon filter and then the porous ceramic filter.

The arrangement of filters in the water purification system of the present invention allows the porous ceramic filter element to remove iron and aluminium contained in the water. The redox alloy filter tends to produce a neutral pH, e.g. a pH of 6.5 to 7.0, at which iron and aluminium precipitate out. The precipitated iron and aluminium can then be removed by the ceramic filter element.

The porous ceramic filter element may be of the type that is conventionally used for the purification of water. These elements typically have pore sizes in the range of from 0.2 to 1.2 μm, particularly in the range of from 0.3 to 0.9 μm, e.g. in the range of from 0.5 to 0.7 μm. By pore size, we are referring to the size across the largest dimension of the pore.

The preferred ceramic filter elements have a total porosity in the range of from 30 to 80 %, more preferably in the range of from 45 to 75 %, particularly in the range of from 50 to 70 % and especially in the range of from 58 to 62 %.

A proportion of the pores in the ceramic element may be partially occluded with water insoluble particles. Typically, these particles will have a mean size across their largest dimension in the range of from 0.1 to 1.0 μm, preferably in the range of from 0.2 to 0.5 μm and particularly in the range of from 0.3 to 0.4 μm, and once resident in the pores will result in pores having an effective mean size across their largest dimension in the range of from 0.09 to 0.7 μm, preferably in the range of from 0.2 to 0.5 μm, more preferably in the range of from 0.2 to 0.4 μm and particularly in the range of from 0.2 to 0.35 μm. In this way, the filtering capability of the ceramic element is enhanced because the partially occluded pores can provide for the removal of foreign materials contained in water down to the effective pore size. Preferably the occluding particles are substantially spherical in shape.

The pores in the ceramic element preferably have a generally eye shaped cross-section and the occluding particles are preferably substantially spherical and of a size that they lodge in the corners of the eye shaped pores in the ceramic element. The partial occlusion of some of the pores in the ceramic element may be achieved by means of a discrete processing step which is conducted before the element is put to its intended use.

The ceramic filter element typically locates in a filter housing comprising inlet and outlet connectors for connection to conduits for conveying water to and from the filter. The ceramic filter element is preferably treated with a sterilising agent, usually silver, to prevent micro-organisms from colonising the element.

The water purification system of the present also preferably comprises a second activated carbon filter which is located downstream of the ceramic filter element so that in use water exiting the ceramic filter element passes onto the second activated carbon filter. The activated carbon which is used in the second activated carbon filter should be of the type which is conventionally used in water purification systems for removing organic compounds and is typically cast into a porous block.

The second activated carbon filter typically locates in a filter housing comprising inlet and outlet connectors for connection to conduits for conveying water to and from the filter.

As stated above, the activated carbon which is used in the second activated carbon filter is typically of a type which is conventionally used in water purification systems for removing organic compounds. However, in conventional water purification systems, the adsorption capacity of this type of activated carbon is typically consumed very quickly, since it also removes chlorine contained in the water. As a result, its effectiveness is very short lived. Moreover, an undesirably high level of organics can be released into the water at the point of failure. The chlorine adsorbs more strongly onto the activated carbon than the organics and therefore will preferentially occupy the adsorption sites at the upstream end of the filter. During use, the volume of adsorption sites occupied by the chlorine steadily increases and the organics are progressively displaced towards the downstream end of the filter. When the adsorption sites are completely occupied, the filter fails and organics are displaced from the filter and released into the water at the rate at which chlorine contained in the incoming water is being adsorbed. The chlorine tends to be present in water at much higher levels than the organics, so at the point of failure, the organics come off the filter at a concentration which is much higher than the concentration of organics in the starting water.

These problems can be avoided with the water purification system of the present invention, since the redox alloy can be used in sufficient quantities to remove all or substantially all of the chlorine so freeing the second activated carbon filter to deal with organics only. These organics include the organic components of any bacteria which decompose within the ceramic element such as endotoxins.

It will be appreciated from the above that a preferred water purification system comprises in sequence a redox alloy filter, an activated carbon filter of the second aspect of the present invention, a ceramic filter element and a second activated carbon filter which are arranged so that in use water passes sequentially through the redox alloy filter, the activated carbon filter of the second aspect of the present invention, the ceramic filter element and finally through the second activated carbon filter. These filters may be incorporated into a single filter cartridge.

The water purification system of the present invention may further comprise an anion resin filter. Where used, this filter will typically be located between the ceramic filter element and the second activated carbon filter.

The water purification system of the present invention may also comprise a pre-filter for removing larger particulate matter, such as sand, grit, rust, soot, peat, moss and algae. These particles will typically have a size in the range of from 1 to 100 μm, e.g. in the range of from 5 to 50 μm.

When used, this pre-filter will provide a first coarse filtration step and, therefore, will be located upstream of the redox alloy filter.

The water purification system of the present invention may also comprise a pump to drive the water to be purified through the arrangement of filters which follow. This pump can be hand operated or powered, e.g. by electricity or an internal combustion engine.

The present invention will now be described by way of example and with reference to the enclosed drawings in which:

Figure 1 is a schematic representation of a water purification system of the present invention showing the arrangement of the various filters. In Figure 1, the water purification system (1) comprises a redox alloy filter (2), an activated carbon filter (3) according to the second aspect of the present invention, a porous ceramic filter element (4) and a porous carbon block filter element (5).

The redox alloy filter (2) and the activated carbon filter (3) are combined in a single, cylindricaily shaped filter cartridge comprising a chamber in which the activated carbon and the redox alloy are arranged as discrete beds so that in use the water being purified flows longitudinally through the filter beds as it passes from one end of the filter cartridge to the other.

The filter cartridge fits into a first, cylindricaily shaped filter housing (not shown) which is provided with inlet and outlet connectors at its ends for connection to conduits for conveying water to and from the filters (not shown). The first filter housing is adapted to convey water to and from the filter cartridge so that it passes firstly through the redox alloy bed and then through the activated carbon bed.

The porous ceramic filter element (4) is cylindricaily shaped and has a tubular configuration. The porous carbon block filter element (5) is also cylindricaily shaped and is located in the central chamber provided by the tubular ceramic filter element (4). The diameter of the porous carbon block filter element (5) is such that it is a close fit inside the ceramic filter element (4) and the complete filter assembly locates inside a second, cylindricaily shaped filter housing (not shown) comprising inlet and outlet connectors for connection to conduits for conveying water to and from the filters (not shown). The second filter housing is adapted so that water is caused to pass laterally through the ceramic filter element (4) and then onto the carbon filter element (5) where it flows in a generally longitudinal direction towards the outiet end of the filter housing.

The first filter housing containing the redox alloy filter (2) and the activated carbon filter (3) and the second filter housing containing the porous ceramic filter element (4) and the porous carbon block filter element (5) are connected in series by an arrangement of conduits (not shown) with the second filter housing being located downstream of the first filter housing so that in use water passes sequentially through the redox alloy filter (2), the activated carbon filter (3), the porous ceramic filter element (4) and the porous carbon block filter element (5). The first filter housing is then connected to a source of water to be purified.

In use the flow of water through the water purification system is generally as shown by the emboldened arrows.

The present invention is now illustrated but not limited with reference to the following examples.

Example 1

The water purification system described above with reference to Figure 1 was used to purify 10,000 litres of tap water contaminated with the following impurities.

Chlorine (Cl₂ ^") ions - 10 mg/1 Iron (Fe²⁺) ions - 50 mg/1 Lead (Pb²⁺) ions - 100 μg/1 Aluminium (Al³⁺) ions - 100 μg/1 Chloroform - 10 μg/1 Trichloroethane - 10 μg/1 Zinc - 0

The activated carbon filter (3) comprised a bed of activated carbon platelets which had a mean thickness of about 0.1 mm, a mean particle size across their largest dimension of about 0.5 mm and an aspect ratio of about 5:1. The activated carbon also had the following properties:

An iodine number of about 1300 mg/g.

A specific surface area as determined in accordance with the nitrogen

BET isotherm of about 1200 m²/g.

A pore volume as determined in accordance with ASTM 3838 of about 0.6 ml/g.

A hardness as determined in accordance with ASTM 3802 of about 99 %.

A K value gold loading as determined in accordance with the AARL method of about 24 mg Au/g.

An R value gold kinetics as determined in accordance with the AARL method of about 58 % .

The redox alloy filter (2) comprised a bed of KDF-55 available from KDF Fluid Systems Inc.

The porous ceramic filter element (4) was a commercially available product available from Fairey Industrial Ceramics under the product code Imperial Supercarb. The filter element comprised generally eye shaped pores having a mean size across their largest dimension of about 0.9 μm. and had a total porosity of about 65 % .

The porous carbon block filter element (5) was a commercially available product available from Ametek Inc under the product code CBC10.

The weight ratio of redox alloy to activated carbon in the redox alloy filter (2) and activated carbon filter (3) was such as to provide 250 g of redox alloy for each 375 g of activated carbon.

The contaminated tap water was passed through the water purification system at a flow rate of 2 littes/minute and samples of the water obtained from the system were collected at 1000 litre intervals and analysed for impurities using a Merck Spectroquant Kit in conjunction with a UN visible spectrophotometer. After passage through the water purification system, the levels of contaminants in the water were below the limits of detection for each sample, including the final sample taken after all the water had passed through the system.

Example 2

The water purification system described above with reference to Figure 1 was used to purify 10,000 litres of tap water contaminated with 10 mg/1 of chlorine (as Cl₂ ^" ions).

An iodine number of about 1300 mg/g. A specific surface area as deteπnined in accordance with the nitrogen BET isotherm of about 1200 m²/g.

A pore volume as determined in accordance with ASTM 3838 of about 0.6 ml/g.

A hardness as determined in accordance with ASTM 3802 of about 99 % . A K value gold loading as determined in accordance with the AARL method of about 24 mg Au/g.

The porous carbon block filter element (5) was a commercially available product available from Ametek Inc under the product code CBC10. The weight ratio of redox alloy to activated carbon in the redox alloy filter (2) and activated carbon filter (3) was such as to provide 250 g of redox alloy for each 375 g of activated carbon.

The contaminated tap water was passed through the water purification system at a flow rate of 2 lifres/minute and samples of the water entering and leaving the activated carbon filter of the present invention were obtained. The zinc concentration of the water entering the activated carbon filter was found to be around 1150 μg/1 owing to the zinc released by the redox alloy filter, while the concentration of the zinc exiting the activated carbon filter was around 190 μg/1.

Claims

Claims:

1. An activated carbon having an ash content as determined in accordance with ASTM 2866 of less than 1 % and which has a zinc adsorption capability such that a filter containing 1.5 g of the activated carbon will adsorb zinc contained in water at a concentration of from 3 to 25 mg/1 so as to reduce the concentration of the zinc to below 3 mg/1.

2. An activated carbon as claimed in claim 1, wherein the zinc adsorption capability of the activated carbon is such that a filter containing

1.5 g of the activated carbon will adsorb zinc contained in water at a concentration of from 3 to 25 mg/1 and reduce the concentration of the zinc in the water to below 1700 μg/1.

3. An activated carbon as claimed in claim 1, wherein the zinc adsorption capability of the activated carbon is such that a filter containing 1.5 g of the activated carbon will adsorb zinc contained in water at a concentration of from 3 to 25 mg/1 and reduce the concentration of the zinc in the water to 15 to 180 μg/1.

4. An activated carbon having an ash content as determined in accordance with ASTM 2866 of less than 1 % and which has a zinc adsorption capability such that a filter containing 1.5 g of the activated carbon will substantially adsorb the zinc released into water by a filter containing 1 g of a redox alloy of zinc and copper when the redox alloy filter and activated carbon filter are fed in sequence with water containing from 0.1 to 10 mg/1 of chlorine at a flow rate of from 0.5 to 4 lifres/minute.

5. An activated carbon as claimed in any one of the preceding claims which is in particulate form.

6. An activated carbon as claimed in claim 5 which is in platelet form.

7. An activated carbon as claimed in claim 6, wherein the platelets of activated carbon have a mean thickness in the range of from 0.02 to 0.2 mm, a mean particle size across their largest dimension in the range of from 0.2 to 0.6 mm and an aspect ratio in the range of from 20:1 to 10:3.

8. An activated carbon as claimed in any one of the preceding claims, wherein the iodine number of the activated carbon is in the range of from 1000 to 1400 mg/g.

9. An activated carbon as claimed in any one of the preceding claims which has a specific surface area as determined in accordance with the nitrogen BET isotherm method in the range of from 1000 to 1400 m²/g.

10. An activated carbon as claimed in any one of the preceding claims which has a pore volume as determined in accordance with ASTM 3838 in the range of from 0.5 to 0.7 ml/g.

11. An activated carbon as claimed in any one of the preceding claims which has a hardness as determined in accordance with ASTM 3802 in the range of from 90 to 100 % .

12. An activated carbon as claimed in any one of the preceding claims which has a K value gold loading as determined in accordance with the AARL method in the range of from 15 to 30 mg Au/g.

13. An activated carbon as claimed in any one of the preceding claims which has an R value gold kinetics as determined in accordance with the AARL method in the range of from 45 to 75 % .

14. A filter comprising the activated carbon as claimed in any one of claims 1 to 13.

15. A water purification system which comprises an activated carbon filter as claimed in claim 14 and a redox alloy filter which is located upstream of the activated carbon filter so that in use water passes sequentially through the redox alloy filter and then through the activated carbon filter.

16. A water purification system as claimed in claim 15, wherein the redox alloy is an alloy of zinc and copper.

17. A water purification system as claimed in claim 15 or claim 16, wherein the redox alloy filter and the activated carbon filter are incorporated into a single filter cartridge.

18. A water purification system as claimed in any one of claims 15 to 17, wherein the weight ratio of redox alloy to activated carbon in the redox alloy filter and activated carbon filter is in the range of from 1 : 1 to 1:10.

19. A water purification system as claimed in claim 18, wherein the weight ratio of redox alloy to activated carbon in the redox alloy filter and activated carbon filter is in the range of from 1 : 1.4 to 1:1.6.

20. A water purification system as claimed in any one of claims 15 to 19, further comprising a porous ceramic filter element which is located downstream of the activated carbon filter and in fluid flow communication therewith so that in use water exiting the activated carbon filter is conveyed to the ceramic filter element for passage therethrough.

21. A water purification system as claimed in claim 20, wherein a proportion of the pores in the ceramic element are partially occluded with water insoluble particles.

22. A method of purifying water containing chlorine which comprises passing the water through a filter containing a redox alloy of zinc and copper so as to substantially remove the chorine and release zinc into the water at a concentration of from 3 to 25 mg/1 and then passing the resulting zinc containing water through an activated carbon filter having an ash content as determined in accordance with ASTM 2866 of less than 1 % so as to reduce the concentration of the zinc to below 3 mg/1.

23. A method as claimed in claim 22, wherein the activated carbon reduces the concentration of the zinc to below 1700 μg/1.

24. A method as claimed in claim 22, wherein the activated carbon reduces the concentration of the zinc to 15 to 180 μg/1.

25. A method of purifying water containing from 0.1 to 10 mg/1 of chlorine which comprises passing the water through a filter containing a redox alloy of zinc and copper so as to substantially remove the chorine and release zinc into the water and then passing the resulting zinc containing water through an activated carbon filter having an ash content as determined in accordance with ASTM 2866 of less than 1 % so as to substantially remove the zinc released into the water by the redox alloy filter.

26. A method as claimed in any one of claims 22 to 25, wherein the water exiting the activated carbon filter is then fed to a porous ceramic filter element.

27. A method as claimed in any one of claims 22 to 25, wherein the water is fed to the redox alloy and activated carbon filters at a flow rate of from 0.5 to 4 litres/minute.