GB2390987A - A carbon containing filtration medium - Google Patents

Abstract

The invention provides a particulate carbon block filtration medium which is suitable for use in gravity fed filters and which comprises powdered activated carbon (PAC) having a particle size in the range of 6 to 325 mesh, and a hydrophilic binder material, in which the effective carbon surface area masked by the binder is less than 10%. The filtration medium of the invention is preferably used in conjunction with a washable or replaceable sediment filter and is capable of achieving a high flow rate in gravity fed applications as well as the removal of particulate contaminants including biological contaminants such as cysts.

Description

C2099 (C) GB FF

FILTRATION MEDIA

FIELD OF THE INVENTION

5 The present invention relates to filters and filtration media and, in particular, to filtration media for use in gravity-fed water filtration units.

BACKGROUND AND PRIOR ART

Fluids such as liquids or gases typically contain contaminants, which include particulates, chemicals and .. Organisms. In many cases such as drinking waters and the like it is desirable to remove the harmful contaminants from 15 the fluids. Importantly, in case of drinking waters, the removal of contaminants is a necessity to maintain proper hygiene and safe conditions for consumption of such water.

It is known to provide devices for filtering water both for 20 industrial and household applications. The requirements for the filtering of water for industrial and domestic application vary and hence the equipment used also varies depending upon the end use of the treated water.

25 Domestic water treatment systems can be categorized as online systems or self contained systems, which process water in batches. Online systems have the advantage of the pressure of the water flow available from online reservoirs and pumps therefore do not usually face problems of 30 achieving the desired flow rate while catering to the basic need of effective filtration.

C209g(C) GB FF On the other hand, self contained filtration systems have to rely only on gravity to force the water through the filtration media and maintain a steady flow. Considering the inherent problem of very little water height in gravity 5 fed units, which have a maximum height of about 10 inches (0.35 psig), filtration media for particulate removal produce either no flow or abysmally low flow rates to be of any practical use when applied to gravity fed systems. It has thus been a real challenge to provide effective 10 filtration media for self contained systems which would on one hand provide the desired flow rate and at the same time cater to the removal of particulate contaminants including biological contaminants such as protozoan cysts like cryptosporidium. Particulate contaminants may include dirt, 15 rust, silt and other particles as well as potentially hazardous microorganisms such as chlorine resistant protozoan cyst like cryptosporidium parvum or giardia or bacteria such as cholera and E.coli.

20 Typically, a self-contained system for domestic application has an upper and a lower chamber separated by a filter cartridge. The water to be treated is poured into the upper chamber and allowed to flow by gravity through the filter cartridge to be stored in the lower chamber for dispensing 25 as and when desired. In such filter cartridges it is known to use activated carbon to remove bad taste and odour from water as well as chlorine and other reactive chemicals. Use of ion exchange resins for removing metal and other ions from water is also known.

C2099(C) GB FF

- 3 It is also known to have gravity fed water filters for domestic use such as carafe filters. In this type of filter, water to be treated enters through a series of small perforations at the wider top of a trapezoid shaped filter 5 and flows through the filter to the narrower bottom, in the process traversing through a porous bed of loose absorbents.

Such gravity flow carafe filters like other gravity filters have some inherent limitations in achieving effective flow rates since the water is required to traverse a deep bed of 10 adsorbent particles and also because the water pressure is low in such gravity systems. Attempts to increase flow rate by the use of larger particulates lead to slower adsorption kinetics i.e. loss of effective filtration. On the other hand use of relatively small particles lead to greater flow 15 restriction.

WO 00/12194 discloses a water filter containing a composite layered filtration medium involving an adsorbent layer and an hydrophilic particulate intercepting layer comprising a 20 mixture of glass micro fibres and an epoxy binder resin.

Apart from the complexity in manufacture of such a layered filtration medium the involvement of glass micro fibres makes it a cost intensive proposal.

25 US 6290848 discloses a filter cartridge for use in gravity fed water treatment systems comprising a hydrophilic porous particulate filter having an open upper end, a lower end and sidewalls therebetween to define an interior volume accommodating a granular filtration medium, such as the 30 loose adsorbent material used in conventional carafe filters. As the hydrophilic porous particulate filter it is

C2099(C) GB FF

- 4 - proposed to use micro-porous filtration media such as glass fibre or carbon block hollowed out to create sidewalls and an open upper end. To counter the problems of flow rate encountered when using glass fibres, it is suggested to use 5 hydrophilic glass binders to alter the hydrophobicity. This proposal involves complications in manufacture due to the shape of the particulate filter, and is costly in terms of the use of glass fibres and the required additives to alter the hydrophobicity of the filtration media.

Given the present day widely felt need for filtered water, which is no longer restricted to urban localities but extends also to remote villages where online systems are not readily available, there exists a need for development of 15 cost-effective gravity-fed systems which would be simple to manufacture and yet take care of the inherent problems of gravity fed systems so as to achieve good flow rate and at the same time effective removal of chemical, biological and particulate contaminants.

SUMMARY OF THE INTENTION

25 The present invention provides a particulate carbon block filtration medium which is suitable for use in gravity fed filters and which comprises powdered activated carbon (PAC) having a particle size in the range of 6 to 325 mesh, and a hydrophilic binder material, in which the effective carbon 30 surface area masked by the binder is less than 10%.

C2099 (C) GB FF

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS

The PAC is preferably selected from bituminous coal, coconut shell, wood, and petroleum tar.

The PAC is preferably in the size range of 30 - 200 mesh, more preferably in the size range of 80 - 120 mesh.

Preferably the PAC has a surface area exceeding 500 sq. m/g, 10 more preferably exceeding 1000 sq. m/g.

Preferably, the PAC has a size uniformity co-efficient of less than 2, more preferably less than 1.5.

15 Preferably, the PAC has a carbon tetrachloride number exceeding 50%, more preferably exceeding 70%.

The PAC can be acid-treated or washed to a wet pH of less than 8.0.

Also, the PAC can have an iodine number greater than 800, more preferably greater than 1000.

The binder used may suitably be a polymer comprising one or 25 more of the repeating groups: olefins, bisphenol. The olefins may suitably be selected from ethylene, propylene, and/or butene.

Preferably the binder used is selected so that the total 30 leachables from the cured binder under flowing water at 50 C is less than 1 mg/L, preferably less than 0.1 mg/L, more preferably less than 0.01 mg/L.

C2099 (C) GB FF

- 6 Preferably, the effective carbon surface area masked by the binder is from O.S to 10% of the carbon surface area, more preferably from 0.5 to 5% of the carbon surface area.

5 The binder can suitably be sized between 10-1000 microns, preferably 10300 microns, more preferably 20-50 microns.

The proportion of binder to PAC is suitably in the range of l:loO to 50:100, preferably 5:100 to 40:100, more preferably 10 10:100 to 30:100.

The particulate carbon block filtration medium of the invention is suitable for gravity fed filtration and can be used along with other known filtration media such as 15 sediment filters for removing fine dust and other micro-particulates. The sediment filter is suitably a washable or replaceable non-woven fabric and is preferably a microporous fabric with hydrophilic character.

20 The particulate carbon block filter medium of the invention is adapted to provide for removal of chemical contaminants and more importantly provide for effective removal of at.

least 99.95% of particulate contaminants of sizes in the 3 to 6 micron range in gravity fed filtration units (including 25 the removal of very small chlorine resistant cysts such as Giardia and Cryptosporidium Parvum) without affecting desired flow rate. The Cryptosporidium cyst can be removed from an input load of lo^5 cysts/L down to 100 cysts/L or lower, thereby giving 99.9% or greater 30 removal efficiency. A combination of water-borne cysts (Cryptosporidium, Giardia and Entamoeba) can be removed to I an efficiency of 99.95% or greater.

C2099(C) GB FF

- 7 - Advantageously, the filtration medium of the invention makes it possible to achieve the desired level of particulate filtration as well as flow rate. The filtration medium is also reusable, since efficacy can be regenerated by simple 5 washing and reverse flow techniques. Reverse flushing can be done under tap water or by reversing the carbon block within the gravity filter device so that the entire head of water in the upper chamber of the device flows through the reversed carbon block. The average flow rate can be 10 improved by 20% or more, whenever the flow rate declines to 50% of the original flow rate under gravity by washing and re-using the sediment filter and by using reverse flow to remove trapped particulates. The sediment filter can be washed and rinsed under flowing tap water or by using a 15 small amount (0.1-10 g/L) of fabric wash detergent in water.

By way of the filtration medium of the invention it is possible to attain an average flow rate of water, from a starting height of 200 mm down to 50 mm, under gravity, of 20 100-1500 ml/min, preferably 200-1500 ml/min, more preferably 300-1200 ml/min without compromising on the requirements of removal of particulate contaminants and chemical contaminants. 25 Preferably the particulate carbon block filtration medium of the invention is used as a component of a filter unit which comprises the carbon block filtration medium of the invention together with a washable or replaceable sediment filter for removing fine dust and other micro- particulates 30 generally above 3 microns. Preferably the filter unit also comprises a base plate with an orifice for water exit, to

C2099(C) GB FF

- 8 which the carbon block is adhered, and a detachable cover to hold the entire filter as one integral unit.

The base plate is preferably made of polypropylene, 5 polyethylene, ABS or SAN.

The detachable cover is preferably made of polypropylene, polyethylene, ABS or SAN.

10 The particulate carbon block filtration medium of the invention may be shaped and sized according to its desired end use. Suitable shapes include a flat circular disc of low thickness, a square disc of low thickness, a low height tapered flat disc, a cylinder, a solid cone or a hollow 15 cone.

The particulate carbon block filtration medium of the invention as described above is suitably prepared by mixing the PAC and hydrophilic binder, placing the mix in a mould 20 of the desired shape which is coated with a mould-release agent, sealing the mould, heating the mould in an oven, cooling the mould and opening to release the carbon block.

For assembling the carbon block thus obtained on a base 25 plate the carbon block is attached to the base plate using an adhesive.

In the above process the binder curing temperature is suitably between 100-500 C, preferably 150-300 C, more 30 preferably 170-250 C.

C2099(C) GB FF

9 The proportion of binder to PAC is suitably 1:100 to 50:100, preferably 5:100 to 40:100, more preferably 10:100 to 30:100.

5 The mixing of the PAC with the binder is preferably carried out in a vessel such as an agitator, a mixer with dulled impeller blades, a ribbon blender, a vibratory mixer, a rotary mixer or any other low shear mixer that does not significantly alter the particle size distribution.

The mould is suitably made from aluminum, cast iron, steel or any engineered polymer capable of withstanding temperatures exceeding 300 C. The shape of the mould is suitably such that a carbon block of one of the following 15 shapes can be produced: a flat circular disc of low thickness, a square disc of low thickness, a low height tapered flat disc, a cylinder, a solid cone, a hollow cone, or any other practical shape as desired for the application.

20 The mould release agent is either silicone oil, aluminum foil or any other commercially available mould release agent that has little or no adsorption onto activated carbon.

The cap used for sealing the mould suitably has a variable 25 inner height, adapted to provide the required degree of compression within the carbon block. For the flat disc carbon block shape, the inner height can suitably be O to 10 mm, preferably O-S mm, more preferably 0-2 mm.

30 The oven used in the process may suitably be a non-

convection, forced air or forced inert-gas convection oven.

The temperature of the oven is suitably maintained at 100

C2099(C) GB FF

- 10 500 C, preferably 150-300 C, more preferably 170-250 C. The duration of heating is suitably 0.1-600 minutes, preferably 10-300 minutes, more preferably 10-180 minutes.

5 The adhesive used to secure the carbon block to a base plate is suitably any food grade hot-melt adhesive (such as Eva bond) that melts at temperature between 60-200 C.

The invention will now be further illustrated by the 10 following nonlimiting Examples: EXAMPLE 1

90 g of PAC (coconut-based, acid washed, 80 - 120 mesh, 60 15 CTC, from Active Carbon (Pvt.) Ltd) was mixed with 22 g of -

Bisphenol-A epoxy resin binder (hydrophilic, finer than 200 mesh, Araldite AT 1-1 from Vantico Polymers) in a rotary blender for 30 minutes.

20 A flat cone aluminum mould having 150 mm internal diameter and a maximum height (at the centre) of 20 mm was manufactured with necessary considerations for shape integrity and mould release, to produce a carbon block as shown in Figure 1. A sheet of thin aluminum foil was fitted 25 inside the mould to aid in mould release.

The PAC-binder mixture was transferred to the mould and the mould was tapped to ensure compact fill of the mixture inside the mould. Then, the mould was sealed using a 30 aluminum plate on the top uniformly on all sides using 4 -

equally spaced screws. The screws were tightened using a wrench (spanner).

C20g9 (C) GB FF The filled and sealed mould was then placed inside a muffle furnace at ambient temperature. Temperature was set to 220 C (curing temperature) and the furnace took about 10 minutes to reach the setpoint temperature. The furnace maintained 5 at 220 C for a period of 2 hours, after which the mould was removed from the furnace and allowed to cool to ambient temperature. Then the top plate was un-screwed and the carbon block was 10 removed from the mould by carefully tapping on the mould and using the aluminum foil as an aid to mould release. The aluminum foil was then carefully peeled off from the carbon block. 15 The carbon block was then affixed to an acrylic base plate with a central hole of 7 mm internal diameter using a hotmelt adhesive that softens between 50-100 C. A non-woven microporous polypropylene fabric (from Pacific Non-wovens, Inc.) was cut to appropriate size to fit the exterior 20 surface of the carbon block and held tightly to the carbon block in such a manner to avoid bypass of water.

The assembled unit as described above was fitted into a rectangular chamber of 250 mm length x 210 mm width x 220 mm 25 height. Municipal tap water in Mumbai was taken and spiked to achieve the following characteristics: 5 NTU turbidity -

generated by adding 40 mg/L of Arizona fine dust of 1-100 microns in size and 500 mg/L of total dissolved solids was achieved by adding Sigma sea salts. -

The spiked municipal tap water was poured to a height of 190 mm in the top chamber fitted with the carbon block filter

C2099(C) GB FF

- 12 and allowed to filter by gravity. Flow rate of the assembled filter was measured as an average over 7 L of water filtered and collected at the bottom. Flow rate was measured periodically and the turbidity of the filtrate (in 5 NTU) was measured at various points during filtration lifetime. The results are reported in Table 1.

C2099 (C) GB FF

TOUSLE 1

Litres Average flow rate Filtrate NTU Filtered (ml/min.) 7 1166

56 1166

105 1000

133 1000 0.47

203 777

259 583

266 583 -

315 538 0.8

399 368

427 269 0.6

497 350 0.6

525 175 sediment filter washed and re-used 532 700 -

602 538

609 466

651 280

693 212 0.54

721 389 Sediment filter rinsed and re-used 812 280 0.66

875 241

_... _ _. _

Table 1 demonstrates the flow rate data over several hundred 5 litres of filtration under gravity, indicating that very high flow rates (in the range 200-1100 ml/mint) can be achieved by the flat cone carbon block filter of the present

C2099(C) GB FF

invention. The declining flow rate trend is expected because of the continuous load of dust particles in turbid water (5 NTU) and 500 mg/L of total dissolved solids (TDS) used for the test. Based on a 23-town survey of water 5 quality in India, the levels of turbidity and TDS used for testing the filter are believed to represent the Soth percentile among Indian drinking water sources. The filter consistently reduces the inlet water turbidity to 0.8 NTU or less, thereby delivering superior quality drinking or 10 packaged water in developing and emerging countries. The flow rate recovered significantly when the sediment filter was washed and re-used. This indicates that the carbon block filter of the invention is further capable of providing and maintaining high flow rates under gravity, via 15 periodic washing or rinsing of the sediment filter.

EXAMPLE 2

Another carbon block filter along with sediment filter was 20 prepared using the same materials and procedure described in Example 1.

The assembled unit as described above was fitted into a rectangular chamber of 2SO mm length x 210 mm width x 220 mm 25 height. Municipal tap water was filled to a height of 190 mm during each filtration cycle. This was allowed to filter by gravity till a total 126 have been filtered. The average flow rate (over 7 L) data are given in Table 2.

C2099(C) GB FF

TABLE 2

Liters Average flow Filtered rate (ml/min.) 7 163 14 538

28 636

as 1166 42 1000

49 1166 -

56 875

63 875

70 1000 -

77 1000

84 1000

91 1000

98 1000

105 1000 -

112 875

119 875

126 875

Cyst removal capacity and efficiency was measured using 3-

micron fluorescent microspheres and was carried out using 5 the following procedure:

C2099 (C) GB FF

Materials Feed Water: 1.23 x 10 Microspheres per liter of BIS/Plain water. (See Preparation section) Filter housing chamber 5 Vacuum Pump Filter assembly 0.45p Millipore filter discs (47 mm) Tween 80 Glassware: Conical flasks (250 ml), Measuring Cylinders (100 10 ml), pipette (1 ml).

Fluorescent Microscope.

Preparation: A concentrated fluorescent polymer microsphere suspension (Catalog no. G0300, reportedly having 7.4 x 10 15 microspheres/ml with each microsphere having 3 micron mean diameter, 0.1 micron standard deviation, from Duke Scientific, Palo Alto, CA 94303, USA) was purchased.

101 of the above solution + 101 Tween 80 + 9.98 ml of 20 distilled water were taken. The stock preparation was vortexed for 10 min to ensure uniform distribution of the microspheres. Tween 80 was used as dispersant. (as per ANSI/NSF 53-2001 protocol). It is important to use the stock solution with 5 days of preparation (as per ANSI/NSF 2S 53-2001 protocol) Feed water was prepared by adding 1 ml of the stock solution to 6 liters of plain water. This resulted in a microsphere concentration of 1.23 x 10 particles/L.

C2099(C) GB FF

- 17 Procedure: Spike: 1. Transfer 6 L of feed water in the top chamber of 5 dimensions described earlier.

2. Collect 100 ml of feed water as inlet sample.

3. Collect 5 liters of the filtrate as 5 samples of one litre each (marked 1 to 5 in the attached report format). 10 4. After collection of 5 samples, drain off remaining spike. 5. Fill the filter chamber with 5 liters of plain municipal tap water.

6. Collect 1 liter of water. This is wash out sample.

15 7. The samples collected above are analyzed as described below. Analysis of the Spike Filtrate 20 1. 100 ml of each collected sample was used and passed through the 0.45p Millipore filter disc using filter assembly and vacuum pump.

2. Each filtered disc was allowed to air dry for at least 5 -

hours under ambient conditions.

25 3. Microspheres were then counted on each filtered disc at 40X magnification using a Fluorescent Microscope. (as per EPA-ICR method 814B-95-003 chapter 6) as follows: Count 20 fields randomly spanning all over the disc.

Ensure a gap of at least 4 fields between each field

30 counted.

C2099(C) GB FF

6. Steps 1-7 were repeated after passing 25 liters of municipal tap water through the filter.

7. The removal efficiency is calculated as follows: Log reduction (X) = log ((740000/6)/(M*4000)) 5 Where M is mean of the mean of microspheres per field

observed in 5 fractions (of 100 ml each) collected during each spike cycle. Since M is based on an average of 20 fields randomly counted (with filter disc having

400 fields), M x 400 gives the total microspheres

10 collected for each disc (100 ml). M x 4000 gives the total microspheres over one L of collected water.

% removal = 100 (1 - (1/lO^X)) The results (averaged per Liter over 6 L cycle) obtained 15 for the carbon block are given in Table 3.

C20g9 (C) GB FF - lg TABLE 3

: inlet Count Piltrate: i' Removal All AIMS Count . Spike 1 1.23 E+05 /L 2.23 E+03 /L 98.18 Washout 4 /L Spike 2 1.23 E+05 /L 2.00 E+03 /L 98. 37 Washout 2 /L TOTAL (2 1.48 E+06 2.54 E+04 98.28

spikes) The above demonstrate that the filter of the invention was capable of producing a consistently higher flow rate when 5 tested under municipal drinking water (without added turbidity or TDS). The same filter delivered a total removal of 98.28% (log 1.76) of the 3-micron microspheres, following the test procedure detailed above. During the two spikes of 3-micron spheres administered during the test, 10 nearly 1.5 million microspheres were loaded in the test system. This corresponds to a cyst load of about loA3 cysts/L continuously applied over 1500 L. This scenario represents an extreme case of poor water quality. The filter gave high removal efficiency even under worse case 15 microsphere load. The accelerated high load microsphere results enable one to predict the lifetime of the gravity filter even under worse case cyst load (10A3/L) in drinking water sources.

20 Under gravity-fed water filter application described in the examples, a removal efficiency of 96.84-99% (1.5-2 logs) for the 3-micron microspheres was found to correspond to a removal of 99.98\ (3.8 logs) for Cryptosporidium Parvum

C2099(C) GB FF

- 20 cysts and to a removal of 99.994% (4.2 logs) of an equi-

number mixture of Cryptosporidium Parvum, Giardia Lamblia and Entamoeba Histolytica cysts.

5 EXAMPLE 3

100 g of PAC (coconut-based, 160 x 200 mesh from Active Carbon (Pvt.) Ltd) was mixed with 45 g of solvent-free epoxy resin + hardener (Lapox C-50 + K-61 hardener from Atul 10 Limited) in a rotary blender for 30 minutes.

The remaining steps followed were identical to those described in Example 1 except curing temperature. The mould was cured at 140 C for 3 hours followed by 150 C curing for 15 a period of 5 hours.

The complete assembled unit thus prepared was tested for flow rate using municipal tap water in the same top-chamber and procedure described in Example 2. The results are given 20 in Table 4.

TABLE 4

Liters Average flow Filtered rate (ml/min.) 7 52 14 58 21 39 Microsphere removal tests were conducted on this block as 25 well using the same procedure described in Example 2.

Results are reported in Table 5 (averaged per Liter over 6 L cycle).

C2099(C) GB FF

- 21 TABLE 5

Inlet Count Filtrate % Removal ::. ,,

Count i : Spike 1 1.23 E+05 /L 1.60 E+03 /L 98.70 Washout 4 /L Spike 2 1. 23 E+05 /L 1.48 E+03 /L 98.80 Washout 3 /L TOTAL (2 1.48 E+06 1.85 E+ 04 98.75

spikes) 5 Example 3 above illustrates a case where the average flow rate is comparatively much lower than in Examples 1 and 2.

This illustrates an important aspect involving the selection of the particle size of the key raw material in making the carbon block filter of the invention. A considerably finer 10 particle size of PAC (160 x 200 mesh) used in Example 3 is the main reason for the lower flow rates observed. This corresponds qualitatively with increased removal of microspheres. The low flow rate filter delivered a removal of about l.9 logs (g8.71%) compared to 1.76 logs (98.28%) 15 from the high flow rate filter. In percentage terms, the difference might not appear to be great, but such differences are significant in terms of remaining viable counts in treated water, during tests with live cysts.

20 The microsphere washout data reported in Tables 3 and 5 are an important indicator of the retention capacity of the carbon blocks filters of the invention. The low washout figures indicate that the carbon block is able to retain

C2099 (C) GB FF

- 22 practically all the microspheres it removed during the spike cycles. This is an important measure that indicates that live cysts, once filtered by the carbon block, are unlikely to be released back into the treated water.

EXAMPLE 4

60 g of PAC (70 x 200 mesh, acid washed, coconut-based) and 12 g of hydrophobic dual-system binder (Product DP-460 from 10 3M) were used to make the carbon block using the process as described in Example 1 except that the curing conditions were 160 C for one hour, as recommended for the binder.

The carbon block was fitted into a rectangular chamber of 15 250 mm length x 210 mm width x 220 mm height. Municipal tap water in Mumbai was filled to a height of 190 mm. This was allowed to filter by gravity. The carbon block did not give any appreciable flow of water through the filter even after 24 hours of submersion under 190 mm of water column.

Example 4 illustrates the other important aspect residing in selecting the binder material used in the carbon block filter of the invention. Under this example, a case where a hydrophobic binder is used to make a carbon block, the 25 filter did not give any appreciable flow under gravity.

This behavior is similar to that expected from carbon blocks as such known and used under high in-line pressures (30-60 psig). This illustrates the importance of choosing the right binder with reduced hydrophobicity and low surface 30 coverage, proportion and method of manufacture in getting to a high flow rate cyst removal filter media for gravity fed filtration in accordance with the present invention.

C2099 (C) GB FF

- 23 Thus the present invention provides for an improvement in filtration media for gravity fed applications involving a selective combination of particulate activated carbon and binder which achieves high flow rate in gravity fed 5 applications compared to the use of carbon block per se, and also avoids the complexities of sheet filters or glass fibre based filters. Additionally, apart from achieving high flow rate under gravity the filter of the invention as also demonstrated above is capable of removal of particulate 10 contaminants including biological contaminants such as cysts like cryptosporidium. Importantly, the filter of the invention provides an additional advantage by way of its adaptability to be regenerated for effective filtration by washing and reverse flow.

Claims (5)

C2099(C) GB FF - 24 CLAIMS

1. A particulate carbon block filtration medium which is suitable for use in gravity fed filters and which 5 comprises powdered activated carbon (PAC) having a particle size in the range of 6 to 325 mesh, and a hydrophilic binder material, in which the effective carbon surface area masked by the binder is less than 10.

2. A filtration medium according to claim 1, in which the PAC is in the size range-of 80 to 120 mesh.

3. A filtration medium according to claim 1 or claim 2, in 15 which the hydrophilic binder material is a polymer comprising one or more of the repeating groups: olefins, bisphenol.

4. A filtration medium according to any one of claims 1 to 20 3, in which the effective carbon surface area masked by the hydrophilic binder material is from 0.5 to 10% of the carbon surface area, more preferably from 0.5 to 5% of the carbon surface area.

25

5. A filtration medium according to any one of claims 1 to 4, which is used as a component of a filter unit together with a washable or replaceable sediment filter.