WO2005094966A1

WO2005094966A1 - Filter media

Info

Publication number: WO2005094966A1
Application number: PCT/EP2005/001774
Authority: WO
Inventors: Sharadchandra Govind Bansode; Dhulipala Ravikumar; Jaideep Chatterjee; Velayudhan Nair Gopa Kumar
Original assignee: Unilever N.V.; Unilever Plc; Hindustan Lever Limited
Priority date: 2004-03-16
Filing date: 2005-02-17
Publication date: 2005-10-13
Also published as: PL380664A1; BRPI0508689A

Abstract

The invention concerns carbon block filter media for use in gravity fed filters comprising powder activated carbon (PAC) having a particle size distribution such that 95 wt% of the particles pass through 50 mesh and not more than 13 % passes through 200 mesh, and a binder material having a Melt Flow Rate (MFR) of less than 5 g/10 min. The invention also provides water filters and a process for the preparation of a carbon block filter medium. The invention finally provides a process for the purification of water whereby the water under the influence of gravity is passed first through a washable or replaceable sediment filter for removing fine dust and other particulates above 3 µm and thereafter through a carbon block filter medium comprising PAC and binder material.

Description

FILTER MEDIA

Field of the invention The present invention relates to a carbon block filter media for use in gravity-fed filtration units for filtering particulate contaminants including microorganisms like cysts, bacteria and viruses from a liquid, as well as for removal of chemical contaminants, while at the same time maintaining high flow rates.

Background and prior art Fluids such as liquids or gases typically contain contaminants, which include particulates, chemicals and organisms. In liquids like water, especially drinking water, it is desirable to remove the harmful contaminants before consumption so as to maintain proper hygiene and safe conditions for maintenance of good health.

Several different methods are known for filtration of water and various devices and apparatus have been designed and are commercially available. These methods and devices vary depending on whether the application is for industrial use or for household use.

Water treatment for household use is basically directed to providing safe drinking water. The methods and devices typically used in households for water treatment can be classified based on whether water is available under pressure, like the high head of water from an overhead tank, or whether water is available in small quantities, like a few litres in which case the pressure available is close to gravity. Filtration of water in the former case is classified under online systems, which have the advantage of the pressure of the water flow to drive the filtration and therefore do not usually face problems of achieving desired flow rate while catering to the basic need of effective filtration.

Filtration of water in the latter is classified under self- contained systems, which process water in batches under low gravity heads. It is a challenge to ensure the necessary removal of the undesirable contaminants by filtration under such low gravity head, while ensuring the desirted high flow rate . Thus, when filters designed for online systems are applied to gravity fed systems they fail to produce the desired flow rates consistently over time and often choke after a few litres are passed through, leading to the need for replacement or cleaning.

It has thus been a real challenge to provide effective filtration media for self- contained systems which would on one hand provide for the desired flow rate and at the same time cater to the much required removal of particulate contaminants including biological contaminants, such as protozoan cysts like cryptosporidium and even smaller organisms like bacteria and viruses, in addition to dissolved chemicals like chlorine, organics and pesticide residues .

Typically, a self-contained system for domestic application has an upper and a lower chamber separated by a filter cartridge wherein the water to be treated is poured in the upper chamber and allowed to flow by gravity through the cartridge to the store in the lower chamber. The treated water in the lower chamber can be dispensed for consumption 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 resin for removing metal and other ions from water is also known.

It is also known to have gravity fed water filters for domestic use such as carafe filter in which water enters the filter element through a series of small perforations at the wider top of a trapezoid shaped filter 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 absorbent particles and the water pressure is low in such systems, unlike in online systems which have the advantage of high pressure and thus achieve high flow rates . Attempts to increase flow rate in gravity fed systems by using larger particulates lead to slower adsorption kinetics and thus loss of effective filtration.

Conversely, use of relatively small particles lead to greater flow restriction.

US5505892 describes a process for manufacturing a filter unit made as a moulded absorbent element permeable to gases and liquids, comprising (i) mixing granules of absorbent medium and a granular organic thermoplastic binder medium (ii) placing in a mould and compressing it (iii) heating to a temperature of about 330 to 350 °C until partial coking of the binder medium occurs and (iv) cooling. The publication describes use of carbon of the granular type i.e of granule size 2 mm to 3 mm.

EP 0345381 (American Cyanamid, 1989) describes a filter structure for use in the purification of a liquid comprising activated carbon particles entrapped within a porous plastic matrix wherein the size of the activated carbon particles is 5-150 μm. The extremely fine carbon particles are bonded with thermoplastic bonding agents of sizes substantially greater than on average, the particle size of the activated carbon particles. The activated carbon particles are uniformly distributed throughout the porous plastic matrix structure.

The above prior art describe filter units comprising carbon particles of either very high granularity or very fine carbon dust and would not be suitable to meet the high demands of microparticle especially microorganism separation from water filtered under low gravity head while consistently maintaining the desired high flow throughput .

US4753728 (Amway 1988) describes a carbon particle filter comprising carbon particles bonded into a filter block by a low melt index polymeric material having a melt index of less than 1 gram per 10 minutes as determined by ASTM D1238 at 190 degree C, and 15 kilograms load, whereby said polymeric material will tackify at elevated temperatures without becoming sufficiently liquid to substantially wet the carbon particles.

The above prior art does not provide for the desired high flow rate of the water under gravity flow conditions over long period of time while consistently removing the contaminants desired to be removed .

Thus it has been a challenge to provide gravity-fed filters which would have effective flow rates and at the same time desired filterability and acceptable adsorption kinetics .

Summary of the invention

The present invention provides carbon block filter media comprising powdered activated carbon of a selected particle size distribution and binder material having selected melt flow characteristics and specified particle size distribution. The filter media have been engineered to have desired particle size distribution profile across height of the bed. The inventors have also developed improved process for preparing the above filter media.

It is thus an object of the present invention to provide filtration media for use in gravity fed filtration units which provides the desired particulate removal including micro-organisms like cysts, bacteria and virus while giving the desired flow rate. Thus, the filter mediaprovide for effective filtering leading to up to 99.9% removal of even chlorine resistant cysts such as cryptosporidium parvum and Giardia Lamblia which are in the size range of 3 to 6 μm and other microorganisms like bacteria which have size in the range of 1 to 0.1 μm with a removal efficiency of more than 99.99% and even smaller organisms like virus up to 99% removal, without affecting flow rate. Furthermore, the filter media provide removal of chemicals including pesticides and removal of bad odour. It is another object of the invention to provide a process for preparing the filter media outlined above.

Furthermore it is an object of the invention to provide a process for purifying water using the filtermedia outlined above .

Detailed desciption of the invention

In one aspect the invention provides carbon block filter media for use in gravity fed filters comprising: (a) powder activated carbon (PAC) having a particle size distribution such that 95 wt% of the particles pass through 50 mesh and not more than 13% passes through 200 mesh and (b) a binder material having a Melt Flow Rate (MFR) of less than 5g / lOmin.

Preferably, 95% of the PAC particles also passes through 60 mesh. Also preferably not more than 12% passes through 200 mesh. Across their height the particle size distribution in the carbon block filter media is preferably not uniform and it is preferred that 55 to 80 wt % of the PAC particles in the particle size range of 100 to 200 mesh is localised in the lower 50 volume% of the carbon block filter media. It is further preferred that the carbon block filter media have 55 to 95 wt% of the PAC particles in the size range smaller than 200 mesh localised in the lower 50 vol% of the carbon block filter media In another aspect the invention provides water filters for use in gravity fed water filtration equipment comprising: (a) a washable or replaceable sediment filter for removing fine dust and other particulates above 3 μm, (b) a carbon block filter medium comprising the PAC and the binder material as herein described;

(c) a base plate with an orifice for water exit, to which the carbon block is attached; (d) a housing or cover to hold the entire filter as one integral unit.

The cover or housing is preferably detachable.

In yet another aspect the invention provides a process for the preparation of a carbon block filter medium comprising the steps of (a) intimately mixing powder activated carbon (PAC) having a particle size distribution such that 95 wt% of the particles pass through 50 mesh and not more than 13% passes through 200 mesh with binder material having a Melt Flow Rate (MFR) of less than 5 in a mixer (b) compacting the mix in a vibratory compactor (c) further compacting the mix in a mould of desired shape and size by applying a pressure of not more 20 kg/cm² (d) heating the mould to a selected temperature (e) cooling the mould and releasing the carbon block from the mould.

The PAC is preferably prepared form carbon sources selected from bituminous coal, coconut shell, wood, or petroleum tar. The surface area of the PAC is preferably above 500 m²/9 more preferably exceeds 1000 m²/g. The particle size of the PAC is selected such that 95 wt% of the particles pass through 50 mesh, preferably 60 mesh, and on the other hand not more than 13%, preferably not more than 12%, more preferably not more than 10% passes through 200 mesh.

Preferably, the PAC has a size uniformity co-efficient of less than 2, or more preferably less than 1.5, a carbon tetrachloride number exceeding 50%, more preferably exceeding 60%. The PAC preferably has an Iodine number greater than 800, more preferably greater than 1000.

As outlined above, the carbon block filter media preferably have a particle size distribution profile across their height. It is preferred that the PAC particles are distributed across the height of the carbon block such that 55 to 80 wt%, preferably 55 to 70 wt% of the PAC particles in the particle size range of 100 to 200 mesh are present in the lower 50 volume% of the carbon block filter media.

It is also preferred that the PAC particles are distributed across the height of the carbon block such that 55 to 95%, more preferably 60 to 95% of the PAC particles in the particle size range smaller than 200 mesh are present in the lower 50 vol% of the carbon block filter media.

The bulk density of the binder material used in the invention is preferably not more than 2.5 g/cm³, more preferably < 0.6 g/cm³, even more preferably ≤ 0.5 g/cm³, most preferably < 0.25 g/cm³.

The binder material is selected to have a melt flow rate (MFR) of less than 5 gram/ 10 minutes, preferably less than 2 gram/ 10 minutes, more preferably less then lg/ lOmin. The binder material preferably has a particle size distribution similar to that of the the PAC but the amount of particles passing 200 mesh is preferably less than 40 wt%, more preferably less than 30 wt%. Preferably the particle size distribution of the binder is substantially the same as that of the PAC.

The melt -flow rate (MFR) is measured using the ASTM D 1238 (ISO 1133) test. The test measures the flow of a molten polymer through an extrusion plastometer under specific temperature and load conditions. The extrusion plastometer consists of a vertical cylinder with a small die of 2 mm at the bottom and a removable piston at the top. A charge of material is placed in the cylinder and preheated for several minutes. The piston is placed on top of the molten polymer and its weight forces the polymer through the die and on to a collecting plate. The time interval for the test ranges from 15 seconds to 15 minutes in order to accommodate the different viscosities of plastics. Temperatures used are 190, 220, 250 and 300 °C (428, 482 and 572°F). Loads used are 1.2, 5, 10 and 15 kg. For the present invention the tests are done at 190 °C at 15 kg load.

The amount of polymer collected after a specific interval is weighed and normalized to the number of grams that would have been extruded in 10 minutes: melt flow rate is expressed in grams per reference time.

The binder material is preferably a thermoplastic polymer having a MFR value above described. Suitable examples include ultra high molecular weight polymer preferably polyethylene or polypropylene which have these low MFR values. The molecular weight is preferably in the range of 10⁶ to 10⁹. Binders of this class are commercially available under the trade names HOSTALEN (from Tycona GmbH) , GUR, Sunfine (from Asahi) , Hizex (from Mitsubishi) 5 and from Brasken Corp (Brazil) . Other suitable binders include LDPE sold as Lupolen (from Basel Polyolefins) and LLDPE from Qunos (Australia) .

The proportion of the binder material to the PAC particles 10 by weight is preferably chosen between 1:1 and 1:10, more preferably between 1: 2 and 1:6.

The above disclosed carbon block filter media of the invention are able remove chemical contaminants and more 15 importantly at least 99.9% of cysts such as Giardia La blia, Cryptospordirium Parvu and Entamoeba Histolica, 99.99% of bacteria and 99% of viruses without considerably affecting the flow rate.

20 Unlike thew filter media of the prior art, the filter media of the present invention do not require washing and reverse flow flushing at regular intervals to ensure high flow rates. However, reverse flushing can be done under tap water or by reversing the carbon block within the gravity

25 filter device to bring about minor improvement.

By way of the above filtration media of the invention it is possible to attain average flow rate of water, from a starting height of 200 mm down to 50 mm, under gravity of 30 100-300 ml/min. , preferably 120-200 ml/min. , without compromising on the requirements of removal of particulate matter including microorganisms, and chemical contaminants. The water filters according to the invention comprise a carbon block filter medium of the invention. The water filters also comprise a sediment filter which may be washable or replaceable and is preferably a woven or non- woven fabric, more preferably a non-woven fabric having micropores. This sediment filter is used as a prefilter and has a pore size suitable to retain particles generally above 3μm. The sediment filter can be washed and rinsed under flowing tap water or by using a small amount (0.1-10 g/L) of fabric wash detergent in water. This use of the sediment filter facilitates wide and extensive application of the carbon block filter media of the invention by preventing the filter media from becoming choked with sedimen ..

According to a preferred embodiment of the invention the carbon block filter medium is attached to a base plate with an orifice for the water exit and additionally comprises a detachable cover. The base plate is preferably made of plastic such as polypropylene, polyethylene, ABS, SAN. The detachable cover is preferably also made of: polypropylene, polyethylene, ABS, SAN.

The filter medium can be of any desired shape and size. Suitable shapes include flat circular disc of low thickness, square disc of low thickness, low height tapered flat disc, cylinder, solid cone, hollow cone, solid or hollow hemisphere, etc.

It is further preferred to include a bed of granular adsorbent particles in said water filter, such that the water to be filtered passes through said bed of granular adsorbent particles before passing through the carbon block filter medium. The granular adsorbent particles are preferably granular activated carbon. The granular adsorbent particles preferably have a particle size in the range of 200 to 5000μm, more preferably 200 to 2000μm, most preferably 500 to 1500μm.

Thus, according to a preferred aspect of the invention, there is provided a water filter for use in gravity fed applications comprising: (a) a washable or replaceable sediment filter for removing fine dust and other particulates above 3μm; (b) a bed of granular adsorbent particles (c) a carbon block filter medium comprising the PAC and the binder material as herein described (d) a base plate with an orifice for water exit, to which the carbon block is attached; (e) a cover or housing to hold the entire filter as one integral unit.

Preferably, the bed of granular absorbent particles is provided so as to enable quick and easy replacement of the particles, either by providing them in a separate housing which may be removed, and if desired emptied and filled with a new charge of particles, and refitted again, or in a housing attached to the sediment filter to be removed and replaced with the sediment filter, or combined in one housing with the carbon block folter medium.

The inclusion of the granular adsorbent particles in the water filter enables filtration of a significantly higher amount of input water over an extended time period thereby ensuring more efficient utilization of the carbon block filter media of the invention. Additionally, the granular adsorbent particles in the water filter enables effective filtration of highly contaminated water containing high amounts of fine particles like dust and dissolved impurities like iron and aluminium salts.

As outlined above, the invention also provides a process for the preparation of carbon block filter media comprising the steps of intimately mixing PAC with binder material in a mixer, compacting the mixture in a vibratory compactor, further compacting the mix in a mould, heating the mould, cooling the mould and releasing the carbon block from the mould.

For the step of mixing the PAC and the binder material any low shear mixer that does not significantly alter the particle size distribution is suitable, such as a mixer with dulled impeller blades, ribbon blender, rotary mixer. The mixing is carried out to prepare a uniform mix of the PAC and the binder material and is preferably carried out for at least 15 minutes, more preferably 20 to 60 minutes.

The compaction of the mix is carried out in a vibratory compactor to obtain the desired particle size distribution profile across the height of the carbon block.

The vibratory compaction is preferably carried out in a vibrator having a frequency in the range of 30 to 100 Hz. This process step is preferably carried out for a period of at least one minute, more preferably for 3 to 30 minutes. The compacted mass is then placed in a mould of preselected size and shape and subjected to a pressure of not more than 20 kg/cm², preferably not more than 10 kg/cm². The pressure is preferably applied using either a hydraulic press or a pneumatic press, more preferably a hydraulic press .

The mould is made from aluminum, cast iron, steel or any suitable material capable of withstanding temperatures exceeding 400 °C.

A mould release agent is preferably coated on the inside surface of the mould. The mould release agent is preferably selected from either silicone oil, aluminum foil or any other commercially available mould release agent that has little or no adsorption onto activated carbon or the binder material .

The mould is then heated to a temperature of 150 to 400 °C, preferably in the range of 180 to 320 °C, depending on the binder material that is used. The mould is kept heated for a period to time of more than 60 minutes, preferably between 90 and 300 minutes depending on the size and the shape of the mold, and sufficient to ensure uniform heating of the contents of the mould. The mould is preferably heated in an oven such as a non-convection, forced air or forced inert-gas convection oven.

The mould is then cooled and the carbon block released from the mould.

Finally, the invention provides a process for the purification of water whereby the water under the influence of gravity is passed first through a washable or replaceable sediment filter for removing fine dust and other particulates above 3 μm and thereafter through a carbon block filter medium comprising PAC and binder material as herein above described. Preferably the water is also passed through a a bed of granular adsorbent particles as herein above described between passing through the sediment filter and the carbon block filter medium. The starting pressure before entering the sediment filter is preferably not more that 3000 mm water column, more preferably at most 1000mm water column, most preferably does not exceed 500mm water column.

The details of the invention, its objects and advantages are explained hereunder in greater detail in the non- limiting examples:

Examples

Example 1 :

A carbon block was prepared by taking 100 gram PAC from Active Carbon (India) and 30 gram of ultra high molecular weight polyethylene with MFR - 0 and bulk density of 0.22 g/cm³ from Asahi (Japan) . The PAC had 6.5 wt% of the particles passing 200 mesh and 98% of the particles passing through 50 mesh. The powders were mixed in a ribbon mixer for 30 minutes and transferred to a mould. The mould was then vibrated on a vibrator for 5 minutes and then subjected to a hydraulic pressure of 10 kg/cm². The mould was then heated to 260 °C for 150 minutes and then cooled.

Comparative Example A: A carbon block as per Example 1 was prepared except that the binder used was HDPE from Sri Lanka having an MFR of 25. The above two sample carbon blocks were subjected to filtration of water under a gravity head of 75 mm. The filtration characteristics and flow rate over time is summarized in Table-1. The removal efficiency of the microorganisms is measured using the test procedure summarized below Table-1.

Table-1

The data in Table-1 indicate that the carbon block filter media according to the invention (Example-1) provides for very high microorganism removal efficiency while providing for consistently high flow rate under gravity head conditions .

Procedure for determining cyst removal efficiency:

Test of cyst removal capacity and efficiency was measured using 3 -micron fluorescent microspheres and was next carried out using the following procedure:

Materials

Feed Water: 1.23 x 10⁵ Microspheres per liter of BIS/Plain water. (See Preparation section) , filter housing chamber, vacuum Pump, filter assembly, 0.45μ Millipore filter discs (47 mm), Tween 80, Glassware: Conical flasks (250 ml), Measuring Cylinders (100 ml) , pipette (1 ml) , fluorescent microscope . 5 Preparation:

A concentrated fluorescent polymer microsphere suspension (Catalog no. G0300, reportedly having 7.4 x 10⁸ 10 microspheres/ml with each microsphere having 3 micron mean diameter, 0.1 micron standard deviation, from Duke Scientific, Palo Alto, CA 94303, USA) was used. lOμl of the above solution + lOμl Tween 80 + 9.98 ml of 15 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 20 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 10⁵ particles/L.

25 Procedure: Spike : 1. Transfer 6 L of feed water in the top chamber of filter assembly. 2. Collect 100 ml of feed water as inlet sample.

30 3. Collect 5 liters of the filtrate as 5 samples of one litre each (marked 1 to 5 in the attached report format) . Analysis of the spike filtrate

1. 100 ml of each collected sample was used and passed through the 0.45μ 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.

3. Microspheres were then counted on each filtered disc at 40X magnification using a Fluorescent Microscope, (as per EPA-ICR method 814-B-95-003 chapter 6) as follows:

4. Count 20 fields randomly spanning all over the disc. Ensure a gap of at least 4 fields between each field counted.

The removal efficiency is calculated as follows: Log reduction (X) = log ( (740000/6) / (M*4000) ) 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 collected for each disc (100 ml) . M x 4000 gives the total microspheres over one L of collected water. % removal = 100 (1 - (1/10^ΛX))

Procedure to prepare bacterial sample:

The model organism chosen for the test was E. Coli. The following are required for the bacteriological testing: m- Endo or MacConkeys's or VRBA Agar (DIFCO) , APHA Buffer water, Tryptone saline, or PBS, Trypticase soy broth and Agar (DIFCO), deionized water, 0.22 and 0.45 □ sterile millipore membrane filters with filter holders and sterile syringes and forceps . Other requirements for both bacterial and viral testing: RO water, dechlorinated municipal water, AR grade CaCl₂, MgCl₂, CaS0₄, MgS0₄, NaHC0₃, Na₂C0₃, NaOH, freshly prepared sodium hypochlorite (1% solution with 10000 ppm average chlorine) , bottles, pipettes, pH, TDS, conductivity, and Turbidity meters.

Procedure :

1. The test organisms were washed and suspended in tryptone/phosphate buffered saline before addition to the test water.

2. The culture is to be passed once every 24 hours in TSB for two successive days and incubated at 37 °C.

3. On the third day, 0.1 ml of a 24 hour culture is to be added to fresh TSB, agitated and incubated for more than 24 hours at 37 °C.

4. The resultant suspension is to be centrifuges for 10 minutes at 3000 rpm.

5. The supernatant is discarded and the cells washed three times with sterile tryptone or PBS and suspended in PBS.

6. The final concentration for immediate use ( - 5 x 106 cfu/100 ml in the challenge water) is to be determined spectrophotometrically

7. In addition the actual count is to be determined by plate count on TSA/ MacKonkey plates.

Procedure to prepare virus sample: MS2 culture Host strain used is E Coli ATCC 15597.

The host strain medium is prepared using tryptone (10 g) yeast extract (8 g) , NaCl (8 g) , distilled water (1 liter) and agar (15 g) . The sample is filtered, sterilised and added aseptically to the sterile medium. The growth conditions are maintained at 37 °C and in aerobic and stationary conditions .

Procedure for MS2 quantification: 1. Prepare an actively growing ATCC broth culture of the MS2 host by innoculating it into 25 ml of the medium under static and aerobic conditions for 24 hours at 37 °C. 2. Pour the basal agar plates with Escherichia medium agar and maintain at 37 °C.

3. Prepare tubes containing 3.5 ml soft agar and maintain at - 60 °C.

4. Serially dilute to 10^"12 in steps of 1:10.

5. For each of the dilutions pipette out 1 ml of the virus sample in the sugar tube with 3 ml soft agar maintained at 60 °C and then add 250 ml of host culture.

6. Mix and pour to base agar and transfer to plates and allow to set for an hour before incubating for 24 hours at 37°C. 7. Count the number of plaques observed in each plate against dark background. 8. Calculate the number of virus in pfu/ml .

Comparative Example B An experiment as per Example 1 was carried out except that the step of vibratory compaction was not carried out. The data on the microbial removal efficiency is given in Table-2 Table-2

The data in Table-2 indicate that the carbon block prepared using the process according to the invention (Example 1) , which ensures a particle size distribution profile across the height of the block, provides for vastly improved microorganism removal as compared to that prepared by the process of the prior art. (Comparative Example B) .

Water filter-1:

Water filter-1 as depicted in Figure-1 was prepared. The water filter as per figure- 1 comprises a sediment filter (SF) , a carbon block filter medium (CB) prepared as per Example 1 which is adhered to a base plate (BP) .

Water filter-2 :

Water filter-2 as depicted in Figure-2 was prepared. The water filter as per figure-2 comprises a sediment filter (SF) , a carbon block filter medium (CB) prepared as per Example 1 which is adhered to a base plate (BP) . The space between the carbon block filter media and the sediment filter is filled with granular activated carbon (GAP) with a particle size in the range of 500 to 1500 μm.

Examples 2 and 3 :

A test water having 15 mg/1 of fine Arizona test dust and additionally 0.47 ppm of dissolved aluminium cations and 0.4 ppm of dissolved iron cations was prepared. This test water was filtered through the water filters of Figure- 1 (Example 2) and Figure-2 (Example 3) with a constant head of 160mm. The flow rates obtained over extended period of use of the filters are summarized in Table-3. Acceptable quality of filtered water was obtained with both filters.

Table-3

The data in table-3 indicates the high output flow rates of filtered water obtained with the use of the water filter comprising the carbon block filter media according to the invention (figure-1) . Increased flow rate of water was obtained with the use of the water filter according to the preferred aspect of the invention comprising granular adsorbent particles and the carbon block (Figure-2)

Examples 4 and 5 :

A further highly contaminated torture test water having 130 mg per litre of fine Arizona test dust and additionally 4.7 ppm of dissolved aluminium cations and 4 ppm of dissolved iron cations was prepared. This test water was filtered through the water filters of Figure-1 and Figure-2 with a constant head of 160mm. The flow rates obtained over extended period of use of the filters is summarized in Table-4. Acceptable quality of filtered water was obtained with both filters. Table-4

Examples 6 to 10

Further experiments were conducted to determine the particle size distribution obtained across the height of the carbon block using the process of the invention. Five samples of PAC meeting the specification according to the invention were taken and vibrated on a vibratory compactor for 5 minutes at 50 Hz. The top 50 vol% of the sample and bottom 50 vol% of the samples were separated and each sample was then separately sieved on a 200 mesh sieve. The weights of the oversize and the undersize fractions were measured. The data are presented in Table-5.

Table-5

Note: Fines in the above table is the fraction having particle size smaller than 200 mesh. In all the samples from examples 6 to 10 the percentage of the fines passing 200 mesh which was in the bottom 50 vol% of the carbon block was in the range of 55 to 95 wt% of the total of fines passing 200 mesh. All these samples were found to provide good carbon blocks as per the invention.

Representative photographs (magnification of xl50) of the top and bottom surface of the block are shown in Figures 3 and 4 respectively. The photographs clearly indicate the different particle size distributions obtained as a result of the process step of vibratory compaction.

The invention thus provides carbon block filter media, a process for preparing the same, water filters which can be prepared using such carbon blocks and a process for purifying water using the carbon blocks.

Claims

1. A carbon block filter medium for use in gravity fed filters comprising: (a) powder activated carbon (PAC) having a particle size distribution such that 95 wt% of the particles pass through 50 mesh and not more than 13 wt% passes through 200 mesh, and (b) a binder material having a Melt Flow Rate (MFR) of less than 5 (in g/ 10 min.) .

2. A filter medium as claimed in claim 1 wherein 55 to 80 wt% of the PAC particles in the particle size range of 100 to 200 mesh is localised in the lower 50 volume% of the filter medium.

3. A filter medium as claimed in claim 1 or claim 2 wherein 55 to 95 wt% of the PAC particles in the size range smaller than 200 mesh are localised in the lower 50 vol% of the filter medium.

4. A filter medium as claimed in any one of the claims 1 to 3 wherein the MFR of the binder material is less than 2.

5. A filter medium as claimed in claim 4 wherein the MFR of the binder material is less than 1.

6. A filter medium as claimed in any one of the preceding claims wherein the binder material has a bulk density less than 2.5 g/cm³

7. A filter medium as claimed in any of the preceding claims wherein the proportion of binder material to PAC is in the range of 1:1 to 1:10

8. A filter medium as claimed in claim 7 wherein the proportion of binder material to PAC is in the range of 1:2 to 1:6.

9. A filter media as claimed in any preceding claim wherein the binder media is selected from ultra high molecular weight polyethylene or polypropylene.

10. A filter medium as claimed in claim 9 wherein the molecular weight of the binder material is in the range of 10⁶ to 10⁹.

11. A water filter for use in gravity fed water filtration equipment comprising: (a) a washable or replacable sediment filter for removing fine dust and other particulates generally above 3 microns (b) a carbon block filter medium comprising the PAC and the binder material as claimed in any one of the preceding claims (c) a base plate with an orifice for water exit, to which the carbon block is attached; (d) a detachable housing or cover to hold the entire filter as one integral unit.

12. A water filter as claimed in claim 11 wherein the water filter comprises a bed of granular adsorbent particles such that the water to be filtred passes through said bed of granular absorbent particles before passing through said carbon block filter.

13. A water filter as claimed in claim 12 wherein the granular absorbent particles are granular activated carbon.

14. A water filter as claimed in claim 12 or 13 wherein the granular adsorbent particles have a particle size in the range of 500 to 1500 μm.

15. A process for the preparation of a carbon block filter medium comprising the steps of: (a) intimately mixing powder activated carbon (PAC) having a particle size distribution such that 95 wt% of the particles pass through 50 mesh and not more than 13% passes through 200 mesh with binder material having a Melt Flow Rate (MFR) of less than 5 in a mixer; (b) compacting the mix in a vibratory compactor; (c) further compacting the mix in a mould of desired shape and size by applying a pressure of not more 20 kg/cm²; (d) heating the mould to a selected temperature; (e) cooling the mould and releasing the carbon block from the mould.

16. A process for the preparation of a carbon block filter medium as claimed in claim 15 wherein the mix is compacted in the mould by applying a pressure of not more than 12 kg/cm².

17. A process for the preparation of a carbon block filter medium as claimed in any one of the claims 15 or 16 wherein the mixing is done for a period of at least 15 minutes .

18. A process for the preparation of a carbon block filter media as claimed in any one of claims 15 to 17 wherein the vibration compaction is carried out for at least 1 minute at a frequency of at least 30 Hz.

19. A process for the preparation of a carbon block filter media as claimed in any one of the claims 15 to 18 wherein the mould is heated to a temperature of 150 to 400 °C.

20. A process for the preparation of a carbon block filter media as claimed in claim 19 wherein the mould is heated to a temperature of 180 to 320 °C.

21. A process for the purification of water whereby the water under the influence of gravity is passed first through a washable or replaceable sediment filter for removing fine dust and other particulates above 3 μm and thereafter through a carbon block filter medium comprising PAC and binder material as described in any one of claims 1-10.

2. A process as claimed in claim 21 wherein between passing through the sediment filter and the carbon block filter medium the water is also passed through a a bed of granular adsorbent particles.