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All of our fridge filters use either granular activated carbon (“GAC”) or extruded activated carbon (“carbon block”), and are the same as we use for our plumbed-in water dispensers. GAC has a labyrinthine, pore-like structure, with up to 1500 square metres of surface area per gram of carbon. That’s about a football pitch of surface area for every 5 grams. Quite literally, chlorine and other gaseous contaminants get trapped (“adsorbed”) within the structure by weak molecular forces known as Van Der Waals forces.
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As the name suggests, Granular Activated Carbon consists of carbon granules. If you shake a GAC filter you can often hear the granules rattling. If these become unevenly distributed within the filter body, then channelling of the water can occur, reducing the effectiveness of the filter. “Carbon block” is made by pulverising the carbon, heating it and then extruding it in block form. The result is a finer, more efficient material, with even greater surface area and with even greater adsorbtion properties than GAC, and without the possibility of channelling.
What about the type of activated carbon used? All other things being equal, carbon from coconut shells is generally the best performing sort. Other sources, such as anthracite are not generally as effective. Since an inline fridge freezer or under-sink filter acts as a sediment filter too, it is generally always the case that a carbon block filter is superior to any sort of GAC filter, regardless of the type of carbon used, unless that is, the basic quality of the extruded carbon block is poor, or the construction is so poor that water is allowed to byass the carbon block, within the housing.
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