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Granulated Activated Carbon
Carbon is a substance that has a long history of being used to absorb impurities and is the most powerful absorbent known to man. One pound of carbon contains a surface area of 125 acres and can absorb thousands of different chemicals. For centuries, sailing vessels used it to store drinking water for long voyages. Carbon is also commonly used as an effective antidote for swallowed poisons.
Activated carbon is carbon which has a slight electro-positive charge added to it, making it even more attractive to chemicals and impurities. Loose granular activated carbon (GAC) is used extensively in most of the commonly encountered water filters today. Most of these filters have ratings as simple taste and odor filters or as pre-filters designed to remove initial dirt, rocks, sediment, etc. Very few of these GAC filters are effective at true filtration (i.e. removal of substances of health concern). Below are some problems:
"Channeling: When water is forced through a substrate of GAC (which has the consistency of sand or coffee grounds) it takes the path of least resistance and makes its own channels through the GAC in the filter. What this means is that the water being filtered is notcontacting the GAC for enough time to allow complete absorption of impurities to take place. Many impurities seep through along with the "filtered" water.
"Bacterial growth: When the water passes through the GAC, some bacteria (primarily heterotrophic) are trapped in the substrate, too. The problem is that these trapped bacteria multiply prolifically in the warm, moist oxygen-containing environment among the grains of GAC. Because the GAC is loose, there is no barrier to keep any bacteria growing inside the filter from coming out with the water pushed through each time the filter is used. There have been extensive tests showing certain filters have thousands more bacteria coming out of them than initially went into them! Some companies add silver nitrate, a known poison, to the GAC to aid in keeping the bacterial growth down, but this method has limited effectiveness. There are also serious questions about adverse health effects from the silver nitrate itself!
"Effectiveness decreases rapidly: Because the amount of GAC is usually minimal in most GAC filters, they quickly become saturated and overwhelmed with normal household usage. The filters must be changed quite often, which also adds to the expense of filter maintenance.
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