Activated Carbon Many of us are familiar with the use of charcoal in the old cornerbox filters of our freshwater days. These usually consisted of small, shiny, irregularly shaped pieces of bone or wood charcoal. In this form the carbon has been subjected to extremely high pressures and temperatures to drive out all impurities and gases leaving behind extremely porous and pure grains of carbon. Therefore activated carbon can be tailored to the specific type of impurities that one wishes to remove. By creating such extremely porous structures within the carbon grains we have, in effect, created a gigantic sponge that can absorb many impurities from the passing water.
Adsorption relies on the fact that many organic molecules are polar in nature.
This means that the two ends of a molecule differ in their affinity for water. When a polar molecule comes close to a polar surface such as GAC, they will become attached to each other, effectively removing the molecule from solution. Perhaps the most common mistake made in using GAC in trickle filters is its placement. When GAC is used in outside power filters the instructions always tell you to place the carbon between pads of floss and to make sure that all the filter water flows through it. The sump of the trickle filter should be designed so that all the water must flow through the GAC chamber.