Granular activated carbon (GAC) is used as a filter for vapor or water treatment. GAC is a highly porous adsorbent material, produced by heating organic matter, such as coal, wood and coconut shell, in the absence of air, which is then crushed into granules. Activated carbon is positively charged and therefore able to remove negative ions from the water such as organic ions, ozone, chlorine, fluorides and dissolved organic solutes by adsorption onto the activated carbon. The activated carbon must be replaced periodically as it may become saturated and unable to adsorb (i.e. reduced absorption efficiency with loading). Activated carbon is not effective in removing heavy metals.

Regenerated carbon is typically of lower quality than so-called “virgin” carbon, and is relegated to uses requiring less sorbent capture ability.  During reactivation, a small amount of carbonaceous char is left inside of the carbon’s pore structure.  Also during reactivation, there is a slight shift in the carbon's pore size distribution which causes a slight loss in surface area.  These losses, along with mechanical attrition, typically result in the loss of between 8-10% of the carbon’s capture efficiency each time it is regenerated.  The following factors should be considered when evaluating GAC off-gas treatment:

• contaminant sorption rate will decrease as the carbon loading increases
• Secondly, the “breakthrough” effect limits the ability to saturate carbon
• Humidity also causes carbon to be ineffectively used
• Compounds which exhibit exothermic adsorption such as methyl-ethyl keytone may result in Autoignition of the carbon bed