Michael Kamrin, Nancy Hayden Center for Environmental Toxicology Barry Christian, Dan Bennack Community Assistance Program in Environmental Toxicology Frank D'Itri Institute of Water Research Activated carbon (AC) filters have been used in home water purification systems primarily to remove taste and odor. Taste and odor, although undesirable, are generally not considered unhealthy. In recent years, however, AC filters have been used to remove some of the contaminants that have been discovered in water supplies. AC is most effective at removing organic compounds such as volatile organic compounds, pesticides and benzene. It can also remove some metals, chloride and radon. As with any treatment system, it cannot remove all possible drinking water contaminants. Because AC systems are limited in the types of compounds they can effectively remove, it is essential that the homeowner determine which water contaminants are present before purchasing such a system. Anyone who suspects they have a water quality problem should first have their water analyzed by their local health department or a reputable laboratory. These analyses are costly, but worth the expense since they are necessary to determine the appropriate home treatment system and how best to operate such a system. A state or local health official can interpret water analysis results. Some laboratories may also provide this service. Note that home water treatment is considered only a temporary solution. The best solutions to a contaminated drinking water problem are to either end the practices causing the contamination or change water sources. ACTIVATED CARBON AC IS A BLACK, solid substance resembling granular or powdered charcoal. It is extremely porous with a very large surface area. One ounce of AC has an estimated 30,000 square yards of surface area. Certain contaminants accumulate on the surface of the AC in a process called adsorption.

The two main reasons that chemicals adsorb onto AC are a "dislike" of the water, and attraction to the AC. Adsorption of most contaminants results from a combination of these reasons. Many organic compounds, such as chlorinated and nonchlorinated solvents, gasoline, pesticides and trihalomethanes can be adsorbed by AC. AC is effective in removing chlorine and moderately effective in removing some heavy metals. AC will also remove metals that are bound to organic molecules. Fluoride, chloride, nitrate, hardness (calcium and magnesium) and most metal ions are not removed by AC to any significant degree. It is important to note that carbon is not necessarily the same as AC. AC removes vastly more contaminants from water than does ordinary carbon. Fig. 1 shows how contaminants adsorb on an activated carbon filter. AC FILTERS HOME AC treatment systems are quite simple. The AC is normally packaged in filter cartridges that are inserted into the purification device. Water needing treatment passes through the cartridge, contacting the AC on its way to the faucet. AC filters eventually become fouled with contaminants and lose their ability to adsorb pollutants. At this time, they need to be replaced. AC treatment systems are typically point-of-use (POU) - installed where they typically treat water used for drinking and cooking only. AC filters can be placed on the end of the faucet, on the countertop, or under the sink. POU systems often have a bypass so that water for purposes other than drinking and cooking can also be dispensed at the tap without being treated. This increases the life of the AC, reducing the time between filter replacements. A POINT-OF-ENTRY (POE) system is more appropriate if a contaminant is present that poses a health threat from general use as well as from consumption. Volatile organic compounds and radon are examples of this type of contaminant. These contaminants may get into the indoor air when water is used for showering and washing. In this case, it is more economical to have a large POE system that treats water as it enters the home than to have POU systems at each tap.