An activated carbon filter cleans water by adsorption, where contaminants stick to the surface of treated carbon. It removes chlorine, bad taste, odor, and many organic compounds. It comes as loose granular carbon or a solid carbon block, and it is common in whole-house units, under-sink filters, and reverse osmosis systems.
An activated carbon filter cleans water by trapping contaminants on carbon. It is one of the most common filters in a home. You will find it in a fridge filter, an under-sink unit, a whole-house tank, and inside a reverse osmosis system.
How adsorption works
Activated carbon is treated so each grain holds a huge internal surface area full of tiny pores. As water passes through, certain contaminants stick to that surface. The University of Nebraska extension describes adsorption as the contaminant being attracted to and held on the surface of the carbon particles. The right word is adsorption, not absorption. The carbon does not soak the water in. It grabs molecules onto its surface as they drift past.
What it removes, and what it does not
Carbon is best at chlorine, taste, and odor. The EPA lists activated carbon for taste- and odor-producing compounds, natural organic matter, volatile organic compounds (VOCs), and disinfection byproduct precursors. It also cuts chloramine and many pesticides. But it does not remove dissolved minerals, salts, or nitrate. Those flow straight through. That is why a water softener or a reverse osmosis membrane handles jobs carbon cannot. See how the two compare, or whether reverse osmosis is worth it for your home.
Types and certification
Carbon comes in two main forms. Granular activated carbon (GAC) is loose grains that water flows through quickly. A carbon block is carbon pressed into a solid cartridge that filters finer and slower. In a reverse osmosis system, a carbon prefilter shields the membrane from chlorine, and a carbon postfilter polishes the taste at the end. When you shop, look for an NSF/ANSI mark. NSF/ANSI 42 covers aesthetic effects like chlorine and odor. NSF/ANSI 53 covers health effects like lead and VOCs. One filter can meet both.