Activated carbon from a water filter used for Carbon filtering in powder and block form

Carbon filtering is a method of filtering that uses a piece of activated carbon to remove contaminants and impurities, utilizing chemical adsorption. Each piece of carbon is designed to provide a large section of surface area, in order to allow contaminants the most possible exposure to the filter media. One pound (454g) of activated carbon contains a surface area of approximately 100 acres (1 km²/kg). This carbon is generally activated with a positive charge and is designed to attract negatively charged water contaminants. Carbon filtering is commonly used for water purification, but is also used in air purifiers.

Carbon filters are most effective at removing chlorine, sediment, and volatile organic compounds (VOCs) from water. They are not effective at removing minerals, salts, and dissolved inorganic compounds.

2000 B.C. Sanskrit text refers to filtering water through charcoal (1905 translation of "Sushruta Samhita" by Francis Evelyn Place).[citation needed] The first modern use of a carbon filter to purify potable water occurred in 1862.[citation needed] Carbon filtration was further advanced in the mid 1970's by H. Allen Rice and Alvin E. Rice when they first manufactured a porous carbon block for drinking water use.[citation needed]

Currently, carbon filters are used in individual homes as point-of-use water filters and, occasionally, in municipal water treatment facilities. They are also used as pre-treatment devices for reverse osmosis systems and as specialized filters designed to remove chlorine-resistant cysts, such as giardia and cryptosporidium.