Charcoal, processed into activated carbon, cleans water primarily through filtration rather than full purification. This medium is derived from carbon-rich sources like wood, coconut shells, or coal. The raw material undergoes high-temperature treatment, creating a vast network of internal pores that make the carbon highly porous. This structure gives activated carbon its capacity to trap impurities from water. The technology is widespread in household filters and commercial water treatment systems, improving the aesthetic quality of drinking water.
The Science of Adsorption
The core mechanism by which activated carbon removes contaminants is called adsorption, which is distinct from absorption. Adsorption is a surface phenomenon where molecules adhere to the exterior surface of the carbon medium, unlike absorption, where the substance is soaked into the material’s bulk. The activation process involves intense thermal treatment of the carbonaceous material, followed by exposure to steam or chemicals. This process creates a massive internal surface area, often exceeding 1,000 square meters per single gram of carbon. Contaminants are then attracted to this vast surface by weak physical forces.
Granular vs. Carbon Block Filters
Activated carbon filters are generally available in two forms: granular activated carbon (GAC), composed of loose, coarse particles, and carbon block (CB) filters, where fine carbon powder is compressed into a dense solid. Carbon block filters typically provide a higher degree of filtration because their fine structure increases contact time with the carbon surface.
Contaminants Successfully Removed
Activated carbon is highly effective at removing a wide range of organic chemicals and compounds that affect the water’s taste and odor. One of the most common targets is chlorine, often used as a disinfectant in municipal water supplies. The carbon chemically reduces free chlorine, which eliminates the unpleasant bleach-like smell and taste. The filter also excels at removing various volatile organic compounds (VOCs), which are often industrial solvents or petroleum products that may enter the water supply. Furthermore, many types of pesticides and herbicides, which are large organic molecules, are readily adsorbed by the carbon’s porous structure.
Contaminants That Remain
Despite its effectiveness against organic compounds, activated carbon is not a comprehensive water purifier and fails to remove several significant classes of contaminants. The most important limitation is its general inability to reliably remove microbiological contaminants, such as bacteria and viruses. Viruses are often too tiny to be reliably trapped by the physical pore structure of the filter.
Inorganic Compounds and Metals
Activated carbon is also ineffective at removing most dissolved inorganic compounds and minerals, like salts, nitrates, and nitrites. These contaminants are present as ions or very small molecules that are not attracted to the carbon surface and pass right through the filter. Dissolved minerals that cause water hardness also remain in the water after carbon filtration. Standard carbon filters are also not designed to remove heavy metals, although some specialized carbon block filters may be certified for lead reduction.
Practical Applications and Filter Maintenance
Activated carbon technology is widely used in various household applications, from simple pitcher filters and refrigerator water dispensers to under-sink units and whole-house filtration systems. In a refrigerator, the filter is often a carbon block designed to reduce chlorine and sediment before the water is dispensed. Pitcher filters, typically GAC, use gravity to pass water slowly over the carbon, which maximizes the contact time for adsorption.
Filter Saturation and Replacement
The necessity of filter replacement is directly tied to the saturation of the carbon’s pore sites. Over time, all the available adsorption sites become filled with trapped contaminants, which causes the filter to lose its efficacy. A spent filter can no longer effectively remove impurities, and the accumulated organic material can even become a breeding ground for bacteria. Filter replacement schedules vary depending on water quality and usage, but manufacturers provide guidelines to ensure continued effectiveness.