The material used to filter water is not the charcoal found in a grill, but a highly specialized substance known as activated carbon. This filtration method has been employed for centuries to improve water quality. Modern science has refined this ancient technique, creating a porous carbon structure that is exceptionally good at removing impurities that affect the taste, odor, and safety of drinking water.
Activated Carbon: Definition and Source
The correct material for water purification is activated carbon, a form of carbon processed to have an extensive network of internal pores. Unlike common charcoal, which is simply the residue of burning organic matter, activated carbon undergoes a treatment process to dramatically increase its surface area. This vast surface area is the feature that allows it to effectively trap contaminants from water flowing through it.
Activated carbon is derived from organic materials rich in carbon, such as wood, bituminous coal, coconut shells, or peat. Coconut shells are a favored source because they yield a carbon with a fine pore structure, effective at removing smaller organic molecules. The “activation” process involves two main steps: first, carbonization, where the source material is heated in an oxygen-deprived environment to high temperatures (often between 600°C and 900°C). This removes non-carbon elements and creates the initial porous structure.
The second step is the activation itself, which further opens and expands these pores, typically using steam or chemical agents at high temperatures. This process creates a microscopic sponge-like structure where a single gram of activated carbon can possess a surface area exceeding 1,000 square meters. This immense internal space transforms ordinary carbon into a powerful filtration medium capable of trapping a wide range of waterborne pollutants.
The Adsorption Process
Activated carbon works through a physical process called adsorption, which is distinct from absorption. Absorption involves a substance soaking up a material, similar to a sponge taking in water. Adsorption is a surface phenomenon where liquid molecules are chemically or physically attracted to and stick onto the solid surface of the carbon.
The massive internal surface area of the activated carbon provides countless sites for this molecular attraction to occur. As water passes over the carbon granules, organic molecules, like those that cause bad taste and odor, are drawn out of the water and held within the carbon’s pores. This mechanism is particularly effective against non-polar organic compounds, which are often the culprits behind issues like chlorine taste, chemical odors, and discoloration.
The process is highly efficient for removing disinfectants like chlorine and chloramine, which are commonly added to municipal water supplies. Chlorine molecules react with the carbon surface to form harmless chloride ions, eliminating the chemical that causes the common swimming pool smell and taste. The removal of these volatile organic compounds (VOCs) and other chemicals significantly improves the aesthetic quality of drinking water.
Physical Forms and Filtration Systems
Activated carbon is manufactured into different physical forms to suit various filtration applications, primarily Granular Activated Carbon (GAC) and Powdered Activated Carbon (PAC). GAC consists of relatively large, irregularly shaped particles packed into filter cartridges for point-of-use systems like pitcher filters and whole-house units. Water flows through GAC beds at a faster rate, making it a good choice for high-volume applications where flow is a consideration.
Powdered Activated Carbon (PAC) is much finer, resembling a fine black dust, and is typically used in large-scale industrial water treatment plants. PAC is often added directly to the water as a slurry, allowing for quick contact with contaminants before being filtered out by a separate process. A common form in residential filtration is the carbon block, made by compressing PAC with a binder to create a solid, dense cylinder.
This compressed carbon block forces water to travel a more tortuous path, increasing the contact time between the water and the carbon surface. The longer contact time, combined with the block’s smaller pore size, enhances filtration efficiency, making it superior for removing finer particles and a wider range of contaminants than loose GAC. The choice between GAC and a carbon block often balances the required flow rate with the desired level of contaminant removal.
What Activated Carbon Does Not Remove
While activated carbon is highly effective for many contaminants, it has distinct limitations and is not a comprehensive water purifier on its own. The adsorption process is largely ineffective against many inorganic pollutants, which tend to be poorly attracted to the carbon surface. These include salts, dissolved minerals that cause water hardness, and most nitrates and nitrites.
Activated carbon filters are also not designed to remove microbial pathogens like bacteria, viruses, and protozoan cysts. The pores in the carbon are generally too large to physically block these microscopic organisms, and the filters do not actively kill them. For water known to contain biological hazards, the carbon filter must be combined with a disinfectant or other purification methods like ultraviolet light or reverse osmosis.
Furthermore, activated carbon’s ability to remove heavy metals like lead, arsenic, and fluoride is inconsistent and often requires specialized, chemically-treated carbon. Standard activated carbon may remove some heavy metals, but not with the high efficiency needed for safety, especially if concentrations are high. As the carbon traps contaminants, its surface sites become full (a state known as saturation), which necessitates regular filter replacement to maintain effective purification.