Carbon Filters

Carbon filters or charcoal filters have been used for several centuries and are still considered one of the most efficient and oldest methods of water purification. Historians have shown evidence that carbon filters may have been used in ancient Egyptian cultures for both air and water decontamination.

Carbon filters are being widely used in the healthcare and industry fields to control the level of contamination in liquid and gaseous materials. Carbon filters remove impurities and contaminant particles or clusters using the chemical adsorption method. An activated carbon or activated charcoal filter uses carbon that has been processed to improve adsorption by creating a number of large internal compartments that increase the surface area and create a maze-like structure that improves the probability of attracting the contaminants. [Page 84]It has been shown that one kilogram of activated carbon can contain a surface area of nearly 2 square kilometers.

Activated carbon is superheated in a controlled environment; in the absence of oxygen, which alters the material by creating tiny nanometer-wide cracks, the result is an immense surface area that attracts gaseous contaminants through a process called adsorption, which is the key in the filtration process. Carbon has a natural affinity for organic pollutants like benzene, which bind to its binding sites, distributed in the surface of its internal compartments. When a material adsorbs something, it attaches to it by chemical attraction, and the huge surface area of activated carbon provides countless bonding sites in which chemicals may be attached to the surface and are trapped. The major force in the trapping interaction is the van der Waals forces, which are relatively weak bonds compared to chemical reactions.

Activated carbons (charcoals) are suitable for trapping other carbon-based impurities, as well as other compounds like chlorine. Chemicals such as sodium, nitrates, and so on, which are not attracted to the trapping sites of the activated carbons, will pass through the carbon filters with no adsorption, so an activated carbon filter acts selectively by removing certain impurities while screening others. It should be noted that once all of the trapping sites are filled, the activated carbon filter becomes saturated and stops working. At this time, the filter must be replaced with a fresh activated filter. The speed of saturation highly depends on the type and concentration of contaminants, as well as on the flow of the carbon filter.

Carbon is usually activated with a positive charge and is designed to attract negatively charged contaminant particles or clusters. Since carbon filters usually have a positive charge, they are suitable for removing negatively charged particles such as chlorine, sediment, and volatile organic compounds (VOCs) from water. However, activated carbons are poor for removing minerals, salts, and dissolved inorganic compounds. Carbon filters can filter particles or clusters ranging from 0.5 to 50 micrometers and are categorized and described based on the range of the particle size that they are able to filter. The efficacy of a carbon filter is based on the flow-rate regulation. When water or air is allowed to flow through the carbon filter at a slower rate, the contaminants are exposed to the internal compartments for a longer time and the chance of trapping increases as the passage of time increases.

...