Charcoal filters water, but the scientific material used is specifically known as activated carbon. This form of carbon is processed to significantly enhance its internal structure, unlike the charcoal used for grilling. The activation process, typically involving high heat or chemicals, creates millions of microscopic pores on the carbon’s surface. This results in an enormous surface area, sometimes exceeding 1,000 square meters per gram, allowing the material to effectively filter water.
The primary function of activated carbon filters is to improve the aesthetic qualities of drinking water, removing compounds that cause unpleasant taste and odor. It is a widely adopted technology in both municipal and consumer filtration systems. Its effectiveness is tied directly to this highly porous structure.
The Adsorption Process
The mechanism by which activated carbon removes contaminants is called adsorption. Unlike absorption, where a substance is soaked up, adsorption involves molecules sticking to the surface of the carbon material. Water flowing through the filter media brings contaminant molecules into contact with the carbon’s vast surface area.
The pores within the carbon act like chemical traps, attracting and holding organic molecules. This attraction is driven by weak intermolecular forces, primarily van der Waals forces, causing organic compounds to adhere to the carbon’s surfaces. The size and shape of the pores, determined during activation, dictate which molecules are effectively captured.
The chemical composition of the carbon surface, which can be tailored, also influences its effectiveness against certain contaminants. For instance, activated carbon can chemically react with chlorine, converting it into harmless chloride ions, which is a form of chemisorption. High porosity ensures prolonged contact time, maximizing the chance for contaminants to be trapped before the water exits the filter.
Contaminants Removed and Those That Remain
What Is Removed
Activated carbon is effective at removing organic contaminants that affect water quality. Its primary function in municipal water is the reduction of chlorine and chloramines, which are disinfectants that contribute to poor taste and smell. The filter is also excellent at eliminating volatile organic compounds (VOCs), which include industrial solvents like benzene and trichloroethylene.
Compounds that cause earthy or moldy tastes and odors, such as Geosmin and 2-methylisoborneol (MIB), are readily adsorbed by the carbon. Activated carbon filters also reduce many pesticides, herbicides, and emerging contaminants like certain pharmaceuticals and the PFAS group of “forever chemicals.” Some specialized carbon filters are certified to reduce heavy metals like lead and copper.
What Is Not Removed
Activated carbon has limitations, particularly regarding inorganic contaminants and microbiological threats. The filter does not effectively remove dissolved minerals, such as calcium and magnesium, which are responsible for water hardness and total dissolved solids (TDS). Consequently, it does not function as a water softener.
Activated carbon is ineffective against nitrates, sodium, and most fluoride, unless the filter is specifically engineered with additional media. It is not a reliable method for removing most microbiological contaminants, including small bacteria and viruses, as these organisms are too small to be physically trapped or chemically adsorbed. Consumers concerned about these substances must use supplemental filtration methods, such as reverse osmosis or UV disinfection.
Granular Versus Block Carbon Filters
Activated carbon is utilized in two main physical configurations for consumer water filtration: granular and carbon block. Granular Activated Carbon (GAC) consists of loose, coarse particles of carbon contained within a filter housing. This structure allows water to flow quickly through the filter, which is advantageous for high-flow applications like whole-house systems or pitcher filters.
However, the loose nature of GAC can sometimes lead to channeling, where water finds paths of least resistance and bypasses sections of the carbon media, reducing contact time and overall effectiveness. Carbon Block (CB) filters are manufactured by compressing fine powdered activated carbon with a binder into a dense, solid block. The tighter structure of the carbon block forces water to travel through a more tortuous path, significantly increasing the contact time between the water and the carbon.
This extended contact time and smaller particle size result in superior removal of fine particulate matter and a wider range of contaminants compared to GAC filters. While carbon block filters typically have a slower flow rate due to their density, they offer better overall performance for high-purity applications, such as under-sink drinking water systems. Both types rely on the same fundamental adsorption science, but the physical structure dictates the performance trade-off between flow rate and contaminant removal efficiency.