Activated carbon filtration is a widely used method for improving water quality in both municipal and household settings. This process utilizes carbon material that has been specifically treated, or “activated,” to enhance its ability to capture impurities from water. The primary purpose of this filtration is to remove contaminants that affect the aesthetics and safety of drinking water, delivering cleaner, better-tasting water.
How Activated Carbon Filtration Works
The mechanism by which activated carbon removes impurities is primarily through adsorption. This is distinct from absorption, which involves one substance soaking up another, like a sponge. Adsorption is a surface phenomenon where liquid contaminants adhere to the solid surface of the carbon material.
Activated carbon is manufactured to be extremely porous, giving it a massive internal surface area—up to 2,000 square meters per gram. This enormous surface area provides countless sites for contaminants to cling to as water passes through the filter. This process is driven by weak intermolecular attractions, specifically van der Waals forces, which cause organic molecules to be drawn out of the water and held onto the carbon surface.
The efficiency of contaminant removal depends on the contact time between the water and the carbon material. A slower flow rate allows water to spend more time interacting with the carbon surface, increasing the likelihood of adsorption.
Key Contaminants Removed by Carbon
Activated carbon is highly effective at removing a wide array of organic compounds, which are molecules containing carbon atoms. These molecules are less soluble in water and have a strong affinity for the non-polar carbon surface.
One of the most common targets is chlorine, which is intentionally added to municipal water as a disinfectant. Carbon filters remove chlorine through catalytic reduction, eliminating the unpleasant taste and odor associated with it. The process also effectively removes chloramines and disinfection byproducts like trihalomethanes, which are formed when chlorine reacts with organic matter in the water.
Activated carbon is the recommended treatment for harmful substances, including Volatile Organic Compounds (VOCs). These include industrial solvents, herbicides, and pesticides that can enter the water supply. Carbon filtration also eliminates compounds like geosmin and methylisoborneol, which are naturally occurring substances that cause earthy or musty tastes and odors in water.
Substances Carbon Filtration Does Not Treat
While carbon filtration is effective against organic compounds, it has significant limitations concerning inorganic contaminants and biological pathogens. Substances that are highly soluble in water or have poor affinity for the carbon surface pass right through the filter.
Standard activated carbon filters do not effectively remove most inorganic minerals, including calcium and magnesium, which cause water hardness. They are also ineffective against dissolved inorganic salts, nitrates, and fluoride. Although specialized carbon filters can be engineered to reduce some heavy metals like lead and mercury, standard filters are not a reliable solution for high levels of these contaminants.
Carbon filters also do not remove microbiological contaminants such as bacteria, viruses, or microbial cysts like Giardia and Cryptosporidium. These organisms are typically too small to be filtered or do not adsorb to the carbon material. Removing these pathogens requires other treatment methods, such as ultraviolet (UV) disinfection or reverse osmosis.
Real-World Use and Filter Lifespan
Activated carbon filters are used in many applications, from large-scale municipal water treatment to small household devices. Systems are categorized as Point-of-Entry (POE), which treat all water entering a home, or Point-of-Use (POU), which treat water at a single tap, such as a kitchen faucet or pitcher filter.
Within these systems, there are two main types of carbon media: Granular Activated Carbon (GAC) and Carbon Block. GAC uses loose carbon granules, which offer a high flow rate and are often used in whole-house POE systems for general taste and odor improvement. Carbon Block filters use finely powdered carbon compressed into a solid form, forcing water to take a “tortuous path” and increasing contact time for superior contaminant removal.
Carbon filters must be replaced regularly because their performance declines as the adsorption sites become full, a state known as exhaustion or saturation. The typical lifespan for a household carbon filter ranges from six to twelve months, depending on the volume of water filtered and the concentration of contaminants present. A noticeable return of chlorine taste or odor is a clear sign that the filter’s capacity has been reached and needs immediate replacement.