Activated Carbon Filters 101

Posted by
John Woodard on November 27, 2023

The detoxification of carbon was first discovered by ancient Egyptians. Today, carbon has become a water treatment method used in practically every home. Utilized in many different forms, activated carbon filters help food and beverages taste and smell their best. In this article, you can learn how activated carbon filters work, the types of carbon filters, and their alternatives.

What is a carbon filter?

A carbon filter is a form of water treatment with remarkable chlorine, foul taste, and bad odor reduction capabilities. It uses a charred form of media, typically wood or coconut shells, to attract contaminants, trapping them as they flow by. Water treatment plants treat water with chlorine and chloramines that linger in drinking water, tainting it with a chemical flavor. Carbon clears water of organic compounds that make your water taste and smell bad.

How does a carbon filter work?

Carbon filters remove contaminants through a process called adsorptionAbsorption soaks up particles like a sponge to water. Adsorption, on the other hand, adheres particles to a surface like a piece of Velcro. Organic compounds bond or stick to the surface of a carbon filter because water and contaminants are both polar compounds that attract one another.

Carbon filters are extremely porous and have a large surface area, making them effective at reducing bad tastes, odors, and other particles in water. A carbon filter acts as a parking lot with pores as parking spaces for contaminants as water flows through. The tiny pores are measured in microns. The smaller the micron, the finer the filtration. Low flow rate and pressure give contaminants more time to adhere to the carbon. The more contact time water has with the surface of a carbon filter, the more efficient the filtration.  

Carbon vs. charcoal filters

Carbon filters are often referred to as charcoal filters, but there are a couple differences between these filter types. First, charcoal filters are only made of wood, while carbon filters can be made from a variety of materials, such as wood, coconut husks, coal, peat, and others. Charcoal filters are generally of worse quality than carbon filters because they contain less carbon than activated carbon filters. Carbon filters are purer than charcoal filters and are overall a better option. However, charcoal filters can still effectively reduce levels of certain contaminants in water.

What is a carbon filter made of?

Activated carbon filters are most commonly made up of one of the following materials:

  • Bituminous coal
  • Wood-based media
  • Coconut shell media

Of these three types of filter media, coconut shell carbon is the most renewable. This type of carbon is made from the shell of a coconut rather than the contents inside, so it doesn't cause allergic reactions or add flavor to water. Wood-based carbon is made from burned wood ground into a granule and resembles what the ancient Egyptians would have used. Bituminous coal is used less frequently today because traces of arsenic have been discovered in the media. 

activated carbon

Types of carbon filters

Granular activated carbon filters

GAC filters contain loose granules of activated carbon that allow water to flow through easily. Water flows in one direction through the cartridge and contacts carbon in the process. 

Advantage: Not as restrictive. Water flows through the carbon at a faster rate. 

Disadvantage: Channeling. Water can cut a path through the carbon and allow contaminants through. 

Carbon block filters

Carbon block is made of fine granules held together with a bonding agent, which only takes up 15% of the surface area. Impure water flows in through the side of the filter and sends filtered water out through the top. As carbon is ground to a finer granule, the surface area increases. 

Advantage: Gives 7-10 times more surface area than GAC filters, and the compact carbon prevents channeling. 

Disadvantage: More flow restrictive.

Radial flow GAC carbon filters 

Radial flow carbon filters combine the surface area of the carbon block with the flow rate of the GAC filter. Water flows to the filter through the side of the cartridge like a carbon block but contains granules for increased flow like the GAC.

Are carbon water filters safe?

Carbon water filters are safe, especially if they've been rated by a third party for material safety. All carbon filters are rated for CTO (chlorine, taste, and odor) removal, and sub-micron carbon blocks remove other contaminants like lead or cysts. Activated carbon block filters with sub-micron ratings go above and beyond to remove additional particles through mechanical filtration. Mechanical filters work like a screen door, keeping unwanted elements out and letting clean water through. Pores of a carbon block filter that measure less than one micron are too small for cysts to pass through.

What do activated carbon filters remove?

Activated carbon filters are best at removing chlorine, bad tastes, and foul odors, but some may be certified to remove other contaminants. Look for the NSF certification on a carbon filter to find out exactly what it can remove.

  • Bad tastes and odors? YES
  • Chlorine? YES
  • Trihalomethanes (THMs)? YES
  • Mercury? YES
  • Pesticides and herbicides? YES
  • Iron or heavy metals? If certified
  • Lead? If certified
  • Bacteria? If certified to remove coliform
  • Hard water? NO
  • Fluoride? NO
  • Total dissolved solids (TDS)? NO

Dissolved minerals like calcium and magnesium flow through a carbon filter undeterred. However, a carbon filter with a pore size smaller than one micron can remove coliform, cysts, lead, arsenic, iron, and other heavy metals through mechanical filtration. Even though these contaminants do not adhere to the carbon, they cannot fit through the small pores. 

Often materials are added to carbon to improve its reduction capabilities. Adding silver to carbon creates a media that kills bacteria. Iron-reduction carbon includes KDF (Kinetic Degradation Fluxion) media made of ground copper and zinc to create a chemical reaction. The KDF converts heavy metals from a ferrous (dissolved) to a ferric (solid) state to capture the solids in the matrix of the carbon. Although certain carbon blends can reduce coliform, a UV system disinfects bacteria-infested water most effectively.

Activated carbon and chlorine

Removing chlorine is the most common reason to use a carbon filter. Chlorine makes your food, beverages, and drinking water nasty and emits a gas that you can inhale in the shower. Chlorine does not adhere to carbon. Instead, a carbon filter removes chlorine through a chemical reaction. Activated catalytic (more reactive) carbon chemically alters the chlorine molecules, converting them into chloride.

Learn more: How to remove chlorine from drinking water

Do carbon filters remove chloramines?

Many water treatment plants use chloramine to disinfect water because it is a stable compound that does not dissipate like chlorine or create by-products like trihalomethane. However, chloramine gives water a foul taste and smell. Chloramines are more difficult to remove than chlorine, so catalytic carbon is used. When chloramine hits the carbon filter, the carbon breaks the ammonia from the chlorine and turns it into chloride. 

Learn more: How does city water treatment work?

Where are carbon filters used?

Carbon filters are used in many applications. They can be used as point-of-entry (POE) systems to filter the whole house or point-of-use (POU) filters to clean water before you drink or cook with it. Refrigerator water filters often contain a blend of sediment and carbon filtration. Some shower heads include carbon filtration to prevent negative chlorine effects on skin and hair. Carbon filters are also part of a reverse osmosis system or an ultrafiltration (UF) system. A carbon filter added to a UF system provides organic and chemical particulate reduction along with lead reduction.

Learn more: What is ultrafiltration?

If you use a water softener to soften water treated by a municipal plant, then you should install a carbon filter before the water softener. If chlorine is removed prior to softening, then the softener resin lasts longer. Chloramines can cause O-rings, gaskets, and rubber seals to deteriorate, so removing chemical disinfectants with catalytic carbon helps water-using appliances, like your tankless water heater, last longer.

Learn more: What is a water softener work and how does it work?

How often should you change your carbon filter?

Change your carbon filter every six months to one year. Never use a carbon filter for longer than one year. Waiting to change the filter could make your water worse than it was before treating it. If the pores of a carbon filter are full, collected particles will begin to break from the carbon and flow into your water supply. 

Alternatives to carbon water filters

Sediment filters

Carbon and sediment filters serve two different functions. Sediment filters catch dirt and debris, while carbon captures organic compounds that cause bad tastes and odors. A sediment filter is a mechanical filter. It has a certain porosity that water can flow through but traps anything larger than the micron rating of the filter. Because a carbon filter removes particles by adsorption, it does not make a good sediment filter. You often need one of each.

Learn more: What is a sediment filter and how does it work?

Ceramic filters

A ceramic filter is another type of mechanical filter. Unlike sediment filters, ceramic filters contain a pore size small enough to block some living organisms. Often, carbon is either blended with ceramic or added to the inner core of a ceramic filter to enhance the scope of reduction or target specific types of contaminants. Adding carbon inside of a ceramic filter gives both carbon and sediment filtration to reduce some chemicals and debris. 

Learn more: What is a ceramic filter and how does it work?

Reverse osmosis systems

Carbon filtration is a part of the reverse osmosis (RO) treatment process. It is present in at least one stage of reverse osmosis systems, but it is also common for an RO system to contain two or three carbon filters. Unlike a simple carbon filter, a reverse osmosis system uses a membrane to separate dissolved inorganic compounds and radionuclides. Contact with chlorine can damage the RO membrane, so carbon filters are used within the system to protect the membrane.

Learn more: What is a reverse osmosis system and how does it work?


If you have any additional questions, please do not hesitate to contact us.

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