Carbon is a commonly-used medium in water filtration processes. In fact, nearly every type of water filter system--whether it's a backpacking water filter, refrigerator filter, shower filter, pitcher filter, reverse osmosis water filtration system or whole-house filter--utilizes carbon filtration in some way.

Carbon filters for water filtration are produced by grinding up a carbon source.

This carbon source could be:

bituminous coal

peat

coconut shells

Of the above-mentioned carbon sources, coconut shells are the most widely used and are highly renewable.

To create the filter, material is heated in the absence of oxygen to 1000 degrees to bake off impurities. The material is then subjected to 1600-degree steam to “activate” the carbon. The steam leaves carbon granules filled with cracks and pores, enabling them to store large amounts of chemicals and contaminants.

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 could 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 a chloride.

Many water treatment plants use this chemical to disinfect water  because it's a stable compound and does not dissipate like chlorine or create by-products like trihalomethane. However, chloramine makes water taste and smell bad. 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.

Activated carbon filters are most used in refrigerators, because it is related to the safety of our drinking water, activated carbon filters are the safest filtration method.

How do Carbon Filters Work?

Carbon filters remove contaminants through adsorption.  Adsorption means that contaminants are attracted to the surface of the activated carbon and held to it, much the same way a magnet attracts and holds iron filings.

Carbon filters also act as a catalyst to change the chemical composition of some contaminants. Activated carbon is ideal for removing chlorine, organic chemicals such as pesticides, THMs like chloroform, and many VOCs that are components of gasoline, solvents and industrial cleaners.

What is a Granular Activated Carbon (GAC) Water Filter?

GAC Granular Activated Carbon Water FilterGAC stands for "granular activated carbon" and is made of tiny, loose granules of carbon. GAC filters are usually used as a "polishing filter' in the water filtration process and are highly effective at improving the taste and odor of drinking water.

While a GAC carbon filter is simply loose pieces of carbon, a carbon block filter is where the loose pieces of carbon are compressed together to form a filter.

What do GAC Water Filters Remove?

Granular activated carbon (otherwise known as GAC) filters have extremely high adsorption capabilities and can remove a wide variety of contaminants (see contaminant list here).

GAC filters are often used to remove VOCs, pesticides, nitrates, hydrogen sulfide, and much more.

Municipal water treatment plants use disinfectants such as chlorine and chloramine which can leave an objectional taste and odor in drinking water. GAC filters are able improve taste and odor, and are therefore commonly used as one or two stages in a reverse osmosis system.

What is a Carbon Block Filter?

A carbon block filter is where the loose pieces of carbon are compressed together to form a filter.

In a carbon block filter, one pound of compressed activated carbon (the amount in a standard ten-inch filter cartridge) has the equivalent surface area of a 160 acre farm, making it one of the most absorbent materials known to man.

Because of the compressed nature of a carbon block filter, water flow rates are lower than that of a GAC filter made of loose medium. And flow rates are impacted by the micron rating of the filter. A carbon block filter is rated to a certain micron rating depending on how much the carbon is compressed. The smaller the micron rating, the finer the filtration and the lower the flow rate.

Carbon Block Filters vs. GAC Carbon Filters  

What is the difference between a carbon block filter and a GAC filter? Carbon block filters and GAC filters are made of the same material, but one is made of ground up loose carbon and the other is the loose carbon that has been compressed.

Many water filter systems use both carbon block AND GAC filters in the water filtration process.

Carbon block filters are made of a solid block of compressed carbon and are often used as a pre-filter in reverse osmosis and other water filtration systems. Carbon block water filters are extremely effective in filtering out a variety of contaminants including very small contaminants or particle size. Water will generally flow slower during this filtration stage due to the compact nature of the compressed carbon.

The granular activated carbon or GAC water filter is made of loose carbon granules. Water flows more freely through this stage, so flow rates are higher for GAC filters than carbon block filters. GAC filters also have very high adsorption properties and can remove contaminants including chlorine, hydrogen sulfide, lead and much more.