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Guide to Activated Carbon Filtration

By: Stephanie Nielsen
Last Updated:
activated carbon filtration

It’s one of the most popular methods of water filtration out there – and for good reason.

Activated carbon technology revolutionized the water purification industry, and this guide will explain what activated carbon is as well as the different types and how they work.

What is Activated Carbon?

Carbon is the element that is found in every organic compound. It’s essential for life itself, hence the moniker carbon-based life forms, and it also makes up many other things including diamonds and fossil fuels.

Activated carbon is produced by taking something that contains carbon and putting it through thermochemical processing. The most common sources of activated carbon for water filtration are coconut shells or coal.

To “activate” these carbon sources, first, the coconut shells or coal chunks are heated without oxygen to 1,100-1,650 degrees Fahrenheit until they burn down into a carbon char. This char is then treated chemically, usually with argon and nitrogen, before once again being heated – this time to 1,100-2,200 degrees.

From there the now-activated carbon can be impregnated with different materials that will boost its filtration properties. You can read more on the process of carbon activation and impregnation here.

Activated carbon as a finished product is different from regular carbon because its surface now has millions of tiny pores. Some of these pores are only microns wide or less, and that gives it an extremely large surface area. The activated carbon surface is now able to interact with and adsorb (trap) contaminants in the water that passes over it.

Applications of Activated Carbon

Activated carbon isn’t just used to purify water – it has widespread industrial applications for reducing gas emissions as well. It’s also used to recover gold and other precious metals from ore, and by far the most widespread application residentially is in water filtration.

Types of Carbon Filters


While the process of making activated carbon remains the same, not all carbon filters are created equal. These are the different types of carbon filters, how they work, and what contaminants they’re best at removing.

Granular Activated Carbon

Granular activated carbon (GAC) filters, also called fixed-bed carbon filters, are a common form of carbon filtration. They can be found as stand-alone water filters or as add-ons to more comprehensive filtration systems, and they’re usually not very expensive.

How It Works

GAC filters work by taking chunks or granules of activated carbon and packing them in a canister or other container. Water then flows through these containers and comes into contact with porous surfaces of the GAC granules. The contaminants in the water can then be physically trapped by the small pore sizes of the granules.

They can also be electrostatically trapped by being more attracted to the surface of the granules than the surrounding water, or contaminants can even form chemical bonds with the activated carbon. While it has relatively little capacity to form chemical bonds, this property is especially pronounced if the GAC granules have been impregnated with other substances during their formation and activation.


While GAC filters are great in theory, there are two main downsides to this carbon filter type: they’re inefficient, and they can harbor bacteria. Water naturally wants to travel down the path of least resistance, and this means that it will often form and follow channels as it flows through the GAC media.

These channels mean that the water overall is coming in contact with less of the GAC surface area than it would otherwise, and GAC already has the smallest surface area of the activated carbon filter types. This also means that there will be places of relatively stagnant water within the filter, which can become a breeding ground for bacteria.

Contaminants Removed

GAC filters work best on compounds that cause foul tastes and odors like hydrogen sulfide and chlorine. They can also reduce or remove many volatile organic compounds (VOCs) as well as synthetic organic compounds including some pesticides, herbicides, and pharmaceuticals. They also work well on compounds with high molecular weights according to the EPA.

Carbon Block

Carbon block filters are similar to GAC filters, but with a couple of extra steps in their production. This makes them even better at filtering out contaminants and more efficient than GAC filters, but you may spend a little extra on them.

How It Works

Once activated carbon has been produced, instead of the granules being packed into the filter housing they’re pulverized into a fine powder. This powdered activated carbon is then combined with a bonding agent, and set into a solid block.

The result is a much higher surface area than GAC with much smaller pore sizes, and because the powdered particles are set in place there is no potential for the water to channel as it passes through the filter. More of the activated carbon surface area comes into contact with the water, which makes this system more efficient and provides a larger surface area for contaminant adsorption.


Because the process of making carbon block filters is more involved, they tend to be more expensive than their GAC counterparts. Finer filtration media also means the water flow is slowed down more significantly, so you’re much more likely to experience a noticeable drop in flow rate with these filters. This makes them generally more suitable for point-of-use systems than as a standalone whole-house filter.

Contaminants Removed

Carbon block filters remove all the contaminants of GAC filters – just more efficiently and in some cases more effectively. Carbon block filters are also able to work on smaller particles thanks to their tight pore structure, and can reduce or remove other contaminants like lead, mercury, asbestos, radon, and microbial cysts.

Catalytic Carbon

Catalytic carbon is yet another twist on the activated carbon production process. This time the intent is to improve the adsorptive properties of the carbon by making it easier to form chemical bonds with contaminants. 

How It Works

Once regular activated carbon has been formed, its surface structure is modified by putting it through gas processing at high temperatures. This causes the electronic structure to change so that the carbon will be more likely to catalyze and participate in chemical reactions with the contaminants it encounters.

Normal GAC and carbon block filters have little inherent catalytic properties, but this special processing of the activated carbon causes much greater catalytic functionality in the finished product. This effect is added while still maintaining the carbon’s ability to adsorb contaminants physically and electrostatically.


Since catalytic carbon is a more refined version of activated carbon, this makes it both less common to find in filtration systems and more expensive when you do.

Contaminants Removed

While GAC and carbon block filters can remove hydrogen sulfide in limited quantities, the ability for catalytic carbon to form chemical bonds with this contaminant means it’s much more effective at reducing it in your water supply. This form of activated carbon works better on trihalomethanes (THMs) as well.

Catalytic carbon also works on chloramines, which are used as disinfectants in some municipal water supplies and can contribute to the foul taste and odor of water. Otherwise it functions similarly to GAC and carbon block filters when it comes to other organic contaminants.

It’s worth noting there’s always going to be some variation in how well carbon filters work on different contaminants based on how the media is produced and the filters are assembled. This also holds true from one GAC filter to another or one carbon block filter to another, and you should always check a particular product’s specifications to see what exact contaminants it removes.

Similarly, you may not need the chloramine-fighting power of catalytic carbon if your home’s municipal water supply doesn’t use it. But how would you know for sure? You can get a water test kit that will tell you exactly what contaminants are present in your water so you can get the filter that will best match your needs.

Is the Carbon in Water Filters Dangerous?


Since activated carbon is the same as activated charcoal, consuming it would give you the same potential side effects: vomiting, constipation or diarrhea, GI blockage, and others. Having said that, the activated carbon itself will be staying out of your filtered water if your filter is working properly. With a well-functioning filter, there is no danger from the activated carbon.

What may pose a real danger is water that is contaminated with bacteria. Carbon filters on their own are not usually rated to handle bacteria, so even though your water may taste fresh and clean it might still be harboring potentially dangerous pathogens. This is especially true of GAC filters that are used past their recommended lifespan.

How Often Do You Need to Replace Carbon Filters?

Every manufacturer is going to have a recommendation for how frequently their filters should be replaced, and some may even come with their own indicators that let you know when the filter is nearing the end of its lifespan. Generally, carbon filters will last anywhere from 6-12 months depending on your household size and average water usage. 

Carbon Filter Alternatives

These are the other filtration methods that are commonly used for home water purification, and the main contaminants each works best on:

Common Filter Types in Under Sink UnitsContaminants Removed
Reverse OsmosisMost microorganisms, minerals, heavy metals, suspended particles,
fluoride, arsenic, nitrates, hexavalent chromium, perchlorate, VOCs,
most radionuclides
Ultraviolet LightMicroorganisms including bacteria, fungi, protozoa, and viruses
Ion ExchangeMinerals (calcium, magnesium), barium, radium, heavy metals
Air injection oxidization (AIO)Iron, manganese, hydrogen sulfide
KDF filterMost heavy metals
DistillationAll inorganic compounds, microorganisms, heavy metals, TDS, and
all organic compounds with boiling points higher than water

When to Use Activated Carbon Filters

Anytime you need to improve the smell and taste of your water, activated carbon is going to be your best bet. It’s also fantastic against organic compounds like herbicides and pesticides, VOCs, semi-VOCs, some pharmaceuticals, and other organic industrial contaminants.

Activated carbon filters also tend to be cheaper than some of the other filter types, which makes them ideal for those who want to improve the quality of their water without breaking the bank.

And finally, if your water tests positive for the contaminants activated carbon works on, then you’ll know it’s the best filter for your home’s needs.

When to Use Another Type of Filter

While activated carbon is great, there are some main contaminants that it doesn’t cover. As mentioned above, you’ll need to have another method of killing bacteria if your water isn’t microbially safe.

Also, even carbon block filtration isn’t the best for heavy metals, especially iron and manganese. Since these are the two most common well water contaminants, those getting their water from a well source should look into another form of filtration to combine with activated carbon.

Activated carbon also won’t reduce total dissolved solids (TDS), which means that those who are looking to reduce the hardness of their water should look at a water softening or conditioning system as well.

And lastly, while catalytic carbon works well on hydrogen sulfide, it’s not as effective as filters like AIO or KDF. If you have exceptionally high levels in your water you may want to consider one of these other filter types.


Activated carbon changed the face of water filtration for the better, and further technological advancements now mean that we have three main types of activated carbon filtration: GAC, carbon block, and catalytic carbon. All three are formed by superheating carbon-containing sources like coconut shells in a multi-step process, and all three work well on a wide variety of organic chemicals as well as chlorine.

Carbon block filters work better on select heavy metals like lead and mercury, while catalytic carbon binds hydrogen sulfide better. There are multiple other kinds of filters out there, however, getting your water tested is the best step you can take to learn exactly what’s in your water and which filter will get it out. Then you can rest assured that you have safe, clean water coming into your home.

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Stephanie Nielsen
Stephanie worked as a department supervisor of kitchen, bath, and appliances at Home Depot, and water filters were part of the inventory she was responsible for assisting clients with so she learned the ins and outs of matching the right filtration device to homeowner’s needs. She also worked closely with Culligan water to educate customers about whole-home water treatment and softener systems. Since leaving Home Depot and pursuing freelance writing, she has used her knowledge about water treatment devices to write about water filtration systems for RVs, water purification methods for backpackers, and home water devices.