Activated carbon filters are the industry standard for specialized industrial and consumer water treatment applications. Activated carbon has been found to be extremely effective in reducing or removing organic chemicals, including chlorine, pesticides, industrial chemicals, THM’s, PCB’s and other similar halogenated organic compounds, as well as bad taste and odor.

Granular Activated Carbon (GAC)

Granular Activated carbon is a form of charcoal that has been treated by high temperatures and steam in the absence of oxygen. This process produces small granules that are extremely porous with a high surface area. GAC filters remove organics from the water by holding them to its surface, a process known as adsorption. As water passes around each little particle of carbon, the organics adhere to the particles. One pound of granular activated carbon has a total surface area of about l25 acres (three football fields), giving each filter the ability to handle large amounts of water before becoming “used up”. When GAC is used in a tall column, most of the unwanted organic pollutants are absorbed in the top layers, and as the water passes downward it progressively contacts cleaner and cleaner carbon. This in-depth filtering “polishes” the water as it passes out of the column and provides a wide margin of organic removal.

While a wide variety of filters are made with GAC, not all will perform equally. Effectiveness varies with the kind of carbon used in the filter, the filter design, the way it is used and maintained, its age, and the condition of the water being treated.

Carbon Block Filters

Also referred to as Extruded Activated Carbon (EAC), this carbon filter is manufactured using a method of extrusion that allows the free formation of porous solids (in this case, carbon) in a highly uniform fashion. Because extruded activated carbon parts are produced on a single tool rather than on large numbers of individual molds, dimensional uniformity of each piece is exceptional. Uniformity, strength, porosity, permeability, sorbent capacity, and dimension accuracy are a few of the advantages to this technology.

Unlike the GAC carbon filter, where water flows around the carbon particles, the carbon block forces the water to penetrate through it. This allows for a more complete contact with and subsequent adsorption of chlorine, THM’s, bad tastes and odors, toxic metals, and more.

The carbon block filter then becomes a reliable sediment filter as well. The pore size of the carbon block filter is variable, and is usually available in a range of l0 micron to sub-micron. When used in tandem, usually with a l0 micron first and a sub-micron second, these filters can provide the most complete removal of organic matter available. As with GAC filters, EAC filters do become “used up” as well, and should be replaced regularly.


The only real drawback associated with carbon filters is that they can grow bacteria and harbor other (usually harmless) matter on the carbon surface. The risk is that over time, this matter can break off and contaminate the water, defeating the whole purpose of water treatment. The carbon can also become “used up” as the adsorption of organic molecules saturates the media. Determining factors for the amount of time this takes are water temperature, sediment content, chlorine levels, organic chemical levels, and total amount of water treated.

Carbon filters will not remove dissolved solids, fluorides, heavy metals or mineral salts. If complete water purification is desired, the removal of these items can only be attained through reverse osmosis, deionization or distillation.

One of the drawbacks of a GAC filter is its tendency to “channel”, where water creates distinct paths through the media. This greatly reduces the available contact area, which shortens the effective life of the filter. It also means that additional prefiltering becomes necessary because the carbon is not at a uniform pore size for this purpose.

Filter Replacement

As mentioned, GAC and EAC cartridges should be replaced on a regular basis. Some determining factors for replacement time are water conditions and temperature, number of gallons treated, chlorine and other organic chemical content of the feed water, sediment and turbidity. For average household usage, depending on the size and type of the carbon filter combined with the family size and the factors listed above, expect to change out the filter every 6 to 12 months. For industrial and commercial use, every 2 weeks-6 months, depending on application. The replacement cost of carbon block filters is usually higher than GAC filters. Prices will vary , as will quality, and the two don’t always go hand in hand. When you purchase a filter system, check with your dealer for the manufacturer’s recommendations.

When purchasing a filtration system, be sure that (i) the filters are replaceable, (ii) the filters are readily available and (iii) that the manufacturer/distributor you are buying from is reputable (and will be around when you need filter replacements).