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Carbon Filters 101

Carbon Filters 101


An activated carbon filter is an air purifier that will remove the larger particles from the air and trap any odors that are passed through it.  Available in a wide range of sizes to fit any ventilation system, these filters are a must for any indoor gardening project.


Activated carbon is a form of carbon that has been processed to make it extremely porous and thus to have a very large surface area available for adsorption.  To undergo this process, temperatures of 1300°F (700°C) are applied to a carbonaceous substance (i.e., coal, wood, or coconut shells) in the absence of air to produce a carbonized char.  The carbonized char is then “activated” at temperatures of 1500°F -1800°F with steam, carbon dioxide or acid to create a highly porous, clean and adsorbent material.  In fact, it is so adsorbent that that each teaspoon of activated carbon has the equivalent surface area of a football field, and one pound of it equals about 125 acres of surface area. These so-called active, or activated, charcoals are widely used to adsorb harmful pathogens and odorous substances from gases or liquids.


The word adsorb is important here. When a material adsorbs something, it attaches to it by chemical attraction, meaning the atoms attract each other and bind together in a molecule. The huge surface area of activated charcoal gives it countless bonding sites. When certain chemicals pass next to the carbon surface, they attach to the surface and are trapped. If broken down, the device is actually comprised of two perforated cylinders with a layer of thick activated charcoal carbon filling both those layers in the cylinder. Air is pushed or pulled through the carbon with a fan, which moves through progressively smaller pathways and comes out clean on the other end.


As adsorption is a surface phenomenon, it is totally reversible (the reverse of adsorption is termed desorption), and this is one of the disadvantages of granular activated carbon. The rate at which activated carbon adsorbs or desorbs is affected most by the temperature and relative humidity of the air stream. Adsorption occurs more readily at lower temperatures and humidity levels while the opposite is true for desorption. At approximately 60% relative humidity (RH), the water content of activated carbon goes up to 25% by weight. Therefore, quick rises in RH can cause activated carbon to desorb gases in order to adsorb water.


To produce activated carbon, carbon-rich materials must be processed. These can include coal, wood, charcoal, petroleum and even coconut shells and bamboo. In the factory, these materials are ground down to a fine powder and then heated up in an oxygen-free chamber to get rid of non-carbon constituents without it igniting. The porous structure comes about when it is heated again with oxygen and steam. Different adjustments to this process can produce different sizes of pores and granules.


Once carbon has been activated, it can adsorb a long list of airborne chemicals, including molds, mildews, alcohols, organic acids, chlorinated hydrocarbons, ethers, esters, ketones, halogens, sulfur dioxide, sulfuric acid, and phosgene, among many other carbon-based impurities (“organic” chemicals).  Many other chemicals are not attracted to carbon at all — sodium, nitrates, etc. — so they pass right through. This means that an activated charcoal filter will remove certain impurities while ignoring others.  For example, the carbon filter may not always adsorb harmful pathogens created by mold or bacteria.  If you want to remove these particles, you may need to add a HEPA filter to your indoor garden as well.


Moisture and odor are adsorbed by the carbon grains, which increase in weight with the amount they adsorb.  As such, activated carbon’s effectiveness will slowly diminish with saturation. This continues until the maximum capacity of the carbon is reached and then the pollutants are no longer adsorbed but are passed through the bed.   This means that once all of the bonding sites are filled, an activated charcoal filter stops working and is no longer effective. At this point or before, the carbon must be replaced or reactivated. Check your filter regularly as it nears the end of its recommended life cycle to ensure it is working properly.