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Old 12-19-2002, 10:16 PM   #1
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Activated carbon

Activated carbon...probably one of the best weapons in our arsenal of chemical filtration...possibly one of the least understood also. There are several different materials used to make activated carbon (AC). Probably the most common is bituminous coal. Lignite and coconut shells are also used. The base product is heated to about 900 deg F. This first heating is done with no air. This results in the material being charred. Then, it is heated a second time with air to a temp of about 1600 deg F. The remaining hydrocarbons are removed with this step and lots of tiny passages and holes are left in the carbon. It is now "activated".

The carbon is much like a sponge...large holes on the outside lead into smaller and smaller passages on the inside. Basically, through a process called adsorption, molecules enter the large holes and are eventually trapped as the passages get too small for them to travel any further. Single molecules can enter through the smaller pores while the larger pores allow complex molecules to enter. A few examples of the stuff that the carbon will remove in this fashion...iodine, phenols (yellow water), amino acids, copper, chromium, iron, mercury, chlorine, chloramine, hydrogen sulfide, malachite green (ich treatment), sulfa, and antibiotics. Some things AC will not remove are ammonia, nitrite, and nitrate.

OK, that's all fine but how do you tell whether you're getting good carbon? First, make sure the container says "Activated Carbon". Some of the cheaper carbon may be plain anthracite coal. A dull black color is preferable to a shiny appearance. The dull color indicates a higher porosity. A rounded particle will enhance water flow and therefore is preferable to a flat sided particle. Another indication that you have a good grade of carbon is, when submerged in water, it gives a hissing sound as the cavities fill with water. It should also float at first...sinking slowly. Also, look for labeling that states that the carbon is "phosphate free". I won't go into the technical terminology that manufacturers use to rate AC. If anyone expresses an interest in this, I'll be happy to go into it in more detail.

The most effective use of carbon in the aquarium is to have it in a mesh bag in a place where it receives a constant slow flow of water. Blasting the carbon with the stream from a high powered pump can wash the contaminants right out of the carbon. Just hanging the bag in the tank results in low adsorption and inefficient use of the AC. If you're not sure how much to use, here's a good starting point. About 1 heaping tsp per 5 gallons of water for a FO tank...about 1/2 that amount for a reef tank. It's better to start with less and see how your system reacts. How long will it last? Impossible to accurately predict. A good system to start with is to have two bags of carbon in the system and change one every3-4 weeks. It's better to change it more often because the carbon, upon reaching saturation, can begin to leach the compounds back into the water. It's especially important to change the carbon quickly if you've used it to remove antibiotics or other medications from the water. I'd run a batch of carbon under these conditions for no more than a week. If your tank begins to get a yellowish tint to it, it's past time for new AC.

It has been said that AC can be regenerated and reused. I have seen several different "recipes" for this. The only way to regenerate AC is to heat it to temperatures in excess of 900 deg F...not a viable alternative for most of us. While there are many different resins available to the aquarist today, activated carbon remains the most versatile and the least expensive. Under ideal conditions, AC can remove as much as 50% of it's weight in organics from the water.
Logan J
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