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Activated carbon means any amorphous form of carbon that has been specially treated to create high adsorption capacities. The two most common forms of activated carbon are granular and powdered. Each form has its own characteristics. However, while powdered carbon is less expensive and has a slightly higher adsorption capacity, it does present other challenges related to handling, regenerating and plugging in the presence of high suspended solids content.

Coal, coconut shells and wood are the most typical raw material sources for activated carbon. The process of converting these raw materials into activated carbon is a combination of dehydration, carbonization and oxidation, respectively. This process creates a product with a high capacity for adsorption due, primarily, to the large surface area created. The carbon adsorbs molecules based on polarity, the higher the polarity the less effective carbon will be in adsorbing contaminates. Thus, certain molecules will adhere to the activated carbon more readily than others.

Contaminate size, water solubility and polarity dictate the overall effectiveness of activated carbon in removing contaminates from either aqueous or vapor streams. As such, activated carbon will not effectively remove contaminates that are highly water soluble. Conversely, larger molecules will adsorb well, but because of the size they occupy large portions of the surface and reduce the further effectiveness of the carbon to absorb other molecules.

While activated carbon can remove a wide variety of organic and inorganic contaminates, the activated carbon particles do have a finite capacity for adsorption. Once this capacity is reached contaminates will simply pass through the activated carbon with no interaction. This is commonly referred to as "media break through". When this occurs, it is necessary to replace the spent carbon with fresh carbon to continue the process of removing contaminates.

There are several manners in which the various waste streams may be brought in contact with the activated carbon. The specific manner selected is predicated on the influent characteristics, discharge requirements flow rate and the economics. Generally speaking, there are four contacting most commonly utilized, as follows:

1. Moving Bed
2. Adsorbers in Series
3. Adsorbers in Parallel
4. Upflow-Expanded

Spent carbon does not become a waste because it can be "regenerated". This is accomplished by thermal means whereby, the adsorbed hydrocarbons are driven off of the surface of the carbon by the high temperatures, thus freeing the pores for future use.

If a large quantity of carbon is used, regeneration becomes economically feasible to recover and reactivate the carbon. As previously stated, thermal reactivation is the most common technique to regenerate activated carbon. This process destroys the adsorbed organic contaminates. The available methods are listed below:

Thermal Regeneration

Thermal regeneration is accomplished in either a rotary kiln or multiple hearth furnaces. To return carbon to its original activity, It is necessary to expose the spent carbon to temperatures in the range of 1600° F to 1800° F with a residence time of at least 30 minutes. It is common practice to also utilize steam in the regeneration process. During this regeneration process it is typical to experience a carbon loss of 5% - 10%, which must be made up with virgin carbon

Other methods exist, but with the exception of steam regeneration, the other available methods are for very specific waste streams and are not typically efficient or applicable for organic contaminate removal. These other methods are:

1. Alkaline Regeneration for Acid Adsorbates
2. Acid Regeneration for Basic Adsorbates
3. Solvent Regeneration
4. Steam Regeneration
5. Biological Regeneration