An Intro to Activated Carbon
Activated carbon is a valuable weapon in the arsenal of water treatment technologies. It is widely used to remove contaminants from water. Activated carbon is an amazing substance which justifies a brief explanation.
What is it?
The primary raw material for activated carbon is any organic material with high carbon content such as wood, coconut shells and coal. Granulated activated carbon is produced by grinding the raw material, adding a suitable binder to give it hardness, re-compacting and crushing to the correct size. The carbon is activated by thermal decomposition in a furnace using a controlled atmosphere and heat. This process produces a product with an incredibly large surface area per unit volume. One quart of carbon provides a surface area equivalent to six football fields. This surface area provides a network of submicroscopic pores where adsorption takes place. Physical adsorption is the process which enables carbon to purify water.
Adsorption is the capability of all solid substances to attract to their surfaces molecules of solutions or gases with which they are in contact. The striking difference between adsorption and absorption is adsorption allows molecules to adhere to surface area whereas absorption allows molecules to penetrate internal past their surface. To efficiently apply the adsorption process it is vital to maximize surface area. Hence activated carbon is an ideal media to employ. The large surface area of carbon has many attractive forces that work to attract other molecules. Thus, contaminants in water are adsorbed to the surface of carbon by attractive forces similar to gravitational forces.
Water contaminants that are effectively adsorbed generally have lower water solubility, are organic (made up of carbon atoms), have a higher molecular weight and are chemically neutral. Besides physical adsorption, chemical reactions can occur on a carbon surface. For instance, chlorine can be removed from water by using carbon as a catalyst. The activated carbon provides electron transfer surface sites where chlorine accepts an electron from the dissolved oxygen in water which reduces chlorine to harmless chloride atoms. The carbon eventually becomes exhausted when these electron transfer sites are no longer available. At this point the chlorine will remain in the water until the water filters are replaced.
Carbon Characteristics which Effect Performance
When choosing carbon to accomplish a specific task for water purification several variables must be considered .One such variable is the Iodine Number. This number represents the efficiency of a carbon type to adsorb organics with lower molecular weights. The iodine number approximates the internal surface area. A high iodine number indicates a carbon with exceptional surface area as measured in square meters/gram. Another important variable is the Mesh Size of the carbon. 12 x 40 mesh is common and simply means that the carbon particle will flow through a size 12 mesh screen and will be trapped at 40 mesh. While selecting mesh size remember this trade off. The smaller particle size increases the rate of diffusion of an organic into the pore which increases the adsorption. This is a desirous effect but small particle size will also significantly increase pressure drop across the carbon bed resulting in restricted flow rates. Another example of an important variable is the carbon’s Molasses Number. This defines the large pore or macro pore volume of carbon. Carbons with a high molasses number have a large pore diameter which makes them better suited for the removal of large color body molecules such as tannins.
Generally, lower flow rates result in greater contact time which allows the organic contaminants to travel deeper into the carbons interstices thus exposing the organic to more pore surfaces which increases adsorption. When contact time is limited consider a carbon with a smaller particle size which will increase the rate of removal as the contaminant has less distance to reach the pores in the center of the carbon particle. This partially explains why carbon block technology is gaining widespread use. The carbon particle is crushed to a power, mixed with binders and processed to an extruded carbon block. These particles are very small which increases the speed of adsorption versus granulated carbon. As carbon block water filter cartridge technology advances the performance and capabilities become more impressive.
Granulated carbon will continue to be in high demand. One reason for this is that a granulated activated carbon bed can withstand the rigors of backwashing which agitates and lifts the carbon bed. This feature is desirous as it allows the bed to be cleaned of dirt and sediment by occasionally reversing the water flow. When carbon is employed in this manner obtain a product with a high abrasion number. This defines the carbons ability to withstand degradation when backwashing.
It is imperative to know that the contaminant size must be smaller than the pore size offered by the carbon employed. For example, carbons with small pore size will be ineffective at removing tannin organics which have a relatively large molecular structure. Although there is a greater adsorption rate with small pore diameter carbons it is many times essential to use carbons that have both fine and wide pore diameters. These are well suited to effectively treat waters with versatile organic contents.
Activated carbon is a media which has impressive applications in the water treatment industry. Its virtues are so great that it seems to purify almost by magic. Hopefully this brief introduction to activated carbon has provided some fundamental insight into something seemingly mystical.
Pavel Water Filtration
Certified Water Specialist