Activated carbon is particles of carbon that have been treated to increase their surface area and increase their ability to adsorb a wide range of contaminants – activated carbon is particularly good at adsorbing organic compounds.
Contaminant reduction in GAC filters takes place by two processes: physical removal of contaminant particles, blocking those that are too large to pass through the pores (obviously, filters with smaller pores are more effective). There is also a process called adsorption by which a variety of dissolved contaminants are attracted to and held (adsorbed) on the surface of the carbon particles. The characteristics of the carbon material (particle and pore size, surface area, surface chemistry, density, and hardness) influence the efficiency of adsorption.
GAC is a highly porous material; therefore, it has an extremely high surface area for contaminant adsorption. One reference mentions “The equivalent surface area of 1 pound of GAC ranges from 60 to 150 acres (over 3 football fields).” Another article states, “Under a scanning electron microscope the activated carbon looks like a porous bath sponge. This high concentration of pores within a relatively small volume produces a material with a phenomenal surface area: one teaspoon of activated carbon would exhibit a surface area equivalent to that of a football field.
GAC is made of tiny clusters of carbon atoms stacked upon one another. The carbon source is a variety of materials, such as peanut shells, coconut husks, or coal. The raw carbon source is slowly heated in the absence of air to produce a high carbon material. The carbon is activated by passing oxidizing gases through the material at extremely high temperatures. The activation process produces the pores that result in such high adsorptive properties.
THE ADSORPTION PROCESS DEPENDS ON THE FOLLOWING 5 FACTORS:
- Physical properties of the GAC, such as pore size distribution and surface area
- The chemical nature of the carbon source
- (the amount of oxygen and hydrogen associated with it)
- Chemical composition and concentration of the contaminant
- The temperature and pH of the water
- The flow rate or time exposure of water to GAC
It is important to replace the cartridge according to the manufacturer’s instructions.
ADVANTAGES OF GAC FILTERS:
- GAC filters are used for aesthetic water treatment, since they can reduce chlorine and
particulate matter as well as improve the taste and odor of the water.
- Loose granules of carbon do not restrict the water flow to the extent of Solid Block Activated Carbon (SBAC) filters, which makes them suitable in situations, like whole house filters, where maintaining a good water flow rate and pressure is important.
- Simple, economical maintenance. Typically an inexpensive filter cartridge needs to be changed every few months to a year, depending on water use and the manufacturer’s recommendation.
- GAC filters do not require electricity, nor do they waste water.
The bottom line is that GAC filters are effective and valuable water treatment devices, but their limitations must be considered. A uniform flow rate, not to exceed the manufacture’s specifications, needs to be maintained for optimal performance and the filter cartridge must be changed after treating the number of gallons the filter is rated for.