The primary raw material used for AC is any organic material with a high carbon content such as coal, wood, peat, coconut shells. Granular AC is most commonly produced by grinding the raw material, adding a suitable binder to give it hardness, re-compacting and crushing it to the correct size. The carbon-based material is converted to AC by thermal decomposition in a furnace using a controlled atmosphere and heat.

The resultant product has an extraordinarily large surface area per unit volume and a network of submicroscopic pores where adsorption takes place. The walls of the pores provide the surface layer molecules essential for adsorption.

AC filtration is most effective in removing organic contaminants from water. Organic substances are composed of two basic elements carbon and hydrogen. Because organic chemicals are often responsible for taste, odour, and colour problems, AC filtration can generally be used to improve aesthetically objectional water. AC filtration will also remove chlorine. AC filtration is recognised by the Water Quality Association as an acceptable method to maintain certain drinking water contaminants within the limits of the EPA National Drinking Water Standards.

AC filtration also removes some organic chemicals that can be harmful if present in quantities above the EPA Health Advisory Level (HAL). Included in this category are trihalomethanes (THM), pesticides, industrial solvents (halogenated hydrocarbons), polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs).

THMs are a by-product of the chlorination process that most public drinking water systems use for disinfection. Chloroform is the primary THM of concern. EPA does not allow public systems to have more than 100 parts per billion (ppb) of THMs in their treated water. Some municipal systems have had difficulty in meeting this standard.

The Safe Drinking Water Act mandates EPA to strictly regulate contaminants in community drinking water systems. As a result, organic chemical contamination of municipal drinking water is not likely to be a health problem. Contamination is more likely to go undetected and untreated in unregulated private water systems. AC filtration is a viable alternative to protect private drinking water systems from organic chemical contamination.

Radon gas can also be removed from water by AC filtration, but actual removal rates of radon for different types of AC filtration equipment have not yet been established.

Similar to other types of water treatment, AC filtration is effective for some contaminants, but not effective for others. AC filtration does not remove microbes, sodium, nitrates, fluoride and hardness. Lead and other heavy metals are removed only by a very specific type of AC filter. Unless the manufacturer states that its product will remove heavy metals, the consumer should assume that the AC filter is not effective in removing them. Please refer to the other circulars in the Treatment Systems for Household Water Supplies series, for information on systems that do remove the contaminants listed above.

Physical adsorption is the primary means by which AC works to remove contaminants from water. Carbon's highly porous nature provides a large surface area for contaminants (adsorbates) to collect. In simple terms, physical adsorption occurs because all molecules exert attractive forces, especially molecules at the surface of a solid (pore walls of carbon), and these surface molecules seek other molecules to adhere to. The large internal surface area of carbon has many attractive forces that work to attract other molecules. Thus, contaminants in water are adsorbed (or held) to the surface of carbon by surface attractive forces similar to gravitational forces. Adsorption from solution occurs as a result of differences in adsorbate concentration in the solution and in the carbon pores. The adsorbate migrates from the solution through the pore channels to reach the area where the strongest attractive forces are

With this understanding of how the adsorption process works, we must then understand why it works, or why water contaminants become adsorbates.

Water contaminants adsorb because the attraction of the carbon surface for them is stronger than the attractive forces that keep them dissolved in solution. Those compounds that are more adsorbable onto activated carbon generally have a lower water solubility, are organic (made up of carbon atoms), have a higher molecular weight and a neutral or non-polar chemical nature. It should be pointed out that for water adsorbates to become physically adsorbed onto activated carbon, they must be both dissolved in water and smaller than the size of the carbon pore openings so that they can pass into the carbon pores and accumulate.

Besides physical adsorption, chemical reactions can occur on a carbon surface. One such reaction is chlorine removal from water involving the chemical reaction of chlorine with carbon to form chloride ions. This reaction is important to POU treatment because this conversion of chlorine to chloride is the basis for the removal of some common objectionable tastes and odours from drinking water.

Water contaminants adsorb because the attraction of the carbon surface for them is stronger than the attractive forces that keep them dissolved in solution.

AC works by attracting and holding certain chemicals as water passes through it. AC is a highly porous material and therefore has an extremely high surface area for contaminant adsorption. One gram of AC can have a surface area in excess of 500 m, with 1500 m being readily achievable. By comparison, a tennis court is about 260 m. Carbon aerogels, while more expensive, have even higher surface areas, and are used in special applications.

Forces of physical attraction or adsorption of contaminants to the pore walls is the most important AC filtration process. The amount and distribution of pores play key roles in determining how well contaminants are filtered. The best filtration occurs when pores are barely large enough to admit the contaminant molecule. Because contaminants come in all different sizes, they are attracted differently, depending on the filters pore size. In general AC filters are most effective in removing contaminants that have relatively large molecules (most organic chemicals). The type of raw carbon material and its method of activation will affect which types of contaminants are adsorbed. This is largely due to the influence that raw material and activation have on pore size and distribution

AC is made of tiny clusters of carbon atoms stacked upon one another. The carbon source is a variety of materials, such as peanut shells 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 oxidising 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 factors: