THE WATER WE DRINK…

Activated Charcoal Filtration

ALSO RESEARCH AND COMPARE ACTIVATED CHARCOAL TO OTHER PURIFICATION PROCESSES:

(AC) EQUIPMENT
(AC) Contaminants REMOVED
(AC) Contaminants NOT REMOVED

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Also:
See: (AC) PROCESS (How It Works)
And: Testing Your Water Supply

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AC works by attracting and holding certain chemicals as water passes through it. AC is a highly porous material; therefore, it has an extremely high surface area for contaminant adsorption. The equivalent surface area of 1 pound of AC ranges from 60 to 150 acres.

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 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 factors: 1) physical properties of the AC, such as pore size distribution and surface area; 2) the chemical nature of the carbon source, or the amount of oxygen and hydrogen associated with it; 3) chemical composition and concentration of the contaminant; 4) the temperature and pH of the water; and 5) the flow rate or time exposure of water to AC.

Physical Properties

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 (Figure 1). Because contaminants come in all different sizes, they are attracted differently depending on pore size of the filter. In general AC filters are most effective in removing contaminants that have relatively large molecules (most organic chemicals). Type of raw carbon material and its method of activation will affect types of contaminants that are adsorbed. This is largely due to the influence that raw material and activation have on pore size and distribution.

Figure 1. Molecular screening in the micropores of an activated carbon filter. (after G. L. Culp and R. L. Culp)

Chemical Properties

Processes other than physical attraction also affect AC filtration. The filter surface may actually interact chemically with organic molecules. Also electrical forces between the AC surface and some contaminants may result in adsorption or ion exchange. Adsorption, then, is also affected by the chemical nature of the adsorbing surface. The chemical properties of the adsorbing surface are determined to a large extent by the activation process. AC materials formed from different activation processes will have chemical properties that make them more or less attractive to various contaminants. For example chloroform is adsorbed best by AC that has the least amount of oxygen associated with the pore surfaces. The consumer can’t possibly determine the chemical nature of an AC filter. However, this does point out the fact that different types of AC filters will have varying levels of effectiveness in treating different chemicals. The manufacturer should be consulted to determine if their filter will adequately treat the consumer’s specific water problem.

Contaminant Properties

Large organic molecules are most effectively adsorbed by AC. A general rule of thumb is that similar materials tend to associate. Organic molecules and activated carbon are similar materials; therefore there is a stronger tendency for most organic chemicals to associate with the activated carbon in the filter rather than staying dissolved in a dissimilar material like water. Generally, the least soluble organic molecules are most strongly adsorbed. Often the smaller organic molecules are held the tightest, because they fit into the smaller pores.
Concentration of organic contaminants can affect the adsorption process. A given AC filter may be more effective than another type of AC filter at low contaminant concentrations, but may be less effective than the other filter at high concentrations. This type of behavior has been observed with chloroform removal. The filter manufacturer should be consulted to determine how the filter will perform for specific chemicals at different levels of contamination.

Water Temperature and pH

Adsorption usually increases as pH and temperature decrease. Chemical reactions and forms of chemicals are closely related to pH and temperature. When pH and temperature are lowered many organic chemicals are in a more adsorbable form.

Exposure Time

The process of adsorption is also influenced by the length of time that the AC is in contact with the contaminant in the water. Increasing contact time allows greater amounts of contaminant to be removed from the water. Contact is improved by increasing the amount of AC in the filter and reducing the flow rate of water through the filter.

Bruce Seelig, Water Quality Specialist, North Dakota Extension Service
Fred Bergsrud, Water Quality Coordinator, Minnesota Extension Service
Russell Derickson, Extension Associate in Water and Natural Resources, South Dakota Extension Service

Further Information

For further information contact your local county Extension Office or State Health Department. Additional information can be found in other publications in this series: Treatment Systems for Household Water Supplies

• AE1030—Iron and Manganese Removal (1992)
• AE1031—Softening (1992)
• AE1032—Distillation (1992)
• AE1045—Identification and Correction (1992)
• AE1046—Chlorination (1992)
• AE1047—Reverse Osmosis (1992)
  

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References

__________. 1989. Recognized treatment techniques for meeting the National Primary Drinking Water Regulations with the application of point-of-use systems. Water Quality Association, Lisle, Il.

__________. 1989. Recognized treatment techniques for meeting the National Secondary Drinking Water Regulations with the application of point-of-use systems.

__________. 1990. Fit to drink? Consumer Reports. pp. 27-43, January.

Caldron, R. L., and E. W. Mood. 1987. Bacteria colonizing point-of-use, granular activated carbon filters and their relationship to human health. Research Project CR-811904-01-0, Health Effects Research Lab., U.S. EPA, Cincinnati, OH. Reprinted by the Water Quality Association, Lisle, IL.

Culp, G. L. and R. L. Culp. 1974. New concepts in water purification. Van Nostrand Reinhold Co., New York.

Ishizake, C., I. Marti, and M. Ruiz. 1983. Effect of surface characteristics of activated carbon on the adsorption of chloroform from aqueous solution. In M. J. McGuire and I. H. Suffet (ed.), pp. 95-106. Treatment of water by granular activated carbon. Advances in Chemistry Series. American Chemical Society, Washington, D.C.

Rodale Press Product Testing Department Staff. 1985. Water treatment handbook – A homeowners quide to safer drinking water. Rodale Press Inc., Emmaus, PA.

Taraba, J. L., L. M. Heaton, and T. W. Ilvento. 1990. Using activated carbon filters to treat home drinking water, IP-6. University of Kentucky Cooperative Extension Service, Lexington, KY.

Temple, Barker, and Sloan Inc. Staff. 1983. Point-of-use treatment for compliance with drinking water standards. Reprinted by the Water Quality Association, Lisle, IL.

Funding for this publication was by the U.S. Department of Agriculture, Extension Service, under project number 90-EWQI-19252.