TABLE OF CONTENTS:

Intro

Physical Characteristics

Uses

Regeneration

Advantages

Disadvantages

Conclusion

References

INTRO

Photo Courtesy of R Summers

Activated carbon is a highly porous carbonaceous substance with a wide range of applications in gas, vapor, and liquid treatment. The use of activated carbon dates back to 1500 BC where its use was discovered in an Egyptians papyrus for medicinal purposes. In the 18 century, Sheele recognized the adsorptive powers of carbons in experiments with gases. During World War I, activated carbon use jumped when the Allies used it in gas masks to filter out chlorine gas (Yehaskel, 1978). Activated carbon is used successfully today, especially in water treatment to remove organic compounds that impart color, taste and odor to the water. Contaminant removal is achieved through a process called adsorption by which contaminants adhere to the surface of the carbon and are thus removed from the water.

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PHYSICAL CHARACTERISTICS

Activated carbon is manufactured from a variety of sources; primarily coal, wood, lignite, and coconut shells. The process includes first carbonizing the raw material at low temperatures, and then activating the carbon in a high temperature steam process (Active Carb, ). Any volatile content inside the carbon is burned, leaving a beehive-like structured carbon with a high volume of pores and a large surface area. PAC (powdered activated carbon) is prepared by a pulverizing action, leaving a very fine powder. GAC (granular activated carbon), is in granular form and has great mechanical strength (Patrick, 1995). The figure below shows granular activated carbon under magnification:

The porosity of activated carbon, which is classified by the size of the diameter of the pores, varies from micropores(2 nm), to mesopores (2-50 nm), to macropores(greater than 50 nm). In water treatment, particles of the same size of the pores tend to get stuck and retained by the carbon. Volatile organic chemicals, metals, and some non-polar inorganic chemicals are captured and held strongly by the carbon. The surface area of activated carbon can range from 500 to 1,400 square meters per gram (Hassler, 1974).

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USES

Applications of activated carbon in the liquid phase include potable water treatment, groundwater treatment, decolorization, industrial water treatment, metals (gold) recovery, and chemical and pharmaceutical treatment. Other applications for air filtration, gas purification, and dechlorination occur in the gas phase. In 1995, the world production of activated carbon was estimated to be 300,000-400,000 tons. About 80% of this carbon is used in liquid-phase applications, and the rest is used in gas-phase applications (Patrick, 1995). Activated carbon is used in water treatment to remove organic compounds. GAC (granular activated carbon), can be added after coagulation and sedimentation as a layer in sand filters to remove organics from the water. The most common way to use GAC in water treatment is in packed-bed parallel downflow adsorbers where water is passed downward through the beds . Other setups include upflow packed bed, upflow expanded bed, and operation in series (An Evaluation, http://www.ul.cs.cmu.edu/books/vol2_drinking_water/0000265.htm).

PAC (powdered activated carbon), is more commonly used than GAC to control taste and odor in drinking water treatment. PAC can be added directly to the water prior to coagulation up until just before the rapid sand filter. The PAC adsorbs contaminants and is then removed by sedimentation or filtration (An Evaluation, http://www.ul.cs.cmu.edu/books/vol2_drinking_water/0000265.htm).

FACTORS AFFECTING ADSORBANCY

VOC removal from a wastestream by activated carbon depends on the flow rate and initial contamination in the wastestream. If an inflow has a very high organic compounds concentration and the activated carbon reaches maximum adsorption, the adsorbed compounds may desorb and come out in the effluent (Active Carb). The Freundlich isotherm is an empirical model that can be used to determine the amount of carbon required to treat a given inflow of water. The mathematical equation is:

X/M = KC_f^1/n

(Novak, 1997)

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REGENERATION

After the activated carbon has reached exhaustion and all the adsorptive sites are filled, it can be regenerated by heating it at a temperature of 820 to 930 degrees C (Ottaway, http://ourworld.compuserve.com/homepages/ottaway/gac.htm). Recovery of the carbon ranges from 90 to 95% (Hassler, 1974). Regeneration is practiced more in Europe than in the US.

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ADVANTAGES

Activated carbon is very successful in the removal of Class I compounds (categorized by the EPA as organic compounds that cause taste and odor and/or color problems). The table below illustrates how effective the carbon is in removal:

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DISADVANTAGES

Activated carbon is expensive, thus making regeneration economically desirable. The prices of filters range from under $100 for a single tap, to more than $1,000 for an entire house unit. One other disadvantage to activated carbon is that the media can become a breeding ground for microorganisms (Ottaway, http://ourworld.compuserve.com/homepages/ottaway/gac.htm). This can be thought of as an advantage too, since the microbes degrade soluble organics. To avoid this from happening, the water should be disinfected prior to going through the carbon media. Postdisinfection should also be used because reactions with GAC can remove aqueous oxidants used in preliminary disinfection. (An Evaluation, http://www.ul.cs.cmu.edu/books/vol2_drinking_water/0000265.htm). Another disadvantage with activated carbon is the high emissions of sulfur dioxide generated from the heating process in manufacturing carbon from coal. One suggestion for a cleaner process is to make activated carbon from pecan nut shells which are produced at a rate of 43,000 tons per year in the US. The shells are first ground into a powder, then heated in the absence of oxygen, and finally reheated in the presence of carbon dioxide (Kleiner, 1997).

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CONCLUSION

Raw, untreated drinking water sources may contain organic compounds that have been proven to be carcinogenic or toxic. To ensure the safety of public health, it is necessary to treat the water and remove as many potentially harmful contaminants as possible. Activated carbon proves to be very effective in organic compound reduction and removal. With the increasing demands for environmental protection, activated carbon technology has quite a future. Other forms of the carbon such as fibers and microbeads are developing and could prove to be very effective.

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REFERENCES

Active Carb General Info.

An Evaluation of Activated Carbon for Drinking Water Treatment. http://www.ul.cs.cmu.edu/books/vol2_drinking_water/0000265.htm

Arnold, Edward. Porosity in Carbons. Hodder Headline Group, London. 1995.

Bansal, Roop C. Active Carbon. Narcel Dekker, Inc. NY. 1988.

Cheremisinoff, Paul N. and Ellerbusch. Carbon Adsorption Handbook. Ann Arbor Science Publishers, Inc. MI. 1980.

Hassler, John W. Activated Carbon. Chemical Publishing Co., Inc., NY. 1974

Kleiner, Kurt. Nutty Idea That’s Clean and Cheap. New Scientist, IPC Magazines Limited. 1997. http://www.newscientist.com/ns/970405/pecan.html

Matterhorn Filer Systems

Novak, Dr. John T. Class Notes. CE 4174. Fall Semester. 1997.

Ottaway, Bill. Activated Carbon http://ourworld.compuserve.com/homepages/ottaway/gac.htm

Yehaskel, Albert. Activated Carbon, Manufacture and Regeneration. Noyes Data Corp., NJ. 1978.

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