What the Heck is Activated Carbon?

Adsorption treatments using a variety of carbon products are effective against organic contaminants in wastewater and drinking water streams.

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Many municipal drinking water and wastewater plants use activated carbons to purify water and air leaving the plant. Activated carbon is not a subject you learn about in school — you learn on the job.

At present, activated carbon in its various forms has more than 2,500 commercial applications. Operators of facilities that use activated carbon can benefit from a better understanding of what it is, how it works and how to use it most effectively.

Trapping contaminants

Activated carbon is an inert, solid, adsorbent material that can remove many dissolved contaminants from water and process gas streams. It can be made from almost any feedstock that contains carbon; for municipal plants these mainly include wood, coconut shells and coal. Activated carbons are inexpensive and readily available. Being highly porous, they provide a large surface area to remove contaminants: One teaspoon has more surface area than a football field.

Activated carbon is especially effective for capture of contaminants that impart taste, odor, color and toxicity. Contaminants adsorb on the surface of the carbon particles in tiny pores and are thus pulled out of solution.

Forms of carbon

Activated carbon manufacturers can provide a variety of pore size distributions by using different feedstocks and process parameters. Proper pore structure selection is the key to effective activated carbon treatment.

Carbons are sold and used in forms that include powders, granules, pellets, blocks and composites. The major difference in coconut, coal and wood activated carbons is the size of their graphitic platelets — honeycombed, six-membered, unsaturated carbon rings. The relative sizes of the graphitic platelets depend on the feedstock. Coconut-based carbons have larger platelets than coal-based carbons, which in turn have much larger platelets than wood-based carbons.

Powdered activated carbon

Powdered, micron-sized activated carbon milled from millimeter granular activated carbon acts faster and has more contaminant removal capacity than larger particles. It can be added to clarification units for sporadic contaminant episodes like algae blooms and industrial spills. Powder also can be used to protect fixed granular activated carbon beds against sudden influent contamination.

Treatment plants that lack the infrastructure to use granular activated carbon or do not have enough granular carbon between the influent and the effluent to remove sporadic contaminant episodes economically can use powder. It is used as a batch process to remove contaminants to acceptable regulated maximum contamination levels (MCLs). It will not necessarily remove the contaminants to zero or non-detected. Powder is a single-use product; it cannot be regenerated.

Granular activated carbon

Millimeter-sized granular activated carbon in beds is more effective than powder; it can remove contaminants to non-detect levels and requires about one-fourth the amount of carbon between the influent and the effluent versus powder. However, the plant needs proper infrastructure to install fresh and remove spent granular activated carbon.

Granular activated carbon is used in continuous processes. It is a multiple-use product in that it can be thermally reactivated. The reactivated carbon (react) costs about half as much as fresh. Water plants in areas with high risk of industrial pollution need more activated carbon in fixed vessels and more powdered carbon available for emergencies.

Pellet activated carbon

Pellets (or extra-large granules) are used to control vapor-phase municipal wastewater hydrogen sulfide (H2S) and other odorous gases. These forms enable gas streams to flow through uninhibited, without the need for energy-consuming fans. Regular and catalytic carbons are used for H2S control. With regular carbon, mobile H2S is oxidized to immobilized sulfur, which accumulates on the carbon surface. Catalytic carbons oxidize H2S to form sulfuric acid, which can be washed from used carbon with water and reused on site many times.

Mass transfer zone

Aqueous- and gas-phase applications develop a fixed, moving-contaminant mass transfer zone (MTZ) as contaminated water or gas passes through a bed. Working carbon beds have three zones. There is a zone where the carbon is completely used, a second zone where contaminants are transferring from the mobile water or air to carbon and immobilized, and a third zone that consists of unused carbon.

Carbon beds are usually 3 to 10 feet deep, use gravity flow, and consist of stratified activated carbon, smaller particles on top and the largest particles on the bottom. Activated carbon removes water-soluble organics and solids; the solids that collect atop the bed are removed by backwashing. Bed stratification must be maintained after backwashing. Used and unused carbon should not be mixed during backwashing. The suspended carbon particles should be allowed to settle slowly after backwashing to maintain the mass transfer zone.

The late beds of activated carbon in the series provide the final polishing to remove trace contaminants. By changing out the earlier exhausted beds with fresh carbon, the later beds function longer as the final polisher and provide a safety margin. When samples are taken to profile a carbon bed, it is preferable to take samples from the top, middle and bottom of the bed. This allows for a more accurate locating of the mass transfer zone and better estimation of the remaining service time.

Multiple carbon beds are typically configured in a sequential series to improve carbon performance and economics. Beds in a series allow complete carbon bed use, where the influent and effluent are equivalent in contaminant concentrations, because remaining backup beds in the series will start another mass transfer zone. This lead-and-lag bed configuration enables treatment of a maximum volume of water per pound of activated carbon before the carbon is replaced.

Spent activated carbon

Activated carbon does not last forever; pores or physical adsorption spaces are eventually filled and can no longer remove contaminants. Spent carbon needs to be periodically changed out with virgin or reactivated carbon.

Carbon pores are heterogeneous and vary in adsorption energy from strong to weak. Carbon graphitic platelets that are close together provide high adsorption potential energies, and wide platelet spacings provide relatively low adsorption energies.

Reactivation of spent carbon uses government-permitted kilns and gas-phase chemistry similar to the original carbon manufacture. The thermal process used for reactivation can change the original pore structures. Gravimetric adsorption energy distribution (GAED) is a test that detects and quantifies the widening of the pore size distribution.

After several reactivation cycles, the efficacy of reactivated carbon diminishes to a point where it needs to be replaced with virgin carbon. Widening of the pore size distribution from reactivation can be beneficial, especially for contaminants with larger molecules and higher molecular weights. However, water-soluble, low-molecular-weight compounds at trace concentrations, like trihalomethanes, may not be as readily adsorbed if a wider pore size distribution is used.

About the Authors

Henry Nowicki, Ph.D., MBA, (henry@pacslabs.com) is president and senior scientist for PACS Activated Carbon Services. George Nowicki, B.A. (george@pacslabs.com) is the company’s laboratory director. The company offers training courses on carbon technology and hosts the annual International Activated Carbon Conference. Wayne Schuliger, P.E., provides a short course on the design, operation and troubleshooting of activated carbon adsorbers.


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