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CHAPTER 14

ACTIVATED CARBON

PROCESSES

Richard D. Brady
Richard Brady & Associates
San Diego, California

Activated carbon is an adsorbent material that provides a surface on which ions or mol-
ecules in the liquid or gaseous phase can concentrate.
The use of activated carbon in water treatment in the United States has been limited
primarily to removing taste- and odor-causing compounds, pesticides, and other organic
contaminants.

CHARACTERISTICS OF ACTIVA TED CARBON

Activated carbon has a random structure that is highly porous, with a broad range of pore
sizes ranging from visible cracks and crevices down to molecular dimensions. Intermol-
ecular attractions in the smallest pores create adsorption forces. These forces cause large
and small molecules of dissolved contaminants to be condensed and precipitated from so-
lution into the molecular-scale pores. Activated carbon is an effective adsorbent because
it provides a large surface area on which the contaminant chemicals can adhere.
Activated carbon is available in two different forms: powdered (PAC) and granu-
lar (GAC). Adsorptive properties of GAC and PAC are fundamentally the same be-
cause they depend on pore size, the internal surface area of the pores, and surface prop-
erties independent of overall particle size. Each brand of commercially available PAC
or GAC has properties making it most suitable for particular applications. Besides ad-
sorptive capacity and selectivity in removal, these properties include the ability to with-
stand thermal reactivation and resistance to attrition losses during transport and han-
dling. Powdered activated carbon is added to water, mixed for a short time, and
removed. Adsorption of molecules occurs while the PAC is in contact with the water.
In both Europe and the United States, PAC is typically fed as a powder using dry feed
equipment or is batched as a slurry and fed with metering pumps or rotodip feeders.
PAC is usually added early in the treatment process and then either settles out with
the floc in the pretreatment basins or is removed from the filter beds during back-
washing.

14.1