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14.2 CHAPTER FOURTEEN

Granular activated carbon is gener~iHy used in adsorption beds or tanks through which
the water passes for treatment. As GAC is used for treatment, surfaces within the pores
gradually become covered with chemical molecules until the carbon is no longer able to
adsorb new molecules. At that point, the old, or spent, carbon must be replaced with new,
virgin, or fresh reactivated carbon. The adsorptive capacity of the spent GAC can be re-
stored by thermal reactivation.
One method of using GAC is to install it as a partial or complete replacement for the
conventional granular media in conventional gravity filters. In this case, GAC acts as a
filtering medium and provides limited adsorbance as water passes through the filters. The
other common type of installation is to locate the GAC contractor as the last step in the
treatment process, commonly with a preozonation step.
The use of PAC adsorption is most appropriate in situations where taste and odor or
organic contaminant, loading episodes are moderate, seasonal, and/or infrequent. The use
of GAC adsorption beds should be considered whenever a system experiences moderate
to severe taste and odor problems or organic contaminant loading. PAC may also be con-
sidered for small to midsized plants to address these conditions.

Physical Properties of Activated Carbon
Surface area within porous carbon structure provides the capacity to adsorb dissolved or-
ganic materials such as natural organic matter (NOM), disinfection by-products (DBPs),
or taste and odor compounds. Carbons used for adsorption in drinking water applications
have a minimum surface area of 73 acre/lb (650 m2/g), with typical surface areas on the
order of 112 acre/lb (1,000 m2/g). A series of tests developed for testing the suitability of
various types and brands of activated carbon are detailed in American Water Works As-
sociation (AWWA) Standard B604, Granular Activated Carbon and B600, Powdered Ac-
tivated Carbon.

Iodine Number and Molasses Number. The quantity of small and large pore volumes
in a sample of activated carbon is described by the iodine number and molasses number.
Adsorption tests are also used to approximate the distribution of pores available for ad-
sorption. Using standard reference adsorbates (such as iodine and a molasses solution) al-
lows the activity characteristics of different carbons to be compared. Iodine's small mo-
lecular size can characterize the small pore volume of a carbon and its ability to adsorb
contaminants small in molecular size. The iodine number is the mass of iodine adsorbed
(in milligrams) from a 0.02 N bulk solution by 1 g of carbon. The molasses number is a
measurement comparing the color (optical density) of the filtrate from a standard acti-
vated carbon with the color of the filtrate from the carbon being investigated.
When activated carbon is in use, the iodine and molasses numbers decrease with time
as adsorption occurs and available adsorption sites are filled. Some water treatment sys-
tems use iodine or molasses numbers to determine when to replace carbon. To use the io-
dine number as a surrogate measure (replacing actual bench-scale testing of the GAC with
the water needing treatment), a correlation between the iodine number and the degree of
carbon exhaustion should be developed for each specific adsorption application.
The AWWA standard indicates a performance requirement that the adsorptive capac-
ity of the granular activated carbon, as measured by the iodine number, shall not be less
than 500 mg/g carbon. The standard procedure for determining the iodine number of ac-
tivated carbon is ASTM D4607.
Carbon Weight. The weight of carbon in air is the apparent density. For water treat-
ment applications, carbon density is described by bulk density or as backwashed and