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ACTIVATED CARBON PROCESSES 14.3

drained. For a filter of known dimensions, density determines the weight (or mass) of
GAC required to fill that filter. Typically, carbon density should be determined on an as-
received basis, and calculations can then be made to correct for moisture content. The
apparent-density test apparatus and the procedure for determining the apparent density are
described in the AWWA standard.
Apparent density is important because, for new GAC systems, the initial quantity of
activated carbon is typically specified either by volume or by weight. Volume (cubic feet
or cubic meters) for first-time installation is specified as backwashed, drained, and in
place. After the initial installation, additional GAC needed to replace lost carbon may be
specified by volume or by weight. The standard specifies that the apparent density of GAC
shall be at least 0.25 g/cm 3.
Moisture Content. The AWWA standard indicates that moisture content of GAC shall
not exceed 8% by weight as packaged or at the time of shipment in the case of bulk
shipments.
Abrasion Resistance. Abrasion resistance is a property describing carbon durability. Ac-
tivated carbon is exposed to abrasion during shipping, installation, backwashing, and re-
generation. If the carbon is not durable enough, abrasion can generate undesirable fines
or crushed carbon. Increased fines can result in increased head losses across the filter bed,
increased loss of carbon, or degradation of water quality. Abrasion resistance is expressed
in terms of the abrasion number; the greater the number, the more resistant the carbon is
to abrasion.
Durability. Although the industry has not yet agreed on a standard test for predicting
carbon durability, the AWWA standard suggests two tests. The stirring abrasion test is
recommended for lignite-based GAC, and the Ro-Tap abrasion test is recommended for
bituminous-based GAC. For either test, the retention of average-size GAC shall be at
least 70%.

Ash Content. Ash content reflects the purity of the carbon. In the United States, most
activated carbons are manufactured from coal. Higher-quality coals, such as metallurgical-
grade bituminous coals, produce carbons with ash contents of approximately 5% to 8%.
Subbituminous coals produce carbons with ash contents of approximately 10% to 15%.
Lower-grade coals, such as lignite, produce activated carbons with the highest ash con-
tent of approximately 20%. Ash found in these coal-based carbons can contain calcium,
magnesium, iron, and silica. These constituents can form precipitates in areas with hard
water supplies. According to AWWA Standard B604, water-soluble ash in GAC should
not exceed 4%.

GAC Particle Size. The particle size of GAC used in a filter affects pressure drop, fil-
tration abilities, requirements for backwash rate, and the rate at which adsorption equi-
librium is reached. Smaller particle sizes increase the pressure drop across the carbon bed
and necessitate lower backwash rates. However, small GAC particles reach equilibrium
more rapidly than large particles because of the smaller (shorter) distance organics must
diffuse to reach the center of the particle and the larger surface area-to-volume ratio of
smaller GAC particles. Mesh size describes the range of particle sizes to be used in a
filter.
The effective size of GAC is defined in the AWWA standard as the size opening
through which only 10% of a sample of representative filter material will pass. For ex-
ample, if the size distribution of the media grains has 10% finer than 0.600, the effective
size of the GAC is 0.600 mm.