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ACTIVATED CARBON PROCESSES 14.31

For a new shipment of GAC, fines are typically specified in the range of 4.0% maxi-
mum, although requiring less than 2.0% is common. The majority of initial carbon fines
are backwashed out of the system within 1 to 2 days after the GAC is placed into service.
During regular backwashing operations, abrasion of particles occurs. The mean panicle
diameter of typical coal-based GAC products is usually reduced by 0.1 to 0.2 mm during
the on-line time of 2 to 4 years.
A loss of GAC bed depth of 0.2 to 1.0 in. (0.5 to 2.5 cm) per year is common. For a
24-in.-deep (0.6-m) bed, this translates to 1% to 4% loss per year. Losses beyond these
values indicate that there may be a problem with the underdrain system or that the ap-
plied backwash rate is excessive, resulting in the migration of carbon over the tops of the
washwater troughs.
Medium for Biological Treatment. Both filter-adsorbers and postfilter adsorbers be-
come biologically active when disinfection occurs after the GAC process. When ozone or
an advanced oxidation process (AOP), such as ozonation and hydrogen peroxide, precedes
the GAC treatment step, the GAC also becomes biologically active. Ozonation makes
some nondegradable compounds biodegradable by breaking the compounds into smaller,
more biodegradable products and thereby provides an additional means of organics
removal.

Practical Design Suggestions
Carbon adsorption system design is not complex. In fact, most of the design elements and
features are similar to those found in the design of a conventional filter system. However,
there are several design considerations worth noting that should be taken into account
when one is preparing a carbon adsorption design.

Design of Adsorber Vessels
• For gravity downflow, open-top concrete adsorbers, consider dual cells to reduce the
backwash rate. In an attempt to limit the total number of contactors, the subsequent
backwash flow rate required for the larger filter area may be too great or require too
much of the instantaneous plant flow for backwashing. Providing dual cells allows each
filter cell to be backwashed separately, reducing the instantaneous backwash flow rate
to one-half. Dual cells reduce the size of the washwater system but increase the num-
ber of valves. In large systems, the number of valves to open, close, operate, and main-
tain is an important consideration.
• If a stainless steel wedge wire underdrain system is used, underdrain laterals should be
at least 1 ft off of the vessel floor to facilitate carbon removal. Sloping the vessel bot-
tom from the back to the front assists in flushing.
• Underdrain systems with nozzles are not recommended, because they are subject to
plugging. Systems with false bottoms or plenums in which carbon fines can accumu-
late also are not recommended. Where nozzles and false bottoms are used in combi-
nation, plugging of the nozzles may create uplift pressures during backwashing, lead-
ing to structural failure of the false bottoms. Allowing carbon fines to collect in the
underdrain system is also undesirable because these fines can provide a habitat for the
growth of microbes and other undesirable organisms.
• Provide a method for periodic evaluation of the carbon bed depths by placing a per-
manent reference mark such as a stainless steel plate, laminated plastic staff gauge, or
painted gauge marks directly on the vessel wail.