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ACTIVATED CARBON PROCESSES 14.9

by inadequate coagulation or sedimentation or high doses of PAC added just before the
filters. The problem is usually eliminated if carbon is fed into the source water intake or
into the rapid mix basin.
Careful attention must be paid to the interaction of PAC with other water treatment
chemicals. Activated carbon chemically reduces substances such as free and combined
chlorine, chlorine dioxide, ozone, and potassium permanganate, and the demand for these
chemicals will be substantially increased. Mixing PAC with chlorine also reduces the ad-
sorption capacity of the activated carbon for selected compounds. Competition between
the two chemicals must be avoided. Adding PAC to a source water supersaturated with
CaCO3 or other precipitates, or after lime softening, may lead to particle coating of the
PAC and a corresponding decrease in adsorption efficiency.
Carbon dust is potentially explosive. Explosion-proof motors should be used if con-
tact with carbon dust is a possibility. Because PAC adsorbs organic compounds, includ-
ing gases that could reduce its effectiveness, PAC storage must be carefully located. Car-
bon should be stored in a separate, climate-controlled storage area. The stock of bagged
PAC should always be stored in a manner that allows stock to be rotated. Storing carbon
outside on pallets under canvas or plastic sheets for long periods is not recommended.
PAC slurry solidifies if not mixed frequently or, preferably, continuously. Feed lines
become clogged, as do the bags of filter dust collectors. Hydraulic eductor lines carrying
slurry also become clogged if they are shut down without being thoroughly flushed. It is
recommended that mixers be set on timers to allow scheduled, unattended operation.
If a slurry feed system is used, carbon should be fed below the water surface in the
slurry tank so that only a small amount of dust is produced during the loading period. A
dust collector should be provided on top of the slurry tank to prevent any dust from es-
caping into the plant.
Some bag storage should be maintained to supplement a slurry-based system for use
if sudden, extremely heavy use exceeds the slurry tank capacity. A bag loader can be pro-
vided on top of the slurry tank to allow the tank to be recharged with PAC without call-
ing for a full tanker truck or to meet lower feed rates. Although this is not recommended
except for worst-case conditions, carbon bags can be dumped directly into the intake well
or elsewhere at the head of the plant.
The PAC feed system should be located as close to the application point as possible.
It is suggested that the system be periodically operated, even without a need for carbon
feed. Working parts of the system should be checked to be sure the system is always ready
for use. Periodic operation also ensures that feed lines are cleaned, carbon is mixed, and
the system is flushed. The discharge piping for any feed device should include a flushing
line, automatically set to flush the pipelines for a period of at least 60 s after carbon feed
is stopped. Because moist PAC is highly corrosive, all metal parts that come in contact
with carbon should be type 316 stainless steel.

DESIGN OF GRANULAR ACTIVA TED
CARBON FACILITIES
As illustrated in Figure 14.2, three basic options are available for locating a GAC treat-
ment step for both new and existing water treatment plants:
• Prefiltration adsorption, ahead of the conventional filtration process (prefilter adsorber)
• Postfihration adsorption, after the conventional filtration process (postfilter adsorber)
• Filtration/adsorption, combining water filtration and GAC filtration into a single pro-
cess (filter-adsorber)