Civil Engineering Reference
In-Depth Information
trough bottoms should be available to permit at least 30 percent expansion of the
carbon layer.
The advanced wastewater treatment (AWT) experience with GAC contactors may
be applied to water purification if some differences in requirements are taken into
account. The required contact time must be determined from pilot plant test results.
Contactors may be designed in a downflow or upflow mode of operation. Upflow
packed beds provide maximum theoretical carbon adsorption efficiency through the
use of countercurrent flow principles. However, because leakage of some carbon fines
(1 to 5 mg / L) in upflow carbon column effluents can cause problems in downstream
processes, downflow carbon beds are more commonly used in municipal water treat-
ment applications. For example, at the Orange County Water Factory 21 (Fountain
Valley, California), upflow beds were converted to downflow beds to correct a problem
with escaping carbon fines. This full-scale plant operating experience indicates that
leakage of carbon fines is not a problem in properly operated downflow GAC contac-
tors.
Single beds or two beds in series may be used. Open gravity beds or closed pressure
vessels are permissible. Structures may be properly protected steel or reinforced con-
crete. In general, small plants will use steel, and large plants may use steel or rein-
forced concrete.
Sand in rapid filters has, in some instances, been replaced with GAC. In situations
where contact times are short and GAC regeneration or replacement cycles are excep-
tionally long (several months or years), as may be the case in taste and odor removal,
this approach may be a solution. However, with the short cycles anticipated for most
organics, conventional concrete-box-style filter beds may not be well suited to GAC
contact; deeper beds may be more economical and provide more efficient use of GAC.
Beds deeper than conventional filter boxes, or contactors with greater aspect ratios of
depth to area, provide much greater economy in capital costs; the contactor cost for
the needed volume of carbon is much less. In water slurry, carbon can be moved from
contactors with conical bottoms easily and quickly and with virtually no labor. Flat-
bottomed filters of a type that require labor to move the carbon unnecessarily add to
carbon transport costs. The labor required to remove carbon from flat-bottomed beds
varies considerably in existing installations from a little labor to a great deal, depending
on the design of the excavation equipment.
For many GAC installations intended for precursor organic removal or synthetic
organic removal, specially designed GAC contactors should be installed. Contactors
should be equipped with flow-measuring devices. Separate GAC contactors are espe-
cially advantageous where GAC treatment is required only seasonally, because the
contactors then can be bypassed when they are not needed, possibly saving unneces-
sary exhaustion and reactivation of GAC.
Carbon Contactor Design: Underdrains
The design of carbon contactor under-
drains requires experienced, expert attention. Although good proven filter underdrain
systems—such as lateral collectors with coarse gravel, screens, and full-floor blocks—
are available, they should be designed for GAC retention. Earlier underdrain designs
have failed in many installations for conventional filter service, and they continue to
be misapplied to GAC contactors as well as filters.
GAC Reactivation or Replacement
One of the principal costs for GAC treatment
is the reactivation frequency required. Organics of concern in water treatment may
