Civil Engineering Reference
In-Depth Information
break through at carbon loadings as low as 0.001 lb organics / lb carbon and can exceed
0.25 lb organic / lb carbon. The actual carbon loading or carbon dosage for a given
case must be determined from pilot plant tests. Generic cost curves, which are plots
of flow (in million gallons per day) versus cost (capital or operation and maintenance
costs), cannot be applied directly to water treatment. Allowance must be made in the
capital costs for the different reactivation capacity needed, and in the operation and
maintenance costs for the actual amount of carbon to be reactivated or replaced.
Carbon life varies greatly depending on the application. A short carbon life (6
months or less between regeneration cycles) will greatly increase the treatment cost.
Long carbon life—exceeding 2 years—is desirable. The designer has little control over
the carbon life, beyond selecting the carbon type and providing some pretreatment to
reduce fouling. Pretreatment, such as enhanced coagulation to reduce the TOC loading
to the GAC, can extend the carbon life. In addition, preoxidation such as ozonation
can change the characteristics of the TOC in the raw water to promote biological
activity and extend GAC life under certain circumstances.
Spent carbon may be removed from contactors and replaced with virgin carbon, or
it may be reactivated either on-site or off-site. The most economical procedure depends
on the quantities of GAC involved. As already discussed, on-site reactivation is more
economical for larger volumes. For small quantities of carbon, replacement or off-site
reactivation will probably be most economical. Several GAC suppliers will pick up
spent carbon and replace it with new carbon on a contract basis.
Carbon may be reactivated thermally to very near virgin activity. However, carbon
burning losses may be excessive under these conditions. Experience in industrial and
wastewater treatment indicates that carbon losses can be maintained at 8 to 10 percent
per cycle with the reactivated carbon capacity (as indicated by the iodine number) at
about 90 percent of the original virgin capacity. For certain organics, there may be no
decrease in actual organics removal despite a 10 percent drop in iodine number.
GAC may be reactivated in a multiple-hearth furnace, a fluidized-bed furnace, a
rotary kiln, or an electric infrared furnace. However, multiple-hearth furnaces are pres-
ently dominating the market. Spent GAC is drained in a screen-equipped tank (40
percent moisture content) or in a dewatering screw (40 to 50 percent moisture) before
being introduced to the reactivation furnace. Dewatered carbon is usually transported
by a screw conveyor. Following thermal reactivation, the GAC is cooled in a quench
tank. The water-carbon slurry may then be transported by means of diaphragm slurry
pumps, eductors, or a blow tank. The activated carbon may contain fines produced
during conveyance; these fines should be removed in a wash tank or in the contactor.
Maximum furnace temperatures and retention times are determined by the amount
(weight organics / weight carbon) and nature (molecular weight or volatility) of the
organics adsorbed.
Off-gases from carbon reactivation present no air pollution problems, provided they
are properly scrubbed. In some cases, an afterburner may also be required for odor
control. Multiple-hearth furnaces are the simplest, most reliable, and easiest to operate
for GAC reactivation. The infrared and fluidized-bed units have virtually disappeared
from the market.
It is necessary with all four types of furnaces to specify top-quality materials to
suit the conditions of service, as well as to see that these materials are properly in-
stalled. Corrosion resistance is important in the furnace itself and especially in all
auxiliaries to the furnace.
