TM 5-814-3/AFM 88-11, Volume III
(3) Pulsed bed. A "pulsed bed" is defined as an upflow carbon adsorption system where a layer of
exhausted carbon is withdrawn from the bottom of the carbon bed, with a regenerated layer being added to
the top of the bed. This technique approximates countercurrent operation and is a nearly-continuous process.
(4) Gravity and pressurized flow. Gravity-flow systems have the advantage of eliminating the need
for pumps and pressure vessels. The restricting factor in gravity flow is head loss. For this reason,
pretreatment for suspended solids removal is required. Gravity-flow systems can either downflow or upflow.
The upflow, expanded-bed configuration will facilitate maintenance of a constant head loss. Pressurized-flow
systems will offer more flexibility in process design by operating at higher flow rates and over wider ranges
of pressure drop.
(5) Series and parallel arrangement. Carbon contacting beds can be arranged as single stages,
independently operated; or as multi-stages, either in series or in parallel. Series configurations achieve more
complete organic removal and will be used when carbon adsorption is required to remove 90 percent of the
total plant organics. Economic studies indicate that two-stage series operation is least expensive in terms of
operating costs. For lower levels of treatment, single-stage, parallel contactors staggered in their status of
operation or degree of exhaustion can produce the desired product by blending of individual effluent.
(6) Regeneration. Activated carbon is regenerated in a step heating process; refer to the EPA Process
Design Manual for Carbon Adsorption for design details. Carbon regeneration systems include preliminary
dewatering of the carbon slurry to a moisture content of 40 to 50 percent, heating in a multiple-hearth furnace
to 1,500-1,700 degrees Fahrenheit, quenching of regenerated carbon, and recycle to contactors. The
regeneration process requires 3,200 British thermal units for burning off the impurities and 1 pound steam
per pound of regenerated carbon. Air pollution control equipment is required, usually an afterburner and a
wet scrubber or bag filter.
(7) Carbon transport. The carbon is usually transported within the system as a slurry at velocities
between 2.5 and 10 feet per second. Lower velocities make the system vulnerable to solids deposition and
higher velocities cause abrasion in pipes. Velocities of 3 to 4 feet per second, with 4 pounds water per pound
of carbon (0.5 gallon water per pound carbon), are recommended. The carbon slurry can be stored before
and after regenera-tion, or it can be transported directly to and from the contactors. The latter arrangement
requires at least two spare contactors and is a significant cost factor in pressurized systems.
(8) Backwashing. Backwashing frequency is determined by head loss buildup, with lower flow rates
usually allowing less frequent backwash. Backwashing is supplemented by surface washing and air scouring,
and the complete operation lasts 15 to 45 minutes. Backwashing should provide 30 to 50 percent bed
expansion while consuming no more than 5 percent of normal feed rate (i.e., 15 to 20 gallons per minute per
square foot). Effluent can be used for backwash and then returned to the primary treatment stage.
b. Process design parameters. Where practical, carbon column studies should be conducted on the waste
to be treated to determine the process design parameters. These studies should use the type of activated
carbon that will be used in operating the full-scale plant.
(1) Pretreatment. Pretreatment will be provided as necessary to keep the suspended solids concentra-
tions below 50 milligrams per liter unless the carbon bed is to be used as a filter also.
(2) Carbon size. The carbon will be 8x 30 mesh, granular carbon unless carbon column studies show
a different size to be more effective.
(3) Contact time. Contact time is the most important design factor affecting organics removal and
should be determined empirically for the particular situation. Typical values range from 18 to 36 minutes.
(4) Hydraulic loadings. Hydraulic loadings between 2 and 10 gallons per minute per square foot are
acceptable; there appears to be little effect on organics removal in this range. The main consideration is with
head loss buildup. Gravity-flow systems are limited to hydraulic loadings less than 4 gallons per minute per
square foot.
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