(1) Pretreatment. Pretreatment will be provided as necessary to keep the
nonfiltrable residue concentrations below 50 mg/l, unless the carbon bed is to be used
as a filter also.
(2) Carbon size. The carbon will be 8 X 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 from 2 to 10 gallons per
minute/square feet (gpm/sq. ft.) are acceptable and effective in organics removal at this
range. The main consideration is with head loss build-up. Gravity-flow systems are
limited to hydraulic loadings less than 4 gpm/sq. ft.
(5) Carbon quantities and adsorption capacity. Carbon requirements range
from 250 to 350 pounds of carbon per million gallons treated. The adsorption capacity
of carbon is affected by several factors and should be determined experimentally for
each particular wastewater to be treated. Factors that influence adsorption include
surface area, nature of the material to be treated, pH, temperature, nature of carbon,
and complexity of material to be absorbed.
d. Considerations. The effluent quality requirement will determine the required
contact time, and this, in turn, will set the approximate total carbon volume. The
hydraulic loading will determine the total cross-sectional area and total carbon bed
depth. The total cross-sectional area can be divided into separate carbon beds in
parallel and the total bed depth can be divided between beds in series. Vessel heights
should provide for bed expansion of 50 percent. Contact tanks should have length-to-
diameter ratios of between 0.75 to 2.0, with carbon depths usually greater than 10 feet.
The tanks should be constructed of concrete or lined carbon steel. Typical coating
materials range from a painted coal tar epoxy to laminated rubber linings. The carbon
transport system must be designed to resist the abrasiveness of carbon slurry.
a. Pretreatment. The wastewater is usually pretreated before undergoing this
chemical and physical process. Skimming tanks are used to separate the wastewater
from the lighter floating objects. Grease traps are used to trap large oily or greasy
discharges. Wastewater is also preaerated before coagulation-sedimentation to
improve treatability, provide grease separation and odor control, provide grit removal,
cause flocculation, increase BOD removal, and promote uniform distribution of
suspended (nonfiltrable) residues and floating residues. Once these pretreatment
operations are complete, coagulation may be commenced.