Civil Engineering Reference
In-Depth Information
m
x
b
Time to exhaustion (in unit days)
11.1
EBCT
(17-7)
design
0
where
b
is the bulk GAC density (lb / ft
3
), EBCT in minutes, and 11.1 is a conversion
factor. The best method for determining the operational and design parameters is to
conduct pilot, bench, or rapid small-scale column tests (RSSCTs). These latter tests
are described next.
RSSCT Testing.
The rapid small-scale column test method was developed by Critten-
den and coworkers as a method to establish the operational parameters and evaluate
treatment efficiency of GAC columns.
26-28
The key to the RSSCT test procedure is to
set up the small-scale test columns to provide hydraulic simultude between the small-
scale test unit and the ultimate full-scale unit to simulate the full-scale performance
of the bed. The hydraulic loading and EBCT of the RSSCT are selected to keep the
dimensionless groups that describe the adsorption process (i.e., the Reynolds numbers)
constant as the full-scale unit is scaled down to the RSSCT.
The RSSCT tests can be conducted in a fraction of the time required for a full-
blown pilot study. Because this approach requires only a small quantity of test water,
RSSCT tests can also be conducted in a laboratory remotely located from the appli-
cation with water shipped to the test laboratory. Full-scale design variables such as
EBCT, use rate, and breakthrough curves can be determined with a high probability
of success.
Crittenden et al.
28
describe the RSSCT test requirements under conditions of ad-
sorbing specific organic compounds, as well as the removal of DOC. The study shows
that the relationship between RSSCT performance of pilot units cannot be determined
a priori because of the complex effect of the water matrix DOC on the adsorption
capacity and kinetics of the carbon. However, for specific organic chemicals, the re-
lationship between the small- and full-scale contactors can be described by the follow-
ing dimensionless expressions:
2
EBCT
R
t
S
S S
(17-8)
EBCT
R
t
L
L
L
v
R
S
L
(17-9)
v
R
L
S
where:
S
,
L
indicators of the small and large contactors, respectively
R
the absorbent particle radius
t
the run time of the reactor
v
the hydraulic loading rate (approach velocity)
In order to determine the true relationship between the RSSCT test and full-scale
implementation, Crittenden et al.
28
recommend that at least one pilot study be con-
ducted to determine the scale-up dimensions for the particular application.
The RSSCT tests will simulate the performance of large-scale GAC contactors quite
well. Figure 17-8 shows a typical comparison of RSSCT and pilot columns for re-
