Civil Engineering Reference
In-Depth Information
For convenience, the isotherm equation above can be converted into logarithmic
form:
x
1
log
log
k
log
C
(17-2)
m
n
where 1 /
n
represents the slope and
k
the intercept of the straight-line isotherm shown
in Figure 17-2. Detailed procedures for establishing the experimental conditions and
conducting and interpreting isotherm adsorption tests are presented elsewhere.
From an isotherm test, it can be determined whether a particular degree of treatment
can be achieved. The test will also show the approximate capacity of the carbon for
the application and provide a rough estimate of the carbon dosage required. Isotherm
tests also afford a convenient means of studying the effects of pH and temperature on
adsorption. Isotherms put a large amount of data into concise form for ready evaluation
and interpretation. Isotherms obtained under identical conditions by using the same
test solutions for two test carbons can quickly and conveniently be compared to reveal
the relative merits of the different carbons.
PHYSICAL CHARACTERIZATION OF ACTIVATED CARBONS
The physical properties of granular carbons that are important to performance are
resistance to breakage, particle size, and density. Resistance to breakage is measured
by using empirical tests, such as the abrasion number and hardness number tests.
Particle size is determined by a screen analysis, from which the mean particle diameter
and effective size can be calculated. Density is simply the weight per unit volume of
the carbon. Typical specifications of an activated carbon suitable for water treatment
applications are given in Table 17-1.
Carbon particle size distribution is important in filter and contactor design for GAC.
If the activated carbon is to replace anthracite coal in a dual-media filter, it should
have similar filtration characteristics and similar backwashing characteristics to the
coal. Fortunately, as shown in Table 17-2, commercial carbons are available with
characteristics that are similar to anthracite coals used as filter media. The typical size
distribution of a commercial carbon is determined by the manufacturing process. For
TABLE 17-1. Typical Activated Carbon Characteristics
Item
Units
Value for GAC
Value for PAC
Total surface area
m
2
/ g
500-1,500
500-1,500
Bulk density
lb / ft
3
(kg/m
3
)
26 (420)
26 (420)
Apparent density
g / cm
3
0.25 minimum
0.20-0.75
Effective size
mm
0.3-2.0*
0.044
Uniformity coefficient
dimensionless
2.1
—†
Iodine number
mg / g
500 minimum
500 minimum
Abrasion number
%
70 minimum
—†
Ash
%
4 maximum
—†
Moisture
%
8 maximum
8 maximum
* Depends on design.
† Not applicable for PAC.
