Civil Engineering Reference
In-Depth Information
moval efficiencies in the range of 50 to 80 percent will be adequate to achieve even
the more stringent of the proposed radon maximum contaminant levels (MCLs). This
removal efficiency can likely be achieved by using a naturally ventilated tray aeration
system or a sparging device placed in a water storage tank.
H
2
S Removal
Aeration is a practical method for H
2
S removal for total hydrogen sulfide concentra-
tions of less than 3-4 mg / L. Higher concentrations may require prolonged aeration
or an initial aeration period in the presence of an atmosphere with a high partial
pressure of CO
2
.
Only the H
2
S form of sulfur is removable via aeration; the hydrolyzed forms are
not affected. Hydrogen sulfide hydrolyzes as follows:
HS
↔
H
HS
k
1
10
(25
C)
7
2
1
HS
disassociates as follows:
HS
↔
H
S
k
1
10
(25
C)
14
2
2
The amount of H
2
S in water may be calculated by using the following equation:
100[H ]
2
%H S
2
[H ]
k
[H ]
kk
1
2
1
2
where
k
1
,
k
2
equilibrium constants
The existence of H
2
S is pH dependent, as indicated in Figure 9-16. The pH must
be below approximately 7.5 to ensure that a high percentage of the sulfur is in the
H
2
S form and, therefore, available for removal via aeration.
100
S
-2
HS
-
80
60
40
H
2
S
20
0
5
6
7
8
9
10
11
pH
Fig. 9-16.
Effect of pH on H
2
S-HS
-S
2
equilibrium
