Civil Engineering Reference
In-Depth Information
TABLE 19-5. Typical Chlorine Dosages at Water
Treatment Plants
Chlorine Compound
Range of Doses
Calcium hypochlorite
0.5-5 mg / L
Sodium hypochlorite
0.2-2 mg / L
Chlorine gas
1-16 mg / L
Source:
See Reference 6.
mixer to create a vacuum and draw the chlorine gas directly into the process water at
the mixer. A schematic of gaseous chlorine feed system is shown in Figure 19-2.
Sodium Hypochlorite.
Sodium hypochlorite can be purchased in bulk in quantities
ranging from 55-gal drums to 4,500-gal truckloads. Bulk loads can be stored in fiber-
glass-reinforced plastic or polyethylene tanks. Sodium hypochlorite solutions degrade
over time and produce an off-gas that tends to accumulate in the piping and pumps.
Design of the feed system should include methods for venting any high points in the
delivery system.
Metering pumps are typically used to feed sodium hypochlorite solution directly
into the process water. The sodium hypochlorite is mixed with the process water with
either a mechanical mixer or induction mixer. Sodium hypochlorite solution is typically
not diluted prior to mixing to reduce scaling problems. If dilution water is required
for mixing, softened water can be used to minimize scaling. Distribution piping can
be acid washed if needed to remove scaling due to sodium hypochlorite dilution. Figure
19-3 is a schematic of a typical hypochlorite feed system.
Calcium Hypochlorite.
Under normal storage conditions, calcium hypochlorite loses
3 to 5 percent of its available chlorine in a year.
7
Calcium hypochlorite solution is
160
10
Giardia cysts
9
140
Viruses
8
120
7
100
6
80
5
4
60
3
40
2
20
1
0
0
0
0.5
1
1.5
2
2.5
3
3.5
Log Pathogen Inactivation Level
Fig. 19-1.
Free chlorine
Giardia
and virus
CT
requirements (
Source:
Reference 2)
