Page 202 - Water and wastewater engineering

CHEMICAL HANDLING AND STORAGE 5-5

c. From Table 5-1 , an interruptible chemical should have a 10-day supply plus 1.5 times
the shipping time of one week. The mass to store is

,
.
))
(3 575 kg/d )(10 d (1 5 )(7 d or 73 287 5 or 73 000 kg
,
.
,
r
d. Using the bulk density from Appendix A, the volume of lime to be stored is
,
.
73 287 5 kg 86 22 or 86 m 3
.
850 kg/m 3
Liquid Chemicals. The majority of storage tanks are located in the lower or basement areas
of the water treatment plants. If they are located out-of-doors, above ground storage rather than
underground storage is preferred as it allows the operator to inspect for leaks. The temperature
regimen of the tanks and the concentration of the solution should be considered carefully for
outside storage because some chemicals will crystallize. For example, a 50.7 percent commercial
liquid alum solution will crystalize at 8.3 C while a 48.8 percent alum solution has a crystal-
lization point of 15.6 C. In climates with severe cold weather, the storage tanks may have to
be heated.
The storage tank must have a liquid level indicator, vent, overflow line, access hatches, and
secondary containment capable of preventing uncontrolled discharge.
The storage tank design volume should be based on the solution strength and percent active
ingredient of the chemical. Characteristics of common liquid chemicals used in water and waste-
water treatment are given in Appendix A.
Secondary containment, that is, an additional tank that completely surrounds the primary
storage vessel, must be provided for liquids ( Figure 5-1 ). Typical secondary containment consists
of a basin with dike walls sufficiently high that the volume of the secondary containment will
hold 100 percent of the volume of the single largest primary storage vessel in the containment

Maximum fill level
Leak

H
Leak
B
C

Recommended relationship: H B C
Volume of containment 100% of storage volume 10% freeboard Too low Too close
(a) Good indoor containment (b) Poor containment
FIGURE 5-1
Secondary containment relationships. Storage tanks placed out-of-doors must either have a roof or provide additional volume
for a 25-year, 24-hour rainfall.