Page 203 - Water and wastewater engineering

5-6 WATER AND WASTEWATER ENGINEERING

area plus a safety factor of 10 percent and freeboard. If the primary storage vessel is located
out-of-doors uncovered, the containment structure should be capable of holding 100 percent of
the volume of the largest vessel plus precipitation from the 25-year, 24-hour rainfall (40 CFR
264.193(e)). The secondary containment structure is constructed of concrete with a chemical
resistant coating. It should not have floor drains or penetrations for personnel entry, piping, con-
trol valves, electrical conduits, or other appurtenances.
The model facility provides secondary containment for bulk and day tanks, pumping equip-
ment, safety valves, and unloading piping associated with each chemical. Incompatible chemicals
are not stored in the same secondary containment structure. Motorized remote control components
should be considered to limit the number of times personnel must enter the containment area.
Day tanks are smaller tanks that are used to supply the chemical feeders or to make dilutions of
the higher concentration solution held in the bulk storage tanks. They allow more accurate measure-
ment of smaller volumes on an hourly, shift, of daily basis. The capacity of the day tank is nominally
sufficient that it can supply the maximum day volume of solution for a 24-hour period (a “day”), so
the operator only needs to service it once a day. Day tanks should hold no more than a 30-hour sup-
ply because of chemical degradation of the diluted solution over time (GLUMRB, 2003).

Example 5-2. Determine the alum storage volume required for the following conditions:
3
Average water demand 0.18 m /s
Maximum dosage 60 mg/L as alum
Shipping time one week
Alum is a noninterruptible chemical

Solution:
a. From Appendix A, alum is shipped as a 50% solution with 100% active ingredient.

b. At the maximum dosage, the daily mass of alum used is
3
3
.
.
,
(60 mg/L )(0 18 m /s )(86 400s/d )(10 L/m 3 )(100 6 kg/mg) 933 12 or 930 kg/d
c. The mass of solution required is
933 12 kg/d
.
1 866 24 or 1 900 kg/d
,
.
,
.
0 50
d. From Table 5-1 , a noninterruptible chemical should have a 30-day supply plus 2 times
the shipping time. The mass to store is
))
2
,
(1 866 24 kg/d )(30 d ( )(7d 82 114 56 or 882 000 kg
,
.
.
,
e. Using the density of alum from Appendix A, the volume of solution to be stored is
,
82 114 56 kg 3
.
.
3 61 28 or 61 m
1 340 kg/m
,