TM 5-813-3/AFM 88-10, Vol 3
and landscape irrigation: 0.44 mgd
Total system design capacity = 2.38 + 0.44 =
2.82 mgd
2.82 mgd = 1,960 gpm = 4.36 cfs
d. Treatment system. Figure B-2 illustrates princi-
pally the design of facilities for presedimentation, fol-
lowed by lime-soda softening. Intake, pumping, meter-
ing, hydraulic profile, filters, chemical feeding, etc.,
were discussed in the preceding example and discus-
sion and calculations pertaining to them are not re-
peated here.
e. Pretreatment. Provide a circular presedimenta-
tion basin equipped for mechanical sludge removal
with a detention time of 3 hours at design flow. With a
side-water depth of 12 feet, the basin overflow rate
will be about 720 gpd per square foot, a satisfactory
value. Basin area will be approximately 3920 square
feet and diameter about 70 feet. Basin effluent should
be collected by a peripheral weir and launder. The
weir's length will be approximately 220 feet, corre-
sponding to a weir loading of about 12,700 gpd per
foot, a satisfactory value. A presedimentation basin
bypass should be provided so that plant operation can
be maintained when the presedimentation basin is out
of service.
b. Population served.
f. Flow-division structures. Refer to figure B-2.
Resident 12,600
Flow division structures, generally similar to those de-
Nonresident 1,200
scribed in the preceding design example, are required
1,200
following the presedimentation basin. These division
-- Effective population =
+ 12,600 =
3
structures insure continuity and efficiency of plant
13,000
operation under emergency conditions; i.e., when a
major unit, such as a solids contact or flocculator-
c. System, design capacity.
Capacity factor: 1.22
clarifier basin, is out of service for repair or mainte-
Design population = (1.22)(13,000) = 15,860
nance. Hydraulic design of the division structures
System design capacity, based on population =
shall be such that with all raw water pumps in oper-
(15,860)(150) = 2,379,000 gpd. Use 2.38 mgd
ation, the full flow can be carried through either half
-- Special design capacity for industrial processes
of the structures.
B-6
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