TM 5-813-3/AFM 88-10, Vol 3
(4) Presedimentation basins. Not required.
(5) Aeration, Not required.
This design example is based on the following condi-
(6) Flow division, Normally, flow division is to be
tions: river water source; no softening required; tur-
maintained through the second stage of treatment.
bidity of raw water is variable, but rarely exceeds 1000
Provide first-stage flow division structure with two
units. See plant flow schematic, figure B-1, showing
identical rectangular weirs which will split flow into
two-stage clarification treatment.
two equal parts. Hydraulic design of division structure
a. Facility to be served. The water treatment plant
should be such that flow, corresponding to maximum
will serve a permanent installation.
pumping capacity (3.3 mgd), can be carried through
b. Population served.
either half. Provide plates or gates so that either half
of flow from division structure can be stopped. Struc-
ture should be designed to permit easy expansion in
the event plant enlargement is required at a future
Effective population = -- = 6000 = 6600
e. First-stage mixing and sedimentation.
c. System design capacity,
(1) Rapid mix. Provide two identical rapid-mix ba-
Capacity factor: 1.42 (based on effective popu-
sins, each providing a detention time of 20 seconds at
50 percent of design flow. Volume each basin is
Design population = (1.42)(6600) = 9372
(20)(0.5)(3.4) or 34 cubic feet. Provide one electric mo-
System design capacity, based on population =
tor-driven, rapid-mix unit, each basin, powered to
(9372)(150) = 1,405,800 gpd. Use 1,41 mgd
yield a G value of approximately 700 see-l at a water
Special design capacity for industrial processes,
independently determined: 0,79 mgd
Total system design capacity = 1.41 + 0.79 =
(2) Flocculation-sedimentation. Provide two me-
chanically-equipped, circular flocculator-clarifiers,
each sized for 50 percent of design flow. These units
= 1530 gpm
will normally operate in parallel.
= 3.40 cfs
-- Detention time in flocculation zone: 30 min-
Intake structure will be difficult to enlarge at a later
date, therefore its hydraulic design should be based on
at least 4.4 mgd, twice plant capacity.
d. Preliminary treatment.
(1) Rack and strainers. Provide coarse rack with 3-
inch clear opening followed by hydraulically cleaned
basket strainers with 3/8-inch clear openings ahead of
each pump. Strainers sized to provide velocity of less
than 2 feet per second through 3/8-inch openings.
(2) Pumps. Provide three pumps rated at 1.10
mgd each. This gives firm pumping capacity of 2.20
mgd. Maximum capacity is 3.30 mgd. Raw water
pumping station design should provide space and pip
ing arrangements that will permit future installation
of larger and additional pumps without major struc-
tural or piping changes.
(3) Meter. Provide Venturi-type flow meter in
pipeline from intake works to treatment plant. Meter
should be sized to cover expected flow range, mini-
mum to maximum. Flow meter should be equipped
with flushing lines and bayonet cleaning rods.