TM 5-813-3/AFM 88-10, Vol 3
CHAPTER 2
WATER TREATMENT PROCESS
2-1.
Process
selection
factors.
parts and should be avoided to the maximum practi-
cable extent. Steam or electric heating, diffusion aera-
The design of treatment facilities will be determined
tion and flow reversal have been used to overcome ice
by feasibility studies, considering all engineering, eco-
problems.
nomic, energy and environmental factors. All legiti-
(4) Disposal of screenings. Project planning must
mate alternatives will be identified and evaluated by
include provision for the disposal of debris removed by
life cycle cost analyses. Additionally, energy use be-
coarse and fine screens.
tween candidate processes will be considered. For the
b. Flow measurement. Water treatment processes,
purpose of energy consumption, only the energy pur-
e.g., chemical application, are related to the rate of
chased or procurred will be included in the usage eval-
flow of raw water, Therefore, it is essential that accu-
uation. All treatment process systems will be com-
rate flow-rate measurement equipment is provided.
pared with a basic treatment process system, which is
Pressure differential producers of the Venturi type are
that treatment process system accomplishing the re-
.
commonly used for measurement of flow in pressure
quired treatment at the lowest first cost. Pilot or labo-
conduits. An alternative selection for pressure con-
ratory analysis will be used in conjunction with pub-
duits is a magnetic flow meter if the minimum velocity
lished design data of similar existing plants to assure
through the meter will be 5 feet per second or more. A
the optimal treatment. It is the responsibility of the
Parshall flume can be used for metering in open chan-
designer to insure that the selected water treatment
nels. Flow signals from the metering device selected
plant process complies with Federal Environmental
should be transmitted to the treatment plant control
Agency, State or local regulations, whichever is more
center.
stringent.
c. Flow division. While not a treatment process,
flow division (flow splitting) is an important treatment
2-2. Preliminary
treatment.
plant feature that must be considered at an early stage
Surface waters contain fish and debris which can clog
of design. To insure continuity of operation during ma-
or damage pumps, clog pipes and cause problems in
jor maintenance, plants are frequently designed with
water treatment. Streams can contain high concentra-
parallel, identical, chemical mixing and sedimentation
tions of suspended sediment. Preliminary treatment
facilities. No rigid rules can be given for the extent of
processes are employed for removal of debris and part
duplication required because a multiplicity of factors
of the sediment load.
influence the decision. Normally, aerators are not pro-
a. Screens.
vided in duplicate. Presedimentation basins may not
(1) Coarse screens or racks. Coarse screens, often
require duplication if maintenance can be scheduled
termed bar screens or racks, must be provided to inter-
during periods of relatively low raw water sediment
cept large, suspended or floating material. Such
load or if the following plant units can tolerate a tem-
screens or racks are made of l/2-inch to 3/4-inch metal
porary sediment overload. If it is determined that pre-
bars spaced to provide 1- to 3-inch openings.
sedimentation at all times is essential for reliable plant
(2) Fine screens. Surface waters require screens or
operation, then the flow division should be made
strainers for removal of material too small to be inter-
ahead of the presedimentation basins by means of
cepted by the coarse rack, These may be basket-type,
ldentical splitting weirs arranged so that flow over
in-line strainers, manually or hydraulically cleaned by
either weir may be stopped when necessary, During
backwashing, or of the traveling type, which are
normal operation, the weirs would accomplish a pre-
cleaned by water jets. Fine-screen, clear openings
cise equal division of raw water, regardless of flow
should be approximately 3/8 inch. The velocity of the
rate, to parallel subsequent units; rapid-mix, slow-mix
water in the screen openings should be less than 2 feet
and sedimentation. The water would then be combined
per second at maximum design flow through the
and distributed to the filters. If presedimentation
screen and minimum screen submergence.
units are not provided, then the flow is commonly split
(3) Ice clogging. In northern areas screens maybe
ahead of the rapid-mix units. If a single treatment
.
clogged by frazil or anchor ice. Exposure of racks or
train is to be provided initially with the expectation of
screens to cold air favors ice formation on submerged
adding parallel units in the future, then the flow-split-
2-1
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