TM 5-813-3/AFM 88-10, Vol 3
ting facilities should be provided as part of the original
proximately 3 hours. Basin depth should be in the ap-
design, with provision for Mocking flow over the weir
proximate range of 10 to 15 feet, corresponding to up-
which is to serve future units.
flow rates of 600 to 900 gallons per day (gpd) per
d. Sand traps. Sand traps are not normally required
square foot for a detention period of 3 hours. Short-cir-
at surface water treatment plants. Their principal ap-
cuiting can be minimized by careful attention to de-
plication is for the removal of fine sand from well wa-
sign of inlet and outlet arrangements. Weir loading
ter, The presence of sand in well water is usually a sign
rates should not exceed approximately 20,000 gpd per
of improper well construction or development. If sand
foot. Where presedimentation treatment is contin-
pumping cannot be stopped by reworking the well, the
uously required, duplicate basins should be provided.
sand must be removed. Otherwise, it will create seri-
Basin bypasses and overflows should also be included.
ous problems in the distribution system by clogging
2-3. Aeration.
service pipes, meters, and plumbing. Centrifugal sand
The term "aeration" refers to the processes in which
separators are an effective means of sand removal.
water is brought into contact with air for the purpose
These cyclone-separator devices are available assem-
bled from manufacturers and require no power other
of transferring volatile substances to or from water.
These volatile substances include oxygen, carbon diox-
than that supplied by the flowing water. They operate
under system pressure; therefore, repumping is not
ide, hydrogen sulfide, methane and volatile organic
necessary. Water from the well pump enters tangen-
compounds responsible for tastes and odor. Aeration is
frequently employed at plants treating ground water
tially into the upper section of the cone and centrifugal
for iron and manganese removal.
force moves the sand particles to the wall of the cone.
They then pass downwater into the outlet chamber.
a. Purpose of aeration. The principle objectives of
Sand is periodically drained to waste from this cham-
aeration are:
ber through a valve that can be manually or automat-
(1) Addition of oxygen to ground water for the
ically operated. The clarified water is discharged from
oxidation of iron and manganese. Ground waters are
normally devoid of dissolved oxygen. The oxygen add-
the top of the cone, These units are available in diam-
eters of 6, 12, 18, 24, and 30 inches. providing a capac-
ed by aeration oxidizes dissolved iron and manganese
ity range from 15 to 4500 gallons per minute (gpm)
to insoluble forms which can then be removed by sedi-
and are suitable for operation up to 150 pounds per
mentation and filtration.
square inch (psi). Pressure drop through the unit
(2) Partial removal of carbon dioxide to reduce the
ranges from 3 to 25 psi, depending on unit size and
cost of water softening by precipitation with lime and
flow rate. These separators will remove up to 99 per-
to increase pH.
(3) Reduction of the concentration of taste-and- -
cent of plus 150 mesh sand and about 90 percent of
plus 200 mesh. The units are rubber lined for protec-
odor producing substances, such as hydrogen sulfides
tion against sand erosion.
and volatile organic compounds.
e. Plain sedimentation. Plain sedimentation, also
(4) Removal of volatile organic compounds which
termed "presedimentation" is accomplished without
are suspected carcinogens, (see para 2-13b.).
b. Types of aerators. Three types of aerators are
the use of coagulating chemicals. Whether plain sedi-
mentation is essential is a judgment decision influ-
commonly employed. These are: waterfall aerators
enced by the experience of plants treating water from
exemplified by spray nozzle, cascade, and multiple-
the same source. Water derived from lakes or im-
tray units; diffusion or bubble aerators which involve
pounding reservoirs rarely requires presedimentation
passage of bubbles of compressed air through the wa-
treatment. On the other hand, water obtained from
ter; and mechanical aerators employing motor-driven
notably sediment-laden streams, such as those found
impellers alone or in combination with air injection de-
in parts of the Middle West, requires presedimenta-
vices. Of the three types, waterfall aerators, employ-
tion facilities for removal of gross sediment load prior
ing multiply trays, are the most frequently used in wa-
to additional treatment. Presedimentation treatment
ter treatment. The efficiency of multiple-tray aerators
should receive serious consideration for water ob-
can be increased by the use of enclosures and blowers
tained from rivers whose turbidity value frequently
to provide counterflow ventilation.
exceeds 1,000 units. Turbidity values of well over
c. Design criteria.
10,000 units have been observed at times on some cen-
(1) Multiple-tray, tower aerators.
(a) Multiple-tray aerators. Multiple-tray aera-
tral U.S. rivers.
(1) Plain sedimentation basins. Plain sedimenta-
tors are constructed of a series of trays, usually three
tion or presedimentation basins may be square, circu-
to nine, with perforated, slot or mesh bottoms. The wa-
lar, or rectangular and are invariably equipped with
ter first enters a distributor tray and then falls from
sludge removal mechanisms.
tray to tray, finally entering a collection basin at the
(2) Design criteria. Detention time should be ap-
base. The vertical opening between trays usually
2-2
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