Civil Engineering Reference
In-Depth Information
TABLE 9-3. Typical Design Values for Tray Aeration
Design Parameter
Value
Hydraulic loading rate
14 to 28 gpm / ft
2
(34 to 68 m / h)
Number of trays
3 to 9
Tray spacing
12 to 30 in. (0.30 to 0.76 m)
Tray area
25 to 50 ft
2
per mgd of capacity
(0.61 to 1.2 m
2
per ML / d of capacity)
Total height
12 to 16 ft (3.6 to 4.8 m)
The trays must be constructed of corrosion-resistant materials, such as stainless steel,
aluminum, rot-resistant wood, concrete, or plastic. An advantage of multiple-tray aer-
ation over packed tower aeration is that it is less susceptible to clogging from iron
and manganese precipitation.
Performance
Tray aeration has lower removal efficiencies compared with packed
tower aeration. For tray aeration, percent reductions for H
2
S and CO
2
are on the order
of 50 to 80 percent.
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The removal of CO
2
by multiple-tray aerators has been approx-
imated by the following equation:
17
C
C
10
kn
n
in
where
C
n
concentration of CO
2
in water after water has passed through
n
trays, mg / L
C
in
initial concentration of CO
2
in water, mg / L
n
number of trays, including the distribution tray
k
experimental coefficient, which is dependent on ventilation, temperature, tur-
bulence, and other characteristics; varies between 0.12 to 0.16
Cascade Aeration
General Description
In cascade aeration (also known as
step aeration
), water flows
in a thin sheet over a series of steps or baffles, dropping from one layer to another. A
common configuration makes use of a concrete step structure.
Cascade aeration is commonly used for oxygenation and for the removal of carbon
dioxide and hydrogen sulfide. For groundwater systems, cascade aeration equipment
is often built into the storage reservoir. This approach is common in Florida for H
2
S
removal.
Design of Cascade Aeration
Typical design values for cascade aeration are listed
in Table 9-4. Increasing the number of steps increases the exposure time. Baffles may
be added to produce turbulence, which increases the air-to-water ratio. In cold climates,
housing of the aeration equipment is required.
Performance
Cascade aeration is less efficient compared with other aeration tech-
nologies. It has been reported to reduce CO
2
concentrations by an amount on the order
of 20 to 45 percent.
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